If you are just using one 12 volt charger you will have to charge them one at a time. Your best bet is to get a 3 bank on-board charger. That way when you get home you just plug it in and forget it until time to go again. You could get by with a 2 bank charger but I always recommend a 3 because most crappie fishermen never run their motor long enough to fully recharge the starting battery.