Why are Gel and AGM batteries so charge sensitive and why is charge voltage so critical?

All lead-acid batteries give off hydrogen from the negative plate and oxygen from the positive plate during charging. VRLA batteries have pressure-sensitive valves. Without the ability to retain pressure within the cells, hydrogen and oxygen would be lost to the atmosphere, eventually drying out the electrolyte and separators. Voltage is electrical pressure. Charge (ampere-hours) is a quantity of electricity. Current (amperes) is electrical flow (charging speed). A battery can only store a certain quantity of electricity. The closer it gets to being fully charged, the slower it must be charged. Temperature also affects charging. If the right pressure (voltage) is used for the temperature, a battery will accept charge at its ideal rate. If too much pressure is used, charge will be forced through the battery faster than it can be stored. Reactions other than the charging reaction occur to transport this current through the battery – mainly gassing. Hydrogen and oxygen are given off faster than the recombination reaction. This raises the pressure until the pressure relief valve opens. The gas lost cannot be replaced. Any VRLA battery will dry out and fail prematurely if it experiences excessive overcharge. Note: It is the pressure (voltage) that initiates this problem — a battery can be “over-charged” (damaged by too much voltage) even though it is not fully “charged.” This is why charging voltage must be carefully regulated and temperature compensated.