Slightly off thread. I located my fourth 12volt bat in a plastic bat box under the quarter berth aft of the nav station. Iwired it to the 12v power panel through its own switch. If I leave the main bat SW in both it won't discharge bat #4.I was a pilot for 40 years so I can't help making back- ups for back- ups. It is charged by its own solar panel. I want to add a pair of 6v deep cycles this year but I'm not sure where yet?