Overcharging a lead acid battery leads to excess charge voltage and current flow into the battery upon reaching full charge. This leads to premature aging of the battery due to decomposition of the water in the electrolyte. At high rates of overcharge a battery will progressively heat up. As it gets hotter it will accept more current, heating up even further. This can lead to thermal runaway, where the battery heats up and can be destroyed in just a few hours.