Solar Battery Maintenance
By Don Casey
Revised by BoatUS editors in April 2012
A lot of underwater corrosion problems stem from leaving a boat plugged into shore power. There are solutions, but if you plug in at the dock just to keep your battery charged, the simplest thing is to unplug and charge your battery with solar power. Unplugging an unattended boat has safety benefits as well.
For a boat on a trailer, solar charging means that when life keeps you off the water for several weekends running, your next outing won't be spoiled by a dead battery. Solar charging works equally well on a mooring or even in a storage rack as long as you can expose the panel to direct sunlight.
The size solar panel you need depends on the size of the battery bank you are maintaining. In sunny climates, about 3.5 watts of solar power will maintain a 100-amp-hour battery. If you live where it is overcast half the time, you may need twice the wattage, i.e. 7 watts per 100 amp-hours of battery capacity. Extra charging capacity is simply wasted, so don't buy a bigger panel than you need. A regulator is not required as long as you don't exceed around 15 watts of solar power for every 100 amp-hours of battery capacity, but you must include a fuse in the positive side of the circuit installed as close to the battery as possible.