3.6.2 Secondary Batteries
A secondary cell after use can be recharged by passing current through it in the opposite direction so that it can be used again. A good secondary cell can undergo a large number of discharging and charging cycles. The most important secondary cell is the lead storage battery (Fig. 3.10) commonly used in automobiles and invertors. It consists of a lead anode and a grid of lead packed with lead dioxide (PbO2 ) as cathode. A 38% solution of sulphuric acid is used as an electrolyte.
The cell reactions when the battery is in use are given below:
Anode: Pb(s) + SO42–(aq) → PbSO4(s) + 2e–
Cathode: PbO2(s) + SO42–(aq) + 4H+(aq) + 2e– → PbSO4 (s) + 2H2O (l)
i.e., overall cell reaction consisting of cathode and anode reactions is:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
On charging the battery the reaction is reversed and PbSO4(s) on anode and cathode is converted into Pb and PbO2, respectively.
Fig. 3.10: The Lead storage battery
Fig. 3.11 A rechargeable nickel-cadmium cell in a jelly roll arrangement and separated by a layer soaked in moist sodium or potassium hydroxide.
Another important secondary cell is the nickel-cadmium cell (Fig. 3.11) which has longer life than the lead storage cell but more expensive to manufacture. We shall not go into details of working of the cell and the electrode reactions during charging and discharging.
The overall reaction during discharge is:
Cd (s) + 2Ni(OH)3 (s) → CdO (s) + 2Ni(OH)2 (s) + H2O (l)
