P.ANIL KUMAR 07N01A0201 CHAPTER-1 BATTERY CHARGER 1.1 INTRODUCTION: A battery charger is a device used to put energy into a secondary cell or (rechargeable) battery by forcing an electric current through it. The charge current depends upon the technology and capacity of the battery being charged. Batteries are used as a source of DC Electric power for different Applications. A battery is a device which supplies DC power by converting the stored chemical energy into electrical energy when it is connected to an external load. A battery set is formed by connecting several individual cells in series or parallel according to the requirement of Voltage and energy. The battery charger consists of separate boost charger and Separate float charger. The boost charger is of silicon diode type and float is of tyristor type. The booster charger is meant for boost the battery .When it is first 1
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P.ANIL KUMAR 07N01A0201
CHAPTER-1
BATTERY CHARGER
1.1 INTRODUCTION:
A battery charger is a device used to put energy into a secondary cell or
(rechargeable) battery by forcing an electric current through it. The charge current depends
upon the technology and capacity of the battery being charged.
Batteries are used as a source of DC Electric power for different Applications. A
battery is a device which supplies DC power by converting the stored chemical energy into
electrical energy when it is connected to an external load. A battery set is formed by
connecting several individual cells in series or parallel according to the requirement of
Voltage and energy.
The battery charger consists of separate boost charger and Separate float charger.
The boost charger is of silicon diode type and float is of tyristor type. The booster charger is
meant for boost the battery .When it is first commissioned, when the battery is discharged
completely. Float Charger is meant for feeding regulated 220v DC supply to DC loads like
breakers, coils, memory circuits, emergency lights, pump sets Etc. Operating on DC voltage
and also to trickle charge the 220v battery both boost and Float charger work on 3-phase,
BATTERY SAFETY EXPLOSIVE HAZARD All storage batteries give off a highly explosive mixture of hydrogen and
Oxygen when gassing. Therefore, never permit sparks, open flame, or lighted
cigarettes near a storage battery. Post "No Smoking" signs where they are clearly
visible to anyone entering the battery room area. A nonmetallic flashlight is
desirable for battery inspection. Use only alcohol thermometers when
taking electrolyte temperatures. Keep all battery connections tight to avoid
sparking. Never lay any metallic object on top of a battery.
A class C 10pound fire extinguisher should be mounted just inside the battery room
door. Carbon dioxide (CO2) is not recommended because of the potential
for thermal shock to the batteries.
ELECTROLYTE HAZARD
When handling electrolyte, wear face shields (face shields should not have
metal reinforcing rims, which could cause a battery short if dropped), rubber
aprons, and rubber gloves; avoid splashes. The electrolyte is injurious to
skin and clothing and must therefore always be handled carefully. The eyes
in particular should be guarded. If acid is splashed into the eyes or
anywhere on the skin, flood with water for at least 15 minutes and get
medical attention. Do not use bicarbonate of soda on the skin, which may
aggravate the burn. For neutralization of acid electrolyte spilled on the floor
or rack, a bicarbonate of soda solution—1 pound per gallon of water—is
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recommended.
For neutralization of ni-cad battery electrolyte (potassium hydroxide), keep
a concentrated solution of 20 ounces of boric acid powder per gallon of
water available for neutralizing spills on skin or clothing. Use plain water to
wash up spills of potassium hydroxide on the cells or racks. Care must be
taken to prevent the solution from getting into the cells.
A combination eye-wash, face, and body spray unit must be located within
25 feet of each battery room or battery system. These units can be
permanently mounted and connected to the facility's potable water system
or can be of a portable pressurized type.
FLAME ARRESTERS PURPOSE AND CLEANING
Article 480-9 of the National Electric Code requires each vented battery cell
to be equipped with a flame arrester designed to prevent destruction of the
cell attributable to an ignition of gases outside the cell.
The diffuser material of flame arresters can become partially clogged from
electrolyte spray if cells are overfilled with water or have been excessively
overcharged. Flame arresters should therefore be inspected annually, and all
arresters having clogged pores should be replaced or cleaned as follows:
Immerse the flame arrester several times in fresh water in a plastic
bucket.
Eject the water after each immersion by vigorous shaking or an air blast.
Dump and refill the bucket with clean water for every 15 flame arresters
that are cleaned.
Do not use any cleaning or neutralizing agents in the water because any
dry residue may clog the pores of the diffuser materials.
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VENTILATION A determination must be made for each battery area as to whether sufficient
ventilation is being provided to ensure adequate diffusion of hydrogen gas
during maximum gas generating conditions. Such determination can be
made from the following data:
1) When the battery is fully charged, each charging ampere supplied to the
cell produces about 0.016 cubic feet of hydrogen per hour from each cell.
2) This rate of production applies at sea level, when the ambient
temperature is about 77 EF, and when the electrolyte is "gassing or
bubbling."
3) Number of battery cells and maximum charging rate (not float rate) can
be obtained from specifications or field inspection.
4) Hydrogen gas lower explosive limit is 4 percent by volume. Good practice
dictates a safety factor of 5, which reduces the critical concentration to
0.8 percent by volume. This large safety factor is to allow for hydrogen
production variations with changes in temperature, battery room
elevation, and barometric pressure and also allows for deterioration in
ventilation systems.
ELIMINATION OF OVER SULFATION
A battery or cell that is "over sulfated" should be charged fully in the regular
way until specific gravity stops rising. Then one of the weakest cells should
be discharged through a load resistor at the normal 8-hour discharge rate to
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a final voltage of 1.75 volts. The battery is not over sulfated if the
representative cell gives normal capacity, that is, about 100 percent rated
capacity for a fairly new battery or down to 80 percent of initial rated
capacity for a battery nearing the end of its expected life.
If the above capacity is not obtained, possible over sulfation should be
treated as follows:
In cases where one or more individual cells have become "over sulfated"
and the rest of the battery is in good condition, these cells should be
treated separately after removing them from the circuit.
Recharge the removed cells at half the 8-hour discharge rate. Record
Hydrometer readings and temperature at regular intervals (3 to 5 hours)
during the charge to determine if rising specific gravity has peaked.
Maintain constant electrolyte level by adding water after each reading.
Do not add water before taking readings.
Continue the charge, recording the readings until no further specific
gravity rise has occurred in any cell for 10 hours. If the temperature
reaches 100 °F, reduce the current or temporarily interrupt the charge so
as not to exceed this temperature. When the specific gravity has reached
maximum, terminate the charge and record the hydrometer reading of
each cell.
The cells must be replaced if they again fail the capacity
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CHAPTER-IV
4.1 STORAGE CELLS-DEFINITION:
The function of ‘storage’ cells is to convert electrical energy into chemical energy
during the process known as ‘charging’ and the reverse of it when ‘discharging’.
During charging of the cell, when current is passed through it, certain chemical
changes take place in the active materials of the cell. Such chemical changes absorb energy
during their formation. When these chemical reactions are completed and the electric current
produces no further chemical changes, the cell is said to be fully charged.
When the cell is next connected to an external circuit, the active materials of the cell
revert to their original condition, thereby reversing the changes which occurred during
charging. In this process of undoing the chemical changes, absorbed energy is released in the
form of electric current, the process being known as discharging.
It should be noted that the cell does not ‘store’ electricity as such but absorbs electric
energy in the form of chemical energy, the whole process being reversible.
We will discuss two types of storage cells or accumulators or secondary cells i.e.
Lead-acid cell and Edison alkali cell.
4.2 CHEMICAL CHANGES:
Following chemical changes take place during the charging and discharging of a lead-
acid cell.
When the cell is fully charged, its positive plate or anode is PbO2 (dark chocolate
brown) and the negative plate or cathode is Pb (slate grey). When the cell discharges i.e. it
sends current through the external load, then H2SO4 is dissociated into positive H2 ions and
negative SO4 ions. As the current within the cell is flowing from cathode to anode, H2 ions
move to anode and SO4 ions move to the cathode.
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At anode (PbO2), H2 combines with the oxygen of PbO2 and H2SO4 attacks lead to
form PbSO4
PbO2 + H2 + H2SO4 PbSO4 + 2H2O
At the cathode (Pb), SO4 combines with it to form PbSO4
Pb+SO4 PbSO4
It will be noted that during discharging,
1. Both anode and cathode become PbSO4 which is somewhat whitish in colour
2. Due to formation of water, specific gravity of the acid decreases.
3. Voltage of the cell decreases.
4. The cell gives out energy.
4.3 TWO EFFICIENCIES OF THE CELL:
The efficiency of a cell be considered in two ways:
1. The quantity or ampere-hour (Ah) efficiency
2. The energy or watt-hour (Wh) efficiency.
The Ah efficiency does not take into account the varying voltages of charge and
discharge. The Wh efficiency does so and is always less Ah efficiency because
average p.d.during discharging is less than that during charging. Usually, during
discharge, the e.m.f.falls from about 2.1V to 1.8V whereas during charges, it rises
from 1.8 volts to about 2.6V.
• Ah.eff. = ampere – hours on discharge /Ampere – hours on charge
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The Ah efficiency of a lead-acid cell is normally between 90 to 95% meaning
that about 100 Ah must be put back into the cell for every 90 – 95 taken out of it.
If Ah efficiency is given, Wh efficiency can be found from the following relation:
Wh efficiency = Ah efficiency X average volts on discharge / Average volts on charge
The Wh efficiency varies between 72-80%
From the above, it is clear that anything that increases the charge volts or reduces the
discharge volts will decrease Wh efficiency. Because high charge and discharge rates will do
this, it is advisable to avoid these.
4.4 ELECTRICAL CHARACTERISTICS OF THE LEAD-ACID CELL:
The three important features of an accumulator, of interest to an engineer, are (i)
voltage, (ii) capacity and (iii) efficiency.
4.4.1 Voltage.
The open-circuit voltage of a fully-charged cell is approximately 2.1 volts. This value
is not fixed but depends on (a) length of time since it was last charged (b) specific
gravity-voltage increasing with increase in specific gravity and vice-versa. If specific
gravity comes near to density of water i.e. 1.00, then voltage of the cell will disappear
altogether (c) temperature-voltage increases (through not much) with increase in
temperature.
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CHARGE AND DISCHARGE VOLTAGE CURVES:
The variation in the terminal p.d. of a cell on charge and discharge are shown in
Fig. The voltage fall depends on the rate of discharge. Rates of discharge are
generally specified by the number of hours during which the cell will sustain the rate
in question before falling to 1.8V. The voltage falls rapidly in the beginning (rate of
fall in the beginning on the rate of discharge), then very slowly up to 1.85 V and
again suddenly to 1.8 V. The voltage should not be allowed to fall to lower than 1.8 V,
other wise hard insoluble lead sulphate is formed on the plates which increases the
internal resistance of the cell.
The general form of the voltage-time curves corresponding to 1-,3-,5- and 10-
hour rates of discharge, are shown in Fig. corresponding to the steady currents which
would discharge the cell in the above mentioned times (in hours). It will be seen that
both the terminal voltage and the rate at which the voltage falls depends on the rate of
discharge. The more rapid fall in voltage at higher rates of discharge is due to rapid
increase in the internal resistance of the cell.
During charging, the p.d. increases (Fig. ). The curve is similar to the discharge
curve reversed but is every where higher due to the increased density of H2SO4 in the
pores of the positive plates.
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4.4.2 Capacity :
It is measured in ampere-hours (Ah). One ampere-hour (Ah) is the amount of
electricity conveyed by one ampere in one hour.
The capacity is always given at a specified rate of discharge (10-hour
discharge in U.K., 8-hour discharge in U.S.A.). The capacity of a cell depends on the
amount of the active material on its plates. In other words, it depends on the size and
thickness of the plates However, for a given battery , the capacity is affected by the
following factors:
(1)_Rate of Discharge. The capacity of a cell, as measured in Ah, depends on
the discharge rate. It decreases with increased rate of discharge. Rapid rate of
discharge means greater fall in p.d. of the cell due to internal resistance of the
cell. Moreover, with rapid discharge, the weakening of the acid in the pores of
the plates is also greater. Hence, the chemical change produced at the plates by
1 amperes for 10 hours is not the same as produced by 2 amperes for 5 hours of
4 amperes for 2.5 hours. It is found that a cell having a 100 Ah capacity at 10-
hour discharge rate, has its capacity reduced to 82.5 Ah at 5-hour rate and
50Ah at 1-hour rate. The variation of capacity with discharge rate is shown in
Fig.
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(2) TEMPERATURE:
Capacity increases with increase in temperature, the increase in capacity being more
marked at higher rates of discharge. This is due to the fact that at higher temperatures
Chemical action is more vigorous, (b) the résistance of the acid decrease and (c) there is better
diffusion of the electrolyte.
With decrease in temperature, available voltage and capacity decrease until at
freezing point, the capacity is zero even when the cell is fully charged.
(3) DENSITY OF ELECTROLYTE:
As the density of electrolyte affects the internal resistance and the vigor of chemical
reaction, it has important effect on the capacity. Capacity increase with the density
4.5 INDICATIONS OF A FULLY – CHARGED CELL:
The indication of a fully-charged cell is:
1. Gassing
2. Voltage
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3. Specific gravity
4. color of plates.
4.5.1 Gassing
When the cell is fully charged, it freely gives off hydrogen at cathode and oxygen
at the anode, the process being known as ‘gassing’. Gassing at both plates indicates
that the current is no longer doing any useful work and hence should be stopped.
4.5.2 Voltage
The voltage ceases to rise when the cell becomes fully charged. The value of
the voltage of a fully-charged cell is a variable quantity being affected by the rate of
charging, the temperature and specific gravity of the electrolyte etc. The approximate
value of the e.m.f. is 2.1 V or so.
4.5.3 Specific Gravity of the Electrolyte
A third indication of the state of charge of a battery is given by the specific
gravity of the electrolyte. We have seen from the chemical equations of that during
discharging, the density of electrolyte decreases due to the production of water,
whereas it increases during charging due to the absorption of water. The value of
density when the cell is fully charged is 1.21 and 1.18 when discharged up to1.8 V.
Specific gravity can be measured with a suitable hydrometer.
4.5.4 Color
The color of plates, on full charge, is deep chocolate brown for positive plates
and slate grey for negative plates and the cell looks quite brisk and alive.
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4.6 APPLICATIONS OF LEAD-ACID BATTERIES:
Storage batteries are, these days, used for a great variety and range of purposes, some
of which are summarized below:
1. In central stations for supplying the whole load during light load periods, also to
assist the generating plant during peak load periods for providing reserve
emergency supply during periods of plant breakdown and finally, to store energy
at times when load is light for use at times when load is at its peak value.
2. In private generating plants both for industrial and domestic use, for much the
same purpose as in Central Stations.
3. In sub-stations, they assist in maintaining the declared voltage by meeting a part
of the demand and so reducing the load on and the voltage drop in the feeder
during peak-load periods.
4. As a power source for industrial and mining battery locomotives and for road
vehicles like cars and trucks.
5. As a power source for submarines when submerged.
6. For petrol motor-car starting and ignition etc.
7. As a low-voltage supply for operating purpose in many different ways such as
high-tension switchgear, automatic telephone exchange and repeater stations,
broadcasting stations and for wireless receiving sets.
8. Semi-sealed portable lead-acid batteries find many applications such as in
electronic cash registers, alarm systems, cordless TV sets, mini-computers and
terminals, electronically-controlled petrol pumps, portable instruments and tools
etc.
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4.7 COMMON BATTERY TERMS:
4.7.1 Cell:
A cell is a device that transforms chemical energy into electrical energy. The simplest
cell is a voltaic cell shown in Fig. 3.5. It consists of a carbon strip and a zinc strip suspended
in a jar containing a solution of water (H2 O) and sulphuric acid (H2SO4).
A cell is fundamental unit of a battery. The cell shown in fig consists of two strips or electrodes placed in the jar which also contains the electrolyte. The electrolyte in a battery can be in the form of either a liquid or a paste.
4.7.2 ELECTRODES:
Electrodes are conductors through which current leaves or returns to the electrolyte.
In a simple cell they are copper and zinc strips that are immersed in the electrolyte, where as
in a dry cell they are a carbon strip in the centre and a zinc container in which the cell is
assembled.
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4.7.3 ELECTROLYTE:
The electrolyte is the solution in which the chemical action called electrolysis
occurs. The electrolyte be a salt (example: sodium chloride), an acid (example: Sulphuric acid
or an alkaline solution. In the simple voltaic cell and in the battery, the electrolyte is in liquid
form, whereas in dry cell, the electrolyte is a paste.
4.8 SERIES AND PARLLEL COMBINATION OF CELLS:
In many cases, a battery operated device may require energy more then what one
cell can provide. The device may require either a higher voltage or higher current or in some
cases both. Under such conditions, it is necessary to connect more cells to meet the
requirement. If a higher is needed, cells are connected in series. They are connected in parallel
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if higher current is desired. To supply both higher voltage and current, they are connected in
combinations of series – parallel networks.
4.8.1 Series connection of cells:
In a series connection, the cathode of the first cell is connected to the anode of the
second cell, the cathode of the second cell to the anode of the third cell and so on
Fig (a)
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Fig (a) pictorial view of series connected cells
(b) Schematic diagram of the series connection
Let n be the number of cells connected in series,
E be the emf of one cell.
r be the internal resistance of one cell, and
R be the load resistance.
Then,
the total emf of the battery of n cells = nE volts
The total internal resistance of n cells = nr ohms
Therefore ,
total resistance of the circuit = R + nr ohms
Hence,
the current in the circuit I = nE / (R + nr) amperes
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In this type of a circuit, the maximum current flow depends on the internal resistance
of the battery.
When the internal resistance is minimum, the current will be maximum.
4.8.2 PARALLEL CONNECTION OF CELLS:
In a parallel connection of cells, all cathodes are connected are connected to one line
and all the nodes to another line, as shown in fig.. Therefore, the emf of the combination is
the same as the emf of one cell. The equivalent internal resistance of the battery is r/n, which
comes in series with the load resistance(R).
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Figure (a) pictorial view of parallel connected cells.
(b) Schematic diagram of the parallel connection
Therefore, the total resistance = R + (r/n) ohms
Hence, the current in circuit I = nE/(nr + r ) A
4.8.3 SERIES – PARLLEL CONNECTION OF CELLS:
Generally, in series – parallel combination of cells, first a certain number of cells
are connected in series and a few such series combinations are then connected in parallel as
shown in fig(a)
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SERIES-PARALLE CONNECTION OF CELLS fig (a)
If n cells are connected in a series circuit, and if m such series circuits are connected in
parallel, then
The internal resistance of each series circuit = nr ohms
The total internal resistance of parallel circuit = nr/m ohms
Therefore
The total resistance of the circuit = (R + nr/ m) ohms
the emf across the circuit = mf of the series circuit
= nE volts
Therefore the current in the circuit = I = nE/(R + nr/m) amperes
i.e. I = mnE/(mR+nr) amperes
from this equation it can be seen that the numerator is a constant . therefore, the
current in the circuit will be maxim only if the denominator is minimum . The denominator
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will have a minimum value if mR=nr or R=nr/m, i.e., when the load resistance is equal to the
internal resistance of the battery
]
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CONCLUSION The battery system is heart of system any electrical control system because the
supply is fed from batteries for protection purpose for emergency ledio pumps etc, during
critical condition i.e., total grid failure.
If the battery system is failed, then the damage in the electrical system will be very
very high so there are to be maintains very carefully. It is also learnt various maintenance free
type battery system available however in power plant only lead-acid maintenance type is
preferred because its reliability, short term rating and rugged construction.
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BIBLIOGRAPHY:
1. Baldsing, et.al.”Lead-Acid Batteries for Remote Area Energy Storage”, CSIRO
Australia, January 1991.
2. Institute of Electrical and Electronics Engineers, ”IEEE Recommended practice for
installation and operation of Lead-Acid Batteries for photovoltaic(pv) systems” ,
ANSI/IEEE Std. 937-1987, New York, NY, 1987.
3. Linden, “Handbook of Batteries and Fuel cells”,Mc Graw Hill, Inc., 1984