Different forms of lead sulfate (PbSO4) are produced in both the cathode and anode in the course of discharging of a lead-acid battery. However, their difference in reaction activity has not been well recognized up to now. From this work, it is shown the cathode product PbSO4(O) due to oxidation of Pb is rather inert and its accumulation could lead to decrease of the battery capacity and life; on the other hand, the anode product PbSO4(R) due to reduction of PbO2 has a much active property and is readily reversible in the charging–discharging recycles. To restore the battery capacity, it is critical to solve the deactivation of cathode by transforming PbSO4(O) into PbSO4(R). For such a purpose, inverse charging is performed, and a procedure from PbSO4(O) to PbO2 and to PbSO4(R) is conducted under a series of measurements with cyclic voltammetry, electrochemical impedance spectroscopy, scanning electronic microscopy and X-ray diffraction spectroscopy. The results of inverse charging tests show that the new capacity of a sulfated battery is more than twice of the initial value, which proves the validity of the mechanism outlined.