How do Gel or AGM (VRLA) batteries recharge and are there any special precautions?

While VRLA batteries accept a charge extremely well due to their low internal resistance, any battery will be damaged by continual under or overcharging. Capacity is reduced and life is shortened. Overcharging is especially harmful to any VRLA battery because of the sealed design. Overcharging dries out or evaporates the electrolyte by driving the oxygen and hydrogen out of the battery through the pressure relief valves. Performance and life are reduced. If a battery is continually undercharged, a power-robbing layer of sulfate will build up on the positive plate, which acts as a barrier to recharging. Premature plate shedding can also occur. Performance is reduced and life is shortened. Therefore, it is critical that a charger be used that limits voltage. The charger must be temperature-compensated to prevent under or overcharging due to ambient temperature changes.