PbO2＋2H2SO4＋Pb ⇔ 2PbSO4＋2H2O (3)
The discharge reaction is shown as equation (3) from left to right，and the charge reaction is shown as (3) from right to left. If the battery is charged more than 100%，(4) and (5) will mix, causing water to be electrolyzed and reduced.
Positive plate H2O→1/2O2＋2H+＋2e- (4)
Negative plate 2H+＋2e-→H2 (5)
Theoretically, if the battery is over charged 1Ah, the water in the battery will be reduced by 0.336g. Water supplement is necessary for flooded lead-acid batteries. However, valve regulated lead-acid (sealed type) batteries, with normal usage electrolyte will not cause to decrease, hence no water supplement needed.
4.2 Absorbing Reaction Principle of Negative Plate
The theory of sealed valve regulated lead-acid battery is as follows: the negative plates will absorb the oxygen generated from the positive plates during charging. VRLA battery uses an absorbent glass mat as separator to contain electrolyte so that no fluid exists inside battery. The oxygen generated from the positive plates during charging will easily migrate through the separator to the negative plates. The negative plates lead will react with oxygen and transform into lead oxide, which reacts with surrounding acid, turning into sulfuric lead and water. The water lost in the process (4) and (5), can be generated again
Furthermore, sulfuric lead generated in negative plates will turn into discharge condition, so no oxygen generated.
Positive plate
