LEAD STORAGE CELL OR LEAD ACCUMULATOR OR ACID STORAGE CELL

LEAD STORAGE CELL OR LEAD ACCUMULATOR OR ACID STORAGE CELL

A lead acid storage is a secondary battery, which can operate both as a voltaic cell and as an electrolytic cell. When it acts as a voltaic cell, it supplies electrical energy and becomes “run down”. When it is recharged, the cell operates as an electrolytic cell.
A lead-acid storage battery consists of a number of (3 to 6) Voltaic cells connected in series to get 6 to 12V battery.
In each cell, the anode is made of lead. The cathode is made of lead dioxide PbO2 or a grid made of lead plates (anodes) are connected in parallel and a number of PbO2 plates (cathodes) are also connected in parallel. Various plates are separated from the adjacent ones by insulators like rubber or glass fibre. The entire combinations is then immersed in dil. H2SO4 (38% by mass) having a density of 1.30 gm/ml.

Cell representation:
Pb │ PbSO4 ││ H2SO4 (aq) │ PbO2 │ Pb

Working (Discharging)
When the lead-acid storage battery operates, the following reaction occurs.
At anode
Pb(s) + SO42‾ (aq) --->PbSO4(s) + 2e¯
(Oxidation of Pb to Pb2+ ions and formation of insoluble PbSO4)

At cathode
PbO2(s) + 4H+ + SO42¯ +2e¯ --->PbSO4(s) + 2H2O
(Reduction of PbO2 to Pb2+ ions, combination of insoluble PbSO4)

Overall cell reactions during use (discharging):
Pb(s) + PbO2(s) + 2H2SO4(aq) ---> 2PbSO4(s) + 2H2O + Energy

A single cell generates 2V and the six such cells in the battery produce a total of 12V.
From the above cell reactions it is clear that, PbSO4 is precipitated at both the electrodes and H2SO4 falls below 1.2 gm/ml. So the battery needs recharging.

Recharging the cell
The cell can be charged by passing electric current in the opposite direction. The electrode reaction gets reversed. As a result, Pb is deposited on anode and PbO2 on the cathode. The density of H2SO4 also increases.

The net reaction during charging is
2PbSO4 (s) + 2H2O +Energy --->Pb (s)+ PbO2(s) +2H2O(aq)

The recharging involves exactly the reverse process of the normal cell reaction. During the period of recharging, a secondary battery is converted from a galvanic cell to an electrolytic cell.
In the discharge of the battery, the sulphuric acid is replaced by water and hence the density of the electrolyte, H2SO4 decreases from the initial value of 1.2 g/cc to 1.0 g/cc. Decreasing density of H2SO4 is an indicative of the weakening of the battery.

Uses
1. Lead storage cell is used to supply current mainly in automobiles such as cars, buses, trucks
etc.
2. It is also used in gas engine ignition, telephone, exchange, hospitals, power stations, etc.