Overload has nothing to do with the batteries being old or new...it has to do with with how much watts you are sending through the unit...if the overload protection fault is on, it is because it is sensing that if the power were to go out the UPS would not hold up the load at all. I do not recommend to load the ups more than half of the load. visit a batteriesplus store near you and they can help you with this in more detail. www.batteriesplus.com