As stated it would depend on how discharged the batteries are. The invertor/chargers in the unit will keep the batteries topped of, BUT re charging a dead battery??? The easy thing would be to try it by connecting your rig to 120 volt source, (adaptors can be used to connect to 20 amp supply). Also, if you have a 12 volt car battery charger, you can re charge the batteries without removing them. Keep the disconect switch off, connect the positive cable to the positive post of one battery, and the negative cable to the other batteries negative post. 4 - 6 hours should completly charge the batteries, keep an eye on it, check the water before and after charging and it should be alright.