Battery University

BU-105: Battery Definitions

Batteries come in all shapes and sizes and there could be as many types as there are species of dog. Rather than giving batteries unique names as we do with pets, we distinguish batteries by chemistry, voltage, size, specific energy (capacity), specific power, (delivery of power) and more. A battery can operate as a single cell to power a cellular phone, or be connected in series to deliver several hundred volts to serve a UPS (uninterruptible power supply system) and the electric powertrain of a vehicle. Some batteries have high capacity but cannot deliver much power, while a starter battery has a relatively low capacity but can crank the engine with 300A.The largest battery systems are used for grid storage to store and delivery energy derived from renewable power sources such as wind turbines and solar systems. A 30-megawatt (MW) wind farm uses a storage battery of about 15MW. This is the equivalent of 20,000 starter batteries and costs about $10 million. One mega-watt feeds 50 houses or a super Walmart store. Let’s now examine each of the battery characteristics further.

Chemistry

The most common chemistries are lead, nickel and lithium. Each system requires its own charging algorithm. Unless provisions are made to change the charge setting, different battery chemistries cannot be interchanged in the same charger. Also observe the chemistry when shipping and disposing of batteries; each type has a different regulatory requirement.

Voltage

The imprinted voltage refers to the nominal battery voltage. Always observe the correct voltage when connecting to a load or a charger. Do not proceed if the voltage differs. The open circuit voltage (OCV) on a fully charged battery can be slightly higher than the nominal; the closed circuit voltage (CCV) represents the battery voltage under load or on charge and the readings will vary accordingly.

Capacity

Capacity represents the specific energy in ampere-hours (Ah). Manufacturers often overrate a battery by giving a higher Ah rating than it can provide. You can use a battery with different Ah (but correct voltage), provided the rating is high enough. Chargers have some tolerance to batteries with different Ah ratings. A larger battery will take longer to charge than a small one.

Cold cranking amps (CCA)

CCA specifies the ability to draw high load current at –18°C (0°F) on starter batteries. Different norms specify dissimilar load durations and end voltages. See Abbreviations / Conversions.

Specific energy and energy density

Specific energy or gravimetric energy density defines the battery capacity in weight (Wh/kg); energy density or volumetric energy density is given in size (Wh/l). A battery can have a high specific energy but poor specific power (load capability), as is the case in an alkaline battery. Alternatively, a battery may have a low specific energy but can deliver high specific power,

as is possible with the supercapacitor. Specific energy is synonymous with battery capacity and runtime.

Specific power

Specific power or gravimetric power density indicates the loading capability, or the amount of current the battery can provide. Batteries for power tools exhibit high specific power but have reduced specific energy (capacity). Specific power is synonymous with low internal resistance and the delivery of power.

C-rates

C-rates specify charge and discharge currents. At 1C, the battery charges and discharges at a current that is par with the marked Ah rating; at 0.5C the current is half, and at 0.1C it is one tenth. On charge, 1C charges a good battery in about one hour; 0.5C takes 2 hours and 0.1C 10 to 14 hours. Read more aboutWhat is the C-rate?

Load

A load draws energy from the battery. Internal battery resistance and depleting state-of-charge cause the voltage to drop. Physical work over time is energy measured in Watt-hours (Wh).

Watts and Volt-amps (VA)

Power drawn from a battery is expressed in watts (W) or volt-amps (VA). Watt is the real power that is being metered; VA is the apparent power that determines the wiring sizing and the circuit breakers. On a purely resistive load, watt and VA readings are alike; a reactive load such as an inductive motor or florescent light causes a drop in the power factor (pf) from the ideal one (1) to 0.7 or lower. For example, a pf of 0.7 has a power efficiency of 70.

BU-201: Lead-based Batteries

Invented by the French physician Gaston Planté in 1859, lead acid was the first rechargeable battery for commercial use. Despite its advanced age, the lead chemistry continues to be in wide use today, and there are good reasons for its popularity; lead acid is dependable and inexpensiveon cost-per-watt base. There are few other batteries that deliver bulk power as cheaply as lead acid, and this makes the battery cost-effective for automobiles, golf cars, forklifts, marine and uninterruptible power supplies (UPS).But lead acid has disadvantages; it is heavy and is less durable than nickel- and lithium-based systems when deep-cycled. A full discharge causes strain and each discharge/charge cycle permanently robs the battery of a small amount of capacity. This loss is small while the battery is in good operating condition, but the fading increases once the performance drops to half the nominal capacity. This wear-down characteristic applies to all batteries in various degrees.Depending on the depth of discharge, lead acid for deep-cycle applications provides 200 to 300 discharge/charge cycles. The primary reasons for its relatively short cycle life are grid corrosion on the positive electrode, depletion of the active material and expansion of the positive plates. These changes are most prevalent at elevated operating temperatures and high-current discharges. [see BU-804: How to Restore Lead-acid Batteries]Charging a lead acid battery is simple but the correct voltage limits must be observed, and here there are compromises. Choosing alow voltage limit shelters the battery but this produces poor performance and causes a buildup of sulfation [see BU-804b: Sulfation and How to Prevent it] on the negative plate. A high voltage limit improves performance but form grid corrosion [see BU-804a: Corrosion, Shedding and Internal Short] on the positive plate. While sulfation can be reversed if serviced in time, corrosion is permanent. [see BU-403: Charging Lead Acid]Lead acid does not lend itself to fast charging and with most types, a full charge takes 14 to16 hours. The battery must always be stored at full state-of-charge. Low charge causes sulfation, a condition that robs the battery of performance. Adding carbon on the negative electrode reduces this problem but this lowers the specific energy. [see BU-202: New Lead Acid Systems]Lead acid has a moderate life span and is not subject to memory as nickel-based systems are. Charge retention is best among rechargeable batteries. While NiCd loses approximately 40 percent of its stored energy in three months, lead acid self-discharges the same amount in one year. Lead acid work well at cold temperatures and is superior to lithium-ion when operating in subzero conditions.

Sealed Lead Acid

The first sealed, or maintenance-free, lead acid emerge in the mid-1970s. The engineers argued that the term “sealed lead acid” is a misnomer because no lead acid battery can be totally sealed. This is true and battery designers added a valve to control venting of gases during stressful charge and rapid discharge. Rather than submerging the plates in a liquid, the electrolyte is impregnated into a moistened separator, a design that resembles nickel- and lithium-bases system. This enables to operate the battery in any physical orientation without leakage.The sealed battery contains less electrolyte than the flooded type, hence the term “acid-starved.” Perhaps the most significant advantage of the sealed lead acid is the ability to

combine oxygen and hydrogen to create water and prevent water loss. The recombination occurs at a moderate pressure of 0.14 bar (2psi). The valve serves as safety vent if gases buildup during over-overcharge or stressful discharge. Repeated venting would lead to an eventual dry out. [see BU-804c: Water Loss, Acid Stratification and Surface Charge] Driven by these advantages, several types of sealed lead acid have emerged and the most common aregel, also known as valve-regulated lead acid (VRLA), and absorbent glass mat (AGM). The gel cell contains a silica type gel that suspends the electrolyte in a paste. Smaller packs with capacities of up to 30A are called SLA (sealed lead acid). Packaged in a plastic container, these batteries are used for small UPS, emergency lighting, ventilators for healthcare and wheelchairs. Because of economical price, dependable service and low maintenance, the SLA remains the preferred choice for biomedical and healthcare in hospitals and retirement homes. The VRLA is the larger gel variant used as power backup for cellular repeater towers, Internet hubs, banks, hospitals, airports and other sites.The AGM is a newer design and suspends the electrolytein aspecially designed glass mat. This offers several advantages to lead acid systems, including faster charging and instant high load currents on demand. AGM works best as a mid-range battery with capacities of 30 to 100Ah and is less suited for large systems, such as UPS. Typical uses are starter batter for motorcycles, start-stop function [see BU-801a: How to Rate Battery Runtime] for micro-hybrid cars, as well as marine and RV that need some cycling.With cycling and age, the capacity of AGM fades gradually; gel, on the other hand, has a dome shaped performance curve and stays in the high performance range longer but then drops suddenly towards the end of life. AGM is more expensive than flooded, but is cheaper than gel.(Gel would be too expensive for start/stop use in cars.) [see BU-201a: Absorbent Glass Mat (AGM)]Unlike the flooded, the sealed lead acid battery is designed with a low over-voltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging causes gassing, venting and subsequent water depletion and dry out. [see BU-804c: Water Loss, Acid Stratification and Surface Charge] Consequently, gel, and in part also AGM, cannot be charged to their full potential and the charge voltage limit must be set lower than that of a flooded. The float charge on full charge must also be lowered. In respect to charging, the gel and AGM are no direct replacements to the flooded type. If no designated charger is available with lower voltage settings, disconnect the charger after 24 hours of charge. This prevents gassing due to a float voltage that is set too high. [see BU-403: Charging Lead Acid]The optimum operating temperature for a VRLA battery is 25°C (77°F); every 8°C (15°F) rise above this temperature threshold cuts battery life in half. [see BU-806a: How Heat and Loading affect Battery Life] Lead acid batteries are rated at a 5-hour (0.2C) and 20-hour (0.05C) discharge. The battery performs best when discharged slowly and the capacity readings are notably higher at a slow discharge rate. Lead acid can, however, deliver high pulse currents of several C if done for only a few seconds. This makes the lead acid well suited as a starter battery, also known as starter-light-ignition (SLI). The high lead content and the sulfuric acid make lead acid environmentally unfriendly.The following paragraphs look at the different architectures within the lead acid family and explain why one battery type does not fit all.

Starter and Deep-cycle Batteries

The starter battery is designed to crank an engine with a momentary high power burst; the deep-cycle battery, on the other hand, is built to provide continuous power for a wheelchair or golf car. From the outside, both batteries look alike; however, there are fundamental differences in design. While the starter battery is made for high peak power and does not like deep cycling, the deep-cycle battery has a moderate power output but permits cycling. Let’s examine the architectural difference between these batteries further.Starter batteries have a CCA rating imprinted in amperes; CCA refers to cold cranking amps, which represents the amount of current a battery can deliver at cold temperature. SAE J537 specifies 30 seconds of discharge at –18°C (0°F) at the rated CCA ampere without dropping below 7.2 volts. (SAE stands for Society of Automotive Engineers.)Starter batteries have a very low internal resistance, and the manufacturer achieves this by adding extra plates for maximum surface area (Figure 1). The plates are thin and the lead is applied in a sponge-like form that has the appearance of fine foam. This method extends the surface area of the plates to achieve low resistance and maximum power. Plate thickness isless important here because the discharge is short and the battery is recharged while driving;the emphasis is on power rather than capacity.

Figure 1: Starter batteryThe starter battery has many thin plates in parallel to achieve low resistance with high surface area. The starter battery does not allow deep cycling.Courtesy of Cadex

Deep-cycle lead acid batteries for golf cars, scooters and wheelchairs are built for maximum capacity and high cycle count. The manufacturer achieves this by making the lead plates thick (Figure 2). Although the battery is designed for cycling, full discharges still induce stress, and the cycle count depends on the depth-of-discharge (DoD). Deep-cycle batteries are marked in Ah or minute of runtime.

Figure 2: Deep-cycle batteryThe deep-cycle battery has thick plates for improved cycling abilities. The deep-cycle battery generally allows about 300 cycles.Courtesy of Cadex

A starter battery cannot be swapped with a deep-cycle battery and vice versa. While an inventive senior may be tempted to install a starter battery instead of the more expensive deep-cycle on his wheelchair to save money, the starter battery won’t last because the thin sponge-like plates would quickly dissolve with repeated deep cycling. There are combination starter/deep-cycle batteries available for trucks, buses, public safety and military vehicles, but these units are big and heavy. As a simple guideline, the heavier the battery is, the more lead it contains, and the longer it will last. Table 3 compares the typical life of starter and deep-cycle batteries when deep-cycled.Depth of Discharge Starter Battery Deep-cycle Battery100% 12–15 cycles 150–200 cycles

50%30%

100–120 cycles130–150 cycles

400–500 cycles1,000 and more cycles

Table 3: Cycle performance of starter and deep-cycle batteries. A discharge of 100% refers to a full discharge; 50% is half and 30% is a moderate discharge with 70% remaining.Lead is toxic and environmentalists would like to replace the lead acid battery with another chemistry. Europe succeeded to keep nickel-cadmium batteries out of consumer products, and authorities try to do it with the starter battery. The choices are NiMH and lithium-ion, but at a price tag of $3,000 for Li-ion, this will not fly. In addition, Li-ion has poor performance at sub-freezing temperature. Regulators hope that advancements in the electric powertrain will lower the cost, but such a large price reduction to match the low-cost lead acid may not be possible. Lead acid will continue to be the battery of choice to crank the engines.Table 4 spells out the advantages and limitations of common lead acid batteries in use today.

Advantages

Inexpensive and simple to manufacture; low cost per watt-hourLow self-discharge; lowest among rechargeable batteriesHigh specific power, capable of high discharge currentsGood low and high temperature performance

Limitations

Low specific energy; poor weight-to-energy ratioSlow charge; fully saturated charge takes 14 hoursMust be stored in charged condition to prevent sulfationLimited cycle life; repeated deep-cycling reduces battery lifeFlooded version requires wateringTransportation restrictions on the flooded typeNot environmentally friendly

Table 4: Advantages and limitations of lead acid batteries. Dry systems have advantages over flooded but are less rugged.

BU-202: New Lead Acid Systems

Discover advancements made in lead acid batteries and how they benefit industry.Lead acid batteries continue to hold a leading position, especially in wheeled mobility and stationary applications. This strong market appeal entices manufacturers to explore ways to make the batteries better. Improvements have been made and some claims are so promising that one questions the trustworthiness. It is no secret that researchers prefer publishing the positive attributes while keeping the negatives under wraps. The following information on lead acid developments was obtained from available printed resources at the time of writing.

Firefly Energy

The composite plate material of the Firefly Energy battery is based on a lead-acid variant that is lighter, longer living, and has higher active material utilization than current lead acid systems. It is also one of the few lead-acid batteries that can operate for extended time in partial-states-of-charge. The battery includes carbon-foam electrodes for the negative plates, which gives it a performance that is comparable to NiMH but at lower manufacturing costs. Firefly Energy was a spin-off of Caterpillar and in 2010 went into bankruptcy. The company was revived under separate ownership. Today, Firefly International Energy manufactures the Oasis line of batteries in limited quantities in the US.

Altraverda Bipolar

Similar to the Firefly Energy battery, the Altraverda battery is based on lead. It uses a proprietary titanium sub-oxide ceramic structure, called Ebonex®, for the grid and an AGM separator. The un-pasted plate contains Ebonex® particles in a polymer matrix that holds a thin lead alloy foil on the external surfaces. With 50–60Wh/kg, the specific energy is about one-third larger than regular lead acid and is comparable with NiCd. Based in the UK, Altraverda works with East Penn in the USA, and the battery is well suited for higher voltage applications.

Axion Power

The Axion Power e3 Supercell is a hybrid battery/ultracapacitor in which the positive electrode consists of standard lead dioxide and the negative electrode is activated carbon, while maintaining an assembly process that is similar to lead acid. The Axion Power battery offers faster recharge times and longer cycle life on repeated deep discharges than what is possible with regular lead acid systems. This opens the door for the start-stop application in micro-hybrid cars. The lead-carbon combination of the Axion Power battery lowers the lead content on the negative plate, which results in a weight reduction of 30 percent compared to a regular lead acid. This, however, also lowers the specific energy to 15–25Wh/kg instead of 30–50Wh/kg, which a regular lead acid battery normally provides.

CSIRO Ultrabattery

The CSIRO Ultrabattery combines an asymmetric ultracapacitor and a lead acid battery in each cell. The capacitor enhances the power and lifetime of the battery by acting as a buffer during charging and discharging, prolonging the lifetime by a factor of four over customary lead acid systems and producing 50 percent more power. The manufacturer also claims that the battery is 70 percent cheaper to produce than current hybrid electric vehicle (HEV)

batteries. CSIRO batteries are undergoing road trials in a Honda Insight HEV and show good results. Furukawa Battery in Japan licensed the technology. The CSIRO battery is also being tested for start-stop applications in micro-hybrid cars to replace the lead acid starter battery. This battery promises extended life when exposed to frequent start-stop conditions and is able to take a fast charge.

EEStor

This is the mystery battery/ultracapacitor combination that receives much media attention. The battery is based on a modified barium titanate ceramic powder and claims a specific energy of up to 280Wh/kg, higher than lithium-ion. The company is very secretive about their invention and releases only limited information. Some of their astonishing claims are: One-tenth of the weight of a NiMH battery in a hybrid application, no deep-cycle wear-down, three- to six-minute charge time, no hazardous material, similar manufacturing costs to lead acid, and a self-discharge that is only 0.02 percent per month, a fraction of that of lead acid and Li-ion.

BU-203: Nickel-based Batteries

The following section describes nickel-based batteries, and we begin with nickel-cadmium (NiCd), an older chemistry for which extensive data is available. Much of these characteristics also apply to nickel-metal-hydride (NiMH), as these two systems are close cousins. The toxicity of NiCd is limiting this solid and robust battery to specialty applications.

Nickel-cadmium (NiCd)

The nickel-cadmium battery, invented by Waldmar Jungner in 1899, offered several advantages over lead acid, but the materials were expensive and the early use was restricted. Developments lagged until 1932 when attempts were made to deposit the active materials inside a porous nickel-plated electrode. Further improvements occurred in 1947 by trying to absorb the gases generated during charge. This led to the modern sealed NiCd battery in use today.For many years, NiCd was the preferred battery choice for two-way radios, emergency medical equipment, professional video cameras and power tools. In the late 1980s, the ultra-high-capacity NiCd rocked the world with capacities that were up to 60 percent higher than the standard NiCd. This was done by packing more active material into the cell, but the gain was met with the side effects of higher internal resistance and shorter cycle.The standard NiCd remains one of the most rugged and forgiving batteries but needs proper care to attain longevity. It is perhaps for this reason that NiCd is the favorite battery of many engineers. Table 1 lists the advantages and limitations of the standard NiCd.

Advantages Fast and simple charging even after prolonged storageHigh number of charge/discharge cycles; provides over 1,000 charge/discharge cycles with proper maintenanceGood load performance; rugged and forgiving if abusedLong shelf life; can be stored in a discharged stateSimple storage and transportation; not subject to regulatory

controlGood low-temperature performanceEconomically priced; NiCd is the lowest in terms of cost per cycleAvailable in a wide range of sizes and performance options

Limitations

Relatively low specific energy compared with newer systemsMemory effect; needs periodic full dischargesEnvironmentally unfriendly; cadmium is a toxic metal and cannot be disposed of in landfillsHigh self-discharge; needs recharging after storage

Table 1: Advantages and limitations of NiCd batteries

Nickel-metal-hydride (NiMH)

Research of nickel-metal-hydride started in 1967; however, instabilities with the metal-hydride led scientists to develop the nickel-hydrogen battery (NiH) instead. Today, NiH is mainly used in satellites. New hydride alloys discovered in the 1980s offered better stability and the development of NiMH advanced in earnest. Today, NiMH provides 40 percent higher specific energy than a standard NiCd, but the decisive advantage is the absence of toxic metals.The advancements of NiMH are impressive. Since 1991, the specific energy has doubled and the life span extended. The hype of lithium-ion may have dampened the enthusiasm for NiMH a bit but not to the point to turn HEV makers away from this proven technology. Batteries for the electric powertrain in vehicles must meet some of the most demanding challenges, and NiMH has two major advantages over Li-ion here. These are price and safety. Makers of hybrid vehicles claim that NiMH costs one-third of an equivalent Li-ion system, and the relaxation on safety provisions contribute in part to this price reduction.Nickel-metal-hydride is not without drawbacks. For one, it has a lower specific energy than Li-ion, and this is especially true with NiMH for the electric powertrain. The reader should be reminded that NiMH and Li-ion with high energy densities are reserved for consumer products; they would not be robust enough for the hybrid and electric vehicles. NiMH and Li-ion for the electric powertrain have roughly one-third less capacity than consumer batteries.NiMH also has high self-discharge and loses about 20 percent of its capacity within the first 24 hours, and 10 percent per month thereafter. Modifying the hydride materials lowers the self-discharge and reduces corrosion of the alloy, but this decreases the specific energy. Batteries for the electric powertrain make use of this modification to achieve the needed robustness and life span.There are strong opinions and preferences between battery chemistries, and some experts say that NiMH will serve as an interim solution to the more promising lithium systems. There are many hurdles surrounding Li-ion also and these are cost and safety. Li-ion cells are not offered to the public in AA, AAA and other popular sizes in part because of safety. Even if they were made available, Li-ion has a higher voltage compared to nickel-based batteries.

Consumer Application

NiMH has become one of the most readily available and low-cost rechargeable batteries for portable devices. NiMH is non-toxic and offers a higher specific energy than NiCd. Battery

manufacturers, such as Sanyo, Energizer, Duracell and GP, have recognized the need for a durable and low-cost rechargeable battery for consumers and offer NiMH in AA and AAA sizes. The battery manufacturers hope to persuade buyers to switch to rechargeable batteries and reduce the environmental impact of throwaway primary cells.The NiMH battery for the consumer market can be viewed as an alternative to the failed reusable alkaline that appeared in the 1990s. Limited cycle life and poor loading characteristics hindered its success.What is of ongoing concern to the consumer using rechargeable batteries is the high self-discharge, and NiMH behaves like a leaky basketball or bicycle tire. A flashlight or portable entertainment device with a NiMH battery gets “flat” when put away for only a few weeks. Having to recharge the device before each use does not sit well. The Eneloop NiMH by Sanyo has reduced the self-discharge by a factor of six. This means that you can store the charged battery six times longer than a regular NiMH before a recharge becomes necessary. The drawback is a slightly lower specific energy compared to a regular NiMH. Other NiMH manufacturers such as ReCyko by GP claim similar results.Table 2 summarizes the advantages and limitations of industrial-grade NiMH. The table does not include the Eneloop and equivalent consumer brands.

Advantages

30–40 percent higher capacity than a standard NiCdLess prone to memory than NiCdSimple storage and transportation; not subject to regulatory controlEnvironmentally friendly; contains only mild toxinsNickel content makes recycling profitable

Limitations

Limited service life; deep discharge reduces service lifeRequires complex charge algorithmDoes not absorb overcharge well; trickle charge must be kept lowGenerates heat during fast-charge and high-load dischargeHigh self-discharge; chemical additives reduce self-discharge at the expense of capacityPerformance degrades if stored at elevated temperatures; should be stored in a cool place at about 40 percent state-of-charge

Table 2: Advantages and limitations of NiMH batteries

BU-204: Lithium-based Batteries

Pioneer work with the lithium battery began in 1912 under G.N. Lewis, but it was not until the early 1970s that the first non-rechargeable lithium batteries became commercially available. Attempts to develop rechargeable lithium batteries followed in the 1980s but the endeavor failed because of instabilities in the metallic lithium used as anode material.Lithium is the lightest of all metals, has the greatest electrochemical potential and provides the largest specific energy per weight. Rechargeable batteries with lithium metal on the anode (negative electrodes)* could provide extraordinarily high energy densities; however, it was discovered in the mid 1980s that cycling produced unwanted dendrites on the anode. These growth particles penetrate the separator and cause an electrical short. When this occurs, the cell temperature rises quickly and approaches the melting point of lithium, causing thermal runaway, also known as “venting with flame.” A large number of rechargeable metallic lithium batteries sent to Japan were recalled in 1991 after a battery in a mobile phone released flaming gases and inflicted burns to a man’s face.The inherent instability of lithium metal, especially during charging, shifted research to a non-metallic solution using lithium ions. Although lower in specific energy than lithium-metal, Li-ion is safe, provided cell manufacturers and battery packers follow safety measures in keeping voltage and currents to secure levels. Read more about Protection Circuits. In 1991, Sony commercialized the first Li-ion battery, and today this chemistry has become the most promising and fastest growing on the market. Meanwhile, research continues to develop a safe metallic lithium battery.The specific energy of Li-ion is twice that of NiCd, and the high nominal cell voltage of 3.60V as compared to 1.20V for nickel systems contributes to this gain. Improvements in the active materials of the electrode have the potential of further increases in energy density. The load characteristics are good, and the flat discharge curve offers effective utilization of the stored energy in a desirable voltage spectrum of 3.70 to 2.80V/cell. Nickel-based batteries also have a flat discharge curve that ranges from 1.25 to 1.0V/cell.In 1994, the cost to manufacture Li-ion in the 18650** cylindrical cell with a capacity of 1,100mAh was more than $10. In 2001, the price dropped to $2 and the capacity rose to 1,900mAh. Today, high energy-dense 18650 cells deliver over 3,000mAh and the costs have dropped further. Cost reduction, increase in specific energy and the absence of toxic material paved the road to make Li-ion the universally accepted battery for portable application, first in the consumer industry and now increasingly also in heavy industry, including electric powertrains for vehicles.In 2009, roughly 38 percent of all batteries by revenue were Li-ion. Li-ion is a low-maintenance battery, an advantage many other chemistries cannot claim. The battery has no memory and does not need exercising (deliberate full discharge) to keep in shape. Self-discharge is less than half that of nickel-based systems. This makes Li-ion well suited for fuel gauge applications. The nominal cell voltage of 3.60V can directly power cell phones and digital cameras, offering simplifications and cost reductions over multi-cell designs. The drawbacks are the need for protection circuits to prevent abuse, as well as high price.

Types of Lithium-ion Batteries

Similar to the lead- and nickel-based architecture, lithium-ion uses a cathode (positive electrode), an anode (negative electrode) and electrolyte as conductor. The cathode is a metal oxide and the anode consists of porous carbon. During discharge, the ions flow from

the anode to the cathode through the electrolyte and separator; charge reverses the direction and the ions flow from the cathode to the anode. Figure 1 illustrates the process.

Figure 1: Ion flowin lithium-ion battery.When the cell charges and discharges,ions shuttle between cathode (positive electrode) and anode (negative electrode). On discharge, the anode undergoes oxidation,or loss of electrons,and the cathode seesa reduction, or a gainof electrons. Charge reverses the movement.

Li-ion batteries come in many varieties but all have one thing in common — the catchword “lithium-ion.” Although strikingly similar at first glance, these batteries vary in performance, and the choice of cathode materials gives them their unique personality.Common cathode materials are Lithium Cobalt Oxide (or Lithium Cobaltate), Lithium Manganese Oxide(also known as spinel or Lithium Manganate), Lithium Iron Phosphate, as well as Lithium Nickel Manganese Cobalt (or NMC)*** and Lithium Nickel Cobalt Aluminum Oxide (or NCA). All these materials possess a theoretical specific energy with given limits. (Lithium-ion has a theoretically capacity of about 2,000kWh. This is more than 10 times the specific energy of a commercial Li-ion battery.)Sony’s original lithium-ion battery used coke as the anode (coal product). Since 1997, most Li-ion manufacturers, including Sony, have shifted to graphite to attain a flatter discharge curve. Graphite is a form of carbon that is also used in the lead pencil. It stores lithium-ion well when the battery is charged and has long-term cycle stability. Among the carbon materials, graphite is the most commonly used, followed by hard and soft carbons. Other carbons, such as carbon nanotubes, have not yet found commercial use. Figure 2-8 illustrates the voltage discharge curve of a modern Li-ion with graphite anode and the early coke version.

Figure 2: Voltage discharge curve of lithium-ionA battery should have a flat voltage curve in the usable discharge range. The modern graphite anode does this better than the early coke version.Courtesy of Cadex

Developments also occur on the anode and several additives are being tried, including silicon-based alloys. Silicon achieves a 20 to 30 percent increase in specific energy at the cost of lower load currents and reduced cycle life. Nano-structured lithium-titanate as an anode additive shows promising cycle life, good load capabilities, excellent low-temperature performance and superior safety, but the specific energy is low.Mixing cathode and anode material allows manufacturers to strengthen intrinsic qualities; however, enhancing one attribute may compromise another. Battery makers can, for example, optimize the specific energy (capacity) to achieve extended runtime, increase the specific power for improved current loading, extend service life for better longevity, and enhance safety to endure environmental stresses. But there are drawbacks. A higher capacity reduces the current loading; optimizing current loading lowers the specific energy; and ruggedizing a cell for long life and improved safety increases battery size and adds to cost due to a thicker separator. The separator is said to be the most expensive part of a battery.Manufacturers can attain a high specific energy and low cost relatively easily by adding nickel in lieu of cobalt, but this makes the cell less stable. While a start-up company may focus on high specific energy to gain quick market acceptance, safety and durability cannot be compromised. Reputable manufacturers place high integrity on safety and longevity.Table 3 summarizes the characteristics of Li-ion with different cathode material. The table limits the chemistries to the four most commonly used lithium-ion systems and applies the short form to describe them. The batteries are Li-cobalt, Li-manganese, Li-phosphate and NMC. NMC stands for nickel-manganese-cobalt, a chemistry that is relatively new and can be tailored for applications needing either high capacity or high loading capabilities. Lithium-ion-polymer is not mentioned as this is not a unique chemistry and only differs in construction. Li-polymer can be made in various chemistries and the most widely used format is Li-cobalt.

Specifications Li-cobaltLiCoO2 (LCO)

Li-manganeseLiMn2O4 (LMO)

Li-phosphateLiFePO4 (LFP)

NMC1

LiNiMnCoO2

Voltage 3.60V 3.80V 3.30V 3.60/3.70V

Charge limit 4.20V 4.20V 3.60V 4.20V

Cycle life2 500–1,000 500–1,000 1,000–2,000 1,000–2,000

Operating temperature Average Average Good Good

Specific energy 150–190Wh/kg 100–135Wh/kg 90–120Wh/kg 140-180Wh/kg

Loading (C-Rate) 1C 10C, 40C pulse 35C continuous 10C

Safety

Average. Requires protection circuit and cell balancing of multi cell pack. Requirements for small formats with 1 or 2 cells can be relaxed

Very safe, needs cell balancing and V protection.

Safer than Li-cobalt. Needs cell balancing and protection.

Thermal. runaway3

150°C(302°F)

250°C(482°F)

270°C(518°F)

210°C(410°F)

Cost Raw material high

Moli Energy, NEC, Hitachi, Samsung

High High

In use since 1994 1996 1999 2003

Researchers, manufacturers

Sony, Sanyo, GS Yuasa, LG Chem Samsung Hitachi, Toshiba

Hitachi, Samsung, Sanyo, GS Yuasa, LG Chem, ToshibaMoli Energy, NEC

A123, Valence, GS Yuasa, BYD, JCI/Saft, Lishen

Sony, Sanyo, LG Chem, GS Yuasa, Hitachi Samsung

Notes

Very high specific energy, limited power; cell phones, laptops

High power, good to high specific energy; power tools, medical, EVs

High power, averagespecific energy, safest lithium-based battery

Very high specific energy, high power; tools, medical, EVs

Table 3: Characteristics of the four most commonly used lithium-ion batteriesSpecific energy refers to capacity (energy storage); specific power denotes load capability.

1 NMC, NCM, CMN, CNM, MNC and MCN are basically the same. The stoichiometry is usually Li[Ni(1/3)Co(1/3)Mn(1/3)]O2. The order of Ni, Mn and Co does not matter much.2 Application and environment govern cycle life; the numbers do not always apply correctly.3 A fully charged battery raises the thermal runaway temperature, a partial charge lowers it.

Never was the competition to find an ideal battery more intense than today. Manufacturers see new applications for automotive propulsion systems, as well as stationary and grid storage, also knows as load leveling. At time of writing, the battery industry speculates that the Li-manganese and/or NMC might be the winners for the electric powertrain.Industry’s experience has mostly been in portable applications, and the long-term suitability of batteries for automotive use is still unknown. A clear assessment of the cycle life, performance and long-term operating cost will only be known after having gone through a

few generations of batteries for vehicles with electric powertrains, and more is known about the customers’ behavior and climate conditions under which the batteries are exposed. Table 4 summarizes the advantages and limitations of Li-ion.

AdvantagesHigh energy densityRelatively low self-discharge; less than half that of NiCd and NiMHLow maintenance. No periodic discharge is needed; no memory.

Limitations

Requires protection circuit to limit voltage and currentSubject to aging, even if not in use (aging occurs with all batteries and modern Li-ion systems have a similar life span to other chemistries)Transportation regulations when shipping in larger quantities

Table 4: Advantages and limitations of Li-ion batteries * When consuming power, as in a diode, vacuum tube or a battery on charge, the anode is positive; when withdrawing power, as in a battery on discharge, the anode becomes negative.** Standard of a cylindrical Li-ion cell developed in the mid 1990s; measures 18mm in diameter and 65mm in length; commonly used for laptops. Read more about Battery Formats.*** Some Lithium Nickel Manganese Cobalt Oxide systems go by designation of NCM, CMN, CNM, MNC and MCN. The systems are basically the same.

BU-212: Experimental Rechargeable Batteries

Experimental batteries live mostly in sheltered laboratories, communicating to the outside world through rosy reports generated for investors. Some systems show good potential, but many are years away from becoming commercially viable. Others disappear from the scene and die gracefully in the lab without hearing of their passing. Below are the most promising experimental batteries worth mentioning in alphabetical order.

Lithium-air (Li-air)

Li-air batteries borrow the idea from zinc-air and the fuel cell in that they breathe air. The battery uses a catalytic air cathode that supplies oxygen, an electrolyte and a lithium anode. Scientists anticipate an energy storage potential that is 5 to 10 times larger than that of Li-ion but speculate it will take one to two decades before the technology can be commercialized. Depending on materials used, Li-ion-air will produce voltages in between 1.7 and 3.2V/cell. IBM, Excellatron, Liox Power, Lithion-Yardney, Poly Plus, Rayovac and others are developing the technology. The theoretical specific energy of lithium-air is 13kWh/kg; aluminum-air has similar qualities, with an 8kWh/kg theoretical specific energy.

Lithium-metal (Li-metal)

Most lithium-metal batteries are non-rechargeable. Moli Energy of Vancouver was first to mass-produce a rechargeable Li-metal battery for mobile phones, but occasional shorts from lithium dendrites caused thermal runaway conditions and the batteries were recalled in 1989. Li-metal has a very high specific energy. In 2010, a trial Li-metal-polymer with a capacity of 300Wh/kg was tested in an experimental electric vehicle (this compares to 80Wh/kg for the Nissan Leaf). DBM Energy, the German manufacturer of this battery, claims 2,500 cycles, short charge times and competitive pricing if the battery were mass-produced. Safety remains a major issue.

Lithium-sulfur (Li-S)

By virtue of the low atomic weight of lithium and the moderate weight of sulfur, lithium-sulfur batteries offer a very high specific energy of 550Wh/kg, about three times that of Li-ion, and a specific power potential of 2,500W/kg. During discharge, the lithium dissolves from the anode surface, and reverses itself when charging by plating itself back onto the anode. Li-S has good cold temperature discharge characteristics and can be recharged at –60°C (–76°F). The challenges are limited cycle life of only 40 to 50 charges/discharges and poor stability at high temperature. Since 2007, Stanford engineers have been experimenting with nanowire and this technology offers promise. Li-S has a cell voltage of 2.10V and is environmentally friendly. Sulfur as the main ingredient is abundantly available.

Silicon-carbon Nanocomposite Anodes for Li-ion

Researchers have developed a new high-performance anode structure for lithium-ion batteries based on silicon-carbon nanocomposite materials. The material contains rigid and robust silicon spheres with irregular channels to promote the access of lithium ions into the particle mass. With graphite anodes, researchers have achieved stable performance and capacity gains of five times that of regular Li-ion. Manufacturing is said to be simple and low-cost, and the battery is safe and broadly applicable. However, the cycle life is limited due to structural problems when inserting and extracting lithium-ion at high volume.

BU-301a: Types of Battery Cells

Compare the pros and cons of the cylindrical cell, button cell, prismatic cell and pouch

Early batteries were in jars, but mass production changed the packaging to the cylindrical design. The year 1896 pioneered the large F cell for lanterns; the D cell followed in 1898, the C cell in 1900, and the popular AA was introduced in 1907. [BU-301: A look at Old and New Battery Packaging] Design criteria and cost considerations required new battery formats that offer distinct advantages over the cylindrical design.

Cylindrical Cell

The cylindrical cell continues to be one of the most widely used packaging styles for primary and secondary batteries. The advantages are ease of manufacture and good mechanical stability. The tubular cylinder has the ability to withstand internal pressures without deforming. Figure 1 shows a cross section of a cell.

Figure 1: Cross section of alithium-ion cylindrical cellThe cylindrical cell design has good cycling ability, offers a long calendar life, is economical but is heavy and has low packaging density due to space cavities.Courtesy of Sanyo

Typical applications for the cylindrical cell are power tools, medical instruments and laptops. Nickel-cadmium offers the largest variety of cell choices, and some popular formats have spilled over to nickel-metal-hydride. To allow variations within a given size, manufacturers use fractural cell length, such as half and three-quarter formats.The established standards for nickel-based batteries did not catch on with lithium-ion and the chemistry has established its own formats. One of the most popular cell packages is the 18650, as illustrated in Figure 2. Eighteen denotes the diameter and 65 is the length of the cell in millimeters. The Li-manganese version 18650 has a capacity of 1,200–1,500mAh; the Li-cobalt version is 2,400–3,000mAh. The larger 26650 cells have a diameter of 26mm with a length of 65mm and deliver about 3,200mAh in the manganese version. This cell format is used in power tools and some hybrid vehicles.

Figure 2: Popular 18650 lithium-ion cellThe metallic cylinder measure 18mm in diameter and 65mm the length. The larger 26650 cell measures 26mm in diameter.Courtesy of Cadex

Lead acid batteries come in flooded and dry formats; portable versions are packaged in a prismatic design resembling a rectangular box made of plastic. Some lead acid systems also use the cylindrical design by adapting the winding technique, and the Hawker Cyclone is in this format. It offers improved cell stability, higher discharge currents and better temperature stability than the conventional prismatic design.Cylindrical cells include a venting mechanism that releases excess gases when pressure builds up. The more simplistic design utilizes a membrane seal that ruptures under high pressure. Leakage and subsequent dry-out may occur when the membrane breaks. The re-sealable vents with a spring-loaded valve are the preferred design. Cylindrical cells make inefficient use of space, but the air cavities that result with side-by-side placement can be used for air-cooling.

Button Cell

Smaller devices required a more compact cell design, and in the 1980s the button cell met this need. The desired voltage was achieved by stacking the cells into a tube. Early cordless telephones, medical devices and security wands at airports used these batteries.Although small and inexpensive to build, the stacked button cell fell out of favor, and newer designs reverted to more conventional battery configurations. A drawback of the button cell is swelling if charged too rapidly. Button cells have no safety vent and can only be charged at a 10- to 16-hour charge. However, newer designs claim rapid charge capability. Most button cells in use today are non-rechargeable and can be found in medical implants, watches, hearing aids, car keys and memory backup. Figure 3 illustrates the button cells with accompanying cross section.

Figure 3: Button cells

Button cells, also known as coin cells, offer small size and ease of stacking but do not allow fast charging. Most commercial button cells are non-rechargeable.Courtesy of Sanyo and Panasonic

Prismatic Cell

Introduced in the early 1990s, the prismatic cell satisfies the demand for thinner sizes and lower manufacturing costs. Wrapped in elegant packages resembling a box of chewing gum or a small chocolate bar, prismatic cells make optimal use of space by using the layered approach. These cells are predominantly found in mobile phones with lithium-ion. No universal format exists and each manufacturer designs its own. If the housing design allows a few millimeters extra in a cell phone or laptop, the manufacturer designs a new pack for the sake of higher capacity. High volume justifies this move.Prismatic cells are also making critical inroads into larger formats. Packaged inwelded aluminum housings, the cells deliver capacities of 20 to 30Ah and are primarily used for electric powertrains in hybrid and electric vehicles. Figure 4shows the prismatic cell.

Figure 4: Cross sectionof a prismatic cellThe prismatic cell improves space utilization and allows flexible design but it can be more expensive to manufacture, less efficient in thermal management and have a shorter cycle life than the cylindrical design.Courtesy of Polystor Corporation

The prismatic cell requires a slightly thicker wall size to compensate for the decreased mechanical stability from the cylindrical design, resulting in a small capacity drop. Optimizing use of space makes up this loss. Prismatic cells for portable devices range from 400mAh to 2,000mAh.

Pouch Cell

In 1995, the pouch cell surprised the battery world with a radical new design. Rather than using a metallic cylinder and glass-to-metal electrical feed-through for insulation, conductive foil tabs welded to the electrode and sealed to the pouch carry the positive and negative terminals to the outside. Figure 5 illustrates such a pouch cell.

Figure 5: The pouch cellThe pouch cell offers a simple, flexible and lightweight solution to battery design. Exposure to high humidity and hot temperature can shorten service life.Courtesy of Cadex

The pouch cell makes the most efficient use of space and achieves a 90 to 95 percent packaging efficiency, the highest among battery packs. Eliminating the metal enclosure reduces weight but the cell needs some alternative support in the battery compartment. The pouch pack finds applications in consumer, military, as well as automotive applications. No standardized pouch cells exist; each manufacturer builds the cells for a specific application.Pouch packs are commonly Li-polymer. Its specific energy is often lower and the cell is less durable than Li-ion in the cylindrical package. Swelling or bulging as a result of gas generation during charge and discharge is a concern. Battery manufacturers insist that these batteries do not generate excess gases that can lead to swelling. Nevertheless, excess swelling can occur and most is due to faulty manufacturing, and not misuse. Some dealers have failures due to swelling of as much as three percent on certain batches. The pressure from swelling can crack a battery cover, and in some cases break the display and electronic circuit board. Manufacturers say that an inflated cell is safe. While this may be true, do not puncture a swollen cell in close proximity to heat or fire; the escaping gases can ignite. Figure 6 shows a swelled pouch cell.

Figure 6: Swelling pouch cellSwelling can occur as part of gas generation. Battery manufacturers are at odds why this happens. A 5mm (0.2”) battery in a hard shell can grow to 8mm (0.3”), more in a foil package.Courtesy of Cadex

To prevent swelling, the manufacturer adds excess film to create a “gas bag” outside the cell. During the first charge, gases escape into the gasbag, which is then cut off and the pack resealed as part of the finishing process. Expect some swelling on subsequent charges; 8 to 10 percent over 500 cycles is normal. Provision must be made in the battery compartment to allow for expansion. It is best not to stack pouch cells but to lay them flat side by side. Prevent sharp edges that could stress the pouch cell as they expand.

Summary of Packaging Advantages and Disadvantages

A cell in a cylindrical metallic case has good cycling ability, offers a long calendar life, is economical to manufacture, but is heavy and has low packaging density.

The prismatic metallic case has improved packaging density but can be more expensive to manufacture, is less efficient in thermal management and may have a shorter cycle life.

The prismatic pouch pack is light and cost-effective to manufacture. Exposure to high humidity and hot temperature can shorten the service life. A swelling factor of 8–10 percent over 500 cycles is normal.

BU-302: Serial and Parallel Battery Configurations

Battery packs achieve the desired operating voltage by connecting several cells in series, with each cell adding to the total terminal voltage. Parallel connection attains higher capacity for increased current handling, as each cell adds to the total current handling. Some packs may have a combination of serial and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve 14.4V and two strings of these 4 cells in parallel (for a pack total of 8 cells) to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4S2P, meaning 4 cells are in series and 2 strings of these in parallel. Insulating foil between the cells prevents the conductive metallic skin from causing an electrical short. The foil also shields against heat transfer should one cell get hot.Most battery chemistries allow serial and parallel configuration. It is important to use the same battery type with equal capacity throughout and never mix different makes and sizes. A weaker cell causes an imbalance. This is especially critical in a serial configuration and a battery is only as strong as the weakest link.Imagine a chain with strong and weak links. This chain can pull a small weight but when the tension rises, the weakest link will break. The same happens when connecting cells with different capacities in a battery. The weak cells may not quit immediately but get exhausted more quickly than the strong ones when in continued use. On charge, the low cells fill up before the strong ones and get hot; on discharge the weak are empty before the strong ones and they are getting stressed.

Single Cell Applications

The single-cell design is the simplest battery pack. A typical example of this configuration is the cellular phone battery with a 3.6V lithium-ion cell. Other uses of a single cell are wall clocks, which typically use a 1.5V alkaline cell, as well as wristwatches and memory backup.The nominal cell voltage of nickel is 1.2V. There is no difference between the 1.2V and 1.25V cell; the marking is simply preference. Whereas consumer batteries use 1.2V/cell as the nominal rating, industrial, aviation and military batteries adhere to the original 1.25V. The alkaline delivers 1.5V, silver-oxide 1.6V, lead acid 2V, primary lithium 3V, Li-phosphate 3.3V and regular lithium-ion 3.6V. Li-manganese and other lithium-based systems sometimes use 3.7V. This has nothing to do with electrochemistry and these batteries can serve as 3.6V cells. Manufacturers like to use a higher voltage because low internal resistance causes less of a voltage drop with a load. Read more: Confusion with Voltages

Serial Connection

Portable equipment needing higher voltages use battery packs with two or more cells connected in series. Figure 3-8 shows a battery pack with four 1.2V nickel-based cells in series to produce 4.8V. In comparison, a four-cell lead acid string with 2V/cell will generate 8V, and four Li-ion with 3.6V/cell will give 14.40V. If you need an odd voltage of, say, 9.5 volts, you can connect five lead acid, eight NiMH/NiCd), or three Li-ion in series. The end battery voltage does not need to be exact as long as it is higher than what the device specifies. A 12V supply should work; most battery-operated devices can tolerate some over-voltage.

Figure 1: Serial connection of four NiCd or NiMH cells Adding cells in a string increases the voltage; the current remains the same.Courtesy of Cadex

A higher voltage has the advantage of keeping the conductor size small. Medium-priced cordless power tools run on 12V and 18V batteries; high-end power tools use 24V and 36V. The car industry talked about increasing the starter battery from 12V (14V) to 36V, better known as 42V, by placing 18 lead acid cells in series. Logistics of changing the electrical components and arcing problems on mechanical switches derailed the move. Early hybrid cars run on 148V batteries; newer models have batteries with 450–500V. Such a high-voltage battery requires 400 nickel-based cells in series. Li-ion cuts the cell count by three.High-voltage batteries require careful cell matching, especially when drawing heavy loads or when operating in cold temperatures. With so many cells in series, the possibility of one failing is real. One open cell would break the circuit and a shorted one would lower the overall voltage.Cell matching has always been a challenge when replacing a faulty cell in an aging pack. A new cell has a higher capacity than the others, causing an imbalance. Welded construction adds to the complexity of repair and for these reasons, battery packs are commonly replaced as a unit when one cell fails. High-voltage hybrid batteries, in which a full replacement would be prohibitive, divide the pack into blocks, each consisting of a specific number of cells. If one cell fails, the affected block is replaced.Figure 2 illustrates a battery pack in which “cell 3” produces only 0.6V instead of the full 1.2V. With depressed operating voltage, this battery reaches the end-of-discharge point sooner than a normal pack and the runtime will be severely shortened. The remaining three cells are unable to deliver their stored energy when the equipment cuts off due to low voltage. The cause of cell failure can be a partial short cell that consumes its own charge from within through elevated self-discharge, or a dry-out in which the cell has lost electrolyte by a leak or through inappropriate usage.

Figure 2: Serial connection with one faulty cellFaulty “cell 3” lowers the overall voltage from 4.8V to 4.2V, causing the equipment to cut off prematurely. The remaining good cells can no longer deliver the energy.Courtesy of Cadex

Parallel Connection

If higher currents are needed and larger cells with increased ampere-hour (Ah) ratings are not available or the design has constraints, one or more cells are connected in parallel. Most chemistries allow parallel configurations with little side effect. Figure 3 illustrates four cells connected in parallel. The voltage of the illustrated pack remains at 1.2V, but the current handling and runtime are increased fourfold.

Figure 3: Parallel connection of four cellsWith parallel cells, the current handling and runtime increases while voltage stays the same.Courtesy of Cadex

A high-resistance cell, or one that is open, is less critical in a parallel circuit than in serial configuration, however, a weak cell reduces the total load capability. It’s like an engine that fires on only three cylinders instead of all four. An electrical short, on the other hand, could be devastating because the faulty cell would drain energy from the other cells, causing a fire hazard. Most so-called shorts are of mild nature and manifest themselves in elevated self-discharge. Figure 4 illustrates a parallel configuration with one faulty cell.

Figure 4: Parallel/connection with one faulty cellA weak cell will not affect the voltage but will provide a low runtime due to reduced current handling. A shorted cell could cause excessive heat and become a fire hazard.Courtesy of Cadex

Serial/Parallel Connection

The serial/parallel configuration shown in Figure 5 allows superior design flexibility and achieves the wanted voltage and current ratings with a standard cell size. The total power is the product of voltage times current, and the four 1.2V/1000mAh cells produce 4.8Wh. Serial/parallel connections are common with lithium-ion, especially for laptop batteries, and the built-in protection circuit must monitor each cell individually. Integrated circuits (ICs) designed for various cell combinations simplify the pack design.

Figure 5: Serial/ parallel connection of four cellsThis configuration provides maximum design flexibility.Courtesy of Cadex

Simple Guidelines for Using Household Primary Batteries

Keep the battery contacts clean. A four-cell configuration has eight contacts (cell to holder and holder to next cell); each contact adds resistance.

Never mix batteries; replace all cells when weak. The overall performance is only as good as the weakest link in the chain.

Observe polarity. A reversed cell subtracts rather than adds to the cell voltage.

Remove batteries from the equipment when no longer in use to prevent leakage and corrosion. While spent alkaline normally do not leak, spent carbon-zinc discharge corrosive acid that can destroy the device.

Don’t store loose cells in a metal box. Place individual cells in small plastic bags to prevent an electrical short. Don’t carry loose cells in your pockets.

Keep batteries away from small children. If swallowed, the current flow of the battery can ulcerate the stomach wall.The battery can also rupture and cause poisoning.

Do not recharge non-rechargeable batteries; hydrogen buildup can lead to an explosion. Perform experimental charging only under supervision.

Simple Guidelines for Using Household Secondary Batteries

Observe polarity when charging a secondary cell. Reversed polarity can cause an electrical short that can lead to heat and fire if left unattended.

Remove fully charged batteries from the charger. A consumer charger may not apply the optimal trickle charge and the cell could be stressed with overcharge.

BU-401: All about Chargers

The performance and longevity of rechargeable batteries are to a large extent governed by the quality of the charger. In a price-competitive world, battery chargers are often given low priority, especially as consumer products. Choosing a quality charger is important considering the cost of battery replacement and the frustration poorly performing batteries create. The charger should serve as a quintessential master and guardian angel to protect the environment and save money by extending battery life.There are two varieties of chargers: the personal chargers and the fleet chargers. For cell phones, laptops, tablets or digital cameras, manufacturers include personal chargers. These are made for one battery type, are economically priced and perform well when used for the application intended.The fleet charger serves employees in a team environment and often has multiple bays. The original equipment manufacturer (OEM) sells the chargers and third parties also provide them. While the OEMs meet the basic requirements, third-party manufacturers often include special features, such as a discharge function for battery conditioning and calibration.Some manufacturers of third-party chargers have become creative and offer advanced charge methods for lead- and nickel-based batteries. While pulse charging may be beneficial for nickel-based batteries, this method is not recommended for Li-ion. The voltage peaks are too high and cause havoc with the protection circuit. Battery manufacturers do not support alternative charging methods and say that pulse charging could shorten the life of Li-ion.There are many valuable additional features for chargers, and hot- and cold-temperature protection is one. Below freezing, the charger lowers or prevents charge depending on the type of battery. When hot, the charger only engages when the battery temperature has normalized to a safe level. Advanced lead acid chargers offer temperature-controlled voltage thresholds, as well as adjustments to optimize charging for aging batteries.Some chargers, including Cadex chargers, feature a wake-up feature or “boost” to allow charging Li-ion batteries that have fallen asleep. This can occur if a Li-ion battery is stored in a discharged condition and self-discharge has depressed the voltage to the cut-off point. Regular chargers read these batteries as unserviceable and the packs are discarded. The boost feature applies a small charge current to activate the protection circuit to 2.20–2.90V/ cell, at which point a normal charge commences. Caution should be applied not to boost lithium-based batteries back to life that have dwelled below 1.5V/cell for a week or longer.There are two common charge methods, which are voltage limiting (VL) and current limiting (CL). Lead- and lithium-based chargers cap the voltage at a fixed threshold. When reaching the cut-off voltage, the battery begins to saturate and the current drops while receiving the remaining charge on its own timetable. Full charge detection occurs when the current drops to a designated level. [see BU-403: Charging Lead Acid ].Nickel-based batteries, on the other hand, charge with a controlled current and the voltage is allowed to fluctuate freely. This can be compared to lifting a weight with an elastic band. The slight voltage drop after a steady rise indicates a fully charged battery. The voltage drop method works well in terminating the fast charge, however, the charger should include other safeguards to respond to anomalies such as shorted or mismatched cells. Most batteries and chargers also include temperature sensors to end the charge if the temperature exceeds a safe level. [see BU-407: Charging Nickel-cadmium ].

A temperature rise is normal, especially when nickel-based batteries move towards full-charge state. When in “ready” mode, the battery must cool down to room temperature. Heat causes stress and prolonged exposure to elevated temperature shortens battery life. If the temperature remains above ambient, the charger is not performing right and the battery should be removed when “ready” appears. Extended trickle charge also inflicts damage, and nickel-based batteries should not be left in the charger for more than a few days.A lithium-based battery should not get warm in a charger and if this happens, the battery or charger might be faulty. Discontinue using the battery and/or charger. Li-ion chargers do not apply a trickle charge and disconnect the battery electrically when fully charged. If these packs are left in the charger for a few weeks, a recharge may occur when the open circuit voltage drops below a set threshold. It is not necessary to remove Li-ion from the charger when full; however, if not used for a week or more, it is better to remove them and recharge before use.A mobile phone charger draws about 2 watts on charge, while a laptop on charge takes close to 100 watts. The standby current must be low and Energy Star offers mobile phone chargers drawing 30mW or less five stars for high efficiency; 30–150mW earns four stars, 150–250mW three stars, and 250–350mW two stars. The industry average is 300mW on no-load consumption and this gets one star; higher than 500mW earns no stars. Low standby wattage is only possible with small chargers, such as the four billion mobile phone chargers that are mostly plugged in.

Simple Guidelines When Buying a Charger

Use the correct charger for the battery chemistry. Most chargers serve one chemistry only.

The battery voltage must agree with the charger. Do not charge if different.

Within reason, the Ah rating of a battery can be higher or lower than specified. A larger battery will take longer to charge than a smaller one and vice versa.

The higher the amperage of the charger, the shorter the charge time will be. There are limitations as to how fast a battery can be charged.

Accurate charge termination and correct trickle charge prolong battery life.

When fully saturated, a lead acid charger should switch to a lower voltage; a nickel-based charger should have a trickle charge NiMH; a Li-ion charger provides no trickle charge.

Chargers should have a temperature override to end charge on a malfunctioning battery.

Observe the temperature of the charger and battery. Lead acid batteries stay cool during charge; nickel-based batteries elevate the temperature towards the end of charge and should cool down after charge; Li-ion batteries should stay cool throughout charge.

Slow Charger

Also known as an “overnight charger”, the slow charger goes back to the old nickel-cadmium days and applies a fixed charge of about 0.1C (one-tenth of the rated capacity) as long as the battery is connected. Slow chargers are very simple; they have no full-charge detection, the charge current is always engaged, and the charge time on an empty battery is 14 to 16 hours.When fully charged, a slow charger keeps NiCd lukewarm to the touch. Some overcharge is acceptable and the battery does not need to be removed immediately when ready. However, the pack should not stay in the charger for more than a day or two because of “memory,” also known as crystalline formation. [see Memory: Myth or Fact?].A problem arises when charging a battery with a lower mAh rating than specified. Although the slow charger will charge the battery normally at first, higher than 0.1C current for this smaller battery will heat up the pack towards the full-charge state. Because there is no provision to lower the current or terminate the charge, excessive heat will shorten the life of this pack. Observe the battery temperature while charging and remove the battery when warm to the touch. Most slow chargers have no “ready” light.The opposite can also occur when the slow charger charges a larger battery. In this case, the battery may never reach full charge and remains cold. Performance is poor because the battery does not receive a full charge. A nickel-based battery that is undercharged will eventually lose the ability to accept a full charge due to crystalline formation.Slow chargers are found in cordless phones, electric toothbrushes and children’s toys. A slow charger works well for these products because the battery and charger are harmonized. Chargers servicing a broader range of batteries need some intelligence to supervise the charge, control the current when full, and provide safety if an anomaly occurs.

Rapid Charger

The rapid charger falls between the slow and fast chargers and services nickel- and lithium-based batteries. Unless specially designed, the rapid charger cannot service both nickel- and lithium-based chemistries on the same platform; it needs a designated platform.The rapid charger is most commonly used for consumer products. The charge time of an empty pack is 3 to 6 hours (less for a partially charged battery), and when the battery is full, the charger switches to “ready.” Most rapid chargers include temperature protection to safeguard against failures. This and other features offer improved service over the slow charger, and batteries tend to perform better. Although they are more expensive to build, high-volume production makes the rapid charger available at a moderate price.

Fast Charger

The fast charger offers several advantages, and the obvious one is shorter charge times.The need for a larger power supply and more complex control circuits reserve fast chargers mostly for commercial use, such as medical, military, communications and power tools.Faster charge times demand tighter communication between the charger and battery. At a 1C charge rate, which the fast charger typically uses, an empty NiCd and NiMH charges in a little more than an hour. [seeBU-402: What is C-rate? ] As a battery approaches full charge, some nickel-based chargers reduce the charge current to adjust to lower charge acceptance, and when the battery is full the charger switches to trickle charge, also known as maintenance charge.

Most nickel-based fast chargers accommodate NiCd and NiMH batteries on the same algorithm, but not Li-ion. To service nickel- and Li-ion-based chemistries in the same charger, a provision is needed to select the correct charge algorithm. In many ways, Li-ion batteries are easier to charge than NiCd and NiMH. The charge to 70 percent at 1C occurs in less than an hour, the rest of the time is devoted to topping charge.Lead acid batteries cannot be fast-charged and the term “fast-charge” is a misnomer. Most lead acid chargers charge the battery in 14 hours; anything slower may be a compromise. As with all chemistries, lead acid can be charged relatively quickly to 70 percent; the all-important saturation charge takes up the remaining time. A partial charge at a high rate is fine provided the battery receives a fully saturated charge once every few weeks to prevent sulfation.

Simple Guidelines on Chargers

Turn the portable device off while charging. A parasitic load confuses the charger.

If possible, charge at a moderate rate. Ultra-fast charging causes undue stress.

Fast and ultra-fast charge fills the battery only partially. A slower topping charge completes the charge. [see BU-401a: Fast and Ultra-fast Chargers]

Do not apply fast and ultra-fast charge when the battery is cold or hot. Only charge batteries at moderate temperatures.

Do not apply fast and ultra-fast charge to low-performing batteries. Very few chargers are able to assess battery condition and govern a suitable charge accordingly.

Type Chemistry C-rate Time Temperatures Charge termination

Slow charger

NiCdLead acid 0.1C 14h

0ºC to 45ºC(32ºF to 113ºF)

Continuous low charge or fixed timer. Subject to overcharge. Remove battery when charged.

Rapid charger

NiCd, NiMH,Li-ion

0.3-0.5C 3-6h10ºC to 45ºC(50ºF to 113ºF)

Senses battery by voltage, current, temperature and time-out timer.

Fast charger

NiCd, NiMH,Li-ion

1C 1h+10ºC to 45ºC(50ºF to 113ºF)

Same as a rapid charger with faster service.

Ultra-fast charger

Li-ion, NiCd, NiMH

1-10C 10-60 minutes

10ºC to 45ºC(50ºF to 113ºF)

Applies ultra-fast charge to 70% SoC; limited to specialty batteries.

Table 1: Charger characteristics. Each chemistry uses a unique charge termination.

BU-401a: Fast and Ultra-fast Chargers

Nowhere is fast-charging in higher demand than with the electric car. Recharging an EV in minutes replicates the convenience of filling up 50 liters (13 gallons) of gasoline into a tank that is capable of delivering 600kWh of energy. Such large storage of energy in an electrochemical system is difficult to fathom and a battery holding this capacity would weigh 6 tons. However, electric energy from a battery delivers far more efficient and cleaner propulsion than the internal combustion engine.Charging an EV will always take longer than filling a tank with liquid fuel, and the battery will always deliver less energy per weight than fossil fuel. This ratio with current battery technology is roughly 1:100 in favor of fossil fuel. Read more about Net Calorific Value. Breaking the rule and forcing ultra-fast charging would cause undue stress to the battery and strain the power grid by dimming the city. When talking about ultra-fast charging we must remember that the battery is an electrochemical device that is sluggish and loses performance with use and aging. Charging a battery cannot be compared to filling a tank with fuel that contains 12,000Wh of calorific value per liter. Furthermore, while a fuel tank keeps its volumetric dimensions, a battery begins to fade by the time it leaves the factory.

Ultra-fast Chargers

Ultra-fast chargers have been around for many years. Most NiCd and specialty types of Li-ion batteries, can be charged at a very high rate up to 70 percent state-of-charge (SoC). At a rate of 10C (see What is the C-rate?) or 10 times the rated current, a 1A battery could theoretically be charged in six minutes, but there are limits. To apply an ultra-fast charge, the following conditions must be observed:

The battery must be designed to accept an ultra-fast charge. Current handing poses limitation with many pack designs.

Ultra-fast charging only applies during the first charge phase. The charge current must be lowered when the 70 percent state-of-charge threshold is reached.

All cells in the pack must be balanced and in good condition. Older batteries with high internal resistance will heat up; they are no longer suitable for ultra-fast charging.

Ultra-fast charging can only be done under moderate temperatures. Low temperature slows the chemical reaction, and energy that cannot be absorbed causes gassing and heat buildup.

The charger must include temperature compensations and other safety provisions to halt the charge if the battery gets unduly stressed. Failure to heed to these conditions could cause rapid disintegration of the battery and fire.

An ultra-fast charger can be compared to a high-speed train that is capable to travel 300km per hour (188 mph) on a track built for it. The tracks, and not the machinery, govern the maximum speed. Adding power to a charger is relatively simple; the intelligence lies in assessing the condition of the battery and applying the right amount of maximum charge. A

properly designed ultra-fast charger will lower the current when certain conditions occur. In essence, only newer batteries can be ultra-fast charged.Do not ultra-fast charge batteries if possible and charge at a more moderate rate of 1C or less. (A maker of the 18650 Li-ion recommends 0.7C.) Makers of electric cars prefer if EV owners charge at an eight-hour or 16-hour charge, both of which are below 1C. The 30-minute charge with a three-phase 440V outlet charges the battery at above 1C and this method should only be used if no other option exists (1C is the current rating of a battery. A 1C charge or discharge of a battery rated at 1Ah is 1A.) Figure 1 compares the cycle life of a lithium-ion battery when charged and discharged at 1C, 2C and 3C. A 1C charge and discharge cycle causes the capacity drop from 650mAh to 550mAh after 500 cycles, reflecting a decrease to 84 percent. A 2C accelerates capacity fade to 310mAh, representing a decrease to 47 percent, and with 3C the battery fails after only 360 cycles with 26 percent remaining capacity.

Figure 1: Cycle performance of Li-ion with 1C, 2C and 3C charge and dischargeCharging and discharging Li-ion above 1C reduces service life. Use a slower charge and discharge if possible. This applies to most batteries.

Although the battery performs best at a gentle rate of 1C and less, we must keep in mind that some applications require high charge and discharge rates, and the user must take shorter life expectation into account. If full cycles with rapid charge and discharge are the norm, consider using a larger battery. This will not only provide more reserve capacity but it will also lower the C-rate in that a given charge and discharge current is less intrusive on the larger pack. An analogy can be made with an underpowered engine pulling a large vehicle; the stress is too large and the engine will not last.

BU-402: What is the C-rate?

In the late 1700s, Charles-Augustin de Coulomb ruled that a battery that receives a charge current of one ampere (1A) passes one coulomb (1C) of charge every second. In 10 seconds, 10 coulombs pass into the battery, and so on. On discharge, the process reverses. Today, the battery industry uses C-rate to scale the charge and discharge current of a battery.Most portable batteries are rated at 1C, meaning that a 1,000mAh battery that is discharged at 1C rate should under ideal conditions provide a current of 1,000mA for one hour. The same battery discharging at 0.5C would provide 500mA for two hours, and at 2C, the 1,000mAh battery would deliver 2,000mA for 30 minutes. 1C is also known as a one-hour discharge; a 0.5C is a two-hour, and a 2C is a half-hour discharge.The battery capacity, or the amount of energy a battery can hold, can be measured with a battery analyzer. The analyzer discharges the battery at a calibrated current while measuring the time it takes to reach the end-of-discharge voltage. An instrument displaying the results in percentage of the nominal rating would show 100 percent if a 1,000mAh test battery could provide 1,000mA for one hour. If the discharge lasts for 30 minutes before reaching the end-of-discharge cut-off voltage, then the battery has a capacity of 50 percent. A new battery is sometimes overrated and can produce more than 100 percent capacity; others are underrated and never reach 100 percent even after priming.When discharging a battery with a battery analyzer capable of applying different C-rates, a higher C-rate will produce a lower capacity reading and vice versa. By discharging the 1,000mAh battery at the faster 2C, or 2,000mA, the battery should ideally deliver the full capacity in 30 minutes. The sum should be the same as with a slower discharge since the identical amount of energy is being dispensed, only over a shorter time. In reality, internal resistance turns some of the energy into heat and lowers the resulting capacity to about 95 percent or less. Discharging the same battery at 0.5C, or 500mA over two hours, will likely increase the capacity to above 100 percent.To obtain a reasonably good capacity reading, manufacturers commonly rate lead acid at 0.05C, or a 20-hour discharge. Even at this slow discharge rate, the battery seldom attains a 100 percent capacity. Manufacturers provide capacity offsets to adjust for the discrepancies in capacity if discharged at a higher C-rate than specified. Figure 1 illustrates the discharge times of a lead acid battery at various loads as expressed in C-rate.

Figure 1: Typical discharge curves of lead acid as a function of C-rateSmaller batteries are rated at a 1C discharge rate. Due to sluggish behavior, lead acid is rated at 0.2C (5h) and 0.05C (20h).While lead- and nickel-based batteries can be discharged at a high rate, a safety circuit prevents Li-ion with cobalt cathodes from discharging above 1C. Manganese and phosphate can tolerate discharge rates of up to 10C and the current threshold is set higher accordingly.

Learn how to optimize charging conditions to extend service life.We now study various charging methods and examine why some systems work better than others. We focus on closed-loop techniques that communicate with the battery and terminate charge when certain responses occur.Lead acid charging uses a voltage-based algorithm that is similar to lithium-ion. The charge time of a sealed lead acid battery is 12–16 hours, up to 36–48 hours for large stationary batteries. With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less; however, the topping charge may not be complete. Lead acid is sluggish and cannot be charged as quickly as other battery systems.Lead acid batteries should be charged in three stages, which are [1] constant-current charge, [2]topping charge and [3] float charge. The constant-current chargeapplies the bulk of the charge and takes up roughly half of the required charge time; the topping charge continues at a lower charge current and provides saturation, and the float charge compensates for the loss caused by self-discharge. Figure 4-4 illustrates these three stages.

Figure 4-4: Charge stages of a lead acid batteryThe battery is fully charged when the current drops to a pre-determined level or levels out in stage 2. The float voltage must be reduced at full charge.Courtesy of CadexDuring the constant-current charge, the battery charges to 70 percent in 5–8 hours; the remaining 30 percent is filled with the slower topping charge that lasts another 7–10 hours. The topping charge is essential for the well-being of the battery and can be compared to a little rest after a good meal. If deprived, the battery will eventually lose the ability to accept a full charge and the performance will decrease due to sulfation. The float charge in the third stage maintains the battery at full charge.The switch from Stage 1 to 2 occurs seamlessly and happens when the battery reaches the set voltage limit. The current begins to drop as the battery starts to saturate, and full charge is reached when the current decreases to the three percent level of the rated current. A battery with high leakage

may never attain this low saturation current, and a plateau timer takes over to initialize the charge termination.The correct setting of the charge voltage is critical and ranges from 2.30 to 2.45V per cell. Setting the voltage threshold is a compromise, and battery experts refer to this as “dancing on the head of a needle.” On one hand, the battery wants to be fully charged to get maximum capacity and avoid sulfation on the negative plate; on the other hand, an over-saturated condition causes grid corrosion on the positive plate and induces gassing.To make “dancing on the head of a needle” more difficult, the battery voltage shifts with temperature. Warmer surroundings require slightly lower voltage thresholds and a cold ambient prefers a higher level. Chargers exposed to temperature fluctuations should include temperature sensors to adjust the charge voltage for optimum charge efficiency. If this is not possible, it is better to choose a lower voltage for safety reasons. Table 4-5 compares the advantages and limitations of various peak voltage settings. 

2.30V to 2.35V/cell 2.40V to 2.45V/cell

AdvantagesMaximum service life; battery stays cool; charge temperature can exceed 30°C (86°F).

Higher and more consistent capacity readings; less sulfation.

Disadvantages

Slow charge time; capacity readings may be inconsistent and declining with each cycle. Sulfation may occur without equalizing charge.

Subject to corrosion and gassing. Needs constant water. Not suitable for charging at high room temperatures, causing severe overcharge.

Table 4-5: Effects of charge voltage on a small lead acid batteryCylindrical lead acid cells have higher voltage settings than VRLA and starter batteries.Once fully charged through saturation, the battery should not dwell at the topping voltage for more than 48 hours and must be reduced to the float voltage level. This is especially critical for sealed systems because these systems are less able to tolerate overcharge than the flooded type. Charging beyond what the battery can take turns the redundant energy into heat and the battery begins to gas. The recommended float voltage of most low-pressure lead acid batteries is 2.25 to 2.27V/cell. (Large stationary batteries float at 2.25V at 25°C (77°F.) Manufacturers recommend lowering the float charge at ambient temperatures above 29°C (85°F).Not all chargers feature float charge. If your charger stays on topping charge and does not drop below 2.30V/cell, remove the charge after 48 hours of charge.Whereas the voltage settings in Table 4-5 apply to low-pressure lead acid batteries with a pressure relief valve of about 34kPa (5psi), cylindrical sealed lead acid, such as the Hawker Cyclon cell, requires higher voltage settings and the limits should be set according to the manufacturer’s specifications. Failing to apply the recommended voltage will cause a gradual decrease in capacity due to sulfation. The Hawker Cyclon cell has a pressure relief setting of 345kPa (50psi) and this allows some recombination of the gases generated during charge.Aging batteries pose a challenge when setting the optimal float charge voltage because each cell has its own age-related condition. Weak cells may go into hydrogen evolution as part of overcharge early on, while the stronger ones undergo oxygen recombination in an almost starved state. Connected in a string, all cells receive the same charge current and controlling individual cell voltages is almost impossible. A float current that is too high for the faded cell might starve the strong neighbor and cause sulfation due to undercharge. Companies have developed cell-balancing devices, which are

placed on the battery and compensate the differences in cell voltages that occur as a result of cell imbalance.Ripple voltage imposed on the voltage of large stationary batteries also causes a problem. The voltage peak constitutes an overcharge, causing hydrogen evolution, while the valleys induce a brief discharge that creates a starved state that results in electrolyte depletion. Manufacturers typically limit the ripple to five percent, or 5A for a 100Ah battery.Much has been said about pulse charging of lead acid batteries. There are apparent advantages in reducing sulfation; however, manufacturers and service technicians are divided on the benefits, and the results are inconclusive. If sulfation could be measured with accuracy and the pulses applied as a corrective service, then the remedy could be beneficial. Assumptions without knowing the underlying results can be harmful.Most stationary batteries are kept on float charge. To reduce stress, the so-called hysteresis charge disconnects the float current when the battery is full. As the terminal voltage drops due to self-discharge, an occasional topping charge replenishes the lost energy. In essence, the battery is only “borrowed” from time to time for brief moments. This mode works well for installations that do not draw a load when on standby.Lead acid batteries must always be stored in a charged state. A topping charge should be applied every six months to prevent the voltage from dropping below 2.10V/cell. With AGM, these requirements can be somewhat relaxed.Measuring the open circuit voltage (OCV) while in storage provides a reliable indication as to the state-of-charge of the battery. A voltage of 2.10V at room temperature reveals a charge of about 90 percent. Such a battery is in good condition and needs only a brief full charge prior to use. If the voltage drops below 2.10V, the battery must be charged to prevent sulfation. Observe the storage temperature when measuring the open circuit voltage. A cool battery lowers the voltage slightly and a warm one increases it. Using OCV to estimate state-of-charge works best when the battery has rested for a few hours, because a charge or discharge agitates the battery and distorts the voltage.Some buyers do not accept shipments of new batteries if the OCV at incoming inspection is below 2.10V per cell. A low voltage suggests partial charge due to long storage or a high self-discharge induced by a possible micro-short. Battery users have indeed found that a pack arriving at a lower than specified voltage has a higher failure rate than the others. Although in-house service can often bring such batteries to full performance, the time and equipment required adds to operational costs. (Please note that the 2.10V/cell acceptance threshold does not apply to all lead acid types.)

Watering

Watering is the single most important step in maintaining a flooded lead acid battery, a requirement that is all to often neglected. The frequency of watering depends on usage, charge method and operating temperature. A new battery should be checked every few weeks to determine the watering requirement. This prevents the electrolyte from falling below the plates. Avoid exposed plates at all times, as this will sustain damage, leading to reduced capacity and lower performance.Exposed plates will sustain damage, leading to reduced capacity and lower performance. If the plates are exposed, immediately fill the battery with distilled or de-ionized water to cover the plates, and then apply a charge. Do not fill to the correct level before charging as this could cause an overflow during charging. Always top up to the desired level after charging. Never add electrolyte as this upsets the specific gravity and induces rapid corrosion. Watering systems eliminate low electrolyte levels by automatically adding the right amount of water.

Simple Guidelines for Charging Lead Acid Batteries

Charge in a well-ventilated area. Hydrogen gas generated during charging is explosive. Please see Health Concerns with Batteries. 

Choose the appropriate charge program for flooded, gel and AGM batteries. Check manufacturer’s specifications on recommended voltage thresholds. 

Charge lead acid batteries after each use to prevent sulfation. Do not store on low charge. 

The plates of flooded batteries must always be fully submerged in electrolyte. Fill battery with distilled or de-ionized water to cover the plates if low. Tap water may be acceptable in some regions. Never add electrolyte. 

Fill water level to designated level after charging. Overfilling when the battery is empty can cause acid spillage. 

Formation of gas bubbles in a flooded lead acid indicates that the battery is reaching full state-of-charge (hydrogen on negative plate and oxygen on positive plate). 

Reduce float charge if the ambient temperature is higher than 29°C (85°F). 

Do not allow a lead acid to freeze. An empty battery freezes sooner than one that is fully charged. Never charge a frozen battery. 

Do not charge at temperatures above 49°C (120°F).

BU-404: Equalizing Charge

Stationary batteries are almost exclusively lead acid and some maintenance is required, one of which is equalizing charge. Applying an equalizing charge every six months or after 20 cycles brings all cells to similar levels by increasing the voltage to 2.50V/cell, or 10 percent higher than the recommended charge voltage. An equalizing charge is nothing more than forced overcharge. It removes sulfation that may have formed during low-charge conditions. Battery manufacturers recommend first measuring sulfation. One method is to apply a saturated charge and then to compare the specific gravity readings (SG) on the individual cells of a flooded lead acid battery. Only apply equalization if the SG difference between the cells is 0.030. During equalizing charge, check the changes in the SG reading every hour and disconnect the charge when the gravity no longer rises. This is the time when no further improvement is possible, and a continued charge would cause damage. The battery must be kept cool and under close observation for unusual heat rise and excessive venting. Some venting is normal and the hydrogen emitted is highly flammable. The battery room must have good ventilation.Equalizing VRLA and other sealed batteries involves guesswork. Good judgment plays a pivotal role when estimating the frequency and duration of the service. Some manufacturers recommend monthly equalizations for 2 to 16 hours. Most VRLAs vent at 34kPa (5psi), and repeated venting leads to the depletion of the electrolyte that can lead to a dry-out condition. 

BU-405: Charging with a Power Supply

With technical knowledge, batteries can be charged manually with a power supply featuring user-adjustable voltage and current limiting. I stress manual because charging cannot be left unattended; charge termination is not automated. You need to observe the state-of-charge according to voltage and current behaviors. Lower the charge voltage or disconnect the charge when the battery is full. Because of difficulties in detecting full charge with nickel-based batteries, I recommend only charging lead acid and Li-ion batteries manually.Before connecting the battery, calculate the charge voltage according to the number of cells in series, and then set the desired voltage and current limit. To charge a 12-volt lead acid battery (six cells) to a voltage limit of 2.40V, set the voltage to 14.40V (6 x 2.40). Select the charge current according to battery size. For lead acid this is between 10 and 30 percent of the rated capacity. A 10Ah battery at 30 percent charges at about 3A. Starter batteries charge at lower currents, and an 80Ah pack would charge at about 10 percent of the rating, or 8A. Higher currents are possible.Observe the battery temperature, voltage and current during charge. Charge only at ambient temperatures in a well-ventilated room. Once the battery is fully charged and the current has dropped to three percent of the rated Ah, the charge is completed. Disconnect the charge. High self-discharge (soft electrical short) may prevent the current from going to the anticipated low current level when fully charged. Disconnect the charge also when the current has bottomed out and cannot go lower. If you need float charge for operational readiness, lower the charge voltage to about 2.25V/cell.You can also use the power supply to equalize a lead acid battery by setting the charge voltage 10 percent higher than recommended. The time in overcharge is critical and must be carefully observed. When using the power supply to perform equalizing, refer to the previous article entitled Equalizing Charge.A power supply can also reverse sulfation but there is no guarantee of success. When applying a charge, a totally sulfated lead acid may draw very little current at first, and as the sulfation layer dissolves the current will gradually increase. If you must increase the charge voltage above the recommended level, set the current limiting to the lowest practical value and observe the battery voltage. If the battery does not accept a charge after 24 hours, restoration is unlikely.Lithium-ion charges similarly to lead acid and you can use the power supply also but use extra caution. Set the voltage threshold to 4.20V/cell and make certain that none of the cells connected in series exceeds this voltage. (The protection circuit in a commercial pack does this.) Full charge is reached when the cell(s) reach 4.20V/cell voltage and the current drops to three percent of the rated current, or has bottomed out and cannot go down further. Once fully charged, disconnect the battery. Never allow a cell to dwell at 4.20V for more than a few hours. Read more about Charging Lithium-ion.I do not recommend charging nickel-based batteries with a power supply. Full-charge detection is difficult to assess because the voltage signature varies with the applied charge current. If you must charge, use the temperature rise on a rapid charge as an indication for full charge. When charging at a low current, estimate the level of remaining charge and calculate the charge time. An empty 2Ah NiMH will charge in three hours at 500mA. The trickle charge must be reduced to 0.05C. Read more about Charging Nickel-based batteries.

BU-406: Battery as a Buffer

The main purpose of a stationary battery is to provide power during power outage. A battery bank can also provide supplementarypowerduring high-traffic periods. In essence, the battery acts as a buffer to assist the AC power supply when so needed. The term “AC power supply” refers to the unit that provides electrical power to the system and charges stationary batteries.Cellular repeater towers are an example where the battery serves as a buffer to bridge heavy usage times. During off-peak periods, the batteries are fully charged, and at peak times when the load exceeds the capacity of the power supply, the batteries kick in to provide the extra power. A starter battery in a vehicle works in a similar way. While the motor is on idle at a traffic light, the battery complements the power to run the lights, windshield wipers and other accessories. Driving at highway speed replenishes the borrowed power.When relying on the battery as buffer, make certain that the battery has enough time to charge between peak periods. The net charge must always be greater than what was drawn from the battery. Avoid deep discharges and make sure that the float charge voltage is set correctly. Stationary and starter batteries are not made for deep cycling. If excessively cycled, the battery will experience unwanted stresses that will shorten the life. 

BU-409: Charging Lithium-ion

Charging and discharging batteries is a chemical reaction, but Li-ion is claimed as an exception. Here, battery scientists talk about energies flowing in and out as part of ion movement between anode and cathode. This claim has merits, but if the scientists were totally right then the battery would live forever, and this is wishful thinking. The experts blame capacity fade on ions getting trapped. For simplicity, we consider aging a corrosion that affects all battery systems.The Li-ion charger is a voltage-limiting device that is similar to the lead acid system. The difference lies in a higher voltage per cell, tighter voltage tolerance and the absence of trickle or float charge at full charge. While lead acid offers some flexibility in terms of voltage cut-off, manufacturers of Li-ion cells are very strict on the correct setting because Li-ion cannot accept overcharge. The so-called miracle charger that promises to prolong battery life and methods that pump extra capacity into the cell do not exist here. Li-ion is a “clean” system and only takes what it can absorb. Anything extra causes stress.Most cells charge to 4.20V/cell with a tolerance of +/–50mV/cell. Higher voltages could increase the capacity, but the resulting cell oxidation would reduce service life. More important is the safety concern if charging beyond 4.20V/cell. Figure 1 shows the voltage and current signature as lithium-ion passes through the stages for constant current and topping charge.

Figure 1: Charge stages of lithium-ion. Li-ion is fully charged when the current drops to a predetermined level or levels out at the end of Stage 2. In lieu of trickle charge, some chargers apply a topping charge when the voltage drops to 4.05V/cell (Stage 4).Courtesy of CadexThe charge rate of a typical consumer Li-ion battery is between 0.5 and 1C in Stage 1, and the charge time is about three hours. Manufacturers recommend charging the 18650 cell at 0.8C or less. Charge efficiency is 97 to 99 percent and the cell remains cool during charge. Some Li-ion packs may experience a temperature rise of about 5ºC (9ºF) when reaching full charge. This could be due to the protection circuit and/or elevated internal resistance. Full charge occurs when the battery reaches the voltage threshold and the current drops to three

percent of the rated current. A battery is also considered fully charged if the current levels off and cannot go down further. Elevated self-discharge might be the cause of this condition.Increasing the charge current does not hasten the full-charge state by much. Although the battery reaches the voltage peak quicker with a fast charge, the saturation charge will take longer accordingly. The amount of charge current applied simply alters the time required for each stage; Stage 1 will be shorter but the saturation Stage 2 will take longer. A high current charge will, however, quickly fill the battery to about 70 percent.Li-ion does not need to be fully charged, as is the case with lead acid, nor is it desirable to do so. In fact, it is better not to fully charge, because high voltages stresses the battery. Choosing a lower voltage threshold, or eliminating the saturation charge altogether, prolongs battery life but this reduces the runtime. Since the consumer market promotes maximum runtime, these chargers go for maximum capacity rather than extended service life.Some lower-cost consumer chargers may use the simplified “charge-and-run” method that charges a lithium-ion battery in one hour or less without going to the Stage 2 saturation charge. “Ready” appears when the battery reaches the voltage threshold at Stage 1. Since the state-of-charge (SoC) at this point is only about 85 percent, the user may complain of short runtime, not knowing that the charger is to blame. Many warranty batteries are being replaced for this reason, and this phenomenon is especially common in the cellular industry.Avoiding full charge has benefits, and some manufacturers set the charge threshold lower on purpose to prolong battery life. Table 2 illustrates the estimated capacities when charged to different voltage thresholds with and without saturation charge. 

Charge V/cellCapacity at

cut-off voltageCharge time

Capacity with full saturation

3.803.904.004.104.20

60%70%75%80%85%

120 min135 min150 min165 min180 min

~65%~75%~80%~90%100%

Table 2: Typical charge characteristics of lithium-ion. Adding full saturation at the set voltage boosts the capacity by about 10 percent but adds stress due to high voltage.When the battery is first put on charge, the voltage shoots up quickly. This behavior can be compared to lifting a heavy weight with an elastic band. The lifting arm moves up quickly but the weight lags behind. The voltage of the charging battery will only catch up when the battery is almost fully charged (see Figure 3. This charge characteristic is typical of all batteries.

Figure 3: Capacity as a function of charge voltage on a lithium-ion batteryThe capacity trails the charge voltage, like lifting a heavy weight with an elastic band.Courtesy of Cadex

Relying on the closed circuit voltage (CCV) to read the available capacity during charge is impractical. Theopen circuit voltage (OCT) can, however, be used to predict state-of-charge after the battery has rested for a few hours. The rest period calms the agitated battery to regain equilibrium. Similar to all batteries, temperature affects the OCV. Read "How to Measure State-of-Charge".Li-ion cannot absorb overcharge, and when fully charged the charge current must be cut off. A continuous trickle charge would cause plating of metallic lithium, and this could compromise safety. To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible.Once the charge is terminated, the battery voltage begins to drop, and this eases the voltage stress. Over time, the open-circuit voltage will settle to between 3.60 and 3.90V/cell. Note that a Li-ion battery that received a fully saturated charge will keep the higher voltage longer than one that was fast-charged and terminated at the voltage threshold without a saturation charge.If a lithium-ion battery must be left in the charger for operational readiness, some chargers apply a brief topping charge to compensate for the small self-discharge the battery and its protective circuit consume. The charger may kick in when the open-circuit voltage drops to 4.05V/cell and turn off again at a high 4.20V/cell. Chargers made for operational readiness, or standby mode, often let the battery voltage drop to 4.00V/cell and recharge to only 4.05V/cell instead of the full 4.20V/cell. This reduces voltage-related stress and prolongs battery life.Some portable devices sit in a charge cradle in the on position. The current drawn through the device is called the parasitic load and can distort the charge cycle. Battery manufacturers advise against parasitic load while charging because it induces mini-cycles, but this cannot always be avoided; a laptop connected to the AC main is such a case. The battery is being charged to 4.20V/cell and then discharged by the device. The stress level on the battery is high because the cycles occur at the 4.20V/cell threshold.A portable device must be turned off during charge. This allows the battery to reach the set threshold voltage unhindered, and enables terminating charge on low current. A parasitic load confuses the charger by depressing the battery voltage and preventing the current in the saturation stage to drop low. A battery may be fully charged, but the prevailing conditions prompt a continued charge. This causes undue battery stress and compromises safety.Battery professionals agree that charging lithium-ion batteries is simpler and more straightforward than nickel-based systems. Besides meeting the voltage tolerances, the charge circuits are relatively simple. Limiting voltage and observing low current in triggering “ready” is easier than analyzing complex signatures that may change with age. Charge currents with Li-ion are less critical and can vary widely. Any charge will do, including energy from a renewable resource such as a solar panel or wind turbine. Charge absorption is very high and with a low and intermittent charge, charging simply takes a little longer without negatively affecting the battery. The absence of trickle charge further helps simplify the charger.

Overcharging Lithium-ion

Lithium-ion operates safely within the designated operating voltages; however, the battery becomes unstable if inadvertently charged to a higher than specified voltage. Prolonged charging above 4.30V forms plating of metallic lithium on the anode, while the cathode material becomes an oxidizing agent, loses stability and produces carbon dioxide (CO2). The cell pressure rises, and if charging is allowed to continue the current interrupt device (CID) responsible for cell safety disconnects the current at 1,380kPa (200psi).Should the pressure rise further, a safety membrane bursts open at 3,450kPa (500psi) and the cell might eventually vent with flame. The thermal runaway moves lower when the battery is fully charged; for Li-cobalt this threshold is between 130–150C°C (266–302°F), nickel-manganese-cobalt (NMC) is 170–180°C (338–356°F), and manganese is 250°C (482°F). Li-phosphate enjoys similar and better temperature stabilities than manganese.Lithium-ion is not the only battery that is a safety hazard if overcharged. Lead- and nickel-based batteries are also known to melt down and cause fire if improperly handled. Nickel-based batteries have also been recalled for safety concerns. Properly designed charging equipment is paramount for all battery systems.

Over-discharging Lithium-ion

Li-ion should never be discharged too low, and there are several safeguards to prevent this from happening. The equipment cuts off when the battery discharges to about 3.0V/cell, stopping the current flow. If the discharge continues to about 2.70V/cell or lower, the battery’s protection circuit puts the battery into a sleep mode. This renders the pack unserviceable and a recharge with most chargers is not possible. To prevent a battery from falling asleep, apply a partial charge before a long storage period.Battery manufacturers ship batteries with a 40 percent charge. The low charge state reduces aging-related stress while allowing some self-discharge during storage. To minimize the current flow for the protection circuit before the battery is sold, advanced Li-ion packs feature a sleep mode that disables the protection circuit until activated by a brief charge or discharge. Once engaged, the battery remains operational and the on state can no longer be switched back to the standby mode.Do not recharge lithium-ion if a cell has stayed at or below 1.5V for more than a week. Copper shunts may have formed inside the cells that can lead to a partial or total electrical short. If recharged, the cells might become unstable, causing excessive heat or showing other anomalies. Li-ion packs that have been under stress are more sensitive to mechanical abuse, such as vibration, dropping and exposure to heat.

Charging Lithium-ion Polymer

Charging Li-ion polymer, also referred as Li-polymer, is very similar to a regular lithium-ion battery and no changes in algorithm are necessary. Most users won’t even know if their battery is Li-ion or Li-polymer. The word “polymer” has been used as promotional hype and does not reflect special attributes other than to know that the battery is built in a different way to a standard Li-ion.Most polymer batteries are based on a hybrid architecture that is a cross between Li-ion and Li-polymer. There are many variations within the polymer family, and the true dry polymer battery for the consumer market is still years away. Also know as the “plastic battery,” this system was first announced in early 2000 but was never able to attain the conductivity needed for most applications at ambient temperatures. Read more about the Lithium-polymer battery and the Pouch Cell.

Simple Guidelines for Charging Lithium-based Batteries

A portable device should be turned off while charging. This allows the battery to reach the threshold voltage unhindered and reflects the correct saturation current responsible to terminate the charge. A parasitic load confuses the charger. 

Charge at a moderate temperature. Do not charge below freezing. 

Lithium-ion does not need to be fully charged; a partial charge is better. 

Chargers use different methods for “ready” indication. The light signal may not always indicate a full charge. 

Discontinue using charger and/or battery if the battery gets excessively warm. 

Before prolonged storage, apply some charge to bring the pack to about half charge. 

Over-discharged batteries can be “boosted” to life again. Discard pack if the voltage does not rise to a normal level within a minute while on boost. 

BU-410: Charging at High and Low Temperatures

Rechargeable batteries operate in a wide temperature range but this does not give license to charge them at extreme temperatures. Extreme cold and high heat reduce charge acceptance, and the battery must be brought into moderate temperature conditions before charging.Older battery technologies, such as lead acid and NiCd, have higher charging tolerances than newer systems and can be charged below freezing at a reduced 0.1C rate. This is not possible with most NiMH and lithium-ion systems. Table 1 summarizes the permissible charge and discharge temperatures of common lead acid, NiCd, NiMH and Li-ion. We exclude specialty batteries designed to charge outside these parameters. 

Battery Type Charge Temperature Discharge Temperature Charge Advisory

Lead acid –20°C to 50°C(–4°F to 122°F)

–20°C to 50°C(–4°F to 122°F)

Charge at 0.3C or lessbelow freezing.

Lower V-threshold by 3mV/°C when hot.

NiCd, NiMH0°C to 45°C

(32°F to 113°F)–20°C to 65°C(–4°F to 149°F)

Charge at 0.1C between –18 and 0°C.

Charge at 0.3C between 0°C and 5°C.Charge acceptance at 45°C is 70%. Charge acceptance at 60°C is

45%.

Li-ion 0°C to 45°C(32°F to 113°F)

–20°C to 60°C(–4°F to 140°F)

No charge permitted below freezing.

Good charge/discharge performance at higher

temperature but shorter life.

Table 1: Permissible temperature limits for various batteries. Batteries can be discharged over a large temperature range but charge temperature is limited. For best results, charge between 10°C and 30°C (50°F and 86°F). Lower the charge current when cold.

Low-temperature Charge

Fast charging of most batteries is limited to a temperature of 5 to 45°C (41 to 113°F); for best results consider narrowing the temperature bandwidth to between 10°C and 30°C (50°F and 86°F). Nickel-based batteries are most forgiving in accepting charge at low temperatures, however, when charging below 5°C (41°F), the ability to recombine oxygen and hydrogen diminishes. If NiCd and NiMH are charged too rapidly, pressure builds up in the cell that will lead to venting. Not only do escaping gases deplete the electrolyte, the hydrogen released is highly flammable. The charge current of all nickel-based batteries should be reduced to 0.1C below freezing.Nickel-based chargers with NDV full-charge detection offer some protection when fast-charging at low temperatures. The resulting poor charge acceptance mimics a fully charged battery. This is in part due to the pressure buildup caused by gas recombination problems. Pressure rise and a voltage drop at full charge appear to be synonymous.

To enable fast-charging at all temperatures, some industrial batteries include a thermal blanket that heats the battery to an acceptable temperature; other chargers adjust the charge rate to prevailing temperatures. Consumer chargers do not have these provisions and users should make all attempts to only charge batteries at room temperatures.Lead acid is reasonably forgiving when it comes to temperature extremes, as we know from the starter batteries in our cars. Part of this tolerance is their sluggish behavior. The recommended charge rate at low temperature is 0.3C, which is almost the same as under normal conditions. At a comfortable temperature of 20°C (68°F), gassing starts at 2.415V/cell, and by lowering the temperature to –20°C (0°F), the gassing voltage rises to 2.97V/cell.Do not freeze a lead acid battery. This would causes permanent damage. Always keep the batteries fully charged. In the discharged state the electrolyte becomes more water-like and freezes earlier than a fully charged battery. According to BCI, a specific gravity of 1.15 has a freezing temperature of –15°C (5°F). This compares to 1.265 of a fully charged starter battery. Flooded lead acid batteries tend to crack the case and cause leakage if frozen; sealed lead acid packs lose potency and only deliver a few cycles before a replacement is necessary.Li-ion batteries offer reasonably good charging performance at cooler temperatures and allow fast-charging in a temperature bandwidth of 5 to 45°C (41 to 113°F). Below 5°C, the charge current should be reduced, and no charging is permitted at freezing temperatures. During charge, the internal cell resistance causes a slight temperature rise that compensates for some of the cold. With all batteries, cold temperature raises the internal resistance.Many battery users are unaware that consumer-grade lithium-ion batteries cannot be charged below 0°C (32°F). Although the pack appears to be charging normally, plating of metallic lithium can occur on the anode during a subfreezing charge. The plating is permanent and cannot be removed with cycling. Batteries with lithium plating are known to be more vulnerable to failure if exposed to vibration or other stressful conditions. Advanced chargers, such as those made by Cadex, prevent charging Li-ion below freezing.Manufactures continue to seek ways to charge Li-ion below freezing and low-rate charging is indeed possible with most lithium-ion cells; however, it is outside the specified (and tested) limits of most manufacturers’ products. Low-temperature charging would need to be addressed on a case-by-case basis and would be manufacturer and application dependent. According to information received from university research centers, the allowable charge rate at –30°C (–22°F) is 0.02C. At this low current, a 1,000mAh Li-ion could only charge at 20mA, and this would take more than 50 hours to reach full charge.Some Li-ion cells developed for power tool and EV applications can be charged at temperatures down to –10°C (14°F) at a reduced rate. To charge at a higher rate, Li-ion systems for automotive propulsion systems require a heating blanket. Some hybrid cars circulate warm cabin air through the batteries to raise the battery temperature, while high-performance electric cars heat and cool the battery with a liquid agent.

High-temperature Charge

Heat is the worst enemy of most batteries, including lead acid. Adding temperature compensation on a lead acid charger to adjust for temperature variations prolongs battery life by up to 15 percent. The recommended compensation is 3mV per cell per degree Celsius applied on a negative coefficient, meaning that the voltage threshold drops as the temperature increases. For example, if the continued float voltage were set to 2.30V/cell at 25°C (77°F), the recommended setting would be 2.27V/cell at 35°C (95°F) and 2.33V/cell at 15°C (59°F). This represents a 30mV correction per cell per 10°C (18°F). Table 2 indicates the optimal peak voltage at various temperatures when charging lead acid batteries. The table also includes the recommended float voltage while in standby mode. 

Battery status 0°C (32°F) 25°C (77°F) 40°C (104°F)Voltage limit 2.55V/cell 2.45V/cell 2.35V/cell

on rechargeFloat voltageat full charge 2.35V/cell or lower 2.30V/cell or lower 2.25V/cell or lower

Table 2: Recommended voltage limits when recharging and maintaining stationary lead acid batteries on float charge. Voltage compensation prolongs battery life when operating at temperature extremes.Charging nickel-based batteries at high temperatures lowers oxygen generation, which reduces charge acceptance. Heat fools the charger into thinking that the battery is fully charged when it’s not.NiCd has the largest pool of published information on this subject, and Figure 3 demonstrates a strong decrease in charge efficiency above 30°C (86°F). At 45°C (113°F), the battery can only accept 70 percent of its full capacity; at 60°C (140°F) the charge acceptance is reduced to 45 percent. NDV for a full-charge detection becomes unreliable at higher temperature and temperature sensing is essential for backup. Newer type NiMH batteries perform better at elevated temperatures than NiCd.

Figure 3: NiCd charge acceptance as a function of temperature. High temperature reduces charge acceptance. At 55°C, commercial NiMH has a charge efficiency of 35–40%; newer industrial NiMH attains 75–80%.Courtesy of CadexLithium-ion performs well at elevated temperatures; however, prolonged exposure to heat reduces longevity. The charge efficiency is 97 to 99 percent, regardless of temperature. In fact, high temperature increases charge effectiveness slightly by improving the internal resistance.While other chemistries can tolerate stepping outside set boundaries once in a while, there are limitations with Li-ion. Safety concerns dictate that Li-ion remains within specified limits because of possible thermal runaway if stressed. A fully charged Li-ion is more sensitive to a thermal runaway than an empty one; the thermal runaway temperature moves lower with higher charge. In spite of this, specialty Li-ion batteries serve in applications that go to momentary high temperatures, and surgical tools that undergo steam sterilization at 137°C (280°F) are such an example. Other uses that reach similar temperatures are batteries in drilling bits for mining. 

Caution: In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with

water for 15 minutes and consult a physician immediately.

BU-415: How to Charge and When to Charge?

Batteries have unique needs and Table 1 explains how to satisfy these desires based of common batteries. Because of similarities within the battery family, we only list lead, nickel and lithium systems. Although each chemistry has its own requirements, there are common denominators that affect the life of all batteries. These are:

Keep a moderate temperature. As food stays fresher when refrigerated, so also does cool temperature retard battery corrosion, a life-robbing adversary of any battery. 

Control discharge. Each cycle wears the battery down by a small amount. A partial discharge before charge is better than a full discharge. Apply a deliberate full discharge only to calibrate a smart battery and to prevent “memory” on a nickel-based pack. 

Avoid abuse. Like a machine that is exposed to strenuous work, a battery wears down more quickly if discharged harshly and if force-charged with high currents. Strenuous demands cannot always be prevented, but the user has the choice of selecting the right battery size, keeping the temperature moderate and following life-extending service guidelines.

Batteries for the electric powertrain have changed the philosophy of battery manufacturers from designing packs for maximum energy density, as demanded by the consumer market, to focusing on optimal safety and longevity. Batteries on the road are exposed to extreme environmental hazards; they must perform at maximum duty under severe heat, cold, shock and vibration. Storing energy of several kilowatts, batteries for the electric powertrain can be dangerous if stressed beyond normal conditions. Furthermore, vehicular batteries are expensive and must last for the life of the car.Pampering a battery to achieve an extended service life, as is sometimes possible with a laptop or cell phone pack, is more difficult with a large battery in a vehicle that must deliver high load currents on command and is exposed to freezing temperatures in the winter and blistering summer conditions. The user has limited control as to the care and attention of the battery. This task is passed over to an intelligent battery management system (BMS), which takes over the command and does the supervising. The BMS assumes the duty of a lead commander who must make sure that the troops in a large army are well organized and that all soldiers are marching in the same direction.While a battery in a portable device can have its own personality and occasionally slack off, this liberty does not exist in a large battery system where all members must be of equal strength. Managing fading and failing cells as the battery ages is a complex issue that the BMS must address effectively. Monitoring and eventual replacing the cells or battery groups is far more complex than getting a new pack for a portable device when the old one becomes a nuisance.

Frequentlyasked question

Lead acid(Sealed, flooded)

Nickel-based(NiCd and NiMH)

Lithium-ion(Li-ion, polymer)

How should I prepare a new battery?

Battery comes fully charged. Apply topping charge

Charge 14–16h. Priming may be needed

Apply a topping before use. No priming needed

Can I damagea battery with incorrect use?

Yes, do not store partially charged, keep fully charged

Battery is robust and the performance will improve with use

Keep some charge. Low charge can turn off protection circuit

Do I need to apply a Yes, partial charge Partial charge is fine Partial charge better

full charge? causes sulfation, than a full charge

Can I disrupt a charge cycle?

Yes, partial charge causes no harm

Interruptions can cause heat buildup

Partial chargecauses no harm

Should I use upall battery energy before charging?

No, deep discharge wears battery down. Charge more often

Apply scheduled discharges only to prevent memory

Deep discharge wears the battery down

Do I have to worry about “memory”?

No, there is no memory

Discharge NiCd every 1–3 months

No memory

How do I calibrate a “smart” battery?

Not applicableApply discharge/charge when the fuel gauge gets inaccurate. Repeat every 1–3 months

Can I charge with the device on?

Some UPS systems simultaneous charge and deliver current.

It's best to turn the device off during charge; parasitic load can alter full-charge detection and overcharge battery or cause mini-cycles

Must I remove the battery when full?

Depends on charger; needs correct float V

Remove after a few days in charger

Not necessary; charger turns off

How do I storemy battery?

Keep cells above 2.10V, chargeevery 6 months

Store in cool place;a total discharge causes no harm

Store in cool place partially charged, do not fully drain

Is the battery allowed to heat up during charge?

Battery may get lukewarm towards the end of charge

Battery gets warm but must cool down on ready

Battery may get lukewarm towards the end of charge

How do I charge when cold?

Slow charge (0.1): 0–45°C  (32–113°F)Fast charge (0.5–1C): 5–45°C (41–113°F)

Do not chargebelow freezing

Can I charge at hot temperatures?

Above 25°C, lower threshold by 3mV/°C

Battery will not fully charge when hot

Do not chargeabove 50°C (122°F)

What should I know about chargers?

Charger should float at 2.25–2.30V/cell when ready

Battery should not get too hot; should include temp sensor

Battery must stay cool; no trickle charge when ready

Table 1: Best charging methods. Strenuous demands cannot always be prevented.

BU-501: Basics About Discharging

The purpose of a battery is to store and release energy at the desired time and in a controlled manner. This section examines discharges under different C-rates and evaluates the depth to which a battery can safely be depleted. Chapter 5 also observes different discharge signatures and explores how certain patterns can affect battery life. But first, let’s look at charge and discharge rates, also known as C-rate.

Depth of Discharge

The end-of-discharge voltage for lead acid is 1.75V/cell; nickel-based system is 1.00V/cell; and most Li-ion is 3.00V/cell. At this level, roughly 95 percent of the energy is spent and the voltage would drop rapidly if the discharge were to continue. To protect the battery from over-discharging, most devices prevent operation beyond the specified end-of-discharge voltage.When removing the load after discharge, the voltage of a healthy battery gradually recovers and rises towards the nominal voltage. Differences in the metal concentration of the electrodes enable this voltage potential when the battery is empty. An aging battery with elevated self-discharge cannot recover the voltage because of the parasitic load.A high load current lowers the battery voltage, and the end-of-discharge voltage threshold should be set lower accordingly. Internal cell resistance, wiring, protection circuits and contacts all add up to overall internal resistance. The cut-off voltage should also be lowered when discharging at very cold temperatures; this compensates for the higher-than-normal internal resistance. Table 1 shows typical end-of-discharge voltages of various battery chemistries. 

End-of-discharge Li-manganese Li-phosphate Lead acid NiCd/NiMHNormal loadHeavy load

3.00V/cell2.70V/cell

2.70V/cell2.45V/cell

1.75V/cell1.40V/cell

1.00V/cell0.90V/cell

Table 1: Recommended end-of-discharge voltage under normal and heavy loadThe lower end-of-discharge voltage on a high load compensates for the losses induced by the internal battery resistance.  Some battery analyzers apply a secondary discharge (recondition) that drains the battery voltage of a nickel-based battery to 0.5V/cell and lower, a cut-off point that is below what manufacturers specify. These analyzers (Cadex) keep the discharge load low to stay within an allowable current while in sub-discharge range. A cell breakdown with a weak cell is possible and reconditioning would cause further deterioration in performance rather than making the battery better. This phenomenon can be compared to the experience of a patient to whom strenuous exercise is harmful.

What Constitutes a Discharge Cycle?

Most understand a discharge/charge cycle as delivering all stored energy, but this is not always the case. Rather than a 100 percent depth of discharge (DoD), manufacturers prefer rating the batteries at 80 percent DoD, meaning that only 80 percent of the available energy is being delivered and 20 percent remains in reserve. A less-than-full discharge increases service life, and manufacturers argue that this is closer to a field representation because batteries are seldom fully discharged before recharge.There are no standard definitions of what constitutes a discharge cycle. A smart battery that keeps track of cycle count may require a depth of discharge of 70 percent to define a discharge cycle; anything less does not count as a cycle. There are many other applications that discharge the battery less. Starting a car, for example, discharges the battery by less than 5 percent, and the depth of discharge in satellites is 6 to 10 percent before the onboard batteries are being recharged during the satellite day. Furthermore, a hybrid car only uses a fraction of the capacity during acceleration before the battery is being recharged.

Discharge Signature

A classic discharge is a battery that delivers a steady load at, say, 0.2C. A flashlight is such an example. Many applications demand momentary loads at double and triple the battery’s C-rating, and GSM (Global System for Mobile Communications) of a cellular phone is such an example (Figure 2). GSM loads the battery with up to 2A at a pulse rate of 577 micro-seconds (µs). This is a large demand for a small 1,000mAh battery; however, with a high frequency the battery begins to behave like a capacitor and the characteristics change.

Figure 2: GSM Pulse of a cellular phoneThe 577 microsecond pulses adjust to field strength and can reach 2 amperes.Courtesy of Cadex

In terms of cycle life, a moderate current at a constant discharge is better than a pulsed or momentary high load. Figure 3 shows the decreasing capacity of a NiMH battery at different load conditions and includes a gentle 0.2C DC discharge, an analog discharge and a pulsed discharge. The cycle life of other battery chemistries is similar under such load conditions.

Figure 3:Cycle life of NiMH under different operating conditionsNiMH performs best with DC and analog loads; digital loads lower the cycle life. Li-ion behaves similarly.Source: Zhang  (1998)

Figure 4 examines the number of full cycles a Li-ion battery with a cobalt cathode can endure when discharged at different C-rates. At a 2C discharge, the battery exhibits higher stress than at 1C, limiting the cycle count to about 450 before the capacity drops to half level.

Figure 4:Cycle life ofLi-ion with cobalt cathode at varying discharge levelsThe wear-and-tear of a battery increases with higher loads.Source: Choi et al (2002)

For a long time, Li-ion had been considered fragile and unsuitable for high loads. This has changed, and today many lithium-based systems are more robust than the older nickel and lead chemistries. Manganese and phosphate-type Li-ion permit a continuous discharge of 30C. This means that a cell rated at 1,500mAh can provide a steady load of 45A, and this is being achieved primarily by lowering the internal resistance through optimizing the surface area between the active cell materials. Low resistance keeps the temperature down, and running at the maximum permissible discharge current, the cells heat up to about 50ºC (122ºF); the maximum temperature is limited to 60°C (140°F).One of the unique qualities of Li-ion is the ability to deliver continuous high power. This is possible with an electrochemical recovery rate that is far superior to lead acid. The slow electrochemical reaction of lead acid can be compared to a drying felt pen than works for short marking but needs rest to replenish the ink.

Simple Guidelines for Discharging Batteries

The battery performance decreases with cold temperature and increases with heat. 

Heat increases battery performance but shortens life by a factor of two for every 10°C increase above 25–30°C (18°F above 77–86°F). 

Although better performing when warm, batteries live longer when kept cool. 

Operating a battery at cold temperatures does not automatically permit charging under these conditions. Only charge at moderate temperatures. 

Some batteries accept charge below freezing but at a much-reduced charge current. Check the manufacturer’s specifications. 

Use heating blankets if batteries need rapid charging at cold temperatures. 

Prevent over-discharging. Cell reversal can cause an electrical short. 

Deploy a larger battery if repetitive deep discharge cycles cause stress. 

A moderate DC discharge is better for a battery than pulse and aggregated loads. 

A battery exhibits capacitor-like characteristics when discharging at high frequency. This allows higher peak currents than is possible with a DC load. 

Lead acid is sluggish and requires a few seconds of recovery between heavy loads.

BU-501a: Calculating the Battery Runtime

If the battery were a perfect power source and behaved linearly, the discharge time could be calculated according to the in-and-out flowing currents. “What is put in should be available as an output in the same amount” goes the argument, and “a one-hour charge at 5A should deliver a one-hour discharge at 5A, or a 5-hour discharge at 1A." This is not possible because of intrinsic losses. The output is always less than what has been put in, and the losses escalate with increasing load. High discharge currents make the battery less efficient. To learn about the coulomb counter, see Inner Workings of a Smart Battery.The efficiency factor of a discharging battery is expressed in the Peukert Law. W. Peukert, a German scientist (1897), was aware of this loss and devised a formula that expresses the loss at a given discharge rate in numbers. Because of sluggish behavior of lead acid, the Peukert numbers apply mostly to this battery chemistry and help in calculating the capacity when loaded at various discharge rates.The Peukert Law takes into account the internal resistance and recovery rate of a battery. A value close to one (1) indicates a well-performing battery with good efficiency and minimal loss; a higher number reflects a less efficient battery. The Peukert Law of a battery is exponentialand the readings for lead acid are between 1.3 and 1.4. Nickel-based batteries have low numbers and lithium-ion is even better. Figure 1 illustrates the available capacity as a function of ampere drawn with different Peukert ratings.

Figure 1: Available capacity of a lead acid battery at Peukert numbersof 1.08–1.50A value close to1 has the smallest losses; higher numbers deliver lower capacities.Source: von Wentzel (2008)

The lead acid battery prefers intermittent loads to a continuous heavy discharge. The rest periods allow the battery to recompose the chemical reaction and prevent exhaustion. This is why lead acid performs well in a starter application with brief 300A cranking loads and plenty of time to recharge in between. All batteries require recovery, and with nickel- and lithium-based system, the electrochemical reaction is much faster than with lead acid. Read more about the Basics About Charging.The runtime of batteries in portable devices relates to the specific energy marked in Ah (mAh in personal devices). Ah as a performance indicator works best at low discharge currents. At higher loads, the internal resistance begins to play a larger role in the ability to deliver power. Resistance acts as the “gatekeeper.” Energy in Ah presents the available storage capacity of a battery and is responsible for the runtime; power governs the load current. These two attributes are critical in digital devices that require long runtimes and must deliver high-current pulses.

Ah alone is not a reliable runtime indicator and the relationship between capacity and the ability to deliver current can best be illustrated with the Ragone Chart. Named after David V. Ragone, the Ragone chart evaluates batteries not on energy alone but also represents power. Figure 2 illustrates the Ragone chart on a digital camera that is powered by an Alkaline, Lithium (Li-FeS2) or NiMH battery drawing 1.3W. (1.3W at 3V draws 433mA.) The horizontal axis displays energy in Watt/hours and the vertical axis displays power in Watts. The scale is logarithmic to allow a wide selection of battery sizes.

Figure 2: Ragone chart illustrates battery performance with various load conditions.Digital camera loads NiMH, Li-FeS2 and Alkaline with 1.3W pulses according to ANSI C18.1 (dotted line). The results are:- Li- FeS2 690 pluses- NiMH 520 pulses- Alkaline 85 pulsesEnergy = Capacity x VPower = Current x VCourtesy of Exponent

The dotted line represents the power demand of the digital camera. All three batteries have similar Ah rating: NiMH delivers the highest power but has the lowest specific energy. This battery works well at high loads such as power tools. The Lithium Li-FeS2 offers the highest specific energy but has moderate loading conditions. Digital cameras and personal medical instruments suit the system well. Alkaline offers an economic solution for lower current drains such as flashlights, remote controls and wall clocks, but a digital camera is stretching the capability of Alkaline. Read more about the Choices of Primary Batteries. ReferencePresentation by Quinn Horn, Ph.D., P.E. Exponent, Inc. Medical Device & Manufacturing (MD&M) West, Anaheim, CA, 15 February 2012

BU-502: Discharging at High and Low Temperatures

Like humans, batteries function best at room temperature, and any deviation towards hot and cold changes the performance and/or longevity. Operating a battery at elevated temperatures momentarily improves performance by lowering the internal resistance and speeding up the chemical metabolism, but such a condition shortens service life if allowed to continue for a long period of time. Some manufacturers of lead acid batteries make use of the improved performance at warmer temperatures and specify the batteries at a toasty 27°C (80°F).Cold temperature increases the internal resistance and diminishes the capacity. Batteries that would provide 100 percent capacity at 27°C (80°F) will typically deliver only 50 percent at –18°C (0°F). The capacity decrease is linear with temperature. The capacity decrease is momentary and the level of decline depends on the battery chemistry.Li-ion also performs better at high temperatures than at low ones. Heat lowers the internal resistance but this stresses the battery. Warming a dying flashlight or cellular phone battery in your jean pocket might provide additional runtime in the winter. As all drivers in cold countries know, a warm battery cranks the car engine easier than a cold one.The dry solid polymer battery uses heat to promote ion flow in what is called a “true plastic battery.” The battery requires a core temperature of 60 to 100°C (140 to 212°F) to become conductive. The dry solid polymer has found a niche market for stationary power applications in warm climates where heat serves as a catalyst rather than a disadvantage. Built-in heating elements keep the battery operational at all times. High battery cost and safety concerns have limited the application of this technology. The more common Li-polymer uses moist electrolyte to enhance conductivity, as discussed earlier. Read more about the Lithium-polymer battery.Batteries achieve optimum service life if used at 20°C (68°F) or slightly below, and nickel-based chemistries degrade rapidly when cycled at high ambient temperatures. If, for example, a battery operates at 30°C (86°F) instead of a more moderate room temperature, the cycle life is reduced by 20 percent. At 40°C (104°F), the loss jumps to a whopping 40 percent, and if charged and discharged at 45°C (113°F), the cycle life is only half of what can be expected if used at 20°C (68°F).The performance of all battery chemistries drops drastically at low temperatures. At –20°C (–4°F) most nickel-, lead- and lithium-based batteries stop functioning. Although NiCd can go down to –40°C (-40°F), the permissible discharge is only 0.2C (5-hour rate). Specially built Li- ion brings the operating temperature down to –40°C, but only on discharge and at a reduced discharge. With lead acid we have the danger of the electrolyte freezing, which can crack the enclosure. Lead acid freezes more easily with a low charge when the specific gravity of the electrolyte is more like water.Cell matching by using cells of similar capacity plays an important role when discharging at low temperature under heavy load. Since the cells in a battery pack can never be perfectly matched, a negative voltage potential can occur across a weaker cell on a multi-cell pack if the discharge is allowed to continue beyond a safe cut-off point. Known as cell reversal, the weak cell suffers damage to the point of developing a permanent electrical short. The larger the cell-count, the greater the likelihood that a cell might reverse under load. Over-discharge at a heavy load and low temperature is a large contributor to battery failure of cordless power tools, especially nickel-based packs; Li-ion packs come with protection circuits and the failure rate is lower. Read about Cell Mismatch and Balancing.Users of electric vehicles need to understand that the driving distance specified per charge is given under normal temperature; frigid cold will sharply reduce the available mileage. Using electricity for cabin heating is not the only cause for the shorter driving distance between charging; the battery performance is reduced when cold. 

BU-503: Determining Power Deliver by the Ragone Plot

Choose and characterize the best lithium-ion system for long-term reliability

Nature offers many means to produce power. Most are through combustion, mechanical movement, photosynthesis or electrochemical reaction as in a battery. An electrochemical reaction produces a voltage potential, and multiplying the voltage by the current that flows when closing the electrical circuit provides power. Power is measured in watts in honor of James Watt, the eighteenth-century developer of the steam engine.

The most simplistic manifestation of a battery is a lemon. Driving a zinc-plated nail and a copper coin into a lemon creates a voltage. This quasi battery does not deliver much power; its current delivery system is very poor and any electrical load causes the voltage to collapse.

All power sources have limitations and the energy drawn must be harnessed carefully so as not to exceed the permitted loading. An analogy is a bicycle rider who chooses the best gear ratio to transfer energy into propulsion. On a flat road a high gear provides fast movement with moderate pedal action, and this can be compared to high voltage. Climbing a hill with the same pedaling action increases the torque, and in our analogy this corresponds to high current. The pedal force the rider exerts relates to power in watt (W); the endurance before exhaustion defines energy in watt/hours (Wh).

Figure 1: Analogy of a bicycle rider.

Energy is the product of power and time, measured in Watt-hours (Wh); power is the flow of energy at any one time, measured in Watts.

A battery is rated in ampere/hours (Ah); it specifies how much current a pack can deliver in an hour. Like fluid in a container, the energy can be dispensed slowly over a long period of time or rapidly in a short time. The amount of liquid a container holds is analogous to the energy in a battery; how quickly the liquid is dispensed is analogous to power.

An alkaline battery has low power with a relatively high specific energy (capacity). See [BU-106: Primary Batteries] This lends itself well for a flashlight or a similar light load. In comparison, most rechargeable batteries have high load capabilities to drive power tools and crank internal combustion engines but these batteries have lower capacities than the primary counterpart.

The relationship between energy and power can best be represented in a Ragone plot. Named after David V. Ragone, the Ragone plot places the energy in Wh on the horizontal x axis and power in W on the vertical y axis. The derived power curve provides a clear demarcation line of what level of power a battery can deliver. The Ragone plot is logarithmic, which enables displaying performance profiles of extremely high and low power. Some table may reverse the W and Wh positioning.

Figure 2 illustrates the Ragone plot reflecting the discharge energy and discharge power of four lithium-ion systems packaged in 18650 cells. The diagonal lines across the field disclose the length of time the battery cells can deliver energy at various loading conditions. The battery chemistries featured are the most common power-

based lithium-ion systems, which include lithium-iron phosphate (LFP), lithium-manganese oxide (LMO), and nickel manganese cobalt (NMC)

.Figure 2: Ragone plot reflects Li-ion 18650 cells.  (Courtesy of Exponent)Four Li-ion systems are compared for discharge power and energy as a function of time.

Legend: The A123 APR18650M1 is a lithium iron phosphate (LiFePO4) with 1,100mAh and a continuous discharge current of 30A. The Sony US18650VT and Sanyo UR18650W are manganese–based Li-ion cells of 1500mAh each with a continuous discharge current of 20A. The Sanyo UR18650F is a 2,600mAh cell for a moderate 5A.discharge. This cell provides the highest discharge energy but has the lowest discharge power.

The physical dimensions of a battery are specified by volume in liter (l) and weight in kilogram (kg). Adding dimension and weight enables rating a battery in specific energy in Wh/kg, power density in Wh/l and specific power in W/kg. Most batteries are rated in Wh/kg, revealing how much energy a given weight can generate. Wh/l denotes watt/hours per liter.

The Sanyo UR18650F has the highest specific energy and can power a laptop or e-bike for many hours at a moderate load. The Sanyo UR18650W, in comparison, has a lower specific energy but can supply a current of 20A. The A123 has the lowest specific energy but offers the highest power capability by delivering 30A of continuous current.

The Ragone plot helps choosing the best Li-ion system to satisfy maximum discharge power and optimal discharge energy as a function of discharge time. If an application calls for very high discharge current, the 3.3 minute diagonal line on the chart points to the A123 (Battery 1) as a good pick; it can deliver up to 40 Watts of power for 3.3 minutes. The Sanyo F (Battery 4) is slightly lower and delivers about 36 Watts. Focusing on discharge time and following the 33 minute discharge line further down, Battery 1 (A123) only delivers 5.8 Watts for 33 minutes before the energy is depleted whereas the higher capacity Battery 4 (Sanyo F) can provide roughly 17 Watts for the same time; its limitation is lower power.

Battery manufacturers take the Ragone snapshot on new cells, a condition that is only valid for a short time.

When calculating power and energy thresholds, design engineers must consider battery fade caused by cycling and aging. Design battery operated systems that still provide full function with a battery that has faded to 70 or 80 percent. A further consideration is temperature as batteries lose power when cold. The Ragone plot does not show these discrepancies and the design engineer must take these less-than-ideal-conditions into consideration by studying the manufacturer’s specifications.  

It should be noted that loading a battery to its full power handling increases stress and shortens the life. When a high current draw is needed continuously, the battery pack should be made larger. Tesla does this with their Model S cars by doubling and tripling the battery size compared to other EVs; BMW i3 uses a smaller but more rugged Li-ion system. An analogy can be drawn with a heavy truck that is fitted with a large diesel engine to provide long and durable service as opposed to installing a souped-up sports car engine with similar horsepower.

The Ragone plot is also suitable to calculate power requirements of other energy sources and storage devices, such as capacitors, flywheels, flow batteries and fuel cells. Fuel cells and internal combustion engines drawing fuel from a tank causes a conflict in that energy-delivery can be made continuous. This distorts the Wh measurements of a self-contained battery (or the bicycle rider) to determine the available intrinsic energy before recharging is required.

Similar plots are also deployed to establish the optimal energy/power ratio and loading condition of renewable power sources such as solar cells and wind turbines. An example of such a chart is the maximum power point tracking (MPPT) used on charge controllers to charge batteries from renewable resources. See [BU-413: Charging with Solar, Turbine]  MPPT allows optimal power transfer without overloading the source when the supply is low during fringe conditions Also see Figure 2 in specific energy and specific power of rechargeable batteries table. See [BU-103: Global Battery Markets]

BU-701: How to Prime Batteries

Caring for the Battery

In many ways, a battery behaves like a human being; it senses the kindness offered and delivers on the care and attention given. Looking after the battery well will return the benevolence bestowed and deliver good performance over a long life. There are exceptions, however, as any parent raising a large family will know, and the generosity given may not always deliver the anticipated returns.To become a good custodian, we must understand the basic needs of a battery, a subject that is not taught in school. This section teaches what to do when the battery is new, how to feed it the right “food” and what to do when putting the pack aside for a while. The next articles also look into restrictions when traveling with batteries by air and how to dispose of them when their useful life has passed.Just as we cannot predict a person’s life expectancy at birth, we cannot date-stamp a battery. Some packs live to a great old age while others die young.Incorrect charging, harsh discharge loads and exposure to heat are the battery’s worst enemies. Although we have waysto protect a battery, the ideal situation is not always attainable, and as battery custodians we try to do our best. This chapter discusses how we can get the most from our batteries.

Priming a new Battery

Rechargeable batteries may not deliver their full rated capacity when new and will require formatting. While this applies to most battery systems, manufacturers of lithium-ion batteries disagree. They say that Li-ion is ready at birth and does not need priming. Although this may be true, users have reported some capacity gains by cycling these batteries after long storage.What’s the difference between formatting and priming? Both address capacities that are not optimized and can be corrected with cycling. Formatting completes the manufacturing process and occurs naturally during early usage when the battery is being cycled. Priming, on the other hand, is a conditioning cycle that is applied as a service tool to improve battery performance during usage or after prolonged storage. Priming relates mainly to nickel-based batteries.Formatting of lead acid batteries occurs by applying a charge, followed by a discharge and recharge as part of regular use. Do not strain a new battery by giving it extra-heavy duty right away. Gradually work it in with moderate discharges like an athlete trains for weight lifting or long-distance running. Lead acid typically reaches the full capacity potential after 50 to 100 cycles. Do not over-cycle on purpose; this would wear the battery down too quickly.Manufacturers advise to trickle charge a nickel-based battery for 16 to 24 hours when new and after a long storage. This allows the cells to adjust to each other and bring them to an equal charge level. A slow charge also helps to redistribute the electrolyte to eliminate dry spots on the separator that might have developed by gravitation.Nickel-based batteries are not always fully formatted when they leave the factory. Applying several charge/discharge cycles through normal use or with a battery analyzer completes the formatting process. The number of cycles required to attain full capacity differs between cell manufacturers. Quality cells perform to specification after 5 to 7 cycles, while others may need 50 or more cycles to reach acceptable capacity levels. Lack of formatting might cause a problem when the industrial user expects a new battery to work to specification right out of the box. Organizations using batteries for critical applications often verify performance through a discharge/charge cycle as part of quality control. Automated analyzers (Cadex) apply as many cycles as needed to achieve full capacity.Cycling also restores lost capacity when a nickel-based battery has been stored for six months or longer. Storage time, state-of-charge and the temperature under which the battery was stored govern the recovery. The longer the storage and warmer the temperature, the more cycles will be required to regain full capacity. Battery analyzers help in the priming functions.Some scientists believe that with use and storage, a passivation layer builds up on the cathode of a lithium-ion cell. Also known as interfacial protective film (IPF), this layer restricts ion flow and increases the internal

resistance. In the worst cases, the phenomenon can lead to lithium plating. Charging, and more effectively cycling, is known to dissolve the layer. Scientists do not fully understand the nature of this layer, and the few published resources on this subject only speculate that performance restoration with cycling is connected to the removal of the passivation layer. Some scientists deny outright the existence of the IPF, saying that the idea is highly speculative and inconsistent with existing studies. Another layer is the solid electrolyte interphase (SEI), which is said to form at the anode on the initial charge. SEI is an electric insulation yet provides sufficient ionic conductivity for proper function.Whatever the truth may be, there is no parallel to “memory” of NiCd batteries, which require periodic cycling. The symptoms may appear similar but the mechanics are different. Nor can the effect be compared to sulfation of lead acid batteries.Lithium-ion is a very clean system and does not need formatting when new, nor does it require the level of maintenance that nickel-based batteries do. The first charge is no different than the fifth or the 50th. Formatting makes little difference because the maximum capacity is available right from the beginning. Nor does a full discharge improve the capacity once faded. In most cases, a low capacity signals the end of life. A discharge/charge may be beneficial for calibrating a “smart” battery, but this service only addresses the digital part of the pack and does nothing to improve the electrochemical battery. Instructions to charge a new battery for eight hours are seen as “old school” from the nickel battery days. 

Caution:

When charging an SLA with over-voltage, current limiting must be applied to protect the battery. Always set the current limit to the lowest practical setting and observe the battery voltage and temperature during charge.In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.Wear approved gloves when touching electrolyte, lead and cadmium. On exposure to skin, flush with water immediately.

BU-702: How to Store Batteries

Learn about storage temperatures and state-of-charge conditions.

The recommended storage temperature for most batteries is 15°C (59°F); the extreme allowable temperature is –40°C to 50°C (–40°C to 122°F) for most chemistries. While lead acid must always be kept at full-charge during storage, nickel- and lithium-based chemistries should be stored at around a 40 percent state-of-charge (SoC). This minimizes age-related capacity loss while keeping the battery operational and allowing for some self-discharge.

Do not recharge lithium-ion if a cell has stayed at or below 1.5V for more than a week. Copper shunts may have formed inside the cells that can lead to a partial or total electrical short. If recharged, the cells might become unstable, causing excessive heat or showing other anomalies. Li-ion packs that have been under stress are more sensitive to mechanical abuse, such as vibration, dropping and exposure to heat. [BU-409, Charging Lithium-ion]

Finding the 40 percent SoC level is difficult because the open circuit voltage (OCV) of batteries is only estimation. For lack of better methods, voltage is nevertheless used as a rough fuel gauge indicator. The SoC of  Li-ion at 40 percent is commonly 3.82V/cell. To get the correct reading, rest the battery for 90 minutes before measurement. If this is not practical, overshoot the voltage on discharge and discharge by 50mV. Discharge will go to 3.77V/cell and charge to 3.87V/cell. The rubber band effect will correct the reading. Figure 1 shows the typical discharge voltage of a Li-ion battery.

Figure 1: Discharge voltage as a function of capacity on a lithium manganese oxide battery at 25°C.Temperature affects the voltage, so does agitation by charge and discharge. For accurate measurements, allow the battery to rest for 90 minutes.

SoC on nickel-based batteries is especially difficult to measure. A flat discharge curve, agitation after charge and discharge, and voltage change on temperature contribute to the fluctuations. Since no other estimation tool exists that is practical, and the charge level for storage is not all too critical for this chemistry, simply apply some charge if the battery is empty, and then make sure that the battery is kept in a cool and dry storage.

Storage will always cause batteries to age. Low temperature and partial SoC only slow the effect. Table 2

illustrates the recoverable capacity of lithium- and nickel-based batteries at various temperatures and charge levels over one year. The recovered capacity is defined as the available battery capacity after storage with a full charge. 

Temperature Lead acidat full charge

Nickel-basedat any charge

Lithium-ion (Li-cobalt)40% charge 100% charge

0°C25°C40°C60°C

97%90%62%38%

(after 6 months)

99%97%95%70%

98%96%85%75%

94%80%65%60%

(after 3 months)Table 2: Estimated recoverable capacity when storing a battery for one yearElevated temperature hastens permanent capacity loss. Depending on battery type, lithium-ion is also sensitive to charge levels. Lithium-ion batteries are often exposed to unfavorable temperatures, and these include leaving a cell phone in the hot sun or operating a laptop on the power grid. Elevated temperature and allowing the battery to sit at the maximum charge voltage for expended periods of time explains the shorter than expected battery life. Elevated temperature and excessive overcharge also stresses lead and nickel-based batteries. All batteries must have the ability to relax after charged, even when kept on float or trickle charge.Nickel-metal-hydride can be stored for about three years. The capacity drop that occurs during storage can partially be reversed with priming. Nickel-cadmium stores well, even if the terminal voltage falls to zero volts. Field tests done by the US Air Force revealed that NiCd stored for five years still performed well after priming cycles. It is believed that priming becomes necessary if the voltage drops below 1V/cell. Primary alkaline and lithium batteries can be stored for up to 10 years with minimal capacity loss.You can store a sealed lead acid battery for up to two years. Since all batteries gradually self-discharge over time, it is important to check the voltage and/or specific gravity, and then apply a charge when the battery falls to 70 percent state-of-charge. This is typically the case at 2.07V/cell or 12.42V for a 12V pack. (The specific gravity at 70 percent charge is roughly 1.218.) Some lead acid batteries may have different readings and it is best to check the manufacturer’s instruction manual. Low charge induces sulfation, an oxidation layer on the negative plate that inhibits current flow. Topping charge and/or cycling may restore some of the capacity losses in the early stages of sulfation. See [BU-804, Sulfation]Sulfation may prevent charging small sealed lead acid cells, such as the Cyclone by Hawker, after prolonged storage. If seemingly inactive, these batteries can often be reactivated by applying a higher than normal voltage. At first, the cell voltage under charge may go up to 5V and absorb only a small amount of current. Within two hours or so, the charging current converts the large sulfate crystals into active material, the cell resistance drops and the charge voltage gradually normalizes, and at a voltage of 2.10–2.40V the cell is able to accept a normal charge. To prevent damage, set the current limit to a very low level. Do not attempt to perform this service if the power supply does not allow setting current limiting. See [BU-405, Charging with a Power Supply]

Simple Guidelines for Storing Batteries

Primary batteries store well. Alkaline and primary lithium batteries can be stored for 10 years with moderate loss capacity. 

Remove battery from the equipment and store in a dry and cool place. 

Avoid freezing. Batteries freeze more easily if in discharged state. 

Charge lead acid before storing and monitor the voltage or specific gravity frequently; apply a boost if below 2.10V/cell or an SG below 1.225. 

Nickel-based batteries can be stored for five years and longer, even at zero voltage; prime before use. 

Lithium-ion must be stored in a charged state, ideally 40 percent. This assures that the battery will not drop below 2.50V/cell with self-discharge and fall asleep. 

Discard Li-ion if the voltage has stayed below 2.00/V/cell for more than a week.

Caution:

When charging an SLA with over-voltage, current limiting must be applied to protect the battery. Always set the current limit to the lowest practical setting and observe the battery voltage and temperature during charge.In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately.Wear approved gloves when touching electrolyte, lead and cadmium. On exposure to skin, flush with water immediately.

Last Updated: 20141122

BU-701a: How to Calibrate Batteries

Batteries for critical devices are made smart and the preferred protocol is the System Management Bus (SMBus). SMBus batteries provide state-of-charge (SoC) estimations and historic battery data, but much of this information is hidden in the background and can only be made available to the user through a host device or battery analyzer capable of displaying the data.

A “smart” battery should be self-calibrating, but in real life a battery does not always get a full discharge at a steady current followed by a full charge. The discharge may be in form of sharp pulses that are difficult to capture; the pack may then be partially recharged and stored at high temperature, causing elevated self-discharge that cannot be tracked.

To correct the tracking error that occurs, a “smart battery” in use should be calibrated once every three months or after 40 partial discharge cycles. [See Battery Calibration: BU-603]  This can be done by a deliberate discharge of the equipment or externally with a battery analyzer. A full discharge sets the discharge flag and the subsequent recharge establishes the charge flag.

A “smart” battery can be seen as consisting of two parts: the electrochemical battery and the digital battery. The electrochemical battery is known as the actual energy storage vessel and the digital battery is the circuitry that predicts state-of-charge (SoC) and monitors other information. The digital battery provides the readouts but the truth lies in the chemical battery.  Figure 1 illustrates the typical drifting away of the digital battery from the electrochemical battery and how periodic calibration corrects the error. The values are assumed and accentuated.

Figure 1: Tracking of Electrochemical and digital battery as a function of timeWith use and time the electro-chemical and digital battery drift apart; calibration corrects the error.Note: The accumulating error is application related; the values on the chart are accentuated.

The user may ask: “What happens if the battery is not calibrated?” Most SMBus chargers obey the dictates of the chemical battery; there is no safety concern if the digital battery is off, only the SoC readings become a nuisance.

The SMBus battery relies exclusively on information obtained through charge and discharge functions. Once in digital hands, clever programmers can make the SoC readout truly stunning and believable but the accuracy is another thing. Operating instructions, such as one for an Apple iPad product says: “For proper reporting of SoC, be sure to go through at least one full charge/discharge cycle per month.”

The SMBus has other limitations, and a major one is the inability to display capacity. This creates a false sense of security by assuming that a recharge will always deliver a full runtime. An older battery with only 50% capacity will give a 100% SoC indication in the same way as a new pack would. The SMBus cannot make the user aware of the shorter runtime when the capacity gets low.

All batteries have losses and the released energy is always less than what has been fed into the pack. Inefficiencies in charge acceptance, especially towards the end of charge, resistive losses that turn into heat, and storage losses in the form of self-discharge reduce deliverable energy. A common flaw with fuel gauge and Battery Management Systems (BMS) is assuming that the battery will always stay young and energetic. Aging takes on many dimensions and some BMS compensate by observing user pattern and environmental conditions to derive a “learn” algorithm that is meant to correct the tracking error. Such modelling is helpful but there are limitations because battery aging cannot be assessed accurately.

BU-801: Setting Battery Performance Standards

A battery is a corrosive device that begins to fade the moment it comes off the assembly line.  The stubborn behavior of batteries has left many users in awkward situations. The British Army could have lost the Falklands War in 1982 on account of uncooperative batteries. The officers assumed that a battery would always follow the rigid dictate of the military. Not so. When a key order was given to launch the British missiles, nothing happened. No missiles flew that day. Such battery-induced letdowns are common; some are simply a nuisance and others have serious consequences.Even with the best of care, a battery only lives for a defined number of years. There is no distinct life span, and the health of a battery rests on its genetic makeup, environmental conditions and user patterns.Lead acid reaches the end of life when the active material has been consumed on the positive grids; nickel-based batteries lose performance as a result of corrosion; and lithium-ion fades when the transfer of ions slows down for degenerative reasons. Only the supercapacitor achieves a virtually unlimited number of cycles, if this device can be called a battery, but it also has a defined life span.Battery manufacturers are aware of performance loss over time, but there is a disconnect when educating buyers about the fading effect. Runtimes are always estimated with a perfect battery delivering 100 percent capacity, a condition that only applies when the battery is new.While a dropped phone call on a consumer product because of a weak battery may only inconvenience the cellular user, an unexpected power loss on a medical, military or emergency device can be more devastating.Consumers have learned to take the advertised battery runtimes in stride. The information means little and there is no enforcement. Perhaps no other specification is as loosely given as that of battery performance. The manufacturers know this and get away with minimal accountability. Very seldom does a user challenge the battery manufacturer for failing to deliver the specified battery performance, even when human lives are at stake. Less critical failures have been debated in court and punished in a harsh way.The battery is an elusive scapegoat; it’s as if it holds special immunity. Should the battery quit during a critical mission, then this is a situation that was beyond control and could not be prevented. It was an act of God and the fingers point in other directions to assign the blame. Even auditors of quality-control systems shy away from the battery and consider only the physical appearance; state-of-function appears less important to them.

BU-801a: How to Rate Battery Runtime

In the past, the battery industry got away with soft standards specifying battery runtimes. Each manufacturer developed their own method, using the lightest load patterns possible to achieve good figures. This resulted in specifications that bore little resemblance to reality. Under pressure from consumer associations, manufacturers finally agreed to standardized testing procedures. The Camera and Imaging Products Association (CIPA) succeeded in developing a standardized battery-life test for digital cameras. Under the test scheme, the camera takes a photo every 30 seconds, half with flash and the other without. The test zooms the lens in and out all the way before every shot and leaves the screen on. After every 10 shots, the camera is turned off for a while and the cycle is repeated. CIPA ratings replicate a realistic way a consumer would use a camera. Most new cameras adapt the CIPA protocol to rate the runtime.The runtime on laptops is more complex to estimate than a digital camera as programs, type of activity, wireless features and screen brightness affect the load. To take these conditions into account, the computer industry developed a standard called MobileMark 2007. Not everyone agrees with this norm, and opponents say that the convention trims the applications down and ignores real-world habits. The setting of brightness is one example. The monitor is one of the most power-hungry components of a modern laptop and at full brightness the screen delivers 250 to 300 nits. MobileMark uses a setting that is less than half of this. Nor does MobileMark include Wi-Fi and Bluetooth; it leaves these peripherals up to the manufacturers to investigate. BAPCO (Business Applications Performance Corporation), the inventor of MobileMark 2007, is led by Intel and includes laptop and chip manufacturers, such as Advanced Micro Devices.Cell phone manufacturers face similar challenges when estimating runtimes. Standby and talk time are field-strength dependent and the closer you are to a repeater tower, the lower the transmit power will get and the longer the battery will last. CDMA (Code Division Multiple Access) takes slightly more power than GSM (Global System for Mobile Communications); however, the more critical power guzzlers are large color displays, touch screens, video, web surfing, GPS, camera, voice dialing and Bluetooth. These peripherals drastically shorten the advertised runtime specifications if used frequently.The insatiable appetite for information and entertainment on the go is devouring the excess energy enjoyed during the past 10 years when we used our cell phones for voice only. Although modern handsets draw considerably less power than older models and the battery capacity has doubled in 12 years, these improvements do not compensate for the modern peripherals, and a new energy crisis is in the making. Figure 1 illustrates the lack of energy with analog cell phones during the 1990s, the sudden excess with the digital phones, and the looming energy shortage when making full use of modern features. These power needs are superimposed on a continuously improving battery. 

Figure 1: Power needs of the past, present and futureThe capacity of Li-ion has doubled in 12 years and the circuits draw less power; however, these improvements do not compensate for the power demand of the new features, and a new energy crisis isin the making.Courtesy of Cadex

Manufacturers of analog two-way radios test the runtime with a scheme called 5-5-90 and 10-10-80. The first number represents the transmit time at high current; the second denotes the receiving mode at a more moderate current; and the third refers to the long standby times between calls at low current. While 5-5-90 simulates the equivalent of a 5-second talk, 5-second receive and 90-seconds standby, the 10-10-80 schedule puts the intervals at a 10-second talk, 10-second receive and 80-second standby. The runtimes of digital two-way radios are measured in a similar way, with the added complexity of tower distance and digital loading requirements that are reminiscent of a cellular phone.

BU-801b: How to Define Battery Life

Most new batteries go through a formatting process during which the capacity gradually increases and reaches optimal performance at 100–200 cycles. After this mid-life point, the capacity gradually begins decreasing and the depth of discharge, operating temperatures and charging method govern the speed of capacity loss. The deeper the batteries are discharged and the warmer the ambient temperature is, the shorter the service life. The effect of temperature on the battery can be compared with a jug of milk, which stays fresh longer when refrigerated.Most portable batteries deliver between 300 and 500 full discharge/charge cycles. Fleet batteries in portable devices normally work well during the first year; however, the confidence in the portable equipment begins to fade after the second and third year, when some batteries begin to lose capacity. New packs are added and in time the battery fleet becomes a jumble of good and failing batteries. That’s when the headaches begin. Unless date stamps or other quality controls are in place, the user has no way of knowing the history of the battery, much less the performance.The green light on the charger does not reveal the performance of a battery. The charger simply fills the available space to store energy, and “ready” indicates that the battery is full. With age, the available space gradually decreases and the charge time becomes shorter. This can be compared to filling a jug with water. An empty jug takes longer because it can accept more water than one with rocks. Figure 1 shows the “ready” light that often lies.

Figure 1: The “ready” light liesThe “ready” light on a charger only reveals that the battery is fully charged; there is no relationship to performance. A faded battery charges faster thana good one. Bad batteries gravitate to the top.Courtesy of Cadex

Many battery users are unaware that weak batteries charge faster than good ones. Low performers gravitate to the top and become available by going to “ready” first. They form a disguised trap when unsuspecting users require a fully charged battery in a hurry. This plays havoc in emergency situations when freshly charged batteries are needed. The operators naturally grab batteries that show ready, presuming they carry the full capacity. Poor battery management is the common cause of system failure, especially during emergencies.Failures are not foreign in our lives and to reduce breakdowns, regulatory authorities have introduced strict maintenance and calibration guidelines for important machinery and instruments. Although the battery can be an integral part of such equipment, it often escapes scrutiny. The battery as power source is seen as a black box, and for some inspectors correct size, weight and color satisfies the requirements. For the users, however, state-of-function stands above regulatory discipline and arguments arise over what’s more important, performance or satisfying a dogmatic mandate.Ignoring the performance criteria of a battery nullifies the very reason why quality control is put in place. In defense of the quality auditor, batteries are difficult to check, and to this day there are only a few reliable devices that can check batteries with certainty. Read about Difficulties with Battery Testing.

BU-801c: How to Know End-of-Battery-Life

Knowing the health of a battery is important, but no practical method exists that can quantify all conditions in a short, comprehensive test. State-of-health (SoH) cannot be measured per se, only estimated to various degrees of accuracies based on available symptoms.

A battery behaves like a living organism that is swayed by conditions such as state-of-charge (SoC), charge and discharge events, rest periods and age. In addition, a battery with low SoC behaves similarly to a pack exhibiting capacity loss and these two symptoms become a blur. Test methods must isolate mood swings and only capture characteristics that relate to SoH. Figure 1 illustrates the usable capacity in form of a liquid that can be dispensed, the “rock content” that presents a permanent loss of capacity and the tap symbolizing power delivery as part of internal resistance.

Figure 1: Conceptual batterySymbolizing the usable capacity, empty portion that can be refilled, permanent capacity loss as “rock content” and the tap symbolizing power delivery as part of internal resistance.

Courtesy Cadex

The leading health indicator of a battery is capacity; a measurement that represents the actual energy storage. A new battery delivers (should deliver) 100 percent of the rated capacity. Lead acid starts at about 85 percent and increases in capacity through use before the long and gradual decrease begins. Lithium-ion starts at peak and begins its decline immediately, albeit very slowly, while nickel-based batteries need priming to reach full capacity when new or after a long storage.

Manufacturers base device specifications on a new battery, but this is only a temporary states and does not represent a battery in real life situations. Performance will decrease with use and time, and the loss will only become visible after the shine of a new device has worn off and daily routines are taken for granted. An analogy is an aging man whose decreased endurance begins to show after the most productive years draw to an end.  Figure 1 demonstrates such an aging process.

Figure 2:Battery aging as an analogy of a man growing old.Few people know when to replace a battery; some are replaced too early but most are kept too long.

When asking battery users: “At what capacity do you replace the battery?” most would reply in confusion: “I beg your pardon?” Few are familiar with the term capacity as a measurement of runtime, and even less as a threshold when to retire them. Performance loss only becomes apparent when breakdowns begin to occur and the battery becomes a nuisance.

Battery retirement depends on the application. Organizations using battery analyzers typically set the replacement threshold at 80 percent. [See Battery Test Equipment: BU-909] There are applications where the battery can be kept longer and a toss arises between “what if” and economics. Some scanning devices in warehouses can go as low as 60 percent and still provide a full day’s work. A starter battery in a car still cranks well at 40 percent. Engine-starting only requires a short discharge that is replenished while driving, but letting the capacity go much lower may get the driver stranded without warning. No one gets hurt if a battery cuts off a

phone call, but a failing medical device can put a patient at risk. Running out of power in an industrial application can also incur high logistic costs.

The best indicator for battery retirement is checking the spare capacity after a full shift. The Cadex battery analyzers (www.cadex.com) do this by applying a discharge before charge. A battery should have 10 to 20 percent spare at the end of a day to cover unknowns and emergencies. If the lowest performing battery in the fleet comes back with 30 percent, then the target capacity can safely be lowered from 80 percent to 70 percent. Knowing the energy requirement creates a sweet spot between risk management and economics.

Let’s take a drone that is specified to fly for 60 minutes with a good battery. Unknown to mission control, the capacity may have dropped to 75 percent, reducing the flying time to 45 minutes. This could crash the $50,000 vehicle when negotiating a second landing approach. With the reserve capacity marked on each pack, batteries delivering close to 100 percent can be assigned for long hauls while older packs may be sent for shorter errands. This allows the full use of each battery and establishes a sound retirement policy based on application. The analyzer’s label print option enables this feature. [See How to Maintain Fleet Batteries: BU-810c]

Many batteries and portable devices include a fuel gauge. [See Battery fuel Gauge: BU-602] While this shows the amount of energy left during use, the readout only measures the remaining charge; capacity estimation is sketchy. SoC always shows 100 percent after a full charge whether the battery is new or faded. This creates a false sense of security by assuming that a fully charged battery will always deliver the anticipated runtime. Runtime data get inaccurate with use and time and the battery needs calibration. [See Battery Calibration: BU-603]

In the absence of maintenance, some device manufacturers mandate to replace a battery on a date-stamp or cycle count. A pack may fail before the appointed time but most last far longer, prompting perfectly good batteries to be discarded prematurely. Dr. Imre Gyuk, manager of the Energy Storage Research Program at DOE, says that “every year roughly one million usable lithium-ion batteries are sent in for recycling with most having a capacity of up to 80 percent.” Lack of suitable battery diagnostics also affects heathcare. An FDA survey says that “up to 50% of service calls in hospitals surveyed relate to battery issues.” Healthcare professionals at AAMI say that “battery management emerged as a top 10 medical device challenge.” (AAMI stand for Association for the Advancement of Medical Instruments.)

Summary

Batteries do not exhibit visible changes as part of usage; they look the same when fully charged or empty, new or old and in need of replacement. A car tire, in comparison, distorts when low on air, shows signs of wear, and indicates end-of-life when the treads are worn. Batteries should receive the same treatment as a critical aircraft part, a medical device and an industrial machine where wear and tear falls under strict maintenance guidelines. Authorities struggle to implement such procedures for batteries, but lack of suitable test technology makes this almost impossible. Bad batteries thus enjoy immunity as they can hide comfortably among the peer. It is no wonder then that batteries escape the scrutiny of vigorous inspection and are declared “uncontrollable.”

Battery analyzers are effective in managing small to mid-sized batteries with a discharge/charge function; rapid-test methods are available for single Li-ion cells. Testing and monitoring technologies are being developed for larger batteries used in vehicles and stationary applications but the advancements seem slow. It appears not much has changed since the invention of the lead acid battery by Gaston Planté in 1859. We don’t even have a reliable method to measure state-of-charge; not to mention attaining accurate capacity assessments as part of rapid-testing. Simply measuring voltage and internal resistance, as was done in the past, is no longer sufficient to estimate SoC and battery capacity today.

BU-801d: Battery Failure, Real or Perceived

Battery manufactures use capacityto specify battery performance, and a new battery should have 100 percent. This means that a 2Ah battery should deliver two amperes for one hour. If the battery quits after 30 minutes, then the capacity is only 50 percent. Manufacturers use capacity to specify warranty obligations. Depending on chemistry and application, the warranty threshold is set between 70 and 80 percent of the specified full capacity.How does the user know when to claim warranty failure on a battery, or when to replace a pack that no longer performs as expected? Battery replacement has been an ongoing problem and the lack of easy-to-use testing procedures is in part to blame. On one hand, an aging battery may be kept too long until it begins affecting operation, while on the other hand perfectly good batteries are being replaced because of equipment problems or operator misapprehension. This commonly occurs with consumer products under warranty, especially cell phones. If the charge on a cell phone does not hold, the user naturally blames the battery when in many cases the fault lies in the device.Cell phone manufacturers say that 90 percent of batteries returned under warranty have no problem, and tests conducted in the Cadex laboratories confirm this finding. Many storefronts replace the batteries on the faintest complaint, and this frivolous battery return policy costs the manufacturers millions of dollars per year. Unrealistic expectations, perceived performance loss and lack of practical testing equipment contribute to this wasteful battery exchange behavior.Generous battery replacement policies are not limited to portable equipment alone: one German manufacturer of luxury cars points out that out of 400 starter batteries returned under warranty, 200 are working well and have no problem. Low charge and acid stratification are the most common causes of the apparent failure. This problem is more frequent with large luxury cars featuring power-hungry accessories than with the more basic models. A genuine factory defect is seldom the cause, and a leading European manufacturer of starter batteries says that factory defects cause less than seven percent of the returned warranty batteries. Similar to the cell phone industry, the manufacturer of the starter battery must take responsibility for a problem that may be customer-induced.Battery failure in Japan is the largest complaint among new owners. Motorists drive an average 13 km (8 miles) per day in congested cities. With the stop-and-go pattern, the battery has little chance to get fully charged and sulfation occurs. North America may be shielded from such battery problems in part because of the long-distance driving. Read more about Sulfation.

BU-802: Capacity Loss

The energy storage of a battery can be divided into three imaginary segments known as the availableenergy, theempty zonethat can be refilled, and the unusable part, or rock content that has become inactive. Figure 1 illustrates these three sections.

Figure 1: Aging batteryBatteries begin fading from the day they are manufactured. A new battery should deliver 100 percent capacity; most packs in use operate at less.Courtesy of Cadex

The manufacturer bases the runtime of a device on a battery that performs at 100 percent; most packs in the field operate at less capacity. As time goes on, the performance declines further and the battery gets smaller in terms of energy storage. Most users are unaware of capacity fade and continue to use the battery. A pack should be replaced when the capacity drops to 80 percent; however, the end-of-life threshold can vary according to application, user preference and company policy.Besides age-related losses, sulfation and grid corrosion are the main killers of lead acid batteries. Sulfation is a thin layer that forms on the negative cell plate if the battery is allowed to dwell in a low state-of-charge. If sulfation is caught in time, an equalizing charge can reverse the condition. [see BU-804: How to Restore Lead-acid Batteries] Grid corrosion can be reduced with careful charging and optimization of the float charge. With nickel-based batteries, the so-called rock content is often the result of crystalline formation, also known as “memory,” and a full discharge can sometimes restore the battery. The aging process of lithium-ion is cell oxidation, a process that occurs naturally as part of usage and aging and cannot be reversed. 

BU-802a: Rising Internal Resistance

High battery capacity is of limited use if the pack cannot deliver the stored energy effectively. To supply power, the battery needs low internal resistance. Measured in milliohms (mΩ), resistance is the gatekeeper of the battery; the lower the resistance, the less restriction the pack encounters. This is especially important on heavy loads such as power tools and electric powertrains. High resistance causes the voltage to collapse on a load, triggering an early shutdown. Figure 1 illustrates low and high resistance batteries in the form of free-flowing and restricted taps.

Figure 1: Effects of internal battery resistanceA battery with low internal resistance delivers high current on demand. High resistance causes the battery voltage to collapse. The equipment cuts off, leaving energy behind.Courtesy of Cadex

Lead acid has a very low internal resistance, and the battery responds well to high current bursts that last for a few seconds. Due to inherent sluggishness, however, lead acid does not perform well on a sustained high current discharge and the battery needs a rest to recover. Sulfation and grid corrosion are the main contributor to the rise of the internal resistance. Temperature also affects the resistance; heat lowers it and cold raises it. Heating the battery will momentarily lower the internal resistance to provide extra run time.Alkaline, carbon-zinc and other primary batteries have a relatively high internal resistance, and this limits its use to low-current applications such as flashlights, remote controls, portable entertainment devices and kitchen clocks. As these batteries discharge, the resistance increases further. This explains the relative short runtime when using alkaline cells in digital cameras. 

BU-802b: Elevating Self-discharge

All batteries are affected by self-discharge. Self-discharge is not a manufacturing defect but a battery characteristic, although poor manufacturing practices and improper handling can increase the problem. Figure 1 illustrates self-discharge in the form of leaking fluid.

Figure 1: Effects of high self-dischargeSelf-discharge increases with age, cycling and elevated temperature. Discard a battery if the self-discharge reaches 30 percent in 24 hours.Courtesy of Cadex

The amount of electrical self-discharge varies with battery type and chemistry. Primary cells such as lithium and alkaline retain the stored energy best and can be kept in storage for several years. Among rechargeable batteries, lead acid has the lowest self-discharge and loses only about five percent per month. With age and usage, however, the flooded lead acid builds up sludge in the sediment trap, which causes a soft short when this semi-conductive substance reaches the plates.Nickel-based rechargeable batteries leak the most and need recharging before use when placing them on the shelf for a few weeks. High-performance nickel-based batteries have a higher self-discharge than the standard versions. Furthermore, self-discharge increases with use and age, and the contributing factors are crystalline formation (memory), permitting the battery to “cook” in the charger or exposing it to repeated harsh deep discharge cycles.Lithium-ion self-discharges about five percent in the first 24 hours and then loses 1 to 2 percent per month; the protection circuit adds another three percent per month. A faulty separator can lead to a high self-discharge and if critical, the electrical current will generate enough heat that can in extreme cases lead to a thermal breakdown.Table 2 shows the typical self-discharge of battery systems.

Battery System Estimated Self-dischargePrimary lithium-metal 10% in 5 years

Alkaline 2-3% per year (7-10 years shelf life)

Lead-acid 5% per month

Nickel-based 10-15% in 24h, then 10-15% per month

Lithium-ion 5% in 24h, then 1-2% per month (plus 3% for safety circuit)

Table 2: Percentage of self-discharge in years and month.Primary batteries have considerably less self-discharge than secondary (rechargeable) batteries.The energy loss is asymptotical, meaning that the self-discharge is highest right after charge and then tapers off. Nickel-based batteries lose 10 to 15 percent of their capacity in the first 24 hours after charge, then 10 to 15 percent per month. Figure 3 shows the typical loss of a nickel-based battery while in storage.

Figure 3: Self-discharge as a function of timeThe discharge is highest right after charge and tapers off. The graph shows self-discharge of a nickel-based battery. Lead- and lithium-based systems have a lower self-discharge.Courtesy of Cadex

The self-discharge on all battery chemistries increases at higher temperature and the rate typically doubles with every 10°C (18°F). A noticeable energy loss occurs if a battery is left in a hot vehicle. High cycle count and aging also increase self-discharge. Nickel-metal-hydride is good for 300-400 cycles, whereas the standard nickel-cadmium lasts for over 1,000 cycles before elevated self-discharge starts interfering with performance. The self-discharge on an older nickel-based battery can get so high that the pack loses its energy through leakage rather than normal use.Under normal circumstances the self-discharge of Li-ion is reasonably steady throughout its service life; however a full state-of-charge and elevated temperature increase the self-discharge. These very same factors also affect longevity, a phenomenon that applies to most batteries. Table 4 shows the self-discharge per month of Li-ion at various temperatures and state-of-charge. The high self-discharge at full state-of-charge may come as a surprise to many. This explains in part the asymptotical self-discharge characteristic when removing a battery from the charger.

Charge condition 0°C (32°F) 25°C (77°F) 60°C (140°F)Full charge

40–60% charge6%2%

20%4%

35%15%

Table 4: Self-discharge per month of Li-ion at various temperatures and state-of-chargeSelf-discharge increases with rising temperature and higher SoC. 

BU-802b: Elevating Self-discharge

All batteries are affected by self-discharge. Self-discharge is not a manufacturing defect but a battery characteristic, although poor manufacturing practices and improper handling can increase the problem. Figure 1 illustrates self-discharge in the form of leaking fluid.

Figure 1: Effects of high self-dischargeSelf-discharge increases with age, cycling and elevated temperature. Discard a battery if the self-discharge reaches 30 percent in 24 hours.Courtesy of Cadex

The amount of electrical self-discharge varies with battery type and chemistry. Primary cells such as lithium and alkaline retain the stored energy best and can be kept in storage for several years. Among rechargeable batteries, lead acid has the lowest self-discharge and loses only about five percent per month. With age and usage, however, the flooded lead acid builds up sludge in the sediment trap, which causes a soft short when this semi-conductive substance reaches the plates.Nickel-based rechargeable batteries leak the most and need recharging before use when placing them on the shelf for a few weeks. High-performance nickel-based batteries have a higher self-discharge than the standard versions. Furthermore, self-discharge increases with use and age, and the contributing factors are crystalline formation (memory), permitting the battery to “cook” in the charger or exposing it to repeated harsh deep discharge cycles.Lithium-ion self-discharges about five percent in the first 24 hours and then loses 1 to 2 percent per month; the protection circuit adds another three percent per month. A faulty separator can lead to a high self-discharge and if critical, the electrical current will generate enough heat that can in extreme cases lead to a thermal breakdown.Table 2 shows the typical self-discharge of battery systems.

Battery System Estimated Self-dischargePrimary lithium-metal 10% in 5 years

Alkaline 2-3% per year (7-10 years shelf life)

Lead-acid 5% per month

Nickel-based 10-15% in 24h, then 10-15% per month

Lithium-ion 5% in 24h, then 1-2% per month (plus 3% for safety circuit)

Table 2: Percentage of self-discharge in years and month.Primary batteries have considerably less self-discharge than secondary (rechargeable) batteries.The energy loss is asymptotical, meaning that the self-discharge is highest right after charge and then tapers off. Nickel-based batteries lose 10 to 15 percent of their capacity in the first 24 hours after charge, then 10 to 15 percent per month. Figure 3 shows the typical loss of a nickel-based battery while in storage.

Figure 3: Self-discharge as a function of timeThe discharge is highest right after charge and tapers off. The graph shows self-discharge of a nickel-based battery. Lead- and lithium-based systems have a lower self-discharge.Courtesy of Cadex

The self-discharge on all battery chemistries increases at higher temperature and the rate typically doubles with every 10°C (18°F). A noticeable energy loss occurs if a battery is left in a hot vehicle. High cycle count and aging also increase self-discharge. Nickel-metal-hydride is good for 300-400 cycles, whereas the standard nickel-cadmium lasts for over 1,000 cycles before elevated self-discharge starts interfering with performance. The self-discharge on an older nickel-based battery can get so high that the pack loses its energy through leakage rather than normal use.Under normal circumstances the self-discharge of Li-ion is reasonably steady throughout its service life; however a full state-of-charge and elevated temperature increase the self-discharge. These very same factors also affect longevity, a phenomenon that applies to most batteries. Table 4 shows the self-discharge per month of Li-ion at various temperatures and state-of-charge. The high self-discharge at full state-of-charge may come as a surprise to many. This explains in part the asymptotical self-discharge characteristic when removing a battery from the charger.

Charge condition 0°C (32°F) 25°C (77°F) 60°C (140°F)Full charge

40–60% charge6%2%

20%4%

35%15%

Table 4: Self-discharge per month of Li-ion at various temperatures and state-of-chargeSelf-discharge increases with rising temperature and higher SoC. 

BU-802c: Premature Voltage Cut-off

Not all stored battery energy can or should be used on discharge, and some reserve is almost always left behind on purpose after the equipment cuts off. There are several reasons for this.Most cell phones, laptops and other portable devices turn off when the lithium-ion battery reaches 3V/cell on discharge, and at this point the battery has about five percent capacity left. Manufacturers choose this voltage threshold to enable some time before recharging. This grace period can be several months until self-discharge lowers the voltage to about 2.5V/cell, at which point the protection circuit opens. Most packs become unserviceable when this occurs. See How to Awaken Sleeping Li-ion.A battery on a hybrid car is seldom fully charged or discharged; most operate between 20 to 80 percent state-of-charge. This is the most effective working bandwidth of a battery; it also delivers the longest service life. A deep discharge causes undue stress, and the charge acceptance above 80 percent diminishes. The emphasis on an electric powertrain and industrial applications is to maximize service life rather than optimize runtime, as it is the case with consumer products.Power tools and medical devices with high current draw tend to push the battery voltage to an early cut-off. This is especially true if one of the cells has a high internal resistance, or if operating at cold temperature. These batteries may still have ample capacity left after the “cut-off;” discharging them with a battery analyzer at a moderate load will often give a residual capacity of 30 percent. Figure 1 illustrates the cut-off voltage graphically.

Figure 1: Illustration of equipment with high cut-off voltagePortable devices do not utilize all available battery power and leave some energy behind.Courtesy of Cadex

Alkaline batteries are not suitable for high load applications because of elevated internal resistance. Cold temperature and a partially depleted cell cause the internal resistance to rise further. This advances the cut-off and much of the energy is left behind. See Function of Primary Batteries.

BU-803: Can Batteries be Restored?

Battery users and entrepreneurs often ask, “Can batteries be restored?” The answer is, “It depends.” Most battery failures are permanent and cannot be repaired, but there are exceptions. Sulfation on lead acid batteries can be removed if caught in time; crystalline formation, also known as “memory,” on nickel-cadmium can be dissolved through deep-cycling. Read more about Memory: Myth or Fact?, and “sleeping” lithium-ion packs can be boosted if they have been over-discharged. Read more about Safety circuits for modern batteries.Permanent battery defects include high internal resistance, elevated self-discharge, electrical short and capacity fade. Poorly designed chargers, exposure to excess heat, harsh charge and discharge cycles, and inappropriate storage contribute to early aging. Let’s examine the cause of these non-correctable battery problems and explore what we can do to minimize them.

Low-capacity Cells

A manufacturer cannot predict the exact capacity when a battery comes off the production line, and this is especially true with lead acid batteries that involve manual assembly. Fully automated cell production in “clean rooms” also causes performance differences, and as part of quality control, each cell is measured and segregated into categories according to their inherent capacity levels. The high-capacity A-cells are reserved for special applications and sold at premium prices; the large mid-range B-group goes to commercial and industrial markets; and the low-grade C-cells may end up as consumer products in department stores. Cycling will not significantly improve the capacity of the low-end cell, and even though the cell may look good, the buyer must be aware of differences in capacity and quality, which often translate into life expectancy.

Cell Mismatch, Balancing

Matching of cells according to capacity is important, especially for industrial batteries. No perfect match is possible, and if slightly off, nickel-based cells adapt to each other after a few charge/discharge cycles similar to the players on a winning sports team. High-quality cells continue to perform longer than the lower-quality counterpart, and the cells degrade at a more even and controlled rate. Lower-grade cells, on the other hand, diverge more quickly with use and time, and failures due to cell mismatch are more widespread. Cell mismatch is a common cause of failure in industrial batteries. Manufacturers of professional power tools and medical equipment are careful in the choice of cells to attain good battery reliability and long life.Let’s look at what a weak cell does in a pack that is strung together with strong ones. The weak cell holds less capacity and is discharged more quickly than the strong brothers. Going empty first, the strong brothers overrun this feeble sibling and the resulting current on a continued discharge pushes the weak cell into reverse polarity. Nickel-cadmium can tolerate a reverse voltage of minus 0.2V and a reverse current of a few milliamps, but exceeding this level will cause a permanent electrical short. On charge, the weak cell reaches full charge first and it goes into heat-generating over-charge while the strong brothers still accept charge and stay cool. The low cell experiences a disadvantage on both charge and discharge. It continues to weaken until finally giving up the struggle.The capacity tolerance between cells in an industrial battery should be +/– 2.5 percent. High-voltage packs designed for heavy loads and wide adverse temperature ranges should have lower tolerances. There is a strong correlation between cell balance and longevity.Li-ion cells share similar deficiencies with nickel-based systems and need management. The mandatory protection circuit supervises the serially connected cells by clamping the voltage when exceeding 4.25 and 4.35V on charge, and disconnecting the pack from discharge when the weakest cell drops to between 2.50 and 2.80V/cell. This prevents the stronger cells from pushing the depleted cell into reverse polarization. The protection circuit acts like a guardian angel that shields the weaker siblings from being bullied by the stronger brothers. This may be help to explain why Li-ion packs for power tools last longer than nickel-based batteries, which normally do not have a protection circuit.The capacity of quality Li-ion cells is consistent and the self-discharge is low. A problem arises when the cells exhibit a discrepancy in self-discharge. This can be attributed to lower-quality cells or high-temperature spots in a

large automotive battery, which hastens aging. Balancing is required and there are two methods: Passive balancing bleeds the high-voltage cells; active balancing shuttles the extra charge from higher-voltage cells to the lower-voltage cells without burning the energy. Active balancing is the preferred method on EVs.With use and time all batteries become mismatched, and this also applies to lead acid. Shorted cells and those having high self-discharge are a common cause of cell imbalance and lead to subsequent failure. Manufacturers of golf cars, aerial work platforms, floor scrubbers and other battery-powered vehicles recommend an equalizing charge of 3–4 hours if the voltage difference between the cells is greater than +/– 0.10V, or if the specific gravity varies more than 10 points (0.010 on the SG scale). An equalizing charge is a charge on top of a charge that brings all cells to full-charge saturation. This service must be administered with care because excessive charging can harm the battery. A difference in specific gravity of 40 points poses a performance problem and the cell is considered defective. A 40-point difference is one cell having an SG of 1.200 and another 1.240. A charge may temporarily cover the deficiency, but the flaw will resurface after a few hours of rest due to high self-discharge.

Shorted Cells

Manufacturers are at a loss to explain why some cells develop high electrical leakage or a short while still new. The culprit might be foreign particles that contaminate the cells during manufacture, or rough spots on the plates that damage the delicate separator. Clean rooms, improved quality control at the raw material level, and minimal human handling during the manufacturing process have reduced the “infant mortality rate.”Applying momentary high-current bursts to repair a shorted NiCd or NiMH cell has been tried but offers limited success. The short may temporarily evaporate but the damage in the separator remains. After service, the repaired cell may charge normally and reach correct voltages; however, high self-discharge will likely drain the battery and the short will return.It is not advised to replace a shorted cell in an aging pack because of cell matching. The new cell will always be stronger than the others. Consider the biblical verses: “No one sews a patch of unshrunk cloth on an old garment. If he does, the new piece will pull away from the old, making the tear worse. And no one pours new wine into old wineskins. If he does, the wine will burst the skins, and both the wine and the wineskins will be ruined” (Mark 2:21, 22 NIV). Replacing faulty cells often leads to battery failures within six months. It’s best not to disturb the cells. Instead, allow them to age naturally as an intact family.Shorts or high leakage in a Li-ion cell are uncommon. If this occurs, the cell becomes unstable and a massive amount of power can dissipate, leading to a possible venting thermal breakdown. Such a leak can be compared to drilling a small pinhole into a high-pressure gas pipeline and holding a match to it. The resulting explosion could rupture the pipe. Similarly, the rushing current in the cell heats up the tiny malfunction, causes a major leak and releases all energy within seconds. (Read more about Safety circuits for modern batteries)Cell disintegration caused by internal disturbances lies outside the safeguarding ability of the protection circuit. Most cell failures occur when the battery has been damaged by shock and vibration, has been overcharged or has been overheated. Li-ion cells for electric powertrains and demanding industrial applications use a heavy-duty separator to reduce the risk of an electrical short. These batteries are larger than consumer-type packs. Saying that Li-ion has twice the energy density of NiCd can be a misnomer; some long-lasting Li-ion cells have a specific energy as low as 60Wh/kg, the same as NiCd. 

Caution:Applying a high current burst works best with nickel-based batteries. Do not use this method for lithium-ion cells.

Loss of Electrolyte

The loss of electrolyte in a flooded lead acid battery occurs through gassing, as hydrogen escapes during charging and discharging. Venting causes the electrolyte to become more concentrated and the balance must be restored by adding clean water. Do not add electrolyte, as this would upset the specific gravity and shorten battery life through excessive corrosion.

Permeation, or loss of electrolyte in sealed lead acid batteries, is a recurring problem that is often caused by overcharging. Careful adjustment of charging and float voltages, as well as operating at moderate temperatures, reduces this failure. Replenishing lost liquid in VRLA batteries by adding water has limited success. Although the lost capacity can often be regained with a catalyst, tampering with the cells turns the stack into a high-maintenance project that needs constant supervision.Nickel-based batteries can lose electrolyte through venting due to excessive pressure during extreme charge or discharge. After repeated venting, the spring-loaded seal of the cells may not seal properly again, and the deposit of white powder around the seal opening is evidence of this. Losses of electrolyte may also occur as part of faulty manufacturing. Dry-up conditions result in a “soft” cell, a defect that cannot be corrected. On charge, the voltage of a “dry” cell goes high because the battery has no clamping action and does not draw current.A properly designed and correctly charged lithium-ion cell should not generate gases, nor should it lose electrolyte through venting. In spite of what advocates have said, lithium-based cells can build up an internal pressure under certain conditions, and a bloated pouch cell is proof of this. Read more about The Pouch Cell. Some cells include an electrical switch that opens if the cell pressure reaches a critical level. Others feature a membrane that releases gases. Many of these safety features are one-way only, meaning that once activated, the cell becomes inoperable. This is done for safety reasons.

BU-804: How to Restore Lead-acid Batteries

A lead acid battery goes through three life phases, called formatting, peak and decline (Figure 1). In the formatting phase, try to imagine sponge-like lead plates that are being exposed to a liquid. Exercising the plates allows the absorption of  more liquid, much like squeezing and releasing a sponge. This enables the electrolyte to better fill the usable areas, an exercise that increases the capacity.Formatting is most important for deep-cycle batteries and requires 20 to 50 full cycles to reach peak capacity. Field usage achieves  this. There is no need to apply added cycles for the sake of priming; however, manufacturers recommend  to go easy on the battery until broken in. Starter batteries are less critical and do not need priming; the full cranking power is present right from the beginning, although the CCA reading will go up slightly with early use.

Figure 1: Cycle life of a batteryThe three phases of a battery are formatting, peak and decline.Courtesy of Cadex

A deep-cycle battery delivers 100–200 cycles before it starts the gradual decline. Replacement should occur when the capacity drops to 70 or 80 percent. Some applications allow lower capacity thresholds but the time for retirement should not fall below 50 percent because the aging occurs rapidly once the battery is past its prime. Apply a fully saturated charge of 14 to 16 hours. Operating at moderate temperatures assure the longest service times. If at all possible, avoid deep discharges; charge more often.The primary reason for the relatively short cycle life of a lead acid battery is depletion of the active material. According to the 2010 BCI Failure Modes Study,* plate/grid-related breakdown has increased from 30 percent five years ago to 39 percent. The report does not give reasons for the increased wear-and-tear, other than to assume that higher demands of starter batteries in modern cars induce added stress.While the depletion of the active material is well understood and can be calculated, a lead acid battery suffers from other infirmities long before plate- and grid-deterioration sound the death knell. The following articles address the most common problems that develop with use and time and what battery users can do to minimize the effect.

These are:

Corrosion / Shedding, Internal Short

Sulfation

Water Loss, Acid Stratification and Surface Charge

                                   *   Every five years, the Battery Council International Technical Subcommittee conducts a study to determine the failure modes of batteries that have been removed from service. 

BU-804a: Corrosion, Shedding and Internal Short

Corrosion occurs primarily on the grid and is known as a softening and shedding of lead off the plates. This reaction cannot be avoided because the electrodes in a lead acid environment are always reactive. Lead shedding is a natural phenomenon that can only be slowed down and not eliminated. A battery that reaches the end of life through this failure mode has met or exceeded the anticipated life span. Limiting the depth of discharge, reducing the cycle count, operating at a moderate temperature and controlling overcharge are key in keeping corrosion in check. To reduce corrosion on long-life batteries, manufacturers keep the specific gravity at a moderate 1.200 when fully charged, compared to 1.265 and greater for high-performing  lead acid batteries Read about How to Measure State-of-charge. A lower specific gravity reduces the capacity the battery can hold.Applying prolonged overcharge is another contributor to grid corrosion. This is especially damaging tosealed lead acid systems. While the flooded lead acid has some resiliency to overcharge, sealed units must operate at a correct float charge. Chargers with variable float voltages adjust to the prevailing temperature to help to keep grid corrosion in check. Such chargers are in common use for stationary batteries.To attain maximum surface area, the lead on a starter battery is applied in a sponge-like form. With time and use, chunks of lead fall off and reduce the performance. Figure 1 illustrates the innards of a corroded lead acid battery.

Figure 1: Innards of a corroded lead acid batteryGrid corrosion is unavoidable

The terminals of a battery can also corrode, and this is often visible in the form of white powder. The phenomenon is a result of oxidation between two different metals connecting the poles. Terminal corrosion can eventually lead to an open electrical connection. Changing the connecting terminals to lead, the same material as the battery pole of a starter battery, will solve most corrosion problems.

Internal Short

The term “short” is commonly used to describe a general battery fault when no other definition is available. As the colloquial term “memory” was the cause for all battery ills in the NiCd days, so do battery users often judge non-functioning lead acid batteries simply as being “shorted.” Let’s take a closer look and see what a shorted lead acid battery truly is.The lead within a battery, especially in deep-cycle units, is mechanically active and when a battery discharges, the lead sulfate causes the plates to expand. This movement reverses during charge and the plates contract. The cells allow for some expansion but over time the growth of large sulfite crystals can result in a soft short that increases self-discharge. This mechanical action also causes shedding of the lead material. On a starter battery, the shedding is manageable because the lead plates are thin and the battery does not go through a deep discharge. On a deep-cycle battery, on the other hand, shedding is a concern.As the battery sheds its lead to the bottom of the container, a conductive layer forms, and once the contaminated material fills the allotted space in the sediment trap, the now conductive liquid reaches the plates and creates a shorting effect. The term “short” is a misnomer and elevated self-discharge or a soft short would be a better term to describe the condition.

“Soft shorts” are difficult to detect because the battery appears normal immediately after a charge and everything seems to function as it should. In essence, the charge has wiped out all evidence of a soft short, except perhaps an elevated temperature on the battery housing. Once rested for 6–12 hours, the battery begins to show anomalies such as a lower open-circuit voltage and reduced specific gravity. The measured capacity will also be low because self-discharge has consumed some of the stored energy. According to the 2010 BCI Failure Modes Study, shorted batteries accounted for 18 percent of battery failures, a drop from 31 percent five years earlier. Improved manufacturing methods may account for this reduction.Another form of soft short is mossing. This occurs when the separators and plates are slightly misaligned as a result of poor manufacturing practices. This causes parts of the plates to become naked. The exposure promotes the formation of conductive crystal moss around the edges, which leads to elevated self-discharge.Lead drop is another cause of short in which large chunks of lead break loose from the welded bars connecting the plates. Unlike a “soft” short that develops with wear-and-tear, a lead drop often occurs early in battery life. This causes a more serious short and is associated with a permanent voltage drop. The shorted cell may have little or no charge and the specific gravity of the electrolyte is close to 1.00. This is mostly a manufacturing defect and cannot be repaired.The most radical and serious form of short is a mechanical failure in which the suspended plates become loose and touch each other. This results in a sudden high discharge current that can lead to excessive heat buildup and thermal runaway. Sloppy manufacturing as well as excessive shock and vibration are the most common contributors to this failure.

BU-804b: Sulfation and How to Prevent it

Sulfation occurs when a lead acid battery is deprived of a full charge. This is common with starter batteries in cars driven in the city with load-hungry accessories. A motor in idle or at low speed cannot charge the battery sufficiently.Electric wheelchairs have a similar problem in that the users might not charge the battery long enough. An eight-hour charge during the night when the chair is free is not enough. Lead acid must periodically be charged 14–16 hours to attain full saturation. This may be the reason why wheelchair batteries last only two years, whereas golf car batteries deliver twice the service life. Longer leisure time allows golf car batteries to get the fully saturated charge.Solar cells and wind turbines do not always provide sufficient charge, and lead acid banks succumb to sulfation. This happens in remote parts of the world where villagers draw generous amounts of electricity with insufficient renewable resources to charge the batteries. The result is a short battery life. Only a periodic fully saturated charge could solve the problem, but without an electrical grid at their disposal, this is almost impossible. An alternative is using lithium-ion, a battery that is forgiving to a partial charge, but this would cost about six-times as much as lead acid.What is sulfation? During use, small sulfate crystals form, but these are normal and are not harmful. During prolonged charge deprivation, however, the amorphous lead sulfate converts to a stable crystalline that deposits on the negative plates. This leads to the development of large crystals, which reduce the battery’s active material that is responsible for high capacity and low resistance. Sulfation also lowers charge acceptance. Sulfation charging will take longer because of elevated internal resistance.There are two types of sulfation: reversible (or soft sulfation), and permanent (or hard sulfation). If a battery is serviced early, reversible sulfation can often be corrected by applying an overcharge to a fully charged battery in the form of a regulated current of about 200mA. The battery terminal voltage is allowed to rise to between 2.50 and 2.66V/cell (15 and 16V on a 12V mono block) for about 24 hours. Increasing the battery temperature to 50–60°C (122–140°F) further helps in dissolving the crystals. Permanent sulfation sets in when the battery has been in a low state-of-charge for weeks or months. At this stage, no form of restoration is possible.There is a fine line between reversible and non-reversible sulfation, and most batteries have a little bit of both. Good results are achievable if the sulfation is only a few weeks old; restoration becomes more difficult the longer the battery is allowed to stay in a low SoC. A sulfated battery may improve marginally when applying a de-sulfation service. A subtle indication of whether a lead acid can be recovered is visible on the voltage discharge curve. If a fully charged battery retains a stable voltage profile on discharge, chances of reactivation are better than if the voltage drops rapidly with load.Several companies offer anti-sulfation devices that apply pulses to the battery terminals to prevent and reverse sulfation. Such technologies tend to lower sulfation on a healthy battery but they cannot effectively reverse the condition once present. Manufacturers offering these devices take the “one size fits all” approach and the method is unscientific. A random service of pulsing or blindly applying an overcharge can harm the battery in promoting grid corrosion. Technologies are being developed that measure the level of sulfation and apply a calculated overcharge to dissolve the crystals. Chargers featuring this technique only apply de-sulfation if sulfation is present and only for the time needed.

BU-804c: Water Loss, Acid Stratification and Surface Charge

During use, and especially on overcharge, the water in the electrolyte splits into hydrogen and oxygen. The battery begins to gas, which results in water loss. In flooded batteries, water can be added but in sealed batteries, water loss leads to an eventual dry-out and decline in capacity. Water loss from a sealed unit can eventually cause disintegration of the separator. The initial stages of dry-out can go undetected and a drop in capacity may not be immediately evident. Early detection of this failure is important. Read about Charging Lead Acid, under Watering.On overcharge, a battery becomes a “water-splitting device” that turns water into oxygen and hydrogen. A parallel can be made with the fuel cell, but this device does the opposite; it turns oxygen and hydrogen back to electricity and produces water. Turning water into hydrogen needs energy and in a battery this is in the form of overcharge. Converting hydrogen and oxygen back to water regenerates energy. Read about the Fuel Cell.

Acid Stratification

The electrolyte of a stratified battery concentrates at the bottom, starving the upper half of the cell. Acid stratification occurs if the battery dwells at low charge (below 80 percent), never receives a full charge and has shallow discharges. Driving a car for short distances with power-robbing accessories contributes to acid stratification because the alternator cannot always apply a saturated charge. Large luxury cars are especially prone to acid stratification. This is not a battery defect per se but the result of use. Figure 1 illustrates a normal battery in which the acid is equally distributed from top to bottom.

Figure 1: Normal batteryThe acid is equally distributed from the top to the bottom of the battery, providing good overall performance.Courtesy of Cadex

Figure 2 shows a stratified battery in which the acid concentration is light on top and heavy on the bottom. The light acid on top limits plate activation, promotes corrosion and reduces the performance, while the high acid concentration on the bottom makes the battery appear more charged than it is and artificially raises the open-circuit voltage. Because of unequal charge across the plates, CCA performance, or the ability to crank the engine, is also reduced.

Figure 2: Stratified batteryThe acid concentration is light on top and heavy on the bottom. This raises the open circuit voltage and the battery appears fully charged. Excessive acid concentration induces sulfation on the lower half of the plates.Courtesy of Cadex

Allowing the battery to rest for a few days, doing a shaking motion or tipping the battery on its side helps correct the problem. Applying an equalizing charge by raising the voltage of a 12-volt battery to 16 volts for one to two hours also helps by mixing the electrolyte through electrolysis. Avoid extending the topping charge beyond its recommended time.Acid stratification cannot always be avoided. During cold winter months, starter batteries of most passenger cars dwell at a 75 percent charge level. Knowing that motor idling and driving in gridlocked traffic does not sufficiently charge the battery, a charge with an external charger may be needed from time to time. If this is not practical, switch to an AGM battery [See BU-201a, Absorbent Glass Mat (AGM)]. AGM does not suffer from acid

stratification and is less subject to sulfation if undercharged than is the case with the flooded version. AGM is a little more expensive than the flooded starter battery but tends to last longer.

Surface Charge

Lead acid batteries are sluggish and cannot convert lead sulfate to lead and lead dioxide quickly enough during charge. As a result, most of the charge activities occur on the plate surfaces. This induces a higher state-of-charge on the outside than in the inner plate. A battery with surface charge has a slightly elevated voltage. To normalize the condition, switch on electrical loads to remove about one percent of the battery’s capacity, or allow the battery to rest for a few hours. Surface charge is not a battery defect but a reversible condition resulting from charging.

Simple Guidelines for Extending Battery Life

Allow a fully saturated charge of 14–16 hours. Charge in a well-ventilated area. 

Always keep lead acid charged. Avoid storage below 2.10V/cell, or at a specific gravity level below 1.190. 

Avoid deep discharges. The deeper the discharge, the shorter the battery life will be. A brief charge on a 1 to 2 hour break during heavy use prolongs battery life. 

Never allow the electrolyte to drop below the tops of the plates. Exposed plates sulfate and become inactive. When low, add only enough water to cover the exposed plates before charging; fill to the correct level after charge. 

Never add acid. This would raise the specific gravity too high and cause excessive corrosion. 

Use distilled or ionized water. Tap water may be usable in some regions. 

When new, a deep-cycle battery may have a capacity of 70 percent or less. Formatting as part of field use will gradually increase performance. Apply a gentle load for the first five cycles to allow a new battery to format. 

New batteries with low capacity many not perform as well as those that begin life with a high capacity. Low performers are known to have a short life. A capacity check as part of acceptance is advisable.  

BU-805: Additives to Boost Flooded Lead Acid

Adding chemicals to the electrolyte of flooded lead acid batteries can reduce the buildup of lead sulfate on the plates and improve the overall battery performance. This treatment has been in use since the 1950s (and perhaps longer) and provides a temporary performance boost for aging batteries. It’s a stopgap measure because in most cases the plates have already been worn out through shedding. Chemical additives cannot replace the active material, nor can cracked plates, corroded connectors or damaged separators be restored with an outside remedy.Extending the service life of an aging battery is a noble desire. The additives are cheap, readily available and worth the experiment of a handyman. Suitable additives are magnesium sulfate (Epsom salt), caustic soda and EDTA. (EDTA is a crystalline acid used in industry.) These salts may reduce the internal resistance of a sulfated battery to give it a few months of extra life. Using Epsom salt, follow these easy steps:Heat up the water to about 66°C (150°F), mix 10 heaping tablespoons of Epsom salt into the water and stir until dissolved. The consistency of the brew should vary according to the extent of the sulfation. Avoid using too much salt because a heavy concentration will increase corrosion of the lead plates and internal connectors. Pour the warm solution into the battery.Be careful not to overfill. Do not place un-dissolved Epsom salt directly into the battery because the substance does not dissolve well. In place of Epsom salt, try adding a pinch of caustic soda. Charge or equalize the battery after service. The results are not instantaneous and it may take a month for the treatment to work. The outcome is not guaranteed.

BU-806: Tracking Battery Capacity and Resistance as part of Aging

Let’s examine the aging mechanism of batteries in terms of fading capacity and increasing internal resistance. Figure 1 shows a battery with high capacity and another that has aged. The capacity loss is illustrated with growing “rock content;” the rocks mark the unusable part of the battery. Figure 2 looks at resistance and illustrates a good battery with a free-flowing tap and a high-resistance one with restricted flow.

             New battery has high capacity       Aged battery has low capacityFigure 1: Battery capacity illustrated as liquid content. Both batteries are fully charged, but the “rock-content” limits the amount of energy being stored.

Battery with high CCABattery with low CCA

Figure 2: Free-flowing and restricted taps representing CCA performance.The cranking current is about 300A. (A golf cart typically draws 56A.)

Automotive technicians are most familiar with CCA, but this reading reflects engine cranking only. Capacity, the energy storage component, remains mostly unknown. Figure 3 illustrates the relationship between CCA and capacity on hand of a fluid-filled container. The liquid represents capacity and the taps symbolize CCA at different loading capabilities.

Figure 3: Relationship of CCA and capacity of a starter batteryCapacity represents energy content and CCA is power delivery. A battery with 40% capacity can still have a healthy crank but the low capacity indicates end-of-life.

Most rechargeable batteries maintain low internal resistance during the service life, and this reflects in a high CCA (cold cranking amps) on starter batteries. Capacity, on the other hand, begins to drop gradually as the battery ages. To study these changes, Cadex measured the capacity and CCA of 20 aging starter batteries. The results are laid out in Figure 4, sorted according to capacity levels in percentage.

Figure 4: Capacity and CCA readings of 20 aging batteries. Batteries 1 to 9 have good CCA and high capacity; the CCA of batteries 10 to 20 remains reasonably strong but suffers from capacity loss. CCA tends to remain high while the capacity drops steadily as part of aging.

Test method: CCA was estimated with the Spectro CA-12 and the capacity was measured with an Agilent load bank by applying full discharges according to BCI standards.Courtesy of CadexBatteries 1 to 9 perform well on capacity and CCA, but batteries 10 to 20 show notable capacity loss while the CCA remains strong. The motorist is unaware of the fading capacity until the car won’t start one morning. This is especially critical during a cold spell, which further reduces the capacity.Capacity is the leading health indicator of a battery, and car manufacturers often use 65 percent as the pass/fail threshold for warranty replacement. This magic level forms a natural bend, a cliff between a high performing battery and one that is beginning to age. Service garages usually take 40 percent as an end-of-life indication. Read more about How to Measure Capacity. Even though a starter battery with 40 percent capacity may still crank well and have 6 to 12 months of service left before it will finally quit, the battery should be replaced. Thrifty drivers, (including this author) prefer to wait, but invariably get caught with a dead battery at the worst possible moment. Evaluating the capacity of a starter battery gives the most accurate end-of-life prediction. Capacity sets the floor upon which CCA and other readings are compared. Without knowing capacity, other measurements mean little.

BU-806a: How Heat and Loading affect Battery Life

Heat is a killer of all batteries and high temperatures cannot always be avoided. This is the case with a battery inside a laptop, a starter battery under the hood of a car and stationary batteries in a tin shelter under the hot sun. As a guideline, each 8°C (15°F) rise in temperature cuts the life of a sealed lead acid battery in half. A VRLA battery for stationary applications that would last 10 years at 25°C (77°F) would only live for five years if operated at 33°C (92°F). The same battery would desist after 2½ years if kept at a constant desert temperature of 41°C (106°F). Once the battery is damaged by heat, the capacity cannot be restored. The life of a battery also depends on the activity and is shortened if the battery is stressed with frequent discharge.According to the 2010 BCI Failure Mode Study, starter batteries have become more heat-resistant over the past 10 years. In the 2000 study, a change of 7°C (12°F) affected battery life by roughly one year; in 2010 the heat tolerance has widened to 12°C (22°F). In 1962, a starter battery lasted 34 months, and in 2000 the life expectancy had increased to 41 months. In 2010, BCI reports an average age of 55 months of use. The cooler North attains 59 months and the warmer South 47 months.Cranking the engine poses minimal stress on a starter battery. This changes in a start-stop function of amicro hybrid. The micro hybrid turns the IC engine off at a red traffic light and restarts it when the traffic flows. This results in about 2,000 micro cycles per year. Data obtained from car manufacturers show a capacity drop to about 60 percent after two years of use in this configuration. To solve the problem, automakers are using specialty AGM and other variations that are more robust than the regular lead acid. Read more about Alternate Battery Systems. Figure 5 shows the drop in capacity after 700 micro cycles. The simulated start-stop test was performed in Cadex laboratories. CCA remains high.

Figure 1:Capacity drop of a flooded starter battery when micro cyclingStart-stop functionon a micro hybrid stresses the battery; the capacity drops to about 50 percent after two years of use. AGM is more robust for this application.Courtesy of Cadex, 2010

Test method:   The test battery was fully charged and then discharged to 70 percent to resemble the SoC of a micro hybrid in real life. The battery was then discharged at 25A for 40 seconds to simulate engine off condition at stoplight with the headlight on, before cranking the engine at 400A and recharging. The CCA readings were taken with the Spectro CA-12.The cell voltages on a battery string must be similar, and this is especially important for higher-voltage VRLA batteries. With time, individual cells fall out of line, and applying an equalizing charge every six months or so should theoretically bring the cells back to similar voltage levels. While equalizing will boost the needy cells, the healthy cell get stressed if the equalizing charge is applied carelessly. What makes this service so difficult is the inability to accurately measure the condition of each cell and provide the right dose of remedy. Gel and AGM batteries have lower overcharge acceptance than the flooded version and different equalizing conditions apply. Always refer to the manufacturer’s specifications.Water permeation, or loss of electrolyte, is a concern with sealed lead acid batteries, and overcharging contributes to this condition. While flooded systems accept water, a fill-up is not possible with VRLA. Adding water has been tried, but this does not offer a reliable fix. Experimenting with watering turns the VRLA into unreliable battery that needs high maintenance.

Flooded lead acid batteries are one of the most reliable systems. With good maintenance these batteries last up to 20 years. The disadvantages are the need for watering and providing good ventilation. When VRLA was introduced in the 1980s, manufacturers claimed similar life expectancy to flooded systems, and the telecom industry switched to these maintenance-free batteries. By mid 1990 it became apparent that the life for VRLA did not replicate that of a flooded type; the useful service life was limited to only 5–10 years. It was furthermore noticed that exposing the batteries to temperatures above 40°C (104°F) could cause a thermal runaway condition due to dry-out.A new lead acid battery should have an open circuit voltage of 2.125V/cell. At this time, the battery is fully charged. During buyer acceptance, the lead acid may drop to between 2.120V and 2.125V/cell. Shipping, dealer storage and installation will decrease the voltage further but the battery should never go much below 2.10V/cell. This would cause sulfation. Battery type, applying a charge or discharge within 24 hours before taking a voltage measurement, as well as temperature will affect the voltage reading. A lower temperature lowers the OCV; warm ambient raises it.  

BU-807: How to Restore Nickel-based Batteries

During the nickel-cadmium years in the 1970s and 1980s, most battery ills were blamed on “memory.” Memory is derived from “cyclic memory,” meaning that a nickel-cadmium battery could remember how much energy was drawn on previous discharges and would not deliver more than was demanded before. On a discharge beyond regular duty, the voltage would abruptly drop as if to rebel against pending overtime. Improvements in battery technology have virtually eliminated the phenomenon of cycling memory.Figure 1 illustrates the stages of crystalline formation that occur on a nickel-cadmium cell if overcharged and not maintained with periodic deep discharges. The first enlargement shows the cadmium plate in a normal crystal structure; the middle image demonstrates full-blown crystalline formation; and the third reveals some form of restoration.

New nickel-cadmium cell.The anode (negative electrode) is in fresh condition. Hexagonal cadmium-hydroxide crystals are about 1 micron in cross section, exposing large surface area to the electrolyte for maximum performance.Cell with crystalline formation.Crystals havegrown to 50 to 100 microns in cross section, concealing large portions of the active material from the electrolyte. Jagged edges and sharp corners can pierce the separator, leading to increased self-discharge or electrical short.Restored cell.After a pulsed charge, the crystals are reduced to 3–5 microns, an almost 100% restoration. Exercise or recondition is needed if the pulse charge alone is not effective.

Figure 1: Crystalline formation on nickel-cadmium cell. Crystalline formation occurs over a few months if battery is overcharged and not maintained with periodic deep discharges.Courtesy of the US Army Electronics Command in Fort Monmouth, NJThe modern nickel-cadmium battery is no longer affected by cyclic memory but suffers from crystalline formation.The active cadmium material is applied on the negative electrode plate, and with incorrect use a crystalline formation occurs that reduces the surface area of the active material. This lowers battery performance. In advanced stages, the sharp edges of the forming crystals can penetrate the separator, causing high self-discharge that can lead to an electrical short. The term “memory” on the modern NiCd refers to crystalline formation rather than the cycling memory of old.When nickel-metal-hydride was introduced in the early 1990s, this chemistry was promoted as being memory-free but this claim is only partially true. NiMH is also subject to memory but to a lesser degree than NiCd. While NiMH has only the nickel plate to worry about, NiCd also includes the memory-prone cadmium negative electrode. This is a non-scientific explanation of why nickel-cadmium is more susceptible to memory than nickel-metal-hydride.Crystalline formation occurs if a nickel-based battery is left in the charger for days or repeatedly recharged without a periodic full discharge. Since most applications fall into this user pattern, NiCd requires a periodic discharge to one volt per cell to prolong service life. A discharge/charge cycle as part of maintenance, known as exercise, should be done every one to three months.Avoid over-exercising as this wears down the battery unnecessarily.

If regular exercise is omitted for six months and longer, the crystals ingrain themselves and a full restoration with a discharge to one volt per cell may no longer be sufficient. However, a restoration is often still possible by applying a secondary discharge called “recondition.” Recondition is a slow discharge that drains the battery to a voltage cut-off point of about 0.4V/cell and lower. Tests done by the US Army indicate that a NiCd cell needs to be discharged to at least 0.6V to effectively break up the more resistant crystalline formations. During this corrective discharge, the current must be kept low to minimize cell reversal and, as discussed earlier, NiCd can tolerate a small amount of cell reversal. Figure 2 illustrates the battery voltage during a discharge to 1V/cell, followed by the secondary discharge to 0.4V/cell.

Figure 2: Exercise and recondition features of a Cadex battery analyzerRecondition restores NiCd batteries with hard-to-remove memory. Recondition is a slow, deep dis-charge to 0.4V/cell.Courtesy of Cadex

Recondition is most effective with healthy batteries and the remedy is also known to improve new packs. Similar to a medical treatment, however, the service should only be applied when so needed because over-use will stress the battery. Automated battery analyzers (Cadex) only apply the recondition cycle if the user-set target capacity cannot be reached.Recondition is only effective on working batteries. Best results in recovery are possible when applying a full discharge every 1–3 months. If exercise has been withheld for 6–12 months, the capacity may not recover fully, and if it does the pack might suffer from high self-discharge caused by a marred separator. Older batteries do not restore well and many get worse with recondition. When this happens, the battery is a ripe candidate for retirement.

Results of Battery Maintenance

After the Balkan War in the 1990s, the Dutch Army began servicing its arsenal of nickel-cadmium batteries that had been used for the two-way radios. The technicians in charge wanted to know the remaining capacity and how many batteries could be restored to full service using battery analyzers (Cadex). The army knew that allowing the batteries to sit in the chargers with only two to three hours of use per day during the war was not ideal, and the tests showed that the capacity on some packs had dropped to a low 30 percent. With the recondition function, however, nine out of 10 batteries could be restored to 80 percent and higher. The army uses 80 percent as a threshold for usability. At time of service, the nickel-cadmiumbatteries were two to three years old.To analyze the effectiveness of battery maintenance further, the US Navy carried out a study to find out how user pattern affects the life of nickel-cadmium batteries. For this, the research team responsible for the program established three battery groups. One group received charge only (no maintenance); another was periodically exercised (discharge to 1V/cell); and a third group received recondition. The 2,600 batteries studied were used for Motorola two-way radios deployed on three US aircraft carriers. Table 3 summarizes the test results, including the cost factor.

Maintenance methodAnnual % of batteries requiring replacement

Annual battery cost(US$)

Charge-and-use onlyExerciseRecondition

45%14%5%

$40,500$13,500$4,500

Table 3: Replacement rates of nickel-cadmium batteriesExercise and recondition prolong battery life by three- and nine-fold respectively.GTE Government Systems, the organization that conducted the test, learned that with charge-and-use the annual percentage of battery failure was 45 percent; with exercise the failure rate was reduced to 15 percent; and with recondition only 5 percent failed. The GTE report concludes that a battery analyzer featuring exercise and recondition costing US$2,500 would return the investment in less than one month on battery savings alone.

BU-808: How to Prolong Lithium-based Batteries

The lithium-ion battery works on ion movement between the positive and negative electrodes. In theory such a mechanism should work forever, but cycling, elevated temperature and aging decrease the performance over time. Since batteries are used in demanding environmental conditions, manufacturers take a conservative approach and specify the life of most Li-ion between 300 and 500 discharge/charge cycles.Counting cycles is not conclusive because a discharge may vary in depth and there are no clearly defined standards of what constitutes a cycle. Read more about What Constitutes a Discharge Cycle?.  In lieu of cycle count, some batteries in industrial instruments are date-stamped, but this method is not reliable either because it ignores environmental conditions. A battery may fail within the allotted time due to heavy use or unfavorable temperature conditions, but most quality packs will last considerably longer than what the stamp indicates.The performance of a battery is measured in capacity, a leading health indicator. Internal resistance and self-discharge also play a role but with modern Li-ion these carry lower significance in predicting the end-of-battery-life. Figure 1 illustrates the capacity drop of 11 Li-polymer batteries that have been cycled at a Cadex laboratory. The 1500mAh pouch cells for smartphones were first charged at a current of 1500mA (1C) to 4.20V/cell and allowed to saturate to 0.05C (75mA) as part of the full charge procedure. The batteries were then discharged at 1500mA to 3.0V/cell, and the cycle was repeated. 

Figure 1: Capacity drop as part of cyclingA pool of new 1500mAh Li-ionbatteries for smartphones is tested on a Cadex C7400 battery analyzer. All 11 pouch packs show a starting capacity of 88–94 percent and decrease in capacity to 73–84 percent after 250 full discharge cycles (2010).Courtesy of Cadex

Although a battery should deliver 100 percent capacity during the first year of service, it is common to see lower than specified capacities, and shelf life may have contributed to this loss. In addition, manufacturers tend to overrate their batteries; knowing that very few customers would complain. In our test, the expected capacity loss of Li-ion batteries was uniform over the 250 cycles and the batteries performed as expected.Similar to a mechanical device that wears out faster with heavy use, so also does the depth of discharge (DoD) determine the cycle count. The shorter the discharge (low DoD), the longer the battery will last. If at all possible, avoid full discharges and charge the battery more often between uses. Partial discharge on Li-ion is fine; there is no memory and the battery does not need periodic full discharge cycles to prolong life, other than to calibrate the fuel gauge on a smart battery once in a while. Read more about Battery Calibration.Table 2 compares the number of discharge/charge cycles Li-ion can deliver at various DoD levels before the battery capacity drops to 70 percent. The number of discharge cycles depends on many conditions and includes charge voltage, temperature and load currents. Not all Li-ion systems behave the same.

Depth of discharge Discharge cycles Table 2: Cycle life as a function ofdepth of dischargeA partial discharge reduces stress and prolongs battery life. Elevated temperature and high currents also affect cycle life.

100% DoD50% DoD25% DoD10% DoD

300 – 5001,200 – 1,5002,000 – 2,5003,750 – 4,700

Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion, a voltage above 4.10V/cell is deemed as high voltage. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling. Table 3 demonstrates capacity loss as a function of temperature and SoC.

Temperature 40% charge 100% charge Table 3: Estimated recoverable capacity when storing Li-ion for one year at various temperaturesElevated temperature hastens capacity loss. The capacity cannot be restored. Not all Li-ion systems behave the same.

0°C25°C40°C60°C

98%96%85%75%

94%80%65%60%

(after 3 months)

Most Li-ions are charged to 4.20V/cell and every reduction of 0.10V/cell is said to double cycle life.  For example, a lithium-ion cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles; 4.00V/cell should deliver 1,200–2,000 and 3.90V/cell 2,400–4,000 cycles. Table 4 summarizes these results. The values are estimate and depend on the type of li-ion-ion battery.

Charge level(V/cell) Discharge cyclesCapacity at full

chargeTable 4: Discharge cycles and capacityas a function of chargeEvery 0.10V drop below 4.20V/cell doubles the cycle; the retained capacity drops accordingly. Raising the voltage above 4.20V/cell stresses the battery and compromises safety.

[4.30]4.204.104.003.92

[150 – 250]300 – 500

600 – 1,0001,200 – 2,0002,400 – 4,000

~[110%]100%~90%~80%~75%

For safety reasons, lithium-ion cannot exceed 4.20V/cell. While a higher voltage would boost capacity, over-voltage shortens service life and compromises safety. Figure 5 demonstrates cycle count as a function of charge voltage. At 4.35V, the cycle count is cut in half.

Figure 5: Effects on cycle life at elevated charge voltagesHigher charge voltages boost capacity but lowers cycle life and compromises safety.Source: Choi et al. (2002)

Chargers for cellular phones, laptops, tablets and digital cameras bring the Li-ion battery to 4.20V/cell. This allows maximum capacity, because the consumer wants nothing less than optimal runtime. Industry, on the other hand, is more concerned about longevity and may choose lower voltage thresholds. Satellites and electric vehicles are examples where longevity is more important than capacity.Charging to 4.10V/cell the battery holds about 10 percent less capacity than going all the way to 4.20V. In terms of optimal longevity, a voltage limit of 3.92V/cell works best but the capacity would only be about half compared to a 4.20V/cell charge (3.92V/cell is said to eliminate all voltage-related stresses).Besides selecting the best-suited voltage thresholds for a given application, Li-ion should not remain at the high-voltage ceiling of 4.20V/cell for an extended time. When fully charged, remove the battery and allow to voltage to

revert to a more natural level like relaxing after exercise. Although a properly functioning Li-ion charger will terminate charge when the battery is full, some chargers apply a topping charge if the battery terminal voltage drops to a given level. Read more about Charging Lithium-ion.

What the user can do

The author of this essay does not depend on the manufacturer’s specifications alone but also listens to user comments. BatteryUniversity.comis an excellent sounding board to connect with the public and learn about reality. This approach might be unscientific, but it is genuine. When the critical mass speaks, the manufacturers listen. The voice of the multitude is in some ways stronger than laboratory tests performed in sheltered environments.Tables 2, 3 and 4 look at cycle life as a function of discharge, temperature and charge level. A summary table should be added that also states the Optimal Battery Energy Factor Over Life. While this would help in selecting the optimal battery, battery makers are hesitant to release such a specification freely, and for good reason. A battery is in constant flux and capturing all of its data is exhaustive. A further criterion is price. Batteries can be built to perform better but this comes at a cost.Let’s look at real-life situations and examine what stresses lithium-ion batteries encounter. Most packs last three to five years. Environmental conditions, and not cycling alone, are a key ingredient to longevity, and the worst situation is keeping a fully charged battery at elevated temperatures. This is the case when running a laptop off the power grid. Under these conditions, a battery will typically last for about two years, whether cycled or not. The pack does not die suddenly but will give lower runtimes with aging.Even more stressful is leaving a battery in a hot car, especially if exposed to the sun. When not in use, store the battery in a cool place. For long-term storage, manufacturers recommend a 40 percent charge. This allows for some self-discharge while still retaining sufficient charge to keep the protection circuit active. Finding the ideal state-of-charge is not easy; this would require a discharge with appropriate cut-off. Do not worry too much about the state-of-charge; a cool and dry place is more important than SoC. Read more about How to Store Batteries.Batteries are also exposed to elevated temperature when charging on wireless chargers. The energy transfer from a charging mat to a portable device is 70 to 80 percent and the remaining 20 to 30 percent is lost mostly in heat that is transferred to the battery through the mat. We keep in mind that the mat will cool down once the battery is fully charged. Read more about Charging Without Wires.Avoid charging a battery faster than 1C; a more moderate charge rate of 0.7C is preferred. Manufacturers of electric powertrains are concerned about super-fast charging of 20 minutes and less. Similarly, harsh discharges should be avoided as also this also adds to battery stress. Read more about Charging Lithium-ion and Ultra-fast Chargers . Commercial chargers do not allow changing the charge voltage limit. Adding this feature would have advantages, especially for laptops as a means to prolong battery life. When running on extended AC mode, the user could select the “long life” mode and the battery would charge to 4.00V/cell for a standby capacity of about 70 percent. Before traveling, the user would apply the “full charge mode” to bring the charge to 100%. Some laptop manufacturers may offer this feature but often only computer geeks discover them.Another way to extend battery life is to remove the pack from the laptop when running off the power grid. The Consumer Product Safety Commissionadvises to do this out of concern for overheating and causing a fire. Removing the battery has the disadvantage of losing unsaved work if a power failure occurs. Heat buildup is also a concern when operating a laptop in bed or on a pillow, as this may restrict airflow. Placing a ruler or other object under the laptop will improve air circulation and keep the device cooler.“Should I disconnect my laptop from the power grid when not in use?” many ask. Under normal circumstances this should not be necessary because once the lithium-ion battery is full the charger discontinues charge and only engages when the battery voltage drops. Most users do not remove the AC power and I like to believe that this practice is safe.Everyone wants to keep the battery as long as possible, but a battery must often operate in environments that are not conducive to optimal service life. Furthermore, the life of a battery may be cut short by an unexpected failure, and in this respect the battery shares human volatility.

To get a better understanding of what causes irreversible capacity loss in Li-ion batteries, several research laboratories* are performing forensic tests. Scientists dissected failed batteries to find suspected problem areas on the electrodes. Examining an unrolled 1.5-meter-long strip (5 feet) of metal tape coated with oxide reveals that the finely structured nanomaterials have coarsened. Further studies revealed that the lithium ions responsible to shuttle electric charge between the electrodes had diminished on the cathode and had permanently settled on the anode. This results in the cathode having a lower lithium concentration than a new cell, a phenomenon that is irreversible. Knowing the reason for such capacity loss might enable battery manufacturers to prolong battery life in the future.                                   

*   Research is performed by the Center for Automotive Research at the Ohio State University in collaboration with Oak Ridge National Laboratory and the National Institute of Standards Technology.

BU-808a: How to Awaken Sleeping Li-ion

Li-ion batteries contain a protection circuit that shields the battery against abuse. This important safeguard has the disadvantage of turning the battery off if over-discharged, and storing a discharged battery for any length of time can do this. The self-discharge during storage gradually lowers the voltage of a battery that is already discharged; the protection circuit will eventually cut off between 2.20 and 2.90V/cell.Some battery chargers and analyzers, including those made by Cadex, feature a wake-up feature or “boost” to reactivate and charge batteries that have fallen asleep. Without this feature, a charger would render these batteries as unserviceable and the packs would be discarded. The boost feature applies a small charge current to first activate the protection circuit and then commence with a normal charge. Do not boost lithium-based batteries back to life that have dwelled below 1.5V/cell for a week or longer. Copper shunts may have formed inside the cells that can lead to a partial or total electrical short. When recharging, such a cell might become unstable, causing excessive heat or showing other anomalies. The “boost” function by Cadex halts the charge if the voltage does not rise normally.

Figure 1: Sleep mode of a lithium-ion batterySome over-discharged batteries can be “boosted” to life again. Discard pack if the voltage does not rise to a normal level within a minute while on boost.

A study done by Cadex to examine failed batteries reveals that three out of ten batteries are removed from service due to over-discharge. Furthermore, 90 percent of returned batteries have no fault or can easily be serviced. Lack of test devices at the customer service level is in part to blame for the high exchange rate. Refurbishing batteries saves money and protects the environment.

BU-809: How to Maximize Runtime

As the author of BatteryUniversity.com, I get many interesting enquiries from battery users. A man writes, “Hi, I am looking for an answer to a perplexing question. A co-worker and I have identical cell phones from the same provider. Moving into a new house, she complained of short battery runtime. I told her she was out of her mind, but then I noticed my battery behaving differently when I travel. Is there some mysterious force that’s draining the battery?”Yes, there is a hidden force that drains the battery but it’s not mystical. When turned on, a cell phone is in constant communication with the tower, transmitting small bursts of power once every second or so to check for incoming calls. To save energy, the signal strength adjusts the transmission power to only what is needed. If the cell phone is close to a repeater tower, the energy required to communicate is very low. Move farther away or enter an area with high electrical noise, such as a shopping mall, hospital or factory, and the required energy increases. An analogy can be made to sitting in a restaurant. When the surroundings are quiet, the voices can be kept low, but as the crowd grows everyone needs to talk a bit louder. Living in sight of a tower has advantages and your cellular battery will last longer between charges. Where you park your cell phone in the house also affects runtime. A manager of a large cellular provider in the UK said his son experienced shorter standby times after moving from the upstairs bedroom to the basement. If possible, leave your cell phone in an upstairs room facing a tower. When traveling by car place it near a window rather than on the floor but avoid direct exposure to the sun.Similar rules apply to TETRA and P25 radio systems, cordless telephones, Wi-Fi and Bluetooth devices. A wireless headset that communicates with the cell phone from belt to ear provides longer runtimes than when placing the handset on the dining-room table while cooking in the kitchen. Although the quality of communication stays the same, the Bluetooth headset needs to work harder when placed farther away from the user.The energy savings only apply when the wireless device is in the “on” position. When “off,” the load on the battery is very low and only provides power for housekeeping functions such as maintaining the clock and monitoring key commands. Housekeeping and self-discharge consume 5 to 10 percent of the available battery energy per month.During the last few years, standby and talk-times on cell phones have improved. Besides increases in the specific energy of lithium-ion, improvements in receiver and demodulator circuits have achieved notable energy savings. Figure 1 illustrates the reduction of power consumption in these circuits since 2002. We must keep in mind that the savings apply mainly to standby and receiving circuits. Transmitting still requires about five times the power of the receiving and demodulation.

Figure 1: Reduction in power consumptionCell phones have achieved notable power savings in the receiver and demodulator circuits. Transmitting needs the most power.Souece: Sieber et al. (2004).

Laptop batteries fare badly in terms of life span. Laptops are demanding bosses that request a steady stream of power under poor working conditions, toiling in an unbearable heat of 40–45°C (104–113°F). In addition, the battery is exposed to a high voltage by being kept at full charge. High heat and dwelling at full state-of-charge, not cycling, cause short battery life in laptops.

Laptop batteries have further demands — they must be small and lightweight. The laptop battery should be invisible to the user and deliver enough power to endure a five-hour flight. In reality, the battery runs for only about 90 minutes. Batteries are getting better; however, the request for higher performance counteracts the capacity gain, resulting in roughly the same runtime with more powerful features.Although users want longer runtimes, computer manufacturers are hesitant to add larger batteries because of increased size, weight and cost. A survey indicates that given the option of a larger size with added weight to gain longer runtimes, most users would settle for what is offered today. For better or worse, we have learned to live with what we have.

Aftermarket Batteries

In the search for low-cost batteries, consumers may inadvertently purchase counterfeit batteries that are unsafe. The label appears bona fide and the buyer cannot distinguish between an original and a forged product. Cell phone manufacturers are concerned about these products flooding the market and advise customers to use approved brands; defiance could void the warranty. Manufacturers do not object to third-party suppliers as long as the aftermarket batteries are well built, safe and approved by a safety agency. Caution also applies to purchasing counterfeit chargers. Some unsafe aftermarket chargers do not terminate the battery correctly and rely on the battery’s internal protection circuit to cut off when full. The need for redundancy is important because a bona fide battery could have a malfunctioning protection circuit that was damaged by a static charge. If, for example, the maker of the counterfeit battery relies on the charger to terminate the charge, and the charger builder has full confidence that the battery will turn off when ready, the combination of these two products can have a lethal effect.Some laptop manufacturers disallow aftermarket batteries by digitally locking the pack with a tamperproof security code. This is done in part for safety reasons, because the potential damage resulting from a faulty laptop battery is many times greater than that of a cell phone.

Simple Guidelines to Prolong Lithium-ion Batteries

Do not discharge Li-ion too low; charge more often. 

A random or partial charge is fine. Li-ion does not need a full charge. 

Limit the time the battery resides at 4.20/cell (full charge), especially if warm. 

Moderate the charge current to between 0.5C and 0.8C for cobalt-based lithium-ion. Avoid ultra-fast charging and discharging. 

If the charger allows, lower the charge voltage limit to prolong battery life. 

Keep the battery cool. Move it away from heat-generating environments. Avoid hot cars and windowsills. 

High heat and full state-of-charge, not cycling, cause short battery life in laptops. 

Remove battery from laptop when used on the power grid.

BU-901: Difficulties with Testing Batteries

A German manufacturer of luxury cars points out that one out of two starter batteries returned under warranty is working and has no problem. It is possible that battery testers used in service garages did not detect the batteries correctly before they were returned under warranty. ADAC* reported in 2008 that 40 percent of all roadside automotive failures are battery-related. In Japan, battery failure is the largest single complaint among new car owners. The average car is driven 13km (8 miles) per day and mostly in congested cities. The most common reason for battery failure is undercharge. Battery performance is important; problems during the warranty period tarnish customer satisfaction.Battery malfunction during the warranty period is seldom a factory defect; driving habits are the culprits. A manufacturer of German-made starter batteries stated that factory defects account for only 5 to 7 percent of warranty claims. The battery remains a weak link, and is evident when reviewing the ADAC 2008 report for the year 2007. The study examines the breakdowns of 1.95 million vehicles six years old or less, and Table 1 provides the reasons. 

Percentage of Failure Cause of Failure

Table 1:Most common car failuresBatteries cause the most common failures requiring road assistance.Source: ADAC 2008

52%15%8%7%7%6%5%

BatteryFlat tireEngineWheelsFuel injectionHeating, coolingFuel systems

The cellular phone industry experiences an even more astonishing battery return pattern. Nine out of 10 batteries returned under warranty have no problem or can easily be serviced. This is no fault of the manufacturers but they pay a price that is ultimately charged to the user.Part of the problem lies in the difficulty of testing batteries at the consumer level, and this applies to storefronts and service garages alike. Battery rapid-test methods seem to dwell in medieval times, and this is especially evident when comparing advancements made on other fronts. We don’t even have a reliable method to estimate state-of-charge — most of such measurements using voltage and coulomb counting are guesswork. Assessing capacity, the most reliable health indicator of a battery, dwells far behind.The battery user may ask why the industry is lagging so far behind. The answer is simple: battery testing and monitoring is far more complex than outsiders perceive it. As there is no single diagnostic device that can assess the health of a person, so are there no instruments that can quickly check the state-of-health of a battery. Like the human body, batteries can have many hidden deficiencies that no single tester is able to identify with certainly. Yes, we can apply a discharge, but this takes the battery out of service and induces stress, especially on large systems. In some cases, even a discharge does not provide conclusive results either. Read more about Discharge Methods.As doctors will examine a patient with different devices, so also does a battery need several approaches to find anomalies. A dead battery is easy to measure and all testers can do this. The challenge comes in evaluating a battery in the 80 to 100 percent performance range. This chapter examines current and futuristic methods and how they stand up. One thing to remember is this: batteries cannot be measured; the appropriate instruments can only make predictions or estimations. This is synonymous with a doctor examining a patient, or the weatherman predicting the weather. All findings are estimations with various degrees of accuracies.                                                

*   The ADAC (Allgemeiner Deutscher Automobil-Club e.V.) originated in Germany in 1903 and is Europe’s largest automobile club, with over 16 million members.

BU-902: How to Measure Internal Resistance

The resistance of a battery provides useful information about its performance and detects hidden trouble spots. High resistance values are often the triggering point to replace an aging battery, and determining resistance is especially useful in checking stationary batteries. However, resistance comparison alone is not effective, because the value between batches of lead acid batteries can vary by eight percent. Because of this relatively wide tolerance, the resistance method only works effectively when comparing the values for a given battery from birth to retirement. Service crews are asked to take a snapshot of each cell at time of installation and then measure the subtle changes as the cells age. A 25 percent increase in resistance over the original reading hints to an overall performance drop of 20 percent.Manufacturers of stationary batteries typically honor the warranty if the internal resistance increases by 50 percent. Their preference is to get true capacity readings by applying a full discharge. It is their belief that only a discharge can provide reliable readings and they ask users to perform the service once a year. While this advice has merit, a full discharge requires a temporary disconnection of the battery from the system, and on a large battery such a test takes an entire day to complete. In the real world, very few battery installations receive this type of service and most measurements are based on battery resistance readings.Measuring the internal resistance is done by reading the voltage drop on a load current or by AC impedance. The results are in ohmic values. There is a notion that internal resistance is related to capacity, and this is false. The resistance of many batteries stays flat through most of the service life. Figure 1 shows the capacity fade and internal resistance of lithium-ion cells.

Figure 1: Relationship between capacity and resistance as part of cyclingResistance does not reveal the state-of-health of a battery. The internal resistance often stays flat with use and aging.Cycle test on Li-ion batteries at 1C:Charge: 1,500mA to 4.2V, 25°CDischarge: 1,500 to 2.75V, 25°CCourtesy of Cadex

To estimate capacity and state-of-charge on the fly involves impedance trending by scanning a battery with frequencies ranging from less than one hertz to several thousand hertz. Read more about Testing Lead Acid Batteries.

What Is Impedance?

Before exploring the different methods of measuring the internal resistance of a battery, let’s examine what electrical resistance means, and let’s differentiate between a pure resistance (R) and impedance (Z) that includes reactive elements such as coils and capacitors. Both values are given in Ohms (W), a measure formulated by the German physicist Georg Simon Ohm, who lived from 1798 to 1854. (One Ohm produces a voltage drop of 1V with a current flow of 1A.) The difference between resistance and impedance lies in the reactance. Let me explain.Most electrical loads, as well as batteries providing power, have internal impedance. Impedance consists of a capacitive reactance component (capacitor) and an inductive reactance component (coil). Capacitive reactance decreases with increasing frequency, while inductive reactance increases with increasing frequency. (To explain resistance change with frequency, we compare an oil damper that has a stiffer resistance when moved fast.). Read more about Watts and Volt-Amps (VA).

A battery has resistive, capacitive and inductive resistance, and the term impedance includes all three in one. The inductive component only shows up at high frequencies when the inductive reactance of the conductors inside the battery becomes a factor in the overall impedance.Impedance can best be illustrated with the Randles model. Figure 2 illustrates the basic model of a lead acid battery, which reflects resistors and a capacitor (R1, R2 and C). The inductive reactance is commonly omitted because it plays a negligible role in a battery, especially at a low frequency.

Figure 2:Randles model of a lead acid batteryThe overall battery resistance consists of ohmic resistance, as well as inductive and capacitive reactance. The schematic and electrical values differ for every battery.

Now that we have learned the basics of internal battery resistance and how they can be applied to rapid-test batteries at different frequencies, this section examines current and future battery test methods. It also discusses advantages and shortfalls.

DC Load Method

Ohmic measurement is one of the oldest and most reliable test methods. The battery receives a brief discharge lasting a few seconds. A small pack gets an ampere or less and a starter battery is loaded with 50A and more. A voltmeter measures the voltage drop and Ohm’s law calculates the resistance value (voltage divided by current equals resistance).DC load measurements work well to check large stationary batteries, and the ohmic readings are very accurate and repeatable. Manufacturers of test instruments claim resistance readings in the 10 micro-ohm range. Many garages use the carbon pile to measure starter batteries, and with experience mechanics familiar with this loading device get a reasonably good assessment of the battery. The invasive test is in many ways more reliable than non-invasive methods.The DC load method has a limitation in that it blends R1 and R2 of the Randles modelinto one combined resistorand ignores the capacitor(see Figure 3). “C” is an important component of a battery that represents 1.5 farads per 100Ah capacity. In essence, the DC method sees the battery as a resistor and can only provide ohmic references.

Figure 3: DC load methodThe true integrity of the Randles model cannot be seen. R1 and R2 appear as one ohmic value.Courtesy of Cadex

The two-tier DC load method offers an alternative method by applying two sequential discharge loads of different currents and time durations. The battery first discharges at a low current for 10 seconds, followed by a higher current for three seconds (see Figure 4), and Ohm’s law calculates the resistance values. Evaluating the voltage signature under the two load conditions offers additional information about the battery, but the values are strictly resistive and do not reveal SoC and capacity estimations.

Figure 4: Two-tier DC loadThe two-tier DC loadfollows the IEC 60285 and IEC 61436standards and provides lifelike test conditions for many battery applications. The load test is the preferred method for batteries powering DC loads.Courtesy of Cadex

AC Conductance

The AC conductance method replaces the DC load and injects an alternating current into the battery. At a set frequency of between 80 and 90 hertz, the capacitive and inductive reactance converge, resulting in a negligible voltage lag that minimizes the reactance. Manufacturers of AC conductance equipment claim battery resistance readings in the 50 micro-ohm range, and these instruments are commonly used in North American car garages. The single-frequency technology as illustrated in Figure 5 sees the components of the Randles model as one complex impedance called the modulus of Z.

Figure 5: AC conductance methodThe individual components of the Randles model are molten together and cannot be distinguished.Courtesy of Cadex

Smaller batteries often use the popular 1000-hertz (Hz) ohm test method. A 1000Hz signal excites the battery, and the Ohm’s law calculates the resistance. It is important to note that the AC method shows different values to the DC load, and both are correct. For example, Li-ion in an 18650 cell produces about 36mOhm with a 1000Hz AC signal and roughly 110mOhm with a DC load. Since both readings are correct, and yet are so far apart, the user needs to consider the application. The pulse DC load method provides the best indication for a DC application such as driving a motor or powering a light, while the 1000Hz method better reflects the performance of a digital load, such as a cellular phone that relies to a large extent on the capacitor characteristics of a battery. Figure 6 illustrates the 100Hz method.

Figure 6: 1000-hertz methodThe IEC 1000-hertz is the preferred method to take impedance snapshots of batteries powering digital devices.Courtesy of Cadex

Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) enables more than resistance readings; it can estimate state-of-charge and capacity. Research laboratories have been using EIS for many years to evaluate battery characteristics, but high equipment cost, slow test times and the need for trained professionals to decipher large volumes of data have limited this technology to laboratory environments. EIS is able to read each component of the Randles model individually; however, analyzing the value at different frequencies and correlating the data is an enormous task. Fuzzy logic and advanced digital signal processor (DSP) technology have simplified this task. Figure 7 illustrates the battery component, which EIS technology is capable of reading.

Figure 7: Spectro™ methodR1, R2 and C are measured separately, which enables state-of-charge and capacity measurements.Courtesy of Cadex

BU-902a: How to Measure CCA (Cold Cranking Amp)

Ever since Cadillac invented the starter motor in 1912, car mechanics explored ways to measure cold cranking amps. CCA assures that the battery has sufficient energy to crank the engine when cold. To do this without “freezing,” testers look at internal resistance, the gatekeeper of a battery. A starter battery with low resistance assures reasonably good cranking, and a CCA reading of 400 to 500A is sufficient for most starter batteries. According to SAE J537, a CCA reading of 500A delivers 500A at -18°C (0°F) for 30 seconds without dropping below 7.2 volts.  Read more about How to Measure Capacity.

Courtesy of BMWGarages seldom do the full-fletched CCA test; this belongs to laboratories. Instead, device manufacturers offer alternatives and the carbon pile introduced in the 1980s is one of the oldest and most reliable methods. To do a pass/fail test, a fully charged starter battery is loaded with half the rated CCA for 15 seconds at a moderate temperature of 10º C (50º F) and higher. The battery will pass if the voltage stays above 9.6V.  Colder temperatures cause the voltage to drop further. The DC load method has the advantage of detecting batteries with a partially shorted cell (low specific gravity) but the device cannot estimate battery capacity.Mechanics prefer small sizes, and instead of applying the prolonged load that is typical of the carbon pile, device manufactures developed handheld testers that induce a high-current pulse. The Ohm’s law calculates the internal resistance based on the load current and voltage drop. The test conditions and results of this device are similar to the carbon pile.Meanwhile, non-invasive test methods emerged, meaning that the battery is no longer loaded for measurements. The AC Conductance method reads CCA by injecting a single frequency of 80–90 Hertz to the battery. The units are smaller than invasive devices and the battery does not need to be fully charged. AC Conductance meters cannot read capacity and a partially shorted cell may pass as good.Critical progress has been made in electrochemical impedance spectroscopy (EIS). Research centers have been using EIS for many years but high equipment cost, long test times and the need for trained professionals to decipher the data have kept this technology in laboratories. Fuzzy logic, advanced digital signal processors and a new algorithm to process the information have simplified this task.Cadex took the EIS concept one step further and developed multi-model electrochemical impedance spectroscopy or Spectroäfor short. Spectro™ gives more accurate CCA estimation than what is possible with single-frequency AC Conductance, but the most important advantage is the ability to estimate capacity, the leading health indicator of a battery. Here is how it works:A control signal ranging from 20 to 2000Hz is injected into the battery as if to capture the topography of a landscape. The scanned imprint is then compared against a matrix to derive at the reading. A matrix can be described as a multi-dimensional lookup table; and text recognition, fingerprint identification and visual imaging operate on a similar principle.CCA works on a basic matrix that covers a broad range of starter batteries. Capacity, on the other hand, requires a complex model. To simplify testing, Cadex has developed a generic matrix that covers most starter batteries, flooded and AGM. The said generic matrix provides pass/fail information based on a capacity setting of 40

percent, which serves as the end-of-life threshold. Battery-specific matrices can be made available that offer numeric capacity values in percent. The test takes 15 seconds and works with a partially charged battery. Figure 1 shows the Spectro CA-12 with Spectro™ technology.

Figure 1: Spectro CA-12 battery testerMulti-frequency concept Spectro™ concept displays capacity, CCA and state-of-charge; test time is 15 seconds.Courtesy Cadex

Patented technology

“How accurate are the readings,” car mechanics ask? This depends on the tester and method used. For example, the Spectro CA-12 with a generic matrix provides a CCA accuracy of 90 percent; capacity is about 80 percent. The single-frequecy AC Conductance, on the other hand, provides a CCA accuracy of roughly 70 percent with no capacity readout. Such low accuracies may come as a surprise to many and service technicians ask for better than 90 percent. This is impossible with lead acid batteries because of inherited inaccuracies. Capacity fluctuations of +/- 12 percent are common even with highly accurate discharge and charge equipment tested in a controlled laboratory environment. Read more about How to Measure Capacity.State-of-charge (SoC) also affects accuracy. Figure 2 compares CCA readings at different SoC, taken with the Spectro CA-12 and a device that is based on AC Conductance. While Spectro shows only a slight decrease with depleting charge, AC Conductance reflects a strong departure form the horizontal line; the readings are only similar at a 70 percent SoC. Since most batteries hover at about 70 percent when the car is brought in for service, the CCA readings of the two methods may appear similar.

Figure 2: CCA accuracy on state-of-chargeThe Spectro CA-12 provides stable CCA readings between a SoC of 100–40% (red); the values on AC Conductance drop rapidly with SoC (blue).

Battery manufacturers are hesitant to endorse new test technologies. It is said that the first digital tester introduced in the early 1990s won approval by agreeing to give slightly higher CCA readings than what a lab test would provide. After all, who knows the true value! Very few service garages would go through a full SAE J537 verification that can take up to a week to complete for one battery. Showing a higher reading will indeed favor market acceptance, but this poses a problem when emerging technologies reveal correct readings that are at lower levels.It so happened that the battery laboratory of a German luxury car manufacturer performed a comparison test as part of product qualification. The battery testers involved were the Spectro CA-12 and a device based on AC Conductance. With a dedicated matrix, the Spectro CA-12 achieved a CCA accuracy of 97%; capacity came in at

87 percent. In comparison, the AC Conductance unit produced a correct CCA prediction of only 51 percent with no capacity reference.One can clearly see that a CCA measurement at a low accuracy provides limited information regarding battery aging and end-of-life prediction. Furthermore, the driver can guess CCA on engine cranking. Capacity is the more reliable health indicator and there is some confusion in differentiating between the two. North America focuses on CCA, and RC (reserve capacity) is usually overlooked. Europe, on the other hand, is more in tune with capacity and their batteries are clearly marked with Ah. [Formula for RC to Ah conversion: RC divided by 2 plus 16]Figure 3 illustrates the bond between capacity and CCA on hand of a fluid-filled container. The liquid represents the capacity, and the tap symbolizes the energy delivery or CCA, best remembered as “pipe size.” While CCA stays stable through most of the battery life, the capacity decreases steadily. The illustration represents the aging process with growing “rock content” that inhibits energy storage. The capacity gradually declines until there won’t be enough “juice” one day to start the engine. Read more about Tracking Battery Capacity and Resistance as part of Aging.

Figure 3: Relationship of CCA and capacity of a starter batteryCapacity represents energy and is shown as liquid. CCA relates to internal resistance and is responsible for energy delivery, best remembered as “pipe size.” CCA tends to stay high while the capacity diminishes as part of aging.

Conclusion

No single instrument can evaluate all battery anomalies and rapid testing only gives a rough estimation. There are battery defects that can only be revealed by applying a heavy load, and a micro short in a cell is such a case. A rapid-test might pass the battery as good even though the short has lowered the specific gravity to almost “empty” due to high self-discharge and the engine won’t crank. A carbon pile or hydrometer is best able to find the anomaly but the test must be done after the battery has been removed from the charger for a few days. A charge will cover up the fault and everything will look normal.There are no ideal battery test instruments; however, scientists predict that the battery industry is moving towards electrochemical impedance spectroscopy to estimate battery performance. While advancements in battery rapid-testing are noteworthy, none is foolproof.

BU-903: How to Measure State-of-charge

Voltage Method

Measuring state-of-charge by voltage is the simplest method, but it can be inaccurate. Cell types have dissimilar chemical compositions that deliver varied voltage profiles. Temperature also plays a role. Higher temperature raises the open-circuit voltage, a lower temperature lowers it, and this phenomenon applies to all chemistries in varying degrees.

The most blatant error of voltage-based SoC occurs when disturbing the battery with a charge or discharge. This agitation distorts the voltage and no longer represents the true state-of-charge. To get accurate measurements, the battery needs to rest for at least four hours to attain equilibrium; battery manufacturers recommend 24 hours. Adding the element of time to neutralize voltage polarization does not sit well with batteries in active duty. One can see that this method is ill suited for fuel gauging.

Each battery chemistry delivers a unique discharge signature that requires a tailored model. While voltage-based SoC works reasonably well for a lead acid battery that has rested, the flat discharge curve of nickel- and lithium-based batteries renders the voltage method impracticable. And yet, voltage is commonly used on consumer products. A “rested” Li-cobalt of 3.80V/cell in open circuit indicates a SoC of roughly 50 percent.

The discharge voltage curves of Li-manganese, Li-phosphate and NMC are very flat, and 80 percent of the stored energy remains in this flat voltage profile. This characteristic assists applications requiring a steady voltage but presents a challenge in fuel gauging. The voltage method only indicates full charge and low charge and cannot estimate the large middle section accurately. Lead acid has diverse plate compositions that must be considered when measuring SoC by voltage. Calcium, an additive that makes the battery maintenance-free, heat raises the voltage by 5–8 percent. Temperature also affects the open-circuit voltage; heat raises it while cold causes it to decrease. Surface charge further fools SoC estimations by showing an elevated voltage immediately after charge; a brief discharge before measurement counteracts the error. Finally, AGM batteries produce a slightly higher voltage than the flooded equivalent.

When measuring SoC by open circuit voltage, the battery voltage must be truly “floating” with no load present. Installed in a car, the parasitic load present makes this a closed circuit voltage (CCV) condition that will falsify the readings. Adjustments must be made when measuring SoC in the CCV state by including the load current in the calculation. In spite of the notorious inaccuracies, most SoC measurements rely on the voltage method because it’s simple. Voltage-based state-of-charge is popular for wheelchairs, scooters and golf cars.

Hydrometer

The hydrometer offers an alternative to measuring SoC, but this only applies to flooded lead acid and flooded nickel-cadmium. Here is how it works: As the battery accepts charge, the sulfuric acid gets heavier, causing the specific gravity (SG) to increase. As the SoC decreases through discharge, the sulfuric acid removes itself from the electrolyte and binds to the plate, forming lead sulfate. The density of the electrolyte becomes lighter and more water-like, and the specific gravity gets lower. Table 1 provides the BCI readings of starter batteries.

Average Open circuit voltage

Approximate state-of-charge

specific gravity

2V 6V 8V 12V

100%75%50%25%0%

1.2651.2251.1901.1551.120

2.102.082.042.011.98

6.326.226.126.035.95

8.438. 308.168.047.72

12.6512.4512.2412.0611.89

Table 1: BCI standard for SoC estimation of a maintenance-free starter battery with antimony. The readings are taken at room temperature of 26°C (78°F); the battery had rested for 24 hours after charge or discharge.

While BCI specifies the specific gravity of a fully charged starter battery at 1.265, battery manufacturers may go for 1.280 and higher. When increasing the specific gravity, the SoC readings on the look-up table will adjust upwards accordingly. Besides charge level and acid density, the SG can also vary due to low fluid levels, which raises the SG reading because of higher concentration. Alternatively, the battery can be overfilled, which lowers the number. When adding water, allow time for mixing before taking the SG measurement.

The specific gravity also varies according to battery type. Deep-cycle batteries use a dense electrolyte with an SG of up to 1.330 to get maximum runtime; aviation batteries have a SG of 1.285; traction batteries for forklifts are at 1.280; starter batteries come in at 1.265 and stationary batteries are at a low 1.225. Low specific gravity reduces corrosion. The resulting lower specific energy of stationary batteries is not as critical as longevity.Nothing in the battery world is absolute. The specific gravity of fully charged deep-cycle batteries of the same model can range from 1.270 to 1.305; fully discharged, these batteries may vary between 1.097 and 1.201. Temperature is another variable that alters the specific gravity reading. The colder the temperature is, the higher (more dense) the SG value becomes. Table 2 illustrates the SG gravity of a deep-cycle battery at various temperatures.

Temperature ofthe Electrolyte Gravity at full charge Table 2: Relation of specific

gravity and temperature of deep-cycle batteryColder temperatures provide higher specific gravity readings.

40°C30°C20°C10°C0°C

104°F86°F68°F50°F32°F

1.2661.2731.2801.2871.294

Errors can also occur if the acid has stratified, meaning the concentration is light on top and heavy on the bottom. High acid concentration artificially raises the open circuit voltage, which can fool SoC estimations through false SG and voltage indication. The electrolyte needs to stabilize after charge and discharge before

taking the SG reading.

Coulomb Counting

Laptops, medical equipment and other professional portable devices use coulomb counting as a SoC indication. This method works on the principle of measuring the current that flows in and out of the battery. If, for example, a battery was charged for one hour at one ampere, the same energy should be available on discharge. This is not the case. Inefficiencies in charge acceptance, especially towards the end of charge, as well as losses during discharge and storage reduce the total energy delivered and skew the readings. The available energy is always less than what had been fed to the battery, and compensation corrects the shortage.

Disregarding these irregularities, coulomb counting works reasonably well, especially for Li-ion. However, the one percent accuracy some device manufacturers advertise is only possible in an ideal world and with a new battery. Independent tests show errors of up to 10 percent when in typical use. Aging causes a gradual deviation from the working model on which the coulomb counter is based. The result is a laptop promising 30 minutes of remaining runtime and all of a sudden the screen goes dark. Periodic calibration by applying a full discharge and charge to reset the flags reduces the error. See Calibration.

There is a move towards electrochemical impedance spectroscopy and even magnetism to measure state-of-charge. These new technologies get more accurate estimation than with voltage and can be used when the battery is under load. Furthermore, temperature, surface charge and acid stratification do not affect the readings noticeably.

BU-904: How to Measure Capacity

The traditional charge/discharge/charge cycle still offers a dependable way to measure battery capacity. Alternative methods have been tried but none deliver reliable readings. Inaccuracies have led users to adhere to the proven discharge methods even if the process is time-consuming and removes the battery from service for the duration of the test.While portable batteries can be discharged and recharged relatively quickly, a full discharge and recharge on large lead acid batteries gets quite involved, and service personnel continue to seek faster methods even if the readings are less accurate. This section explains what’s available in new technologies, but first we look at the discharge method more closely.

Discharge Method

One would assume that capacity measurement with discharge is accurate but this is not always the case, especially with lead acid batteries. In fact, there are large variations between identical tests, even when using highly accurate equipment and following established charge and discharge standards, with temperature control and mandated rest periods. This behavior is not fully understood except to consider that batteries exhibit human-like qualities. Our IQ levels also vary depending on the time of day and other conditions. Nickel- and lithium-based chemistries provide more consistent results than lead acid on discharge/charge tests. To verify the capacity on repeat tests, Cadex checked 91 starter batteries with diverse performance levels and plotted the results in Figure 1. The horizontal x-axis shows the batteries from weak to strong, and the vertical y-axis reflects capacity. The batteries were prepared in the Cadex laboratories according to SAE J537 standards by giving them a full charge and a 24-hour rest. The capacity was then measured by applying a regulated 25A discharge to 10.50V (1.75V/cell) and the results plotted in diamonds (Test 1). The test was repeated under identical conditions and the resulting capacities added in squares (Test 2). The second reading exhibits differences in capacity of +/–15 percent across the battery population. Other laboratories that test lead acid batteries experience similar discrepancies.

Figure 1: Capacity fluctuations on two identical charge/discharge tests of 91 starter batteries. The capacities differ +/–15% between Test 1 and Test 2.Courtesy of Cadex (2005)

Capacity vs. CCA

Starter batteries have two distinct values, CCAand capacity.These two readings are close to each other like lips and teeth, but the characteristics are uniquely different; one cannot predict the other. [BU-806, Changes in Capacity and Resistance]

Measuring the internal battery resistance, which relates to CCA on a starter battery, is relatively simple but the reading provides only a snapshot of the battery at time of measurement. Resistance alone cannot predict the end of life of a battery. For example, at a CCA of 560A and a capacity of 25 percent, a starter battery will still crank well but it can surprise the motorist with a sudden failure of not turning the engine (as I have experienced).The leading health indicator of a battery is capacity,but this estimation is difficult to read. A capacity test by discharge is not practical with starter batteries; this would cause undue stress and take a day to complete. Most battery testers do not measure capacity but look at the internal resistance, which is an approximation of CCA. The term approximationis correct — laboratory tests at Cadex and at a German luxury car manufacturer reveal that the readings are only about 70 percent accurate. A full CCA test is seldom done; one battery can take a week to measure.The SAE J537 CCA test mandates to cool a fully charged battery to -18°C (0°F) for 24 hours, and while at subfreezing temperature apply a high-current discharge that simulates the cranking of an engine. A 500 CCA battery would need to supply 500A for 30 seconds and stay above 7.2V (1.2V/cell) to pass. If it fails the test, the battery has a CCA rating of less than 500A. To find the CCA rating, the test must be repeated several times with different current settings to find the triggering point when the battery passes through 7.2V line. Between each test, the battery must be brought to ambient temperature for recharging and cooled again for testing. (For CCA DIN and IEC norms, please refer to “Test Method” on this essay.)To examine the relationship between CCA and capacity, Cadex measured CCA and capacity of 175 starter batteries at various performance levels. Figure 2 shows the CCA on the vertical y-axis and reserve capacity* readings on the horizontal x-axis. The batteries are arranged from low to high, and the values are given as a percentage of the original ratings.

Figure 2: CCA and reserve capacity (RC) of 175 aging starter batteriesThe CCA of aging starter batteries gravitates above the diagonal reference line. (Few batteries have low CCA andhigh capacity.)Courtesy of Cadex

Test method: The CCA and RC readings were obtained according to SAE J537 standards (BCI). CCA (BCI) loads a fully charged battery at –18°C (0°F) for 30s at the CCA-rated current of the battery. The voltage must stay above 7.2V to pass. CCA DIN and IEC are similar with these differences: DIN discharges for 30s to 9V, and 150s to 6V; IEC discharges for 60s to 8.4V. RC applies a 25A discharge to 1.75V/cell and measures the elapsed time in minutes.The table shows noticeable discrepancies between CCA and capacity, and there is little correlation between these readings. Rather than converging along the diagonal reference line, CCA and RC wander off in both directions and resemble the stars in a clear sky. A closer look reveals that CCA gravitates above the reference line, leaving the lower right vacant. High CCA with low capacity is common, however, low CCA with high capacity is rare. In our table, one battery has 90 percent CCA and produces a low 38 percent capacity; another delivers 71 percent CCA and delivers a whopping 112 percent capacity (these are indicated by the dotted lines).As discussed earlier, a battery check must include several test points. An analogy can be made with a medical doctor who examines a patient with several instruments to find the diagnosis. A serious illness could escape the doctor’s watchful eyes if only blood pressure or temperature was taken. While medical staff are well trained to

evaluate multiple data points, most battery personnel do not have the knowledge to read a Nyquist plot and other data on a battery scan. Nor are test devices available that give reliable diagnosis of all battery ills.                                         

*   North America marks the reserve capacity (RC) of starter batteries in minutes; RC applies a 25A discharge to 1.75V/cell and measures the elapsed time in minutes. Europe and other parts of the world use ampere/hours (Ah). The RC to Ah conversion formula is as follows: RC divided by 2 plus 16.

BU-905: Testing Lead Acid Batteries

Many manufacturers of battery testers claim to measure battery health on the fly. These instruments work well in finding battery defects that involve voltage anomalies and elevated internal resistance, but other performance criteria remain unknown. Stating that a battery tester based on internal resistance can also measure capacity is misleading. Advertising features that are outside the equipment’s capabilities confuses the industry into believing that multifaceted results are attainable with basic methods. Manufacturers of these instruments are aware of the complexity involved, but some like to add a flair of mystery in their marketing scheme, similar to a maker of a shampoo product promising to grow lush hair on a man’s bald head. Here is a brief history of battery testers for lead acid and what they can do.The carbon pile, introduced in the 1980s, applies a DC load of short duration to a starter battery, simulating cranking. The voltage drop and recovery time provide a rough indication of battery health. The test works reasonably well and offers evidence that power is present. A major advantage is the ability to detect batteries that have failed due to a shorted cell (low specific gravity in a cell due to high self-discharge). Capacity estimation, however, is not possible, and a battery that has a low state-of-charge appears as weak. A skilled mechanic can, however, detect a faulty battery based on the voltage signature and loading behavior. To do a CCA pass/fail test, load a fully charged starter battery with half the rated CCA value for 15 seconds. To pass, the voltage must stay above 9.6V at 10º C (50º F) and higher. Colder temperatures cause a large voltage drop.The AC conductance meters appeared in 1992 and were hailed as a breakthrough. The non-invasive method injects an AC signal into the battery to measure the internal resistance. Today, these testers are commonly used to check the CCA of starter batteries and verify resistance change in stationary batteries. While small and easier to use, AC conductance cannot read capacity, and the resistive value gives only an approximation of the real CCA of a starter battery. A shorted cell could pass as good because in such a battery the overall conductivity and terminal voltage are close to normal, even though the battery cannot crank the motor. AC conductance testers are common in North America; Europe prefers the DC load method.Critical progress has been made towards electrochemical impedance spectroscopy (EIS). Cadex took the EIS technology a step further and developed battery specific models that are able to estimate the health of a lead acid battery. Multi-model electrochemical impedance spectroscopy, or Spectroäfor short, reads battery capacity, CCA and state-of-charge in a single, non-invasive test.Figure 1 illustrates the Spectro CA-12 handheld battery tester.

Figure 1: Spectro CA-12 battery testerCompact battery rapid tester displays capacity, CCA and state-of-charge in 15 seconds.Courtesy Cadex

The Spectro CA-12 handheld device, in which the Spectro™ technology is embedded, excites the battery with signals from 20–2000Hz. A DSP deciphers the 40 million transactions churned out during the 15-second test into readable results. To check a battery, the user simply selects the battery voltage, Ah and designated matrix. Tests can be done under a steady load of up to 30A and a partial

charge, however, if the state-of-charge is less than 40 percent, the instrument advises the user to charge and retest.The Spectro method is a further development of EIS, a technology that had been around for several decades. What’s new is the use of multi models and faster process times. Cost and size have also shrunk. Earlier models cost tens of thousands of dollars and traveled on wheels. The heart of Spectro is not so much the mechanics but the algorithm. No longer do modern EIS devices accompany a team of scientist to decipher tons of data. Experts predict that the battery industry is moving towards the multi-model EIS technology to estimate batter performanceNowhere is the ability to read capacity more meaningful than with deep-cycle batteries in golf cars, aerial work platforms and wheelchairs, as well as military and naval carriers. Getting a readout in seconds without putting the vehicles out of commission allows for a quick performance check on a suspect battery before deployment in the field. Figures 2, 3 and 4 show typical battery problems and how modern test technologies can detect them.

Figure 2: Low chargeDrive is sluggish; Spectroäreads low SoC. Capacity estimation is correct in spite of low charge.

Figure 3: Low capacityBattery has good drive but short runtimes. Spectroäreads good impedance but low capacity.

Figure 4: Faulty setSpectroäfinds low performing and shorted blocks in a string. Good batteries can be regrouped and reused.All figures Courtesy of Cadex

Matrices

Measurement devices, such as the Spectro CA-12, are not universal instruments capable of estimating the capacity of any battery that may come along; they require battery specific matrices, also known as pattern recognition algorithm. A matrix is a multi dimensional lookup table against which the measured readings are compared. Text recognition, fingerprint identification and visual imaging operate on a similar principle in that a model exists, with which to equate the derived readings.This book identifies three commonly used measuring methods. The principle in all is to take one or several sets of readings and compare them against known reference settings or images to disclose the characteristics of a battery. The three methods are as follows.

Scalar:     The single value scalar test takes a reading and compares the result with a stored reference value. In battery testing this could be measuring a voltage, interrogating the battery by applying discharge pulses or injecting a frequency and then comparing the

derived result against a single reference point. This is the simplest test, and most DC load and single-frequency AC conductance testers use this method.Vector:    The vector method applies pulses of different currents, or excites the battery with several frequencies, and evaluates the results against preset vector points to study the battery under various stress conditions. Typical applications for this one-dimensional scalar model are battery testers that apply multi-tier DC loads or inject several test frequencies. Because of added complexity in evaluating the different data points and limited benefits, the vector method is seldom used.Matrix:    The matrix method scans a battery with a frequency spectrum as if to capture the image of a landscape and compare the imprint with a stored model of known characteristics. This multi-dimensional set of scalars, which form the foundation of Spectroä, provides the most in-depth information but is complex in terms of evaluating the data generated. With a proprietary algorithm, the Spectroätechnology is able to estimate battery capacity, CCA and SoC.

Matrices are primarily used to estimate battery capacity, however, CCA and state-of-charge also require matrices. These are easier to assemble and serve a broad range of starter batteries. While the Spectroämethod offers an accuracy of 80 to 90 percent on capacity, CCA is 95 percent exact. This compares to 60 to 70 percent with battery testers using the scalar method. Service personnel are often unaware of the low accuracy; verifications are seldom done, as this would involve several days of laboratory testing.A further drawback of scalar battery testers is obtaining a reading that is neither resistance nor CCA. While there are similarities between the two, no standard exists and each instrument gives different values. In terms of assessing a dying battery, however, this method is adequate as it reflects conductivity. The larger disadvantage is not being able to read capacity. Table 5 illustrates test accuracies using scalar, vector and matrix methods. 

Measuring unitsScalar

Single value

VectorOne-dimensional

set of scalars

MatrixMulti-dimensional

set of scalars

CCA 60–70% accurate 90–95% accurate

Capacity N/A 80–90% accurate

SoCVoltage-based; requires rest after charge and

discharge90–95% accurate (with

new battery)

Table 5: Accuracy in battery readings with different measuring methodsScalar and vector provide resistance with references to CCA on starter batteries. The matrix method is more accurate and provides capacity estimations but needs reference matrices.To generate a matrix, batteries with different state-of-health are scanned. The more batteries of the same model but diverse capacity mix are included in the mix, the stronger the matrix will become. If, for example, the matrix consists only of two batteries, one showing a capacity of 60 percent and the other 100 percent, then the accuracy would be low for the batteries in between. Adding a third battery with an 80 percent capacity will solidify the matrix, similar to placing a pillar at the center of a bridge. To cover the full spectrum, a well-developed matrix should include battery samples capturing capacities of 50, 60, 70, 80, 90 and 100 percent. Batteries much below 50 percent are less important because they constitute a fail.It is difficult to obtain aged batteries, especially with newer models. Forced aging by cycling in an environmental chamber is of some help; however, age-related stresses from the field are not represented accurately. It also helps to include batteries from different regions to represent unique environmental user patterns. A starter battery in a Las Vegas taxi has different strains than that of a car driven by a grandmother in northern Germany.Different state-of-charge levels increase the complexity to estimate battery health. The SoC on a new battery can be determined relatively easily with impedance spectroscopy, however, the formula

changes as the battery ages. A battery tester should therefore be capable of examining new and old batteries with a charge level of 40 to 100 percent. With ample data, this is possible because natural aging of a battery is predictable and the scanned information can be massaged to calculate age. This is similar to face recognition that correctly identifies a person even if he or she has developed a few wrinkles and has grown gray hair.Simplifications in matrix development are possible by grouping batteries that share the same chemistry, voltage and a similar capacity range into a generic matrix. This simplifies logistics; however, the readout is classified into categories rather than numbers. Figure 6 illustrates the classification scheme of Good, Low and Poor. Good passes as a good battery; Low is suspect and predicts the end of life; and Poor is a fail that mandates replacement.

Service personnel appreciate the classification method because it gives them an intelligent assessment of what constitutes a usable battery for a given application and eliminates customer interference. If numeric capacity readings are mandatory for a given battery type, a designated matrix can be developed and downloaded into the tester from the Internet. 

Figure 6: Classifying batteries into categoriesThe classification method provides an intelligent assessment of what constitutes a usable battery for a given application. Some classifications have pass/fail; others provide GOOD, LOW and POOR.Courtesy of Cadex

BU-905: Testing Nickel-based Batteries

Nickel-based batteries have unique properties, and Cadex developed a rapid-test method for these battery systems called QuickTest™. The process takes three minutes and uses an inference algorithm. Figure 1 illustrates the general structure of the algorithm applied.

Figure 1:QuickTest™ structureMultiple variablesare fed to the micro controller, “‘fuzzified” and processed by parallel logic. The data is averaged and weighted according to battery application.Courtesy of Cadex

QuickTest™ fuses data from six variables, which are capacity, internal resistance, self-discharge, charge acceptance, discharge capabilities and mobility of electrolyte. A trend-learning algorithm combines the data to provide a dependable state-of-health (SoH) reading in percentage. The system uses battery-specific matrices stored in battery adapters of a designated battery analyzer (Cadex). The user can create a matrix in the field by scanning two or more batteries on the analyzer’s Learn program. The battery must be at least 20 percent charged.Among other parameters, QuickTest™ relies on the internal resistance of a battery pack, and the welding joints between the cells might cause a problem, especially on packs with 10 cells or more. Although seemingly insignificant in terms of added resistance, mechanical linkages behave differently to a chemical cell and this causes an unwanted error. The linkage error is not seen on a conventional discharge test or when doing a resistance check but interferes with rapid-test methods on voltages above 20V. It is also possible that each cell of a multi-cell pack behaves on its own when excited with a common signal and the result gets muddled.   

BU-907: Testing Lithium-based Batteries

With the large number of lithium-ion batteries in use and the population growing rapidly, developing an effective testing method has become an urgent task. QuickSort™ (Cadex) is a further development of QuickTest™ using a generic matrix. The simplification was made possible by limiting the battery population to single-cell Li-ion from 500 to 1,500mAh. (Larger cells and higher voltages will need a different generic matrix.) Rather than capacity readout in percentage, QuickSort™ classifies the battery health as Good, Low or Poor.Electrochemical dynamic response,the method used for QuickSort™, measures the mobility of ion flow between the electrodes on a digital load. The response can be compared with a mechanical arm under load. A strong arm resembling a good battery remains firm, and a weak arm synonymous to a faded battery bends and becomes sluggish under load. Figure 1 illustrates the concept of the technology.

Figure 1:Electrochemical dynamic responseThe electrochemical dynamic response measures the ion flow between the positive and negative plates. This process can be compared to a mechanical arm under load.Courtesy of Cadex

The test takes 30 seconds, is 90 percent accurate regardless of battery cathode material and can be performed with a state-of-charge range of between 40 and 100 percent. QuickSort™ requires the correct mAh, and setting a wrong value does not shift the reading on a linear scale from good to poor, as one would expect, but makes the sorting less accurate. The system does not rely on internal resistance per se. This would produce unreliable readings because modern lithium-ion maintains a low resistance with use and time. Read more about How to Measure Internal Resistance. At the conclusion of the test, however, an overall resistance check is performed.Lithium-ion batteries have different diffusion rates, and in terms of electrochemical dynamic response, Li-ion polymer with gelled electrolyte appears to be faster than Li-ion containing liquefied electrolyte. Li-polymer may need a different matrix to produce accurate readings.Scientists explore new ways to evaluate the health of a battery with scanning frequencies ranging from several kilohertz to milihertz. High frequencies reveal the resistive qualities of a battery, which presents a bird-eye’s view in landscape form. By lowering the frequency, diffusion begins to provide insight into unique battery characteristics that allow capacity estimation, sulfation detection and revealing of dry-out condition.Evaluating batteries at sub one-hertz frequency needs long test times. At one milihertz, for example, a cycle takes 1,000 seconds and several data points are required to assess a battery with certainty. Low-frequency tests can take several minutes for one measurement, however, with clever software simulation, the duration can be shortened to just a few seconds.Research engineers at Cadex are working on a technique called Low Frequency Pulse Train (LFPT), also known as diffusion technology. Diffusion works with most chemistries and the information retrieved provides vital information relating to battery capacity and underlying deficiencies. This knowledge enables the all-important state-of-life estimation, the ultimate goal for advanced battery management systems (BMS).There is a critical need for practical battery testers that can examine the state-of-health of batteries in medical equipment, military instruments, computing devices, power tools and UPS systems. There are currently no instruments that can reliably predict battery state-of-life on the fly, although many device manufacturers may claim their instruments will do so. 

BU-908: How to Monitor a Battery

One of the most urgent requirements for battery-powered devices is the development of a reliable and economical way to monitor battery state-of-function (SoF). This is a demanding task when considering that there is still no dependable method to read state-of-charge, the most basic characteristic of a battery. Even if SoC were displayed accurately, charge information alone has limited benefits without knowing the capacity. The objective is to identify battery readiness, which describes what the battery can deliver at a given moment. SoF includes capacity (the amount of energy the battery can hold), internal resistance (the delivery of power), and state-of-charge (the amount of energy the battery holds at that moment).Stationary batteries were among the first to include monitoring systems, and the most common form of supervision is voltage measurement of individual cells. Some systems also include cell temperature and current measurement. Knowing the voltage drop of each cell at a given load reveals cell resistance. Cell failure caused by rising resistance through plate separation, corrosion and other malfunctions can thus be identified. Battery monitoring also serves in medical, defense and communication devices, as well as wheeled mobility and electric vehicle applications.In many ways, present battery monitoring falls short of meeting the basic requirements. Besides assuringreadiness, batterymonitoring should also keep track of aging and offer end-of-life predictions so that the user knows when to replace a fading battery. This is currently not being done in a satisfactory manner. Most monitoring systems are tailored for new batteries and adjust poorly to aging ones. As a result, battery management systems (BMS) tend to lose accuracy gradually until the information obtained gets so far off that it becomes a nuisance. This is not an oversight by the manufacturers; engineers know about this shortcoming. The problem lies in technology, or lack thereof.Another limitation of current monitoring systems is the bandwidth in which battery conditions can be read. Most systems only reveal anomalies once the battery performance has dropped below 70 percent and the performance is being affected. Assessment in the all-important 80–100 percent operating range is currently impossible, and systems give the batteries a good bill of health. This complicates end-of-life predictions, and the user needs to wait until the battery has sufficiently deteriorated to make an assessment. Measuring a battery once the performance has dropped or the battery has died is ineffective, and this complicates battery exchange systems proposed for the electric vehicle market. One maker of a battery tester proudly states in a brochure that their instrument “Detects any faulty battery.” So, eventually, does the user.Some medical devices use date stamp or cycle count to determine the end of service life of a battery. This does not work well either, because batteries that are used little are not exposed to the same stresses as those in daily operation. To reduce the risk of failure, authorities may mandate an earlier replacement of all batteries. This causes the replacement of many packs that are still in good working condition. Old habits are hard to break, and it is often easier to leave the procedure as written rather than to revolt. This satisfies the battery vendor but increases operating costs and creates environmental burdens.Portable devices such as laptops use coulomb counting that keeps track of the in- and out flowing currents. Such a monitoring device should be flawless, but as mentioned earlier, the method is not ideal either. Internal losses and inaccuracies in capturing current flow add to an unwanted error that must be corrected with periodic calibrations.Over-expectation with monitoring methods is common, and the user is stunned when suddenly stranded without battery power. Let’s look at how current systems work and examine up-and-coming technologies that may change the way batteries are monitored.

Voltage-Current-Temperature Method

The Volkswagen Beetle in simpler days had minimal battery problems. The only management system was ensuring that the battery was being charged while driving. Onboard electronics for safety, convenience, comfort and pleasure have greatly added to the demands on the battery in modern cars since then. For the accessories to function reliably, the state-of-charge of the battery must be known at all times. This is especially critical with start-stop technologies, a mandated requirement on new European cars to improve fuel economy.

When the engine stops at a red light, the battery draws 25–50 amperes of current to feed the lights, ventilators, windshield wipers and other accessories. When the light changes, the battery must have enough charge to crank the engine, which requires an additional 350A. With the engine started again and accelerating to the posted speed limit, the battery begins charging after a 10-second delay.Realizing the importance of battery monitoring, car manufacturers have added battery sensors that measure voltage, current and temperature. Packaged in a small housing that forms part of the positive clamp, the electronic battery monitor(EBM)provides useful information about the battery and provides an accuracy of about +/–15 percent when the battery is new. As the battery ages, the EBM begins drifting and the accuracy drops to 20-30 percent. The model used for monitoring the battery is simply not able to adjust. To solve this problem, EBM would need to know the state-of-health of the battery, and that includes the all-important capacity. No method exists today that is fully satisfactory, and some mechanics disconnect the battery management system to stop the false warning messages.A typical start-stop vehicle goes through about 2,000 micro cycles per year. Test data obtained from automakers and the Cadex laboratories indicate that with normal usage in a start-stop configuration, the battery capacity drops to approximately 60 percent in two years. Field use reveals that the standard flooded lead acid lacks robustness, and carmakers are reverting to a modified version lead acid battery. Read about Environmental Concerns.Automakers want to ensure that no driver gets stuck in traffic with a dead battery. To conserve energy, modern cars automatically turn off unnecessary accessories when the battery is low and the motor stays running at a stoplight. Even with this measure, state-of-charge can remain low if commuting in gridlock conditions because motor idling does not provide much charge to the battery, and with essential accessories like lights and windshield wipers on, the net effect could be a small discharge.Battery monitoring is also important on hybrid vehicles to optimize charge levels. Intelligent charge management prevents stressful overcharge and avoids deep discharges. When the charge level is low, the internal combustion (IC) engine engages earlier than normal and is left running longer for additional charge. On a fully charged battery, the IC engine turns off and the car moves on the electrical motor in slow traffic.Improved battery management is of special interest to the manufacturers of the electric vehicle. In terms of state-of-charge, a discerning driver expects similar accuracies in energy reserve as are possible with a fuel-powered vehicle, and current technologies do not yet allow this. Furthermore, the driver of an EV anticipates a fully charged battery will power the vehicle for the same distance as the car ages. This is not the case and the drivable distance will get shorter with each passing year. Distances will also be shorter when driving in cold temperatures because of reduced battery performance.

BU-909: Battery Test Equipment

Batteries are commonly tested by measuring the capacity through a full discharge. While voltage and internal resistance provide a rough indication of the battery condition, these readings do not disclose the capacity, the leading health indicator of a battery. Voltage and resistance tend to reveal anomalies only when the battery is in a fault mode. Most batteries keep a normal voltage and low resistance while the capacity gradually fades with age. Read about How to Measure Internal Resistance.There is a move towards rapid testing, however, current methods only provide an estimation of the battery performance and the results can be in accurate. Rapid-test methods work best with single-cell Li-ion packs; series and parallel connection of cells can distort the readings. Public safety, medical and defense organizations still apply a periodic full discharge/charge cycles, and this is normally done with a battery analyzers.

Battery Analyzer

Battery analyzers became popular in the 1980s and 1990s to restore nickel-cadmium batteries affected by “memory.” Today, battery analyzers serve in identifying packs that no longer meet requirements; they form a vital part in maintaining fleet batteries. Read about How to Maintain Fleet Batteries. Typical battery analyzers are the Cadex C7000 Series, workhorses that serve a broad range of batteries. These devices accommodate lead-, nickel- and lithium-based batteries, feature automated service programs and operate in stand-alone mode or with PC software.

Figure 1:Cadex C7400ER battery analyzerFour-station battery analyzer services batteries of up to 36V and 6A per station. Custom and universal battery adapters accommodate lead-, nickel- and lithium-based batteries.Courtesy of Cadex

The Cadex analyzers include Custom programs in which the user sets a unique sequence of charge, discharge, recondition, wait and repeat. The Lifecycle program cycles battery until the capacity drops to the preset target capacity while counting the delivered cycles. OhmTest measures the internal battery resistance, and Runtime discharges at three different current levels to test battery runtimes within a simulated user pattern. QuickSort™ sorts lithium-ion batteries in 30 seconds into Good, Low and Poor;Boost reactivates packs that fell asleep due to over-discharge. Further programs include Self-Discharge to measure losses in 24 hours, and Prime to prepare new and stored batteries for field use.Connecting the batteries for service has always been a challenge. Cadex solved the battery interface with the SnapLock™ adapter system consisting of custom adapters for common batteries and universal adapters for specialty packs. The custom adapters are easiest to use as they are designed for a battery type and the pack can go in only one way. The adapters include configuration codes that store up to 10 unique battery types and feature a thermistor to monitor temperature. Installing the adapter configures the analyzer to the correct setting. Editing is possible with analyzer’s menu function or via the PC-BatteryShop software. See Cadex's list of available adapters.With the proliferation of cellular batteries and the need for a quick and simple battery interchange, Cadex developed the RigidArm™. This universal battery adapter features spring-loaded arms that meet the battery contacts from the top down. Read about How to Service Mobile Phone Batteries.  A third option is the Smart Cables (Figure 3) featuring alligator clips and a temperature sensor to monitor battery temperature.

Figure 3: Programmable Smart CableThe cable stores 10 different battery types; programming is by menu or via PC-BatteryShop™ software; a thermistor monitors temperature..Courtesy of Cadex

With PC-BatteryShop™ software (Cadex), the PC becomes the master and the analyzers the slave. Clicking the mouse on any of the 2,000 batteries listed in the database or swiping the bar code on the battery label configures the analyzer to the correct setting. You can extend the library by adding new battery models or downloading the latest listing from the Cadex website. PC-BatteryShop™ software is optional; it displays the readings and real-time graphic and is designed to operate 32 analyzers for simultaneous service of 128 batteries (with most PCs).

Figure 4: PC-BatteryShop™ software provides practical PC-interface to control and monitor Cadex C7000 Series battery analyzers. The monitor provides real-time graphic; the system stores vital data.Courtesy of CadexWhile battery analyzers are primarily used as a service tool, battery test systems provide multi-purpose test functions for research laboratories. Typical applications are life-cycle and stress-testing to verify batteries for field use. Much of this testing can be automated.  The Cadex C8000 (Figure 4) is such an automated battery test system. You can measure the battery runtimes by capturing and storing load signatures from mobile phones, laptops, power tools and electric drivetrain and then replicate the load condition in the lab. A further test involves checking the longevity of a battery under the discharge conditions reminiscent in the field. SMBus capability displays the register settings of a smart battery to read flags and to check for correct function. If higher discharge currents than 10A are needed, the C8000 connects to designated external load banks. The C8000 forms a laboratory system that controls environmental chambers, monitors analog signals and triggers user-set alarm conditions. PC-BatteryLab™ software provides interface to a PC for the control and monitoring of up to 8 units to service 32 batteries independently (with most PCs).

Figure 4:Cadex C8000 Battery Test SystemFour independent channels provide up to 10A each and 36V. Maximum charge power is 400W, discharge is 320W. The discharge power can be enhanced with external load banks.Courtesy of Cadex

The alternate to a battery test system is a programmable power supply controlled by a computer. Such a platform offers high flexibility but requires careful programming to prevent stress to the battery and avoid damage or fire,

should an anomaly occur. A battery test system, such as the Cadex C8000, offers protected charge and discharge programs that will identify a faulty battery and terminate a service safely. The system can be overridden to perform destructive tests, however.

he Secrets of Battery Runtime

NOTE: This article has been archived. Please read our new "Four Renegades of Battery Failure" for an updated version.

Declining Capacity

Energy storage in a battery can conceptually be divided into three imaginary segments of the available energy,the empty zone that can be refilled, and the unusable part (rock content). Figure 1 illustrates these three sections.

Figure 1: Aging batteryBatteries begin fading from the day they are manufactured.A new battery should deliver 100 percent capacity; most packs in use operate at less.Courtesy of Cadex

Although the manufacturer specifies the runtime of portable equipment based on a battery performing at 100 percent, most packs in the field operate at less capacity. As time goes on, the performance declines further and the battery gets smaller in terms of holding capacity. A pack should be replaced when the capacity drops to 80 percent. This is only 20 percent down from 100 percent, and the end-of-life threshold may vary according to application and company policy.Besides age-related losses, sulfation and grid corrosion are the main killers of lead acid batteries. Sulfation is a thin layer that forms on the negative cell plate if the battery is allowed to dwell in a low state-of-charge. If sulfation is caught in time, an equalizing charge can reverse the condition. [BU-804, Sulfation] With nickel-based batteries, the so-called rock content is often the result of crystalline formation, also known as “memory,” and a full discharge can sometimes restore the battery. The aging process of lithium-ion is cell oxidation, a process that occurs naturally as part of usage and aging and cannot be reversed.

Rising Internal Resistance

High capacity has limited use if the battery is unable to deliver the stored energy effectively. To bring the power out, the battery needs low internal resistance. Measured in milliohms (mW), resistance is the gatekeeper of the battery; the lower the value, the less restriction the pack encounters. This is especially important with heavy loads and high current pulses, as elevated resistance causes the voltage to collapse and trigger an earlyshutdown. The device turns off and valuable energy is left behind. Figure 2 illustrates batteries with low and high internal resistance as free-flowing and restricted taps.

Figure 2: Effects of internal battery resistanceA battery with low internal resistance delivers high current on demand. High resistance causes the battery voltage to collapse. The equipment cuts off, leaving energy behind.Courtesy of Cadex

Lead acid has a very low internal resistance, and the battery responds well to high current bursts lasting for only a few seconds. Due to inherent sluggishness, however, lead acid does not perform well with a sustained discharge at high current and the battery needs rest to recover. Sulfation and grid corrosion are the main causes of increased internal resistance. Temperature also affects the resistance; heat lowers it and cold raises it.Alkaline, carbon-zinc and other primary batteries have relatively high internal resistance, and this relegates their use to low-current applications such as flashlights, remote controls, portable entertainment devices and kitchen clocks. As the batteries discharge, the resistance increases further. This explains why regular alkaline cells have a relatively short runtime in digital cameras. The high internal resistance limits most primary batteries to “soft” applications, and using them to drive power tools that draw high amperage is unthinkable.Figures 3, 4 and 5 reflect the talk-time of cellular phones with pulsed discharge loads of 1C, 2C and 3C, which GSM and CDMA demand. All batteries tested are similar in size and have capacities of 113%, 94% and 107% respectively, when checked with a battery analyzer on a DC discharge. The three graphs clearly demonstrate the importance of low internal resistance, which varies from a low 155mΩto a moderate 320mΩ, to a high 778mΩ respectively. 

Figure 3: GSM discharge pulses at 1, 2, and 3C with resulting talk-timeThe capacity of the NiCd battery is 113%; the internal resistance is 155mΩ.

Figure 4: GSM discharge pulses at 1, 2, and 3C with resulting talk-timeThe capacity of the NiMH battery is 94%, the internal resistance is 320mΩ.

Figure 5: GSM discharge pulses at 1, 2, and 3C with resulting talk-timeThe capacity of the Li-ion battery is 107%; the internal resistance is 778mΩ.All three figures courtesy of Cadex

Notes:  The above tests were done on cellular phone batteries before lithium-ion took over as the leading battery type for this application. The internal resistance of a modern cellular battery is between 150 and 350mΩ.The maximum discharge pulse current of GSM is 2.5 amperes. When drawn from an 800mAh pack, this represents a 3C discharge, or three times the rated current.

Elevated Self-discharge

All batteries are affected by self-discharge. Self-discharge is not a manufacturing defect per se, although poor manufacturing practices and improper handling can promote the problem. The amount of electrical leakage varies with chemistry, and primary cells, such as lithium and alkaline, are among the best in retaining the energy. Nickel-based rechargeable systems, in comparison, leak the most and need recharging if the battery has not been used for a few days. High-performance nickel-based batteries are subject to higher self-discharge than the standard versions with lower energy densities. Figure 6 illustrates in the form of leaking fluids the self-discharge of a battery.

Figure 6: Effects of high self-dischargeSelf-discharge increases with age, cycling and elevated temperature. Discard a battery if the self-discharge reaches 30 percent in 24 hours.Courtesy of Cadex

The energy loss is asymptotical, meaning that the self-discharge is highest right after charge and then tapers off. Nickel-based batteries lose 10 to 15 percent of their capacity in the first 24 hours after charge, then 10 to 15 percent per month. Figure 7 shows the typical loss of a nickel-based battery while in storage.

Figure 7: Self-discharge as a function of timeThe discharge is highest right after charge and tapers off. The graph shows self-discharge of a nickel-based battery. Lead- and lithium-based system have a lower self-discharge.Courtesy of Cadex

One of the best batteries in terms of self-discharge is lead acid; it loses only five percent per month. This chemistry also has the lowest specific energy and is ill suited for portable use. Lithium-ion self-discharges about five percent in the first 24 hours and 1 to 2 percent thereafter. The need for the protection circuit increases the discharge by another three percent per month.The self-discharge on all battery chemistries increases at higher temperatures and the rate typically doubles with every 10°C (18°F). A noticeable energy loss occurs if a battery is left in a hot vehicle. Cycling and aging also increase self-discharge. Nickel-metal-hydride is good for 300-400 cycles, whereas the standard nickel-cadmium lasts over 1,000 cycles before elevated self-discharge starts interfering with performance. The self-discharge on an older nickel-based battery can get so bad that the pack loses its energy mainly through leakage rather than normal use during the day. Discard a battery if the self-discharge reaches 30 percent in 24 hours.The self-discharge of Li-ion is reasonably steady throughout the service life and does not increase noticeably with age, unless there is a cell anomaly caused by separator damage when microscopic metal particles group together. Improved manufacturing methods have minimized this problem on newer batteries.Table 8 reveals the self-discharge rate per month at various temperatures and state-of-charge conditions. 

Charge condition 0°C (32°F) 25°C (77°F) 60°C (140°F)Full charge

40–60% charge6%2%

20%4%

35%15%

Table 8: Self-discharge of Li-ion at various temperatures and state-of-chargeSelf-discharge increases with rising temperature and higher SoC.

Once present, the high self-discharge of a flooded lead acid battery cannot be reversed. Factors leading to this failure are sludge buildup in the sediment trap on the bottom of the container. The sludge is semi-conductive, and when substance reaches to the plates a soft short occurs. On nickel-based batteries, a weakened or damaged separator is the cause of high self-discharge. Contributing factors are crystalline formation (memory), permitting the battery to “cook” in the charger or exposing it to repeated deep discharge cycles. A faulty separator also increases the self-discharge in lithium-ion batteries. In extreme cases, heat generated by the electrical leak further weakens the damaged separator. This can lead to a thermal breakdown.

Premature Voltage Cut-off

Not all stored battery energy can or should be used on discharge, and some reserve is almost always left behind when the equipment cuts off. There are several reasons for this.Most cell phones, laptops and other portable devices turn off when the lithium-ion battery reaches 3V/cell on discharge. The manufacturers choose this relatively high voltage threshold to allow for some self-discharge while in storage, giving a grace period before the protection circuit opens at about 2.5V/cell.A hybrid battery on a car never fully discharges and operates on a state-of-charge of 20 to 80 percent. This is the most effective working bandwidth of the battery, and staying within this range delivers the longest service life. A deep discharge with a full recharge causes undue stress to any battery, including Li-ion. Nickel-based batteries are similar, and because of reduced charge acceptance and heat buildup above the 80 percent SoC, the batteries are seldom fully charged. The emphasis on an electric powertrain is on maximizing service life rather than optimizing runtime (as is the case with consumer products).Power tools and medical devices that draw high currents push the battery voltage to an early cut-off. This is especially true if one of the cells has a high internal resistance, or when the battery is operating at cold temperatures. These batteries may still have ample capacity left after the “cut-off” and when discharging at moderate load, a battery analyzer may read a residual capacity of 30 percent. Figure 9 illustrates the cut-off voltage graphically.

Figure 9: Illustration of equipment with high cut-off voltagePortable devices do not utilize all available battery power and leave some energy behind.Courtesy of Cadex

How does Internal Resistance affect Performance?

With the move from analog to digital, new demands are placed on the battery. Unlike analog portable devices that draw a steady current, the digital equipment loads the battery with short, heavy current spikes. 

One of the urgent requirements of a battery for digital applications is low internal resistance. Measured in milliohms, the internal resistance is the gatekeeper that, to a large extent, determines the runtime. The lower the resistance, the less restriction the battery encounters in delivering the needed power spikes. A high mW reading can trigger an early 'low battery' indication on a seemingly good battery because the available energy cannot be delivered in the required manner and remains in the battery

Figure 1 demonstrates the voltage signature and corresponding runtime of a battery with low, medium and high internal resistance when connected to a digital load. Similar to a soft ball that easily deforms when squeezed, the voltage of a battery with high internal resistance modulates the supply voltage and leaves dips, reflecting the load pulses. These pulses push the voltage towards the end-of-discharge line, resulting in a premature cut-off. As seen in the chart, the internal resistance governs much of the runtime. 

Figure 1: Discharge curve on a pulsed load with diverse internal resistance. This chart demonstrates the runtime of 3 batteries with same capacities but different internal resistance levels.

Talk-time as a function of internal resistance 

As part of ongoing research to measure the runtime of batteries with various internal resistance levels, Cadex Electronics examined several cell phone batteries that had been in service for a while. All batteries were similar in size and generated good capacity readings when checked with a battery analyzer under a steady discharge load. The nickel-cadmium pack produced a capacity of 113%, nickel-metal-hydride checked in at 107% and the lithium-ion provided 94%. The internal resistance varied widely and measured a low 155 mOhm for nickel-cadmium, a high 778 mOhm for nickel-metal-hydride and a moderate 320 mOhm for lithium-ion. These internal resistance readings are typical of aging batteries with these chemistries.

Let's now check how the test batteries perform on a cell phone. The maximum pulse current of a GSM (Global System for Mobile Communications) cell phones is 2.5 amperes. This represents a large current from a relatively small battery of about 800 milliampere (mAh) hours. A current pulse of 2.4 amperes from an 800 mAh battery, for example, correspond to a C-rate of 3C. This is three times the current rating of the battery. Such high current pulses can only be delivered if the internal battery resistance is low.

Figures 2, 3 and 4 reveal the talk time of the three batteries under a simulated GSM current of 1C, 2C and 3C. One can see a direct relationship between the battery's internal resistance and the talk time. nickel-cadmium performed best under the circumstances and provided a talk time of 120 minutes at a 3C discharge (orange line). nickel-metal-hydride performed only at 1C (blue line) and failed at 3C. lithium-ion allowed a moderate 50 minutes talk time at 3C. 

Figure 2: Discharge and resulting talk-time of nickel-cadmium at 1C, 2C and 3C under the GSM load schedule. The battery tested has a capacity of 113%, the internal resistance is a low 155 mOhm.

Figure 3: Discharge and resulting talk-time of nickel-metal-hydride at 1C, 2C and 3C under the GSM load schedule. The battery tested has a capacity of 107%, the internal resistance is a high 778 mOhm.

Figure 4: Discharge and resulting talk-time of a lithium-ion battery at 1C, 2C and 3C under the GSM load schedule. The battery tested has a capacity of 94%, the internal resistance is 320 mOhm. 

Internal resistance as a function of state-of-charge

The internal resistance varies with the state-of-charge of the battery. The largest changes are noticeable on nickel-based batteries. In Figure 5, we observe the internal resistance of nickel-metal-hydride when empty, during charge, at full charge and after a 4-hour rest period.The resistance levels are highest at low state-of-charge and immediately after charging. Contrary to popular belief, the best battery performance is not achieved immediately after a full charge but following a rest period of a few hours. During discharge, the internal battery resistance decreases, reaches the lowest point at half charge and starts creeping up again (dotted line).  

Figure 5: Internal resistance in nickel-metal-hydride. Note the higher readings immediately after a full discharge and full charge. Resting a battery before use produces the best results. References: Shukla et al. 1998.

Rodrigues et al. 1999.

The internal resistance of lithium-ion is fairly flat from empty to full charge. The battery decreases asymptotically from 270 mW at 0% to 250 mW at 70% state-of-charge. The largest changes occur between 0% and 30% SoC. 

The resistance of lead acid goes up with discharge. This change is caused by the decrease of the specific gravity, a depletion of the electrolyte as it becomes more watery. The resistance increase is almost linear with the decrease of the specific gravity. A rest of a few hours will partially restore the battery as the sulphate ions can replenish themselves. The resistance change between full charge and discharge is about 40%. Cold temperature increases the internal resistance on all batteries and adds about 50% between +30°C and -18°C to lead acid batteries. Figure 6 reveals the increase of the internal resistance of a gelled lead acid battery used for wheelchairs.  

Figure 6: Typical internal resistance readings of a lead acid wheelchair battery. The battery was discharged from full charge to 10.50V. The readings were taken at open circuit voltage (OCV).Cadex battery laboratories.