When restoring old lead acid batteries it is usually required to add H2O to refill the battery. Usually the refilling is followed by slow process of charging at reasonably high voltage. Normal charge is at 2.35 V/Cell with fast charging going up to 2.4V/cell. At 2.5V/cell electrolysis starts, so care must be taken charging above 2.4V/cell. Desulfators sometime pulse up to 50V into a 6 cell (12V) battery to dissolve hardened PbSO4.
I am thinking.... what if one to "top off" the lead acid battery with dilute H2O2, say 3%, instead of H2O? I am thinking the H2SO5 might help to dissolve some stubborn PbSO4 from the plates?