1. How you will prepare electrolyte for a lead acid battery?
While preparing electrolyte for lead acid battery sulphuric acid is added to distilled
water.
2. How battery capacity is expressed?
Always battery capacity is expressed in Ampere – hour.
3. What is the instrument used to measure the specific gravity?
The instrument used to measure the specific gravity is called Hydrometer.
4. What you mean by SCR?
SCR is meant for silicon controlled rectifier.
5. Define specific gravity and mention the specific gravity of a fully charged lead acid
battery?
Specific gravity of a substance is the comparison of density of the substance with the
density of pure water.
Specific gravity = Density of the substance / density of pure water.
= kg / cm2
kg / cm2
= (No unit)
Specific gravity is only number. It has no unit.
Specific gravity of pure water is one.
Specific gravity of fully charged lead acid battery is 1.215. Specific gravity should
always be corrected to 27°C.
Corrected specific gravity is equal to indicated specific gravity ± (t - 27°C)*0.0007.
Indicated specific gravity = 1.205 and ‘t’ means electrolyte temperature.
6. What are the parts of the battery?
Parts of the battery are
a. Battery container.
b. Battery cover.
c. Positive plate (Pb o2).
d. Negative plate (Pb).
e. Cell connector.
f. Grid.
g. Cell separator (porous material).
h. Sediment chamber.
i. Positive and negative terminals.
j. Vent plugs.
k. Dilutes sulphuric acid (electrolyte).
7. What are the indications of a fully charged cell?
a. The colour of the + ve plates will be dark brown. This can be seen only if the
battery has transparent cover.
b. Voltage per cell will be a 2.15 volts.
c. Gassing in the will electrolyte will indicate. But the current is splitting up water to
H2 and O2. Because the positive and negative plates are fully converted to their
original constituents.
8. What are the difference between primary cell and secondary cell?
Primary cell:
The electrolyte in primary cell is chemically irreversible. That is once
the cell is discharged it cannot be recharged. It should be replaced with a new cell.
The cells can supply only low currents and have low efficiency. They supply
intermittent current. Their internal resistance is more. These cells are comparatively
cheap.
Secondary cell:
These cells are chemically reversible. They can be discharged and
charged. They can supply large currents because their internal resistance is less.
These have high efficiency compare to primary cells. These can supply constant
current. These are comparatively costly.
9. What do you mean by sulphation? And what are the effects of sulphation?
Sulphation:
During normal discharge of battery Pb so4 is formed. This Pb so4 is
chemically reversible by passing current. These split up to their original constituents.
But under certain condition crystalline lead sulphate is formed (Example: under
charging after some time without trickle charging). This Pb so4 is chemically
irreversible. So if the sulphation occurs the battery life decreases. Efficiency
decreases and the active material starts falling of the grid.
10. Write down the equation for Nickel cadmium battery.
Equation for Nickel cadmium battery.
Ni (OH4) + Cd +2 kOH Ni (OH2) + Cd OH2 + kOH (during charging)
(Nickel hydrate + cadmium + potassium hydroxide Nickel hydroxide + cadmium
hydroxide + potassium hydroxide.)
Ni (OH2) + Cd OH2 + kOH Ni (OH4) + Cd +2 kOH (during discharge)
We see that there is no change in electrolyte. It just acts as a catalyst. So there is no
need to change the electrolyte.
11. Write down the theory of lead acid battery.
A simple lead acid battery consists of positive and negative electrodes Immersed in
an electrolyte. The positive electrodes are Pbo2 (lead oxide) and the negative
electrodes are Pb (spongy lead). The electrolyte is dilute sulphuric acid.
On discharging the battery
Pbo2 + Pb +2 H2 so4 Pbso4 + Pbso4 + 2 H2o
Lead acid is converted into lead sulphate. Spongy lead is also converted into lead
sulphate and H2 so4 used up in the process. Only water is remain. So the specific
gravity of the cell comes down.
On charging the battery
Pbso4 + Pbso4 + 2 H2o Pbo2 + Pb +2 H2 so4
Here the products are converted to their original constituents and the acid is released.
So the specific gravity rises as the cell is charged. How much ever larger be the cell
the voltage of each cell will be approx. 2.15 V when fully charged.
The positive plate is made of a paste lead oxide, lead sulphate that is fitted in a mesh
like material and is connected to grid. All positive plates are made common and
connected to a grid.
The negative plate is made of spongy lead also it is in mesh and connected to grid.
These are also grouped together.
These plates are separated by a micro porous separator for the diffusion of
electrolyte.
The electrolyte is prepared by adding sulphuric acid to distilled water drops by drop
and stirring it until the reasoned specific gravity is attained.
Every thing is placed in a container of hard rubber. The cells of a battery are
connected by a cell connector. The container is leak proof.
12. What are the parts of a 48V DC charger?
Main transformer: This steps down the 3Φ 415V supply to the require value of
voltage.
Synchronizing transformer for phase sequence: This gives the synchronizing signal
to the firing card. That is, the pulses from this card if fed to the firing card. The firing
card gives pulses to the SCR of R or Y or B depending upon which phase is positive
maximum.
Half control module: This has a diode and a SCR for each phase. The firing card
controls the firing angle of SCR.
Firing card: This gives the firing pulses to SCR depending on phase sequence and
the feed back from output.
Controller card: This card monitors the output and gives signal to firing card to
conduct at certain angle to maintain constant output voltage.
Power supply card: This gives power supply for the controller card, firing card and
protection.
13. Explain the operation of 48 V battery charger.
The supply for the charger is from MCC. The supply is tapped for power supply to
control card, PF correction capacitor. LC filter is used for suppressing surge voltage.
The main supply is stepped down and given to the half control rectifier module. The
SCR conducts only when gate gets positive pulse. This pulse is given by pulse
transformer, which gets pulses from firing card. Firing card gives pulse to the
respective RYB SCR only when their phases are positive maximum. The freewheel
diode is incorporated to protect the SCR and diodes from back emf when supply to
coils is cut off due to collapsing magnetic field.
The filter is provided to smoothen the ripple output and the bleeder is used for
voltage regulation. It gives improved voltage regulation and acts as a minimum load.
Also it keeps the SCR in conducting state by drawing the minimum current which is
higher than the SCR holding current. Thus there is always output voltage irrespective
of load.
DC CT is used for limiting output current. It works on principle of magnetic
amplifier. There is also provision for smooth rising of output voltage.
14. What is purpose of freewheeling diode and DC filter circuit in the charger?
Freewheeling diode is used to protect the semiconductor components used in the
charger from the back emf, which is induced in the inductive coils of relays when the
supply to the relays is cut off. The magnetic field in the relays collapses and induces
high voltage in reverse direction. This emf is shunted by the freewheeling diode,
which is connected in reverse bios with the output.
DC filter is used to smoothen the output, which has ripple. Ripple frequency is same
as system frequency for half wave rectifier and 2 times of system frequency for full
wave rectifier. The filter, which is a capacitor, will oppose any change in voltage.
Thus the ripple will not be allowed to come to zero.
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