The chemical reaction equation for THANK-U  Battery during Charging and Discharging as follows:

 

Overcharging causes electrolysis of the water content of the electrolyte, which generates O2 gas at the positive plate and H2 gas at the negative plate. These gasses are then discharged to the outside. Since a drop in the electrolyte levels results, adding water is occasionally needed.

The VRLA battery is designed so that the negative plate does not have to be fully charged even when the positive plate is fully charged. Furthermore, no H2 gas is generated from the negative plate although O2 gas is being generated from the overcharged positive plate. O2 generated from the positive plate then reacts with the charged sponge lead (Pb) of the negative plate and turns into lead monoxide (PbO). The lead monoxide, in turn, reacts with sulfuric acid (H2SO4) in the electrolyte to turn into lead sulfate (), allowing the negative plate to discharge. In other words, O2 from the positive plate is absorbed by the negative plate without being expelled to the outside. Since the negative plate develops discharging with the help of O2, there always exists a portion free from discharging. As a result, the negative plate never generates H2. This completely prevents the loss of water.

a. At the termination of charging or when the battery is overcharged, Oxygen is produced on positive plate:
+4H+­2H2O=O2 +4e-（2）
 
b. During discharging, oxygen reaches negative electrodes through the gas path on separator and reacts with active material spong-Pb (in charging state) and sulfuric acid, which, converts a part of active material spong-Pb into lead sulfate (in charging state) and restrains the generation of Hydrogen at the same time.
2Pb+O2=PbO  (3)
PbO+ H2SO4=PbSO4+ H2O  (4)

c. When charging, lead sulfate returns to spong-Pb(in charging state).
PbSO4+2H+ +2e- =Pb+ H2SO4  (5)

Thus, the total reaction on negative electrode plate is: (3)+(4)+(5) that is,
O2+4H+ +4e- =2H2O
This is right the reversal course of the reaction (2) on positive electrode plate.
In general, we can see that the oxygen produced on the positive electrode plate by
electrolyzing water reacts with the active material on the negative electrode plate and
deoxidizes water. During this whole process, there is no loss of water in battery, therefore
THANK-U  Battery can be designed to a sealed structure