A recent Ship Shape TV episode featured Battery Tender products for on-board battery charging. They seemed to have several reasonably priced, well featured products. Might be worth a look.

Welcome to Batterytender.com... Home of the World's Most Advanced Fully Automatic Battery Charger for Longer Battery Life, Dependability, and far Less Maintenance! Guaranteed!

Why the Battery Tender® vs. conventional chargers?
Avoid the potentially damaging effects caused by most trickle chargers! Battery Tender® BTP Microprocessor Technology and quick-connect convenience automatically maintains the batteries in all your vehicles so they are ready to go when you are! You just set it, and forget it!