First of all its a converter not inverter. Inverter changes 12vdc to 120 vac. The converter changes 120vac to 12vdc so you batteries can charge. When plugged into shore power for extended periods it tend to "boil out" the water in the batteries allot sooner. I turn off the breaker for the converter occasionally when plugged into shore power to aviod damaging the batteries. Normall the batteries will charge in a few hours. Yours could be damaged though. If they wont hold a charge they are toast.