what kind of switch is there between the batteries or is there none?? Some boats have a battery disconnected from the rest of the boats electrical system. My old boat with a 2 stroke and a only a 6amp stator was a good example, one battery for the boat and one for the stereo and it got charged only out of the water of for longer trips I had a harness that I could connect to the tow vehicle through the trailer harness to charge it.