 The one way valve on each cell is set at 2 psi, which allows for full recombination to take place within the sealed enclosure.When charging is complete and the battery is allowed to continue charging unregulated, oxygen is created by the overcharging condition on the positive plate.The oxygen then travels through the shrinkage cracks in the gel directly to the negative plate which is made from a high surface area pure sponge lead and causes a reaction that combines the oxygen with the hydrogen that is adsorbed on the surface of the sponge lead metal negative plate to create water that is retained in the cell.