The reactions between Pb<->PbSO4 and PbO <-> PbSO4, which enable electricity to be produced in the battery, are not 100% reversible reactions. For this reason, PbSO4, as a residue of the reaction, accumulates on the plates over time and prevents the plates from reacting.
During the Charge-Discharge, the volumes of the plates constantly grow and shrink, which in time causes the positive plates to become dusty and accumulate on the bottom of the cell and on the separators.
The battery cannot be fully charged and fully discharged because the Lead Sulfate accumulated on the plates prevents the reaction. While the usage time is shorter, the charging time is longer.
Lead sulphate accumulating on the cell floor and on the separators causes a short circuit between the plates and the battery becomes unusable.