How low were they? The water in a battery is on top of the electrolite ( acid) to prevent the acid from evaporating. If you run them low ( and your water layer is gone ...you eventually start losing acid to evaporation....then when you fill your batteries back up with distilled water they are weakened because your acid is less. Never run them so low that the plates are exposed. Battery longivity is increased if you check them often and top the water off before they get too low.