I left my trickle charger on one of my batteries for about a week and a half. I always thought two things....One it would shut off before it hurt the battery because it has an automatic shut off or two... the trickle charger is so low voltage that it could be kept on for a long time. I guess that is wrong cause it was dead as a door nail and will not take a charge now. Battery was only 2 years old!!!!!