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7/26/2019 Alok Mitra notes
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Q.9.8. Compare the advantages and disadvantages of Lead-Acid
Alkaline
ANS.
Batteries.
LEAD-ACID
i
Develops nominal 2 volt per i)
cell, demanding. 12 cells
battery for the normal 24 volt
supply.
Less expensive in initial cost ii)
and more efficient. .
.ALKALINE
Develops 1.2 volt per cell,so
20 cells are required for
normal 24 volt supply.
Costly bus has the longer life
of 15 years compared to 5yrs
of lead-acid.
. Preferred for emergency power
supply being ore suited to long
periods of idle operation on
stand-by. It is regarded as
more reliable.
Electrolyte is aqueous KOH
solution, evolve gas.
continuously during charging.
Hydrogen and Oxygen gases
are evolved during changing
and vented through non-return
values.
Gases are highly explosive and .
alkaline in nature.
vii) Evolution of gas causes water
. loss during changing, which
should be tapped up.
viii) Life starts reducing viii) Can be operated continuously
considerably above 25C. at 45C, without loss of life.
ii
iii) Preferred for regular recycling iii)
duty of essential power
supplies operating on charge /
discharge basis.
iv)
Electrolyte IS dilute H2S04, iv)
evolves gas when nearing the
top of the changing.
Hydrogen & Oxygen gases are v)
evolved due to chemical
breakdown of water. Cells are
rented to allow gases to escape.
Gases are highly explosive and vi)
acidic in nature.
v)
vi)
vii) Evolution of gas causes water
loss which should be topped up
with distilled water.
ix)
Care must be taken not to ix)
cause sparks or produce naked
flame, when inspecting
batteries.
Sparking and naked film are
prohibited in the battery room.
7/26/2019 Alok Mitra notes
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Q.9.9. Give detailed report on Battery Checking, Maintenance And Safety.
What is Voltage Drop Test?
ANS.
1. Cell tops must be kept clean, dry, vents clear, and free of deposits. Terminal
connections should be tight, free of corrosion and coated with petroleum jetty to
prevent corrosion.
2. Electrolyte levels should be checked and topped up with distilled water to cover the
plates, by about 1/8 higher than the top end of plates.
3. Checking of specific gravity or relative density by hydrometer-
For a lead-acid battery, the relative density varies between 1.275-1.285 at 15C for a
fully charged cell, to about 1.11 when fully discharged. The terminal voltage to be
measured when supplying load. The terminal voltage of an idle cell is likely to be high
giving false indication of a fully charged cell. The terminal voltage of a fully charged cell
is about 2.0 volt on load falling to 1.75 V when fully discharged.
For alkaline battery, the relative density does not change, however, as the cell ages,
relative density gradually falls. A new cell will have a relative density of 1.190 at 15C,
whereas when it falls to 1.145, it indicates chemical deterioration has occurred and
electrolyte should be renewed or battery replaced. The terminal voltage of a fully charged
alkaline cell is about 1.2 V falling to 1.09 V when fully discharged.
4. The relative density readings vary with temperature and correction must be made to
arrive at a correct value. The figures specified above are for idea electrolyte at 15C.
Actual readings should be corrected by adding 0.007 for each 10C rise above 15C
and subtracting 0.007 for each 10C fall below 15C. .
5. The end charge is indicated when the terminal voltage of an alkaline cell levels out at
1.7 volt and for lead acid cell, it is about 2.6 volt.
6. Lead acid cells should be charged until gassing freely and charging should continue
for a further period until the charging voltage per cell levels out at 2.6 V.
Overcharging causes overheating, distortion of cell plates and consequent discharging
of plate active materials.
Lead acid cells suffer self- discharge due to spontaneous chemical reaction even when
not in use, so they must be maintained on a trickle charge, a low rate of charging.
7. Alkaline cells should be charged at the recommended rate until gassing freely and the
charging voltage per cell rises to about 1.7 V. Alkaline cells are able to retain their
charge for a considerable period and do not suffer, self-discharge, except at high temp.
Periodically, alkaline batteries should be given' a sort refresher charge by special
arrangement or through shore facility.
Lead acid cells best o~erating temp. range is 15-:-25C and maximum recommended
operating temp. is 45 C. If the temp. goes above, the battery condition deteriorates
and battery life decreases for Alkaline cells, the operating range is 15 - 45C. At
low temp., battery condition does not deteriorate but capacity decreases.
7/26/2019 Alok Mitra notes
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Safety-
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9. The battery capacity is rated in terms of its discharge current at the 10 hour rate. A
250 Amp-Hr battery can supply 25 Amp for 10 hours.
Voltage drop test- The condition of a battery is checked by allowing it to discharge at
the recommended rate for 10 hrs (i.e. for a 150 Amp-Hr battery, @15Amp for 10 Hrs.),
the voltage drop should not exceed 12.5 of the nominal system voltage after the test
The voltage variation should be within the limits of +10 and -12.5 from fully charged
to completion of full performance on load of the prescribed duty.
1. Wearing of protective clothing e.g. rubber apron, rubber gloves and eye goggles,
while checking and topping up of batteries.
2. No naked light, flame should be taken into battery room because of hydrogen gas
evolving from cells which can cause inflammable mixture. Insulated spanners and
non-metallic water jugs should be used to prevent sparking and short circuiting of
cells.
3. Ventilation fan should be kept on.
4. First aid treatment - H2S04 splashes should be washed off with fresh water and
potassium hydroxide should be washed off with boracic powder or boracic acid
solution. .
Q.9.10. a) State how short circuit may develop in a lead-acid battery and how this
can be detected.
b) State five causes of why the rated capacity of a lead-acid battery may fall off
in service.
Ans. a) Causes of short circuit -
i) If a metallic or conducting material falls across the positive & negative terminals of a
cell.
ii) If the battery container or the electrolyte gets heated up above the maximum
recommended temperature, then the electrolyte resistance (internal resistance of the
battery) decreases as it has negative temp. coefficient of resistance, and the cell plates
get shorted causing high current flow and further rise irr temperature.
For every 10C rise in temp,the battery life reduced to half of its remaining life ..
iii) If the separation plates deteriorates and the +ve and -ve plates comes in contact with
each other.
iv) If contaminated water of higher conductivity is added in place of distilled water
topping up.
v) .Accumulation of sulphate at the bottom can also short the plates.
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High current flow, heat generation and high temp. of battery container and buckling of
plates are the indications of short circuit.
b)i)Since the discharge action of the battery depends on the surface area of the plates
dipped in electrolyte, the capacity reduces if the electrolyte level falls off and does not
immerse the plates fully ..
ii) A fall in relative density ofthe electrolyte reduces the rated capacity of the battery.
iii) Corrosion or oxide formation at the battery terminals causes reduction in rated
capacity.
V Temperature below lSoC or in the minus range causes fall in rated capacity.
v) Due to long use and ageing of the plates, the active material deteriorates or sometimes
covered with sulphate deposition.
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Q.9.11. Draw the Wiring Diagram For Two Sets of. Batteries On A Charge
Discharge Basis Br 4 Pole Change Over Switch While One Is Being Charged The
Other Is On Stand By Duty.
I it lJ
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