I have a three battery switches.Port,starboard,and both.Donzi wired my port battery to the starter.My starboard battery goes to the boat.The both switch will charge all,I tried to jump the wrong battery and had to tow it.I found out the hard way.3 hours 40 mins of towing behind my 16 baby outboard.Two new batterys at the marina(could have done that myself and saved money)