during recharging of lead storage battery, why is some battery water added

Dear Student,

The lead storage battery is discharged when the lead(Pb) of anode and lead oxide (PbO_​2) of cathode are converted completely to lead sulphate (PbSO₄). Water is also present in the reaction. Now this cell can be recharged by supplying direct current which reverses the electrode reaction and converts lead sulphate back to lead and lead oxide. In this process, some water ma be lost due to evaporation which will again hinder the reaction. Therefore, some water is added ti avoid this to happen. 
$2{\mathrm{PbSO}}_4\left(\mathrm{s}\right) +\hspace{0.17em}2{\mathrm{H}}_2\mathrm{O} \to \mathrm{Pb}\left(\mathrm{s}\right) + {\mathrm{PbO}}_2\left(\mathrm{s}\right) +\hspace{0.17em}4{\mathrm{H}}^{+} + 2{{\mathrm{SO}}_4}^{2-}$