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My new to me B25 has three batteries. One for each engine and a house battery. There are selection switched on each engine battery and an on off switch for the house battery. I would like to understand how they should be wired. There is also a trickle charger but I have not taken the rear panel off yet to see what that is all about. Its under the stern gunwale behind the panel along with the trim tab motor and reservoir.

It's probably wired right but I want to understand it.

Does anyone have a diagram or input?

You should have a heavy wire going from each starting battery going to each engine battery switch and a heavy positive to each starter. As for the house battery it should be independent to the house battery switch. How your engines are wired to the house battery for charging purposes can be done a couple of ways.

Go to Blue sea systems and look up their ACR automatic charging relays. They have good diagrams with the installation manual that covers a variety of setups.

Also GUEST has some diagrams for their battery switches.

I second Blue Seas.