Adding a Battery

I received a good question from one of our readers yeaterday that needs to be shared because I'll bet a few of you out there are trying to do the same thing. Garry writes: My new fishing boat has a merc 60 hp 4 stroke efi outboard with 18 amps of alternator output.Can I safely connect the starter battery and deep cycle trolling motor battery in series, to charge the deep cycle with the outboard engine? Of course I would install a switch to isolate the deep cycle when in use to avoid draining ...