As a lead-acid battery is charged in the reverse direction, the action described in the discharge is reversed. The lead sulphate (PbSO 4) is driven out and back into the electrolyte (H 2 SO 4). The return of acid to the electrolyte will reduce the sulphate in the plates and increase the specific gravity.
Because the reversed battery is no longer formatted correctly, it will only work to a limited degree. The fact of the matter is, a lead acid battery cannot reverse its own polarity without an external stimulus. It is just not possible. Guilty As Charged Blog Post touching on the battery myth of reverse polarity.
The sulfate (SO 4) combines with the lead (Pb) of both plates, forming lead sulphate (PbSO 4), as shown in Equation. As a lead-acid battery is charged in the reverse direction, the action described in the discharge is reversed. The lead sulphate (PbSO 4) is driven out and back into the electrolyte (H 2 SO 4).
If controlled-current reverse charge is done long enough, the chemistry of the reactions will cause the polarity of the battery to also reverse as the lead compositions coating the plates reverses.
The anode is transformed into lead peroxide (PbO 2) and cathode into the spongy lead (Pb). Water is consumed and sulphuric acid is formed which increases the specific gravity of electrolyte from 1.18 to 1.28. The terminal voltage of each battery cell increases to 2.2 to 2.5V.
The batteries should be charged in a well-ventilated place so that gases and acid fumes are blown away. The lead-acid battery should never be left idle for a long time in discharged condition because the lead sulfate coating on both the positive and negative plates will form into hard crystals that will be difficult to break up on recharging.