Skip to Content

Install A Second Battery On Your Boat The Correct Way In Parallel

I received a good question from one of our readers yesterday 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 the starter battery.


 Garry, the key phrase here is series vs. parallel. No, you don’t want to be series connecting your trolling motor battery to your engine starting battery. That said, a properly connected battery in parallel will work nicely. Just understand that the starting battery will recharge much more quickly than your deep cycle unit.

Also, you don’t tell us what the technology for the deep cycle battery is. If its a gel cell, you probably don’t want to connect it to the outboard at all. If its a flooded cell or an AGM, things should work out here.

So, the diagram at the top of the page comes from a drawing supplied by one of our sponsors here, Blue Sea Systems. This diagram shows how to accomplish what you want to do properly. Just substitute “Trolling” for “house” in the drawing and you’ll be good to go.

Comments are closed.