There have been a few recent threads on the use of small solar panels
to maintain the charge on your battery
bank. I recently built and installed one using inexpensive components.
I found a 30 watt solar
panel on eBay for $30-40. It puts out about 1-1/2 amps in direct sun. This is enough to recharge a battery
but it will take a long while. But it is perfect for maintaining the charge long term.
I bought a cheap
and simple Morningstar Sunsaver 6 amp controller for about $35. I mounted it to the back side of the panel with a bead of 5200. Then I ran two short wires from the panel output to the controller input. I made up a long 12' cable with a cigarette lighter plug
on one end which I connected to the controller's output.
To use it, I set it in a sunny place on the roof and run the wire inside the boat and plug
into a cigarette lighter outlet. Make sure that the breaker that supplies that outlet is on so that the panel can back feed to the batteries. The breaker is typically rated at 15 amps which will protect the 18 gauge wire that I used.
With this setup you have enough capacity to do some recharging and certainly maintain the charge long term. The controller keeps from cooking
the batteries and the cigarette lighter breaker makes it safe. It is much, much cheaper than ready to go solar