look on the harbor freight website and see all the people in the reviews who have left bad reviews about this. this isn't a battery maintainer, it is a trickle charger which can and will ruin your battery if left hooked up for too long. i checked mine with a multimeter when i received it.
anything higher will boil the acid in your battery and ruin your battery. it is not a problem as long as it is not putting out more than 13.6 volts. These things do NOT have an automatic shutoff when you battery is full.
