Accumulators are rechargeable batteries by électrolysis.

The lead storage battery

The accumulator consists of lead electrodes immersed in sulfuric acid. The sulfuric acids attacks the lead plates superficially covering them with a layer of lead sulfate: $2H^+$ $+$ $SO_4^{2-}$ $+$ $Pb$ $\longrightarrow$ $Pb^{2+}SO_4^{2-}$ $+$ $H_2$ The lead sulfate layer est very thin and impermeable. In the following interpretation we shall consider the $Pb^{2+}$ ion alone which is contained in the lead sulfate layer.

Charge     =     Electrolysis



At the cathode: $Pb^{2+}$ $+$ $2e^-$ $\longrightarrow$ $Pb$ ( deposit on the electrode ) At the anode: $Pb^{2+}$ $+$ $2H_2O$ $ -$ $2e^-$ $\longrightarrow$ $4H^+$ $+$ $PbO_2$ (deposit on the electrode )

Discharge     =     Battery



On the side of the reductant $Pb$: $Pb$ $-$ $2e^-$ $\longrightarrow$ $Pb^{2+}$ ($\ominus$ pole) On the side of the oxidant $PbO_2$: $PbO_2$ $+$ $4H^+$ $+$ $2e^-$ $\longrightarrow$ $Pb^{2+}$ $+$ $2H_2O$ ($\oplus$ pole)