When lead storage battery is discharged
 

A lead storage battery can be charged as it is a secondary voltaic cell. The reactions taking place during the recharging process are the reverse of the reactions taking place during the discharge. The electrolyte loses much of its dissolved sulfuric acid.

$2{\mathrm{H}}_2{\mathrm{SO}}_4\left(aq\right)\to 4{\mathrm{H}}^{+}\left(aq\right)+2{\mathrm{SO}}_4^{2-}\left(aq\right)$
$\mathrm{Pb}\left(s\right)+{\mathrm{PbO}}_2\left(s\right)+2{\mathrm{H}}_2{\mathrm{SO}}_4\left(aq\right)\to 2{\mathrm{PbSO}}_4\left(s\right)+2{\mathrm{H}}_2\mathrm{O}\left(l\right)$