Andy et,al, every bassboat I've ever seen has a charger hooked up to the cranking battery. Some will scrimp and only put 1 bank to the TM batteries (2) and it will charge them albeit slowly. There is a system that charges your TM batteries off of your big motor and there is a system that charges all of your batteries from your tow vehicle alternator. There is also a system where if you run down your cranking battery, you can manually switch over to your TM batteries to start the big motor. You will not ruin your cranking battery by hooking up a charger to it. The reason none of the graphs and extra's are on your TM batteries is that the TM creates interference and distorts finders, stereo's, etc. Sure, you can wire anything you want to them because a battery doesn't know any difference.