Six-volt and twelve-volt automobile batteries are obtained by connecting three or six of these cells in series.

When the lead storage battery has run down, most of the lead and lead oxide is in the form of lead sulfate, and the battery fluid is depleted of sulfuric acid.

The fluid then is less dense, which is the reason that the specific gravity (or density) of the battery fluid can be used by a service station attendant as a measure of the state of charge of the battery.

If a direct current is passed through a run-down battery so electrons flow into the anode (originally Pb) and out the cathode (originally PbO₂), the half-reactions are reversed.

Lead sulfate is reconverted to Pb and PbO₂, the battery fluid becomes a more concentrated (and denser) sulfuric acid solution, and electrochemical energy is stored in the cell, ready for later use.