A read storage battery is the most impotant type of secondary cell having a lead anode and a grid of lead packed with `PbO_(2)` as cathode. A `38%` solution of sulphuric acid is used as electrolyte. (Density=1.294 g `mL^(-1))` battery holds 3.5 L of the acid. During the discharge of the battery, the density of `H_(2)SO_(4)` falls to 1.139 g `mL^(-1).(20%H_(2)SO_(4)` by mass)
Write the reaction taking place at the cathode when the battery is in use.
