Batteries & Battery Chargers

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramcosemployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX21102 W.A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Batteries And Battery Chargers

Engineering Encyclopedia Electrical

Batteries and Battery Chargers

Saudi Aramco DeskTop Standards

CONTENTS PAGE

DETERMINING TYPES OF BATTERIES FOR USE IN TYPICALSAUDI ARAMCO APPLICATIONS ...................................................................................1DETERMINING BATTERY SIZE FOR USE IN TYPICAL SAUDIARAMCO APPLICATIONS ..............................................................................................34DETERMINING BATTERY CHARGER SIZE FOR USE IN TYPICALSAUDI ARAMCO APPLICATIONS .................................................................................57WORK AID 1: PROCEDURE AND TECHNICAL AND ECONOMIC

REQUIREMENTS FROM SADP-P-103 ANDESTABLISHED ENGINEERING PRACTICES FORDETERMINING THE TYPE OF BATTERY FOR USE INTYPICAL SAUDI ARAMCO APPLICATIONS......................................71

WORK AID 2: PROCEDURE AND TECHNICAL REQUIREMENTSFROM SADP-P-103 AND ESTABLISHEDENGINEERING PRACTICES FOR DETERMINING THESIZE OF BATTERY FOR USE IN TYPICAL SAUDIARAMCO APPLICATIONS ....................................................................73

WORK AID 3: PROCEDURE, TECHNICAL REQUIREMENTS, ANDFORMULAS FROM SADP-P-103 AND ESTABLISHEDENGINEERING PRACTICES FOR DETERMINING THESIZE OF A BATTERY CHARGER FOR USE INTYPICAL SAUDI ARAMCO APPLICATIONS......................................85

GLOSSARY........................................................................................................................88



Saudi Aramco DeskTop Standards 1

DETERMINING TYPES OF BATTERIES FOR USE IN TYPICAL SAUDI ARAMCOAPPLICATIONS

The process of determining the type of battery for use in a given application requires anevaluation of factors such as the following:

The type and the duration of the connected load. The anticipated frequency and depth of discharge. The ambient temperature at the installation site. The planned life of the installation. The frequency of maintenance. Space limitations. Life cycle cost. Seismic requirements.

The two general types of storage batteries that can be used in Saudi Aramco installations arelead-acid batteries and nickel-cadmium batteries. Each of these general types offers certainadvantages and disadvantages in regard to the previously listed factors. In addition, a numberof individual designs exist within each general type. These individual designs offer furtheradvantages and disadvantages in regard to the previously listed factors.

Because no single battery design can provide the optimum performance that is associated witheach of the previously listed factors, the factors must be weighted as to their importance ineach given installation. The actual battery type is then determined based on the weightedfactors to provide the best available compromise between desirable characteristics andundesirable characteristics.

This section of the Module will provide information on the following topics that are pertinentto determining the type of battery for use in typical Saudi Aramco applications:

Lead-Acid Batteries Nickel-Cadmium Batteries Operational Characteristics Battery Applications




Lead-Acid Batteries

The lead-acid battery is a group of electrochemical cells that are connected in series togenerate a nominal dc voltage that would supply power to a suitably connected electrical load.The number of cells that are connected in series determines the nominal voltage rating of thebattery. The amount of active material that is contained in an individual cell is the factor thatdetermines the discharge capacity rating of the cell; the rated capacity of the individual cells isthe rated capacity of the entire battery. Connecting the individual cells in series does notincrease the capacity rating of the battery.

The electrochemical couple that is used to form lead-acid cells is configured throughplacement of a lead active material in a dilute sulfuric acid electrolyte. This electrochemicalcouple produces a nominal cell voltage of approximately two volts.

All lead-acid cells are constructed through use of the following basic components that areshown in Figure 1:

Element Cell jar Cell cover Electrolyte




Basic Components of a Lead-Acid CellFigure 1




Figure 2 shows an exploded view of the battery element. The battery element is the keycomponent of the cell, and it consists of an assembly of positive and negative plates that areinsulated from each other through use of separators. The interaction that occurs between thebattery element and the electrolyte determines the cell's performance characteristics.

Each of the plates consists of a rigid lead alloy that provides physical support for therelatively porous active materials. The positive and the negative plates are sandwichedtogether in an alternating pattern (e.g., negative-positive-negative-positive-negative) with anegative plate at each end of the assembly. Each positive plate is separated from itsneighboring negative plate by an insulating material. The insulating material typically is athin sheet of microporous rubber or plastic that is ribbed on the side that faces the positiveplate.

Battery ElementFigure 2




The cell jar, which was shown in Figure 1, is usually made of a transparent impact-resistantplastic material. The cell jar must be large enough to enclose the element and still providesufficient reservoir space above and below the element. The upper reservoir space is neededto allow for the gradual lowering of the electrolyte level that occurs during normal operationof the battery. The lower reservoir space serves as a collection basin for the sediment that isshed from the plates during the expected life of the battery. This sediment must be kept awayfrom the element to prevent short-circuiting of the positive and the negative plates.

After the element has been lowered into the empty jar, the cell cover is placed over the top ofthe jar and is sealed. The terminal posts that are connected to the element protrude throughholes that are in the top of the cell cover. The cell cover also is fitted with a flame arrestorvent that allows gases to escape from inside of the cell but that prevents entry of sparks orflames.

The electrolyte that is used in lead-acid batteries is a dilute solution of sulfuric acid and water.The ratio of acid weight to water is measured as specific gravity. Pure water has a specificgravity of 1.000. A typical nominal specific gravity for a lead-acid storage battery is 1.215 at25oC (77oF). The specific gravity of the electrolyte gradually drops as a cell is discharged.When the battery charger recharges the cell, the specific gravity gradually rises back to thenominal value.

If a battery is to be operated at temperatures that exceed 29oC (85oF) for more than 30 daysper year, a tropical (low) specific gravity electrolyte could be used to increase the life of thebattery. A medium or a high specific gravity electrolyte is also available for specialapplications such as UPS systems that, in some cases, will reduce the overall battery size thatis required.

As previously stated, all lead-acid batteries contain the same basic components. The majordifference between the various types of lead-acid batteries is the design of the positive platesthat are used in the battery element. The remainder of this section provides information onthe following specific types of lead-acid batteries:

Plante Lead calcium Lead antimony Sealed




Plante

The Plante lead-acid battery is named after Raymond Gaston Plante, who was the inventor ofthe first practical lead-acid storage battery. The original Plante cell was constructed of twolong strips of lead foil and intermediate layers of coarse cloth that were spirally wound andimmersed in a 10% solution of sulfuric acid. Because the amount of stored energy of theearly Plante cells depended on the corrosion of one lead foil to form lead dioxide, which is theactive material of the positive plate, the early Plante cells had little capacity. However, thecapacity of the early Plante cells did increase after repeated cycling because the cyclingresulted in corrosion of the substrate foil. Such corrosion created more active material and anincreased surface area.

Today, the name "Plante" refers to all lead-acid storage batteries in which the active materialof the positive plates is electro-chemically developed from pure metallic lead. Modern Plantelead-acid batteries are available with two types of positive plate constructions: traditional andManchester. Both types of plates are shown in Figure 3.

Positive Plates Used in Plante Lead-Acid Batteries




Figure 3




The traditional positive plate starts off as a blank (slab) of pure metallic lead. Grooves arethen cut into the surface of the lead blank through use of a combing process. The groovesincrease the surface area of the plate, which increases the capacity of the cell.

The Manchester plate is the most common type of plate that is used in Plante lead-acidbatteries. The Manchester plate is constructed of a heavy antimony alloy grid that is cast withcircular holes. Heavy corrugated strips of high purity lead are then rolled into spiral buttonsor "rosettes" that are forced into the holes that are cast in the grid. Each of the lead buttonsexposes approximately five times more surface area to the electrolyte than a comparable areaon a pasted plate battery. The lead buttons also help to prolong the life of the cell byproviding a reserve supply of unformed lead that is gradually converted to active materialduring operation of the cell.

The negative plates of Plante lead-acid batteries are pasted or "flat" plates with a heavy alloygrid. The construction of these plates will be discussed in more detail in the next section ofthis Module.

The following are the major advantages and disadvantages of Plante lead-acid batteries:

Advantages Disadvantages

Long service life (20 to 25 years) High cost Smallest amount of positive plate growthof all lead-acid cells

Poor energy density and poor powerdensity

Very high reliability Moderate self-discharge rates (3%/month)

Plante lead-acid batteries have the highest capital cost of all types of lead-acid batteries. Thecost of Plante lead-acid cells is approximately 3 to 3-1/2 times that of a comparable leadantimony cell and approximately 2-1/2 to 3 times that of a comparable lead calcium cell.However, in some applications, the high initial capital cost may be justified by the long (25year) service life.

The cyclic performance and cycle life of Plante lead-acid batteries, as is the case with allstationary lead-acid storage batteries, can only be discussed in relative terms. Stationary lead-acid storage batteries are designed to provide a relatively long calendar service life (more thanten years) when they are operated under float charge conditions rather than a Lead-AcidBatteries (Cont'd)

long cycle life. Stationary lead-acid storage batteries are normally not rated or guaranteed todeliver a specific number of equivalent full charge cycles.




When any stationary lead-acid storage battery is subjected to deep and frequent discharges, itscalendar service life will be reduced; however, because of the number of variables that areinvolved, the specific amount of calendar service life reduction cannot easily be quantified.For this reason, the cyclic performance of one type of lead-acid storage battery is generallyonly discussed in terms of comparison with another type of stationary lead-acid storagebattery.

The cyclic performance (e.g., the ability to withstand frequent and/or deep discharges) ofPlante lead-acid batteries is far superior to the cyclic performance of lead calcium batteriesand is approximately equivalent to the cyclic performance of lead antimony batteries. Plantelead-acid batteries are capable of withstanding a moderate amount of cycling with a minimumloss of calendar service life.

Lead Calcium

The positive and the negative plates of lead calcium batteries are pasted plates. Pasted platesare constructed by forcing a thick slurry of active material (e.g., a combination of lead oxidesand sulfuric acid) into an open lattice grid. A typical pasted plate grid is shown in Figure 4.The open lattice grid is cast from an alloy of pure lead and calcium. The addition of thecalcium alloying agent is necessary to increase the mechanical strength of the plate.




Typical Pasted Plate GridFigure 4

After the active material is "pasted" on the open lattice grid, the plates are dried and formed(activated) by an electrochemical process. The completed plate is porous so that the sulfuricacid electrolyte can circulate through the active material. The porous construction greatlyincreases the surface area of active material that is in contact with the electrolyte, whichincreases the capacity of a given sized cell.




The following are the major advantages and disadvantages of lead calcium batteries:


Medium service life (12-15 years). Subject to excessive positive plate gridgrowth.

Better energy density and power Not suitable for deep or density than Plantebatteries.frequent discharges

Low self-discharge rates (1%/month). Low water consumption. Medium cost.

The cost of lead calcium batteries is more than the cost of lead antimony batteries but isconsiderably less than the cost of Plante batteries. On a cursory examination, this mediumrange cost, when coupled with the other advantages of lead calcium batteries, make thisbattery appear to be more attractive for most applications than the Plante battery. However, aclose look at the disadvantages of the lead calcium battery shows that this battery is only moreattractive in non-cycling applications.

The cyclic performance of lead calcium batteries is extremely poor. Of the three major typesof stationary lead acid storage batteries (Plante, lead calcium, and lead antimony), the leadcalcium battery suffers the greatest loss of calendar service life when it is subjected to cyclicservice. This battery should only be used in float charge, shallow cycle applications.

Lead Antimony

The positive and the negative plates of lead antimony batteries are also pasted plates. Theonly real difference between the construction of lead antimony batteries and the constructionof lead calcium batteries is the addition of an antimony alloying material to the grid ratherthan a calcium alloying material. The antimony alloying material, which is similar to thecalcium alloying material, is added to increase the mechanical strength of the plate.

The amount of antimony that is used in the grid varies from about 1.5% to 12% antimony byweight. The percentage of antimony that is used affects the characteristics of the battery.High antimony content provides greater grid strength; however, it also results in higher floatcurrent requirements, an increase in water usage, and an increase in the frequency ofequalizing chargers towards the end of the battery's service life. Low antimony contentprovides more desirable operating characteristics but at the sacrifice of grid strength.




If the antimony content is less than 4%, small amounts of other elements such as seleniummust be added to maintain sufficient grid strength. Saudi Aramco does not allow the use oflead antimony batteries that contain more than 3% antimony.

The following are the major advantages and disadvantages of lead antimony batteries:


Low cost. High self-discharge rates (7%/month). Better energy density and power densitythan Plante batteries.

Short service life (10-12 years).

Can be used in cycling service. High water consumption.

Lead antimony batteries have the least capital cost of all lead-acid batteries. The low capitalcost makes this battery an attractive choice for cycling service applications in which the highwater usage is acceptable.

The cyclic performance of lead antimony batteries is approximately equivalent to the cyclicperformance of Plante batteries. Lead antimony batteries are capable of withstanding amoderate amount of cycling with a minimum loss of calendar service life.

Sealed

Two different versions of sealed lead-acid batteries are available: the gelled electrolyteversion and the absorbed glass mat (AGM) version. The basic technology of both versions ofsealed lead-acid batteries is identical in that they are both recombinant-type batteries that havepositive limited plate groups that operate in the oxygen cycle.

In all types of lead-acid batteries, a lead oxide positive plate and a sponge lead negative plateare placed in a dilute mixture of sulfuric acid. The voltage difference that is produced by thiselectrochemical couple causes electrons to flow from one plate to the other plate when theplates are connected. This electron flow causes a chemical reaction inside of the battery. Thechemical reaction will be discussed in more detail later in this section. When a lead acidbattery is discharged, much of the sulfuric acid electrolyte is changed to water and both of theplates are reduced to lead sulfate. When a lead-acid battery is subsequently charged byforcing electrons to flow in the opposite direction, all of the chemical reactions are reversed.




During recharge, oxygen is released at the positive plate, and, when the negative plate is fullycharged, it releases hydrogen. These gases are formed from the decomposition of the waterthat is mixed with the sulfuric acid. The gases must be vented out of the battery room, andthey must be properly dispersed because of the explosive nature of hydrogen. In floodedlead-acid batteries, this water loss must be replaced on a regular basis to prevent the platefrom drying out.

In sealed lead-acid batteries, the negative plate is designed to be what is known in the industryas "positive limited" because the chemical reaction of discharging is stopped when thepositive plate is exhausted. When a sealed lead-acid battery is recharged, the negative platenever reaches its full charge condition; therefore, hydrogen gas is not released. The oxygenthat is released at the positive plate travels through the void paths that are in the separator tothe negative plate. At the negative plate, the oxygen combines with the lead in the negativeplate to form lead dioxide. The lead dioxide reacts with the sulfuric acid that is in theelectrolyte to form lead sulfate and water. This water replaces the water that was consumed atthe positive plate to make the oxygen; therefore, water never needs to be added to the batteryand the case can be sealed.

The oxygen that is generated at the positive plate creates a positive pressure inside of thebattery that is two to three pounds above the ambient pressure. Because of this positivepressure, sealed lead-acid batteries must contain a relief valve to prevent overpressurization ofthe battery in the event of a malfunction such as a runaway charger. The relief valves aredesigned to automatically reseal once the pressure returns to normal.

The construction of the gelled electrolyte version of sealed lead-acid batteries is similar to theconstruction of flooded lead-acid batteries with a few key differences. In the gelledelectrolyte version, the dilute sulfuric acid mixture is blended with silica to form a gel or apaste-like substance. This gelled electrolyte is then used with a pasted plate element that iscomplete with a microporous separator. The primary difference between a pasted plateelement for a flooded lead-acid battery and the pasted plate element for sealed lead acidbattery is that the element that is used in the sealed lead-acid battery is positive limited. Theother key difference is that the sealed lead-acid battery is constructed with a relief valve ratherthan with a flash arrestor.

In the AGM version, the positive limited element is constructed without the microporousseparator. Instead, the positive plates are separated from the adjoining negative plates by afiberglass mat. The fiberglass mat absorbs the liquid electrolyte so that no free liquid existsinside of the battery. The very fine micro fiber construction of the mat and its relatively thin(approximately 1/8 inch) construction results in a very low internal cell impedance. Becauseof the AGM version's construction, it is generally smaller, lighter, and more energy-efficientfor short discharge periods than the gelled electrolyte version. However, the AGM version ismore prone to plate shorting as a result of small lead filaments bridging the thin glass matseparator.




The following are the major advantages and disadvantages of sealed lead-acid batteries:


Long service life (15 to 20 years) in floatservice.

High cost.

No maintenance. Short service life (< 10 years) in cyclingservice.

Reduced battery room/ventilationrequirements.

Moderate self-discharge rates (3%/month).

Vertical or horizontal mounting. Not suitable for deep or frequentdischarges.

The development of sealed lead-acid batteries for large stationary storage applications is arelatively recent (within the last ten years) technology and, as such, these batteries are moreexpensive than their lead calcium and lead antimony flooded cell counterparts. The cost ofsealed lead-acid batteries is similar to the cost of Plante lead-acid batteries.

Because most sealed lead-acid batteries for stationary storage applications use a lead calciumgrid, their cyclic performance is similar to the cyclic performance of the flooded lead calciumbatteries that were previously discussed. Sealed lead acid batteries do not perform well infrequent or deep cycle application; these batteries should only be used in float charge, shallowcycle applications.

Nickel-Cadmium Batteries

The nickel-cadmium battery also is a group of electrochemical cells that are connected inseries to generate a nominal dc voltage that would supply power to a suitably connectedelectrical load. The number of cells that are connected in series determines the nominalvoltage rating of the battery. The amount of active material that is contained in anindividual cell is the factor that determines the discharge capacity rating of the cell; the ratedcapacity of the individual cells is the rated capacity of the entire battery. Connecting theindividual cells in series does not increase the capacity rating of the battery.

The electrochemical couple that is used to form nickel-cadmium cells is configured throughplacement of a nickel hydroxide positive electrode and a metallic cadmium negative electrodein a dilute potassium hydroxide electrolyte. This electrochemical couple produces a nominalcell voltage of approximately 1.2 volts.




The following types of nickel-cadmium stationary storage batteries will be discussed in thissection:

Pocket Plate Sintered Plate

Pocket Plate

Pocket plate nickel-cadmium batteries get their name from the "pocket" strip that holds theactive material of the plates. The pockets are made from very thin, finely perforated strips ofsteel that are formed into shallow "U" channels as shown in Figure 5. The positive activematerial, which consists of hydrates of nickel oxide and graphite, and the negative activematerial, which consists of cadmium oxide and a small amount of iron oxide, are placed intothe open "U" channel. The open "U" channel is then covered with a similar strip of perforatedsteel and the two strips are crimped together to form a perforated pocket.

A number of these perforated pockets are interlocked edge-to-edge and then cut to theapproximate finished plate width. The interlocked pockets are then rolled to compress theactive material and to form longitudinal indentations in the plate. Each plate group is thenbolted or welded together. The two (positive and negative) plate groups are interleaved andinsulated with plastic rod or mat separators that are inserted into the longitudinal grooves ofthe plates. The insulated plate group or element is then inserted into a plastic container or jar.After the mechanical assembly is complete, the cells are put through an electrochemicalformation process to convert the active materials to their charged condition.




Pocket Plate ConstructionFigure 5




Figure 6 shows a cutaway view of a typical pocket plate nickel-cadmium battery. Thefollowing components are identified in Figure 6:

Vent Cap/Flame Arrestor Cell Container or Jar Plate Tab Plate Group Bus Separating Grids Plate Frame Plate

Typical Pocket Plate Nickel-Cadmium Battery




Figure 6




The vent cap/flame arrestor allows the gases that are generated during normal charging anddischarging to escape from the battery, but it prevents the entry of sparks and foreignmaterial. The vent cap/flame arrestor can be removed to allow access to the interior of thecell.

The cell container or jar is made out of translucent polypropylene. The cell container or jar isthe enclosure that holds the individual components of the cell and that contains the liquidpotassium hydroxide (KOH) electrolyte.

The plate tabs are spot welded to the plate frame and to the upper edge or the pocket plates.The plate tabs provide the electrical connection between the plates and the plate group bus.The plate group bus connects the plate tabs to the terminal posts of the battery. The plate tabsand the terminal posts are welded to opposite sides of the plate group bus.

The separating grids separate the plates and insulate the plate frames from each other. Theseparating grids are porous, and they allow the free circulation of the KOH electrolytebetween the plates.

The plate frames seal the plate pockets, and they serve as current collectors. The plates in thisparticular battery have horizontal pockets of double-perforated steel strips.

Most manufacturers make three general pocket plate designs: thin plates, medium plates, andthick plates. The thin plate design has a low internal resistance, and it is used in high rate,short duration (less than 30 minutes) discharge cells. The medium plate design has an internalresistance that is similar to the internal resistance of general purpose lead-acid cells. Thisplate design is used in median rate, median duration (from 30 minutes to 2 hours) dischargecells. The thick plate design has a high internal resistance, and it is used in very low rate,long duration (more than two hours) discharge cells.

All vented nickel-cadmium batteries have several general advantages over their flooded leadacid counterparts. One of these advantages stems from the KOH electrolyte that is used innickel-cadmium batteries. The freezing point of KOH electrolyte with a typical specificgravity of 1.190 is -32oC (-25oF). Because the specific gravity of the electrolyte in a nickel-cadmium battery remains relatively constant from the fully charged state to the fullydischarged state, the freezing point also remains relatively constant. In contrast, the freezingpoint of a fully discharged lead-acid battery essentially is the same as the freezing point ofwater or 0oC (32oF); therefore, lead acid batteries are much more likely to freeze than nickel-cadmium batteries. Nickel-cadmium batteries also make a significantly higher percentage oftheir current available at lower temperatures than lead-acid batteries.




Another general advantage of nickel-cadmium batteries is that they require much lessmaintenance than flooded lead-acid batteries. Nickel-cadmium batteries can go years withoutbeing watered, and they will not deteriorate when they are left in a discharged condition.Nickel-cadmium batteries have successfully been returned to service after being left "on-the-shelf" for 10 to 15 years.

The other general advantages of nickel-cadmium batteries include a more rugged and durableconstruction (i.e., relatively immune to vibration, shock, and overcharge currents), a longcycle life, and minimal gas generation on charge and discharge. Also, the gaseous vapors thatare emitted from nickel-cadmium batteries are not corrosive to ferrous metals.

The only general disadvantage of nickel-cadmium batteries to flooded lead-acid batteries isthe high cost of nickel-cadmium batteries. The cost (dollars/kilowatt-hour) of nickel-cadmium batteries is four to ten times the cost of flooded lead-acid batteries. Also, becausecadmium is more difficult and expensive to recycle and/or dispose of than lead, the disposalcost of expended nickel-cadmium batteries is higher than it is for expended lead-acidbatteries. In North America, large nickel-cadmium storage batteries are actually being phasedout of production as a result of the disposal issues that surround cadmium.

The advantages of pocket plate nickel-cadmium batteries over sintered-plate nickel-cadmiumbatteries are that pocket plate nickel-cadmium batteries have a lower cost, they have a longcycle life, and they do not suffer a "memory" effect on shallow discharges. The majordisadvantage of pocket plate nickel-cadmium batteries is that they only have about 50% of theenergy density of sintered-plate nickel-cadmium batteries.

In contrast to flooded stationary lead-acid batteries, pocket plate nickel-cadmium batterieshave a long cycle life in addition to a long calendar life. Under normal operating conditions,pocket plate nickel-cadmium batteries can deliver as many as 2000 equivalent full chargecycles. The calendar life of pocket plate nickel-cadmium batteries ranges from 15 to 25years. Because of the large number of cycles that this battery can withstand (more than 80equivalent full charge cycles per year over a 25 year calendar service life), cycling has little orno effect on the calendar service life of pocket plate nickel-cadmium batteries.

Sintered-Plate

Because Saudi Aramco does not permit the use of sintered-plate nickel-cadmium batteries,this section is intended only for general information purposes. The primary applications ofsintered-plate nickel-cadmium batteries are those that require high-power discharge service ina lightweight compact package such as aircraft turbine engine starting circuits.




Sintered-plate nickel-cadmium batteries are constructed similarly to pocket plate nickel-cadmium batteries. The major difference between the two batteries is the way in which theelectrodes (positive and negative plates) are designed. In the sintered-plate design, the activematerials are impregnated into a porous nickel coating that is applied to an iron grid or chargecollector.

Sintered-plate nickel-cadmium batteries have nickel oxyhydroxide positive plates andcadmium hydroxide negative plates. The plates are separated by non-conductive, porousmaterials that act as a gas barrier and an electrical separator. The electrolyte is a dilute KOHsolution, and it completely covers the plates and separators.

Figure 7 shows the construction of a typical sintered-plate nickel-cadmium battery. Thebattery consists of a plate pack that contains the positive plates, the negative plates, and theseparators. A terminal comb is placed over the negative plate tabs to connect the negativeplates. A separate terminal comb is placed over the positive plate tabs to connect the positiveplates. The assembled plate pack is then placed in the cell container. A cell cover is placedon top of the cell container, and it is sealed to the container. A terminal is connected to thepositive terminal comb and to the negative terminal comb. The terminals are used to connectthe battery to the external load. The removable vent cap allows excess gases to escape and ameans to add electrolyte.




Typical Sintered-Plate Nickel-Cadmium BatteryFigure 7




The advantages and disadvantages of sintered-plate nickel-cadmium batteries are shown inFigure 8.

Advantages and Disadvantages of Sintered-Plate Nickel-Cadmium BatteriesFigure 8




As was noted in Figure 8, the sintered-plate nickel-cadmium battery is more expensive thanthe vented pocket plate battery and many times more expensive than the sealed lead-acidbattery.

The average calendar life of a sintered-plate nickel-cadmium battery ranges from three to tenyears, and the cycle life ranges from 500 to 2000 equivalent full charge cycles. The cycle lifeand calendar life are strongly influenced by the method of operation, the ambient temperatureconditions, and the depth of discharge. The best battery life performance is obtained fromoperation at normal temperatures and at moderate (50% average) discharge depths. The lifeof sintered-plate nickel-cadmium batteries drops when the battery is subjected to frequentdeep or shallow discharges.

Operational Characteristics

Each type (lead-acid or nickel-cadmium) of battery has a unique set of operationalcharacteristics that are based on the construction and configuration of the electrochemicalcouple that is used in the battery. The following operational characteristics of batteries will bediscussed in this section:

Electrochemical Reaction Charge Discharge Characteristics Effects of Temperature on Battery Life and Capacity

Electrochemical Reaction

The electrical energy that secondary batteries are capable of delivering is derived from theelectrochemical reaction that occurs between two electrically dissimilar metals or metalliccompounds. In the case of lead-acid batteries, the electrolyte also takes part in theelectrochemical reaction. The electrochemical reaction is essentially reversible and electricalenergy is consumed to restore the battery to a charged condition. The sections that followprovide an explanation of the electrochemical reactions that occur during the discharge andcharge of the following types of secondary storage batteries:

Lead-Acid Nickel-Cadmium

Lead-Acid - In a fully charged lead-acid battery, the positive electrode material is lead dioxide(PbO2), the negative electrode material is pure sponge lead (Pb), and the electrolyte is amixture of sulfuric acid (H2SO4) and water (H2O). As the battery is discharged, both of theelectrodes are converted to lead sulfate (PbSO4) and the electrolyte is consumed, whichproduces water; the process reverses when the battery is charged. This electrochemicalreaction is called the "double-sulfate" reaction.




Figure 9 shows the discharge electrochemical reaction that occurs in a lead-acid battery. Theinitial reaction that occurs on discharge is an ionization process. The 2H2SO4 breaks downinto 2SO2-4 and 4H

+ through the ionization process. The PbO2 breaks down to 4OH- and

Pb4+. After the ionization process, the current producing process begins to produce usableelectric current. The Pb of the negative electrode reacts with the SO2-4 ions that are in theelectrolyte. The product of this reaction is Pb2+ and 2e; the Pb2+ then combines with theSO2-4 ions to produce PbSO4. PbSO4 is the discharged composition of the negativeelectrode.

At the positive electrode, the Pb4+ combines with the SO2-4 ions that are in the electrolyteand with 2e from the load to produce PbSO4. PbSO4 also is the discharged composition ofthe positive electrode. The 4OH- ions from the positive electrode combine with the 4H+ ionsthat are in the electrolyte to produce 4H2O. This discharge reaction continues until the activematerials are effectively depleted.

Discharge Electrochemical ReactionFigure 9




After the battery is discharged, a reverse polarity electric current must be applied to thebattery to reverse the electrochemical reaction, which restores the battery to a chargedcondition. Figure 10 shows a graphical representation of the electrochemical reactionthat occurs inside of a lead-acid cell during a battery charge.

When a reverse polarity current is applied to the electrodes, the PbSO4 that comprisesboth electrodes ionizes to form Pb2+ ions and SO2-4 ions. Also, the water (4H2O)that is in the electrolyte ionizes to form 4H+ ions and 4OH- ions. After ionizationoccurs, the Pb2+ ions of the negative electrode absorb 2e, which converts the negativeelectrode back to Pb. The SO2-4 ions from both of the electrodes combine with the4H+ ions that are in the electrolyte, which forms 2H2SO4. The Pb

2+ ions of thepositive electrode combine with the 4OH- ions that are in the electrolyte, whichconverts the positive electrode back to PbO2 and forms 2H2O. This charge reactioncontinues until the previously depleted active materials are returned to their originalcomposition. The following is the overall discharge/charge equilibrium equation forlead-acid batteries:

Pb + PbO2 + 2H2SO4 _ 2PbSO4 + 2H2O + 2e

Electrochemical Reaction During a Battery ChargeFigure 10




When a lead-acid battery approaches its full charge condition, the majority of thePbSO4 has been converted to Pb or PbO2, which causes the battery voltage on chargeto rise above the gassing voltage. The overcharge reactions then begin. Theovercharge reactions result in the production of hydrogen and oxygen (i.e., gassing)and the resultant loss of water. The following are the equations for the overchargereactions:

Negative Electrode Reaction: 2H+ + 2e _ H2

Positive Electrode Reaction: H2O - 2e _ 1/2O2 + 2H+

Overall Reaction: H2O _ H2 + 1/2O2

In sealed lead-acid batteries, the above reactions are controlled by design to preventhydrogen evolution and the loss of water through recombination of the evolvedoxygen with the negative plate.

Nickel-Cadmium - In a fully charged nickel-cadmium battery, the positive electrodematerial is nickel oxyhydroxide (NiOOH), the negative electrode material is cadmium(Cd), and the electrolyte is a mixture of potassium hydroxide (KOH) and water. Whennickel-cadmium batteries are discharged, the active materials that are contained in theelectrodes change in oxidation with no deterioration in the physical state. The activematerials are present only as solids that are highly insoluble in the KOH electrolyte.Also, the KOH electrolyte does not participate in the electrochemical reaction; theelectrolyte only acts as a current carrying medium.

During a discharge, the cadmium metal of the negative electrode oxidizes to cadmiumhydroxide (Cd(OH)2) and releases electrons to the external circuit. This portion of theelectrochemical reaction is shown in the following equation:

Cd + 2OH- _ Cd(OH)2 + 2e

At the positive electrode, the NiOOH is reduced to the lower valence state of nickelhydroxide (Ni(OH)2) by accepting electrons from the external circuit. This portion ofthe electrochemical reaction is shown in the following equation:

2NiOOH + 2H2O + 2e _ 2Ni(OH)2 + 2OH-




After the battery is discharged, a reverse polarity electric current must be applied to thebattery to reverse the electrochemical reaction, which restores the battery to a chargedcondition. When a reverse polarity current is applied to the electrodes, the Cd(OH)2of the negative plate accepts electrons and returns to its original state as shown by thefollowing equation:

Cd(OH)2 + 2e _ Cd + 2OH-

At the positive electrode, the 2Ni(OH)2 is oxidized back to the its original highervalence state of NiOOH as shown by the following equation:

2Ni(OH)2 + 2OH- _ NiOOH + 2H2O + 2e

The following is the overall discharge/charge equilibrium equation for nickel-cadmiumbatteries:

Cd + 2NiOOH + 2H2O _ Cd(OH)2 + 2Ni(OH)2

Charge/Discharge Characteristics

Figure 11 shows the way in which the following lead-acid battery characteristics changeduring a constant current discharge and subsequent charge:

Specific gravity Ampere-hours discharged/charged Volts per cell

When a lead-acid battery is discharged at a constant, the specific gravity linearly decreases inproportion to the number of ampere-hours that are discharged. In contrast, the volts per cellremain relatively constant at the beginning of the discharge and then the volts per celldecrease at an increased rate as the voltage approaches the end voltage of 1.75 volts. Whenthe battery reaches its end voltage, the discharge should be stopped and the battery should berecharged.

When a lead-acid battery is placed on a constant rate (current) charge, the volts per cell havean initial step jump increase, and then the volts per cell gradually rise and level off toward theend of the charge. The specific gravity increase during the battery charge is not linear and itlags behind the number of ampere-hours charged. The reason that the specific gravityincrease lags the number of ampere-hours charged is because complete mixing of theelectrolyte does not occur until gassing begins. Once gassing begins, the specific gravityquickly rises to its full charge level.




Lead-Acid Battery Charge/Discharge Characteristic Curves Figure 11

Figure 12 shows a typical charge/discharge characteristic curve for a vented nickel-cadmiumbattery. When a constant current discharge is placed on a fully charged nickel-cadmiumbattery, the battery voltage initially drops as it begins to supply current to the load. After theinitial drop in voltage, the voltage remains essentially constant until about 90% to 95% of theampere-hours have been discharged. At this point, the volts per cell quickly drop to the finalvoltage of 1.1 volts. When the battery reaches its end voltage, the discharge should bestopped and the battery should be recharged.

When a vented nickel-cadmium battery is charged at a constant rate, the volts per cell have aninitial step jump increase and then gradually rise and level off toward the end of the charge.The charge curve for a vented nickel-cadmium battery is similar to the charge curve for alead-acid battery except that the volts per cell values are lower.




Because the specific gravity of a nickel-cadmium battery remains essentially constant fromfully charged to fully discharged, the curve for the specific gravity of a nickel-cadmiumbattery is not shown in Figure 12.

Nickel-Cadmium Battery Charge/Discharge Characteristic CurvesFigure 12




Effects of Temperature on Battery Capacity and Service Life

The temperature at which a lead-acid battery is operated has a definite effect on its capacityand its service life. Figure 13 shows the effects of temperature on the capacity of a lead-acidbattery at several different discharge rates. For each discharge rate that is shown (3 to 8 hourrate, 0.5 to 3 hour rate, and 0.5 hour rate), the battery delivers its rated capacity (100%) at anelectrolyte temperature of 25oC, which is the optimum operating temperature for a lead-acidstorage battery. As the operating temperature drops below 25oC, the % rated capacity of thebattery also decreases. The decrease is due to reduced chemical activity and to increasedinternal resistance.

As the operating temperature rises above 25oC, the % rated capacity of the battery alsoincreases; however, such operation should be avoided as the high temperatures also cause asevere reduction in service life. Operation of a lead-acid storage at an electrolyte temperatureof 35oC reduces the service life by 50%.

Effect of Temperature on Lead-Acid Battery CapacityFigure 13




High and/or low electrolyte temperatures have a much smaller effect on nickel-cadmiumbatteries than they do on lead-acid batteries. Pocket plate nickel-cadmium storage batteriesthat contain standard electrolyte concentrations can be used at temperatures as low as -20oCwith little loss of capacity. Batteries that contain concentrated electrolyte can be used attemperatures as low as -50oC.

Pocket plate nickel-cadmium storage batteries can also be continuously operated at elevatedtemperatures with little or no loss of service life. Generally, these batteries can operate withelectrolyte temperatures that range from 45 to 50oC with no long term detrimental effects.

Battery Applications

A number of factors must be considered to select the best battery for a particular application.No single battery provides optimum performance under all operating conditions. Thecharacteristics of each available battery must be weighed against the total equipmentrequirements, and the battery that is selected must best fill these requirements. Generally, thefollowing factors are considered in the selection process:

Ambient temperature Life cycle cost Required ampere-hour capacity and duty cycle Frequency of required maintenance Available space

Because of the effects of electrolyte temperature on the capacity and the life of lead-acidbatteries, the ambient temperature of the battery installation site must be considered in thebattery selection process. If the battery will be required to operate in low ambienttemperatures, a lead-acid battery may still be satisfactory; however, a larger cell size would berequired to make up for the decreased ampere-hour capacity of lead-acid batteries at lowtemperatures. If the battery will be required to operate in high ambient temperatures, a nickel-cadmium battery may be the only acceptable choice because of the severe service lifereduction that is suffered by lead-acid batteries when they are operated in high ambienttemperatures.

The life cycle cost of the different batteries that could be used in a given installation must bedetermined to realistically compare the cost of different battery types. The life cycle cost of agiven battery is the initial capital cost of the battery plus the anticipated cost of maintenanceover the battery's anticipated service life, divided by the anticipated calendar service life ornumber of equivalent full charge cycles, whichever is applicable.




The required ampere-hour capacity and duty cycle must be known to determine the requiredcell size for the application. This topic is discussed in more detail in the next section of thisModule ("Determining Battery Size for use in Typical Saudi Aramco Applications").

The frequency of required maintenance must be considered from the standpoint of both costand practicality. If a battery is going to be installed in a remote, unmanned location, a highcapital cost battery that requires minimal maintenance may have a lower overall cost and bemore practical than a low capital cost, high maintenance battery.

The amount of space that is available must be considered to ensure that the chosen batterywill fit in the installation. If the installation space is limited, the additional cost of a compact,high-performance battery is justified.

The following are typical examples of applications in which each type of battery might beused:

Plante lead-acid batteries are used for UPS installations that have airconditioned battery rooms. This is one of the few applications in whichthe long service life of the Plante battery justifies its cost.

Lead calcium batteries are used in air conditioned applications in whichthe battery is only expected to be subjected to infrequent and shallowdischarges. Lead calcium batteries are not suitable for frequent and/ordeep discharge services. Lead-calcium batteries are suitable for manyUPS installations, emergency lighting installations, and telephoneexchanges.

Lead antimony batteries are used in air conditioned applications inwhich the battery is expected to be subjected to frequent and/or deepdischarges but also in which the high cost of Plante batteries cannot bejustified.

Sealed lead-acid batteries are used in applications that require nomaintenance or that require the battery to be installed in other than ahorizontal configuration.

Because nickel-cadmium batteries are generally more expensive than acomparable lead-acid battery, they should not be used in air conditionedspaces. Because the temperature has only a small effect on the nickel-cadmium battery's life and capacity, the nickel-cadmium battery is idealfor installation in high temperature, remote locations.




DETERMINING BATTERY SIZE FOR USE IN TYPICAL SAUDI ARAMCOAPPLICATIONS

After a specific type of battery is selected for a particular application, the size of the batterymust be determined. Determination of the proper size will ensure that the battery can supplypower to the connected load for the specified duration of time. This section providesinformation on the following topics that are pertinent to determining the correct battery size:

Load Criteria Duty Cycle Battery Voltage Determining Battery Capacity

Load Criteria

Accurate specification of battery size depends on an accurate definition of the system's load.The following data must be considered for each item of connected electrical equipment toaccurately define the system's load:

Voltage Range (Window) Current or kW Draw Load Classification (Duration of Operation) Frequency of Use

Voltage Range (Window)

Stationary battery systems can be designed to meet almost any desired voltage rating. Mostdc-powered electrical equipment falls within one of the following major dc voltage groups:

6/12 volts (emergency lighting units)

24 volts (alarm systems, engine cranking, communications systems)

32 volts (emergency lighting systems, engine cranking, electric clocksystems)

48 volts (switchgear systems, telephone systems, microwave systems,engine cranking)

120 volts (switchgear systems, emergency lighting systems, boiler flamecontrol, communication systems, telemetering, supervisory controlsystems, fire alarm systems, UPS systems, large engine cranking)

240 volts (switchgear systems, UPS systems, large engine cranking)

Higher voltages (UPS systems)




The battery system voltage is based on the number of cells that are connected in series and thenominal voltage of each cell; however, battery systems are normally maintained at voltagesthat are higher than the nominal voltage of the system. For example, a battery system that hasa nominal system voltage of 120 volts is normally operated at 130 to 135 volts. This highervoltage is due to the fact that most battery systems are always operated in a float chargecondition, and the float charge voltage is always higher than the nominal system voltage.

Most dc-powered equipment items are designed to operate on a fairly broad range of dcsupply voltages for the following reasons:

To accommodate the gradual decline of battery voltage as the batterydischarges.

To accommodate the voltage increases that occur when the battery is oncharge.

The minimum and the maximum voltage that is permissible for each item of connectedelectrical equipment must be known to properly size the battery in terms of voltage. Theactual determination of battery voltage is discussed in more detail later in this module.

Current or kW Draw

Each item of electrical equipment is assigned a nominal current rating (ampere rating) or akW rating by its manufacturer. If the equipment is assigned a nominal current rating, thisrating can be used as the equipment's contribution to the overall load on the battery for theperiod of time that the equipment operates. If the equipment is assigned a nominal kW rating,this rating must be divided by the battery system voltage to determine the equipment'scontribution to the overall load on the battery for the period of time that the equipmentoperates. When making this calculation, the engineer must note that, as the battery voltagedecreases during the discharge, the current load must increase to maintain the same kW.

In addition to the normal ampere rating, some electrical equipment has another current factorthat is seldom rated by the manufacturer but that is vitally important to battery size. Thisfactor is the temporary high ampere demand or inrush current that is imposed on the powersource (battery) when equipment such as electric motors are started. Inrush demands must bedetermined or estimated on the high side to ensure that the battery voltage does not dropbelow its specified minimum value during the inrush.




Load Classification (Duration of Operation)

For Saudi Aramco installations, the individual dc loads that are supplied by a battery areclassified as follows:

Continuous loads Essential loads Momentary loads

Continuous loads are indicating lights, alarm systems, and other devices that are necessary forpersonnel safety. The following are the minimum continuous load durations that must beprovided:

Twelve hours for the continuous loads that are located in attendedsubstations or other similar locations.

Eighteen hours for the continuous loads that are located in unattendedsubstations in on-shore locations.

Twenty-four hours for the continuous loads that are located inunattended off-shore locations.

Essential loads are critical motors, emergency lighting, selected communication equipment,emergency shutdown systems (ESD), and any loads that are determined to be life critical bythe Loss Prevention Department. Regardless of the location of the load, the minimumessential load duration that must be provided is three hours.

Momentary loads are random, short duration loads that are considered coincident with thehighest load requirement. Generally, momentary loads provide close or trip power forswitchgear and generator field flashing. UPS systems may also require a momentary load atthe end of the discharge to furnish power for ESD systems to energize motor operated valves.




The design duration of momentary loads is different for lead-acid and nickel-cadmiumbatteries. Because of the time that is required for the battery to respond to an abrupt loadchange, the momentary loads for lead-acid batteries system have a design duration of oneminute. The design duration for nickel-cadmium batteries is one second. The faster responseof nickel-cadmium batteries can result in smaller ampere-hour ratings.

The loads that are included in the three listed categories are not a comprehensive list of all thepossible dc loads at a given installation. The Electrical Engineer must carefully analyze eachsystem to be sure that all possible loads and load variations have been included.

Frequency of Use

Because some of the electrical equipment that is supplied by the battery may be energizedmore than once during the battery discharge, the anticipated frequency of such usage must bespecified. If the frequency of use varies dependent on equipment positions or status when thebattery discharge begins, the maximum number of possible operations should instead bespecified. Such a specification is necessary to ensure that the battery will have sufficientcapacity to handle a worst-case situation.




Duty Cycle

The duty cycle of a battery is defined as the load currents that a battery is expected to supplyfor specified periods of time. Accurate information about the anticipated duty cycle of a newbattery is needed to properly size the battery.

Selection of the proper size of battery depends not only on the nominal current rating or thekW draw and the duration of each load (i.e., the duty cycle), but it also depends on thesequence in which the load are energized. Careful scheduling of the load sequence in theduty cycle helps keep the required cell size to a minimum and reduces the cost of theinstallation.

Most stationary battery installations are multi-load systems rather than single-load systems.When a variety of loads are connected to a battery, sudden increases and decreases in currentdemands are imposed on the battery. If the high-current loads can be scheduled to energize atthe beginning of the duty cycle rather than at the end of the duty cycle, a smaller sized batterycan be used. However, if the load is random in nature and could occur at any time during theduty cycle, the best practice is to assume that the load will energize at the most limiting pointin the duty cycle when its effect will be the most severe.

A duty cycle diagram is a tool that aids in the analysis of the duty cycle and in thedetermination of the required battery capacity. A duty cycle diagram shows the total requiredbattery current at any given time in the duty cycle. To prepare such a diagram, all of the loadsthat are expected during the duty cycle are tabulated along with their anticipated start and stoptimes.

The loads that have known start and stop times are plotted on the duty cycle diagram as theywould occur. The remainder of the loads should be plotted through use of the followingguidelines:

If the start time for a load is known but the stop time is indefinate, theload is assumed to be continuous for the remainder of the duty cycle.

If the load can occur at random, the load is assumed to occur at the mostcritical point in the duty cycle to simulate the worst-case condition. Themost critical point in the duty cycle is the point that controls the size ofthe battery that is needed.




Figure 14 shows an example of the duty cycle diagram that would be plotted from thefollowing hypothetical loads:

L1 represents 50 amps of continouus emergency lighting load for threehours.

L2 represents 100 amps of momentary switchgear operations load and250 amps of momentary motor starting load.

L3 represents 50 amps of noncontinuous motor load for one and a halfhours.

L4 represents 100 amps of noncontinuous load that starts after 30minutes and stops after one hour of operation.

L5 represents 50 amps of momentary switchgear operations load thatoccurs during the last minute of the duty cycle.

L6 represents four 25 amp random momentary loads that can occur atany time during the duty cycle.




Duty Cycle DiagramFigure 14




Battery Voltage

The voltage at which a battery system operates is not constant; rather, it fluctuates betweenminimum and maximum fixed values dependent on the state of charge. The range of voltagesthat lies between the minimum system voltage and the maximum system voltage is thevoltage operating "window" for the battery system. The size (number of cells) of the batterymust be selected to ensure that the battery operates inside of the voltage window.

For batteries that supply dc systems, the minimum and the maximum battery system voltagesare based on the voltage rating of the most limiting load. The minimum system voltage isselected to ensure that sufficient voltage is available to operate the loads at the end of thebattery duty cycle. The maximum system voltage is selected to ensure that the voltage ratingof the loads will not be exceeded when the battery is being recharged. The following are theminimum and the maximum battery system voltages for Saudi Aramco dc systems:

Nominal System Voltage Minimum System Voltage Maximum System Voltage12 vdc 10.5 vdc 15.5 vdc24 vdc 21.0 vdc 30.0 vdc48 vdc 42.0 vdc 58 vdc

120/125 vdc 105 vdc 143 vdc240/250 vdc 210 vdc 286 vdc360/375 vdc 315 vdc 429 vdc

For batteries that supply UPS systems, the minimum and the maximum battery systemvoltages are based on the allowable dc input voltage ratings of the UPS system. Theallowable dc input voltage ratings are specified by the manufacturer of the UPS system.

For batteries that supply a combined dc and UPS system load, the minimum and themaximum system voltages for dc systems should be used.

Because the battery system voltage is the product of the individual cell voltages and thenumber of cells that are connected in series, the minimum and the maximum battery systemvoltage values are used to determine the minimum and the maximum number of cells that canbe used in a battery installation. The following equation (included in Work Aid 2) can beused to calculate the minimum number of cells that can be used in an installation:

For most applications, the final volts per cell for lead-acid batteries is 1.75, and the final voltsper cell for nickel-cadmium batteries is 1.1 volts. If justified by valid engineering andeconomic reasons, other final volts per cell values could be used.

The following equation (included in Work Aid 2) can be used to calculate the maximumnumber of cells that can be used in an installation:




The maximum cell voltage on charge for lead-acid batteries is the equalizing voltage that isrecommended by the manufacturer. If the manufacturer's data are not available, a value of2.33 volts per cell can be assumed. The maximum cell voltage on charge for nickel-cadmiumbatteries is the charging voltage that is recommended by the manufacturer. If themanufacturer's data are not available, a value of 1.55 volts per cell can be assumed.

After the minimum and the maximum number of cells are known, the actual number of cellscan be specified. The actual number of cells that are specified can be any number that liesbetween the minimum and the maximum number. For consistency and ease of specification,SAES-P-103 contains the following information that lists the number of lead-acid or nickel-cadmium cells that should be specified for various nominal battery system voltages:

Nominal BatterySystem Voltage

(vdc)

Number of Cells

Lead-Acid Nickel-Cadmium12 6 1024 12 1948 24 37

120/125 60 92240/250 120 184360/375 180 276

The equations serve as a second check to verify that the number of cells that are listed iscorrect for the application.

For example, if a new lead-acid battery is needed to supply emergency power for a dc systemwith a nominal voltage of 120 vdc, the number of cells that are required is determined asfollows:

From the table that is in SAES-P-103, 60 lead-acid cells are required fora nominal battery system voltage of 120 vdc.

The number of cells that were determined from the table can be verified as follows:

The above calculations verify that 60 lead-acid cells are adequate for this installation.




Determining Battery Capacity

The capacity of a secondary storage battery is usually expressed in amperes and time. Forexample, a battery capacity can be expressed as 42.5A for eight hours or 340 ampere-hours(Ah) at the eight hour rate. Because the temperature of the electrolyte and the minimumallowable cell voltage also affect the number of Ah's that a given battery can deliver, thesevalues must also be included as part of the battery's capacity rating. For example, acompletely defined battery capacity is 42.5A for eight hours at an electrolyte temperature of25oC to a final cell voltage of 1.75 vdc. The following is a summary of relationships thatexist between the various elements of a battery capacity rating:

As the discharge rate (amperes) of a battery increases above thestandard rate, the length of time that elapses before the final cell voltageis reached decreases.

As the discharge rate (amperes) of a battery decreases below thestandard rate, the length of time that elapses before the final cell voltageis reached increases.

If the electrolyte temperature is less than the standard rated electrolytetemperature and the battery is still discharged at its standard rate, thefinal cell voltage rating will be reached before the standard rated timeelapses.

If the final cell voltage that is allowed increases, the number of ampsthat the battery can deliver for the standard rated time period decreasesor the length of time that the battery can deliver its standard rated ampsdecreases.

If the final cell voltage that is allowed decreases, the number of ampsthat the battery can deliver for the standard rated time period increasesor the length of time that the battery can deliver its standard rated ampsincreases.

Each stationary storage battery manufacturer generally designs more than one series of celltypes that vary in plate thickness and separation, in the number of plates, and in the size of thecurrent carrying parts. These variances result in large differences in the performance ofdifferent cells that have similar nominal Ah capacities. For example, the following list showssix different commercially available cells from different manufacturers:




Discharge in Amperes at 25oC to 1.75 v/c(Lead-Acid) and 1.10 v/c (Nickel-Cadmium)

Ah Capacityfor 8 Hours 8 Hours 3 Hours 1 Hour 1 Minute

Plante 760 95 190 330 500Lead-Calcium 752 94 188 369 910Lead-Antimony 752 94 202 411 964Nickel-Cadmium (L) 760 95 237 454 1140Nickel-Cadmium (M) 736 92 232 540 1780Nickel-Cadmium (H) 744 93 241 666 3180

The list shows that although all of the batteries have similar Ah capacities at the eight hourrate, the batteries have very different ampere capabilities for shorter duration discharges. Ifone of the batteries from the table was specified for a given application solely on the basis ofits Ah rating at the eight hour rate, the battery could be too small or too large for theapplication by a factor of two.

Because of all of the variables that can affect the ability of a battery to deliver a given amountof amps for a given period of time, the required battery capacity should not be determinedsolely on the basis of the Ah capacity that is required. Determining the required batterycapacity really involves determining the specific type of cell from a given manufacturer that iscapable of meeting all of the requirements of the duty cycle for which it is intended.

A variety of methods can be used to determine the required battery capacity for a givenapplication. The most common method of determining the required battery capacity for thefollowing Saudi Aramco applications will be discussed in this section:

DC Systems UPS Systems

DC Systems

This section uses an example to explain a method that can be used to determine the requiredcapacity of a battery for a dc system. The actual procedural steps and technical requirementsfor determining the required capacity of a battery for a dc system are located in Work Aid 2A.The Participant should refer to Work Aid 2A as necessary during this discussion.

Before Work Aid 2A can be used to determine the required capacity of a battery for a dcsystem, the following aspects of the duty cycle diagram that were previously shown in Figure14 must be defined:

Period Section




A period is an interval of time in the battery duty cycle during which the current is assumed tobe constant for the purpose of cell sizing calculations. A section is an interval of time fromthe beginning of the duty cycle to the end of a period. A simple duty cycle that contains asingle continuous load has one period and one section. A complex duty cycle has multipleperiods and multiple sections. Figure 15 identifies the periods and the sections of the dutycycle diagram that were previously shown in Figure 14. Figure 15 shows that this particularduty cycle has five periods and five sections.




Periods and Sections of a Duty Cycle DiagramFigure 15




To determine the required battery capacity for a dc system, the following data concerning theinstallation must be obtained:

The type (lead-acid or nickel-cadmium) of battery that is to be installed.

The nominal dc voltage of the system.

A list of all of the loads (including, for each load, the electrical ratingsand the anticipated start and stop times) that are to be powered by thebattery.

The lowest anticipated electrolyte temperature.

The battery manufacturer's technical literature.

The following data are used in the example capacity determination:

The type of battery is lead-acid.

The nominal voltage is 120 vdc.

The lowest anticipated electrolyte temperature is 10oC.

The typical manufacturer's technical literature from Work Aid 2A isused in the example.

The following loads are to be powered by the battery:

Load Type Voltage Range Current Draw Start Time Stop TimeContinuous 96-150 vdc 50 A o min 180 minMomentary 96-150 vdc 350 A 0 min 1 min

Noncontinuous 96-150 vdc 50 A 1 min 90 minNoncontinuous 96-150 vdc 100 A 30 min 90 min

Momentary 96-150 vdc 50 A 179 min 180 minRandom 96-150 vdc 100 A --- ---

Step 1 of the procedure that is in Work Aid 2A is to determine the minimum and themaximum allowable dc voltage for the example system. The nominal dc voltage of theexample system is 120 vdc; therefore, from the table that is in the Technical Requirementssection of Work Aid 2A, the minimum voltage for the example system is 105 vdc and themaximum voltage for the example system is 143 vdc.




Step 2 of the procedure that is in Work Aid 2A is to verify that the system loads are designedto operate within the minimum and the maximum voltage limits. Because the given data statethat all of the loads are designed to operate on dc voltages that range from 96 to 150 vdc andthe actual voltage range will be from 105 to 143 vdc, all of the loads can operate within theminimum and the maximum voltage limits of the example system.

Step 3 of the procedure that is in Work Aid 2A is to determine the number of cells that arerequired in the example system. The nominal dc voltage of the example system is given as120 vdc; therefore, from the table that is in the Technical Requirements section of Work Aid2A, the required number of cells for the example system is 60 cells.

Step 4 of the procedure that is in Work Aid 2A is to verify that the number of cells is correctfor the application. The calculations for this verification were previously explained in thesection titled "Battery Voltage".

Step 5 of the procedure that is in Work Aid 2A is to tabulate the load information into a dutycycle diagram. An explanation of this step was previously provided in the section titled "DutyCycle Diagram". The duty cycle diagram for the example system is the same as the dutycycle diagram that was previously shown in Figure 15.

Step 6 of the procedure that is in Work Aid 2A is to use the duty cycle diagram and the CellSizing Work Sheet to determine the appropriate cell size for the example system. The CellSizing Work Sheet provides a convenient format for performing and recording thecalculational data. The completed Cell Sizing Work Sheet for this example is shown inFigure 16. The remainder of this discussion covers the substeps that are involved incompleting the Cell Sizing Work Sheet.

Substep 6a of the procedure that is in Work Aid 2A is to fill in the information that is at thetop of the work sheet. All of this information can be directly transferred from the given dataexcept for the minimum cell voltage. The minimum cell voltage must be calculated throughuse of the information that was determined in step 1 and step 3 of the procedure. Theminimum cell voltage is calculated as follows:




Example Cell Sizing Work SheetFigure 16




Substep 6b of the procedure that is in Work Aid 2A is to fill in the load amperes that arerequired for each period of the duty cycle. The load amperes for each period of the duty cyclediagram (Figure 15) are recorded in the appropriate block of column (2) in the work sheet(Figure 16). For example, Figure 15 showed that the load for period 1 is 400 amperes;therefore, 400 is written in each of the blocks of column (2) of the work sheet that containsA1. The "A" designates amperes, and the "1" designates period 1. Because the duty cyclediagram only contains five sections, only the first five sections of the work sheet are used.Also, because the amperes of period 3 (A3) are greater than the amperes of period 2 (A2) andbecause the amperes of period 5 (A5) are greater than the amperes for period 4 (A4), onlysections 1, 3, and 5 of the work sheet need to be filled out.

Substep 6c of the procedure that is in Work Aid 2A is to fill in the duration (minutes) of eachperiod of the duty cycle. The duration of each period of the duty cycle diagram (Figure 15) inminutes is recorded in the appropriate block of column (4) of the work sheet (Figure 16). Forexample, Figure 15 showed that the duration of period 1 is one minute; therefore, 1 is writtenin each of the block of column (4) of the work sheet that contains M1. The "M" designatesminutes and the "1" designates period 1.

Substep 6d of the procedure that is in Work Aid 2A is to perform the calculations that areindicated in column (3) of the work sheet (Figure 16). The results of the calculations shouldbe recorded as positive or as negative values. As an example, the following are thecalculations that are performed for section 3 of the work sheet:

A1 - 0 = Change in Load for Period 1400 - 0 = Change in Load for Period 1400 = Change in Load for Period 1

A2 - A1 = Change in Load for Period 2100 - 400 = Change in Load for Period 2-300 = Change in Load for Period 2

A3 - A2 = Change in Load for Period 3200 - 100 = Change in Load for Period 3100 = Change in Load for Period 3




Substep 6e of the procedure that is in Work Aid 2A is to perform the calculations that areindicated in column (5) of the work sheet (Figure 16). The times (T) that are being calculatedare the times from the beginning of the period to the end of the section. As an example, thefollowing are the calculations that are performed for section 3 of the work sheet:

T = M1 + M2 + M3T = 1 + 29 + 60T = 90

T = M2 + M3T = 29 + 60T = 89

T = M3T = 60

Substep 6f of the procedure that is in Work Aid 2A is to record the battery capacity factor (RTor KT from the manufacturer's literature) in column (6) of the work sheet (Figure 16) for eachof the discharge times that were calculated in substep 6e. For this example, the typical RTcurves for a lead-acid battery that are located in the Technical Requirements section of WorkAid 2A are used to complete this substep. The typical RT curves are used as follows:

The curve that corresponds to the time that is indicated in column (5) ofthe work sheet (Figure 16) must be located.

The time curve is followed to the left until it intersects the curve for theminimum cell voltage, which is the 1.75 volt curve for the examplesystem.

An imaginary line that is perpendicular to the x-axis is then drawnthrough the point of intersection of the time curve and the minimum cellvoltage curve such that it intersects the x-axis.

The point at which the imaginary line intersects the x-axis is thecapacity factor (RT or amps per positive plate) for that discharge time.

Substep 6g of the procedure that is in Work Aid 2A is to calculate and record the cell size thatis required for each period of the duty cycle as indicated in column (7) of the work sheet(Figure 16). The results of the calculation that are positive numbers should be recorded in thepositive values column, and the results that are negative numbers should be recorded in thenegative values column. As an example, the following are the calculations that are performedfor section 3 of the work sheet:




(3) / (6) = # of Positive Plates Required400 / 59 = # of Positive Plates Required6.78 = # of Positive Plates Required

(3) / (6) = # of Positive Plates Required-300 / 59 = # of Positive Plates Required-5.08 = # of Positive Plates Required

(3) / (6) = # of Positive Plates Required100 / 73 = # of Positive Plates Required1.37 = # of Positive Plates Required

Substep 6h of the procedure that is in Work Aid 2A is to calculate the algebraic totals andsubtotals for each section of the duty cycle and to record the sums in column (7) of the worksheet (Figure 16). The results for each section are as follows:

Section 1 total is 2.27 positive plates Section 2 total is 3.07 positive plates Section 3 total is 2.82 positive plates

Substep 6i of the procedure that is in Work Aid 2A is to record the maximum section size thatis indicated in column (7) of the work sheet (Figure 16) on line (8) of the work sheet and torecord the random section size on line (9) of the work sheet. These two values are then addedtogether and their sum is recorded on lines (10) and (11) of the work sheet.

Substep 6j of the procedure that is in Work Aid 2A is to select and record the appropriate cellsize temperature correction factor on line (12) of the work sheet (Figure 16). The table of cellsize temperature correction factors is located in the Technical Requirements section of WorkAid 2A. Because the lowest anticipated electrolyte temperature for the example system is10oC, the cell size temperature correction factor is 1.19.

Substeps 6k and 6l of the procedure that is in Work Aid 2A are to respectively record thedesign margin and the aging factor on lines (13) and (14) of the work sheet (Figure 16). Thedesign margin and the aging factor are stated in the Technical Requirements section of WorkAid 2A. The design margin is 110% and the aging factor is 125%.

Substep 6m of the procedure that is in Work Aid 2A is to calculate the product of lines (11),(12), (13), and (14) and to enter this value on line (15) of the work sheet (Figure 16). Thiscalculation is performed as follows:




Line (15) = (11) x (12) x (13) x (14)Line (15) = 3.64 x 1.19 x 1.10 x 1.25Line (15) = 5.96

Substep 6n of the procedure that is in Work Aid 2A is to round the fractional number ofpositive plates that are indicated on line (15) of the work sheet (Figure 16) up to the nexthigher integer. This value is then recorded on line (16) of the work sheet. The next higherinteger for the example system is six.

Substep 6o of the procedure that is in Work Aid 2A is to calculate the total number of platesthat are required. This calculation is performed as follows:

Total Number of Plates = 2 (Number of Positive Plates) + 1Total Number of Plates = 2 (6) + 1Total Number of Plates = 13

Once the total number of plates is known, the manufacturer's typical standard cell data that arein the Technical Requirements section of Work Aid 2A can be used to determine the specifictype of cell that is required for the example system. Because the example system requires acell that has 13 plates, the type of TCCX-825 cell should be used. This cell is rated to deliver825 Ah at the eight hour rate at an electrolyte temperature of 25oC to a final cell voltage of1.75 vdc.

UPS System

Because the only load that should be connected to a UPS system is the UPS system inverter,determining the capacity of UPS system batteries is less involved than determining thecapacity of dc system batteries that supply complex duty cycles. For the purpose ofdetermining the required capacity of a UPS system battery, the UPS system is treated as asingle continuous load with a duration that is equal to the specified battery protection period.The specified battery protection period for most Saudi Aramco UPS systems is 15 minutes or30 minutes.

The remainder of this section explains a method that can be used to determine the requiredcapacity of a UPS system battery through use of an example. The actual procedural steps andtechnical requirements for determining the required capacity of UPS system batteries arelocated in Work Aid 2B. The Participant should refer to Work Aid 2B as necessary duringthis discussion.




To determine the required capacity of a UPS system battery, the following data concerningthe installation must be obtained:

The type (lead-acid or nickel-cadmium) of battery that is to be installed. The nominal ac load voltage and the number of phases. The kVA rating of the UPS system. The power factor of the critical ac load. The efficiency of the inverter. The design battery protection period. The lowest anticipated electrolyte temperature. The battery manufacturer's technical literature.

The following data are used in the example capacity determination:

The type of battery is nickel-cadmium. The nominal ac load voltage is 120 vac, single phase. The kVA rating of the UPS system is 30 kVA. The power factor of the critical ac load is .90. The efficiency of the inverter is 80%. The design battery protection period is 15 minutes. The lowest anticipated electrolyte temperature is 25oC. The typical manufacturer's technical literature from Work Aid 2B is

used in the example.

Step 1 of the procedure that is in Work Aid 2B is to determine the minimum battery voltageand the number of cells that are required based on the nominal ac load voltage. The nominalac load voltage is 120 vac; therefore, from the table that is in the Technical Requirementssection of Work Aid 2B, the minimum battery voltage for the example system is 105 vdc andthe number of cells that are required for the example system is 92.

Step 2 of the procedure that is in Work Aid 2B is to calculate the required battery kW for theexample system, using the formula that is provided in the Technical Requirements section ofWork Aid 2B. This calculation is performed as follows:

Step 3 of the procedure that is in Work Aid 2B is to calculate the minimum required batterycurrent for the example system, using the formula that is provided in the TechnicalRequirements section of Work Aid 2B. This calculation is performed as follows:




Step 4 of the procedure that is in Work Aid 2B is to multiply the battery current that wascalculated in step 3 by the applicable cell size temperature correction fa

Batteries & Battery Chargers

Documents

type of battery

battery size

thesize of battery

battery charger size

technical material

nickelcadmium batteries

work aid

numberof individual