I have mine wired to each of the batteries and leave the selector switch in the off position when the charger is plugged in. If I have the boat in a slip I will leave the switch on only one of the batteries when the charger is plugged in. I know you didn't ask but when out on the water I leave the switch on only one battery. That way if you run it down you still have a fully charged battery in reserve at the flip of the switch.