Page 198 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

Working of battery: It has two groups of positive and negative plates. The number of positive plates is one less
than the number of negative plates. The total number of plates is 11,13,17,23,25 etc. So that each positive plate
can work from both sides, the number of negative plates is kept one more. Both types of plates are made of lead
mesh and a pulp of red-lead (Pb O ) is deposited in them.
3 2
Dilute sulphuric acid H2SO4 is used as electrolyte in the cell. Rests are made in the vessel to keep the plates at
the bottom and separators are used to prevent them from short circuiting.
A vent plug is kept in every cell, through which the gases released in chemical reactions can come out. The
connecting ends of both the plate groups are taken out and the vessel is sealed from the top. Initially both the
electrodes of the lead acid cell are made of lead sulphade (PbSO4), but when the cell is charged by chemical
reactions, the lead sulphade electrodes get converted into spongy lead (Pb) (negative plate) and lead peroxide
(PbO ) (positive plate). Along with this, the electrolyte solution gets concentrated and turns into sulphuric acid
2
(H SO ).
2
4

Chemical Reactions
In a lead-acid battery, chemical reactions are performed in the following stages

1 Forming: This process starts as soon as dilute sulphuric acid is filled in the cell. This work is usually done by
the battery seller.
PbO4 + 2H2SO4 → PbO2 + 2PbSO4 + 2H2O

2 Charging: After 10-12 hours of filling dilute sulphuric acid in the cell, it is connected to a DC source, so that
electrical energy can be collected in the form of chemical reactions. For this work, the positive connector of the
cell is connected to the positive terminal of the source and the negative connector is connected to the negative
terminal of the source and the following chemical reactions take place in the battery.
H O→ 2H + O 2-
+
2
At Anode PbSO + 2H O + SO → PbO + 2H SO
4 2 4 2 2 4
At Cathode PbSO + 2H → Pb + H SO 4
+
2
4
As a result of the charging process, the positive plates of the cell become of land peroxide (PbO ) and the
2
negative plates become of spongy lead (Pb).
Discharging: The discharging process starts when the charged cell is connected to the load.
H O → 2H + O 2-
+
2
At anode PbO + 2H + H SO → PbSO + 2H O
+
2
4
2
2
4
At Cathode Pb + O + H SO → PbSO + H O
2-
2 4 4 2
As a result of the discharging process, both types of plates of the cell become of land sulphade (PbSO ). Due to
4
the formation of water in the chemical reaction, the acid becomes diluted the relative density of the acid decreases.
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CITS : Automotive - Mechanic Diesel - Lesson 57 - 61