+1 what Charlie said. Except that for your application (which is identical to mine, except I have a group 31 start battery), I'd recommend the Xantrex EchoCharge because it's very simple to install, totally automatic, and works like a charm. And, it's half the price
of the Duo-Charge. That's what I have on my boat and it's worked flawlessly for over five years.
Wish I could recommend other Xantrex products, but I can't. I try to avoid them like the plague. Seems there are always problems and Xantrex support-after-sale is mostly non-existent.