What do you mean by a secondary cell? Discuss the function of lead storage battery.

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 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) + SO₄^2–(aq)----> PbSO₄(s) + 2e^–

Cathode: PbO₂(s) + SO₄^2–(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.