Batteries in Parallel outboard charging question.

[T Man]

I would like to run a pair of deep cycle batteries in parallel for the reserve capacity when running lights at night, but I am worried about damaging either the batteries (less likely) or my outboard's charging system (more likely?). I know the outboard wont be able to keep a full charge, but they couldnt when it was one battery. So, will the outboard charging system be alright running a pair of Group 27 marine batteries in parallel?

 

[turbotodd]

Yes it'll be fine. You are correct in your thinking that if the outboard wouldn't charge one battery, it's definitely not going to charge two of them. There is an isolator made for just this purpose. Accessories run off of one battery but the outboard can crank off of the other, and will charge both. I think Minn Kota made it. I installed one (actually 2) of them on a customer's boat a while back. Why he wanted 2 I don't know, when one would have worked fine.