I would fully charge the batteries over 12-24 hrs, charging voltage should read 13.4 v or more. After batteries have rested for several hrs with no charger attached take another reading, you should have 12.6 v or so. Use a multi meter to check at the pos and neg terminal of your batteries. How many batteries and what kind do you have? One 12v 2 6v in series etc? If voltage is good then disconnect the neg terminal and switch your mutimeter to 10 amps and measure your parasitic amp draw if you have several amps or more something is left on drawing your batteries down overnight. Do you have a mutimeter?