Balmar www.balmar.net makes stuff to do this right but it is expensive. I have a large boat with twin diesels so I have it. If my boat was a day-only boat, I'd have a house bank large enough to run all day w/o charging & they charge the house bank with a dedicated A/C charger made for deep cycle batteries when I got back. Use the starting battery for only starting the engines on a dedicated circuit. Charging a Deep Cycle with a starting battery charging system will lead to premature deep cycle battery failure. The issue is what your definition of premature is & how important the issue is to you.