The cell reaction that takes place in a lead storage battery are:
Pb + PbO2 + 4H+ + SO4-2 → 2PbSO4 + 2H2O
In this reaction Pb shows an oxidation state of +4. It gets reduced to PbSO4 which shows an oxidation state of +2.
In this reaction, 2 electrons are necessary for the reduction of PbO2.
Pb4+ + 2e- -----------> Pb2+
Therefore, as 2 electrons were needed for the reduction, the Faraday’s law of electrolysis stated that:
The reduction of 1 mole of PbO2 required 2F electricity.