Why is charging voltage so critical to both gel and sealed lead acid batteries

Why is charging voltage so critical to both gel and sealed lead acid batteries?

 

Charge voltage is critical on these types of batteries because they both are recombinant batteries. This means that the oxygen that is normally produced on the positive plate in all lead acid batteries recombines with the hydrogen given off by the negative plate. The "recombination" of hydrogen and oxygen produces water, which replaces the moisture in the battery. Therefore the battery is maintenance free and never needs watering.

The sealing vent is critical to the performance of a sealed battery. The cell must maintain a positive internal pressure, otherwise the recombination of the gasses will not take place and the cell will dry out and not perform.

In addition, the valve must safely release any excess pressure that may be produced during overcharging, otherwise the cell would be irreparably damaged. It is because of the safety mechanism of the valve that overcharging hurts a sealed battery.