I noticed recently that my deep cycle battery that powers the camper lights and stuff was low on electrolyte , so day before yesterday I topped it offwith distilled water . Walking past it today I hear it "boiling" so I got the DVM out and checked the voltage . It's sitting right on 12.8 or so ... and when I turn the charger off (built-in) the lights go dim . I knew there was a problem somewhere the other night when we lost power and the battery was flat in less than 2 hours . Sound like battery replacement time ? I'd think if it were the charger the voltage would be higher .