My boat is equipped with two deep-cycle batteries and an 11-horsepower diesel with the original alternator. I expect to run the engine only for short periods of less than 30 minutes and my loads will be limited to the autopilot, VHF, stereo, bilge pump, and basic lighting. Can you suggest a best-bet approach to battery charging?
Don Casey responds:
If the boat is tied to a dock, your best bet is an AC battery charger. For a little over $100 at a discount price, you can buy a three-stage charger that will take good care of flooded batteries. If you have gel cells, expect to spend $250 or more.
If the boat is on a mooring in a sunny environment, a small solar panel is the way to go. The output of the panel should be around 0.3 percent of total battery capacity, perhaps a bit higher if your climate is less sunny. In your case, that works out to around 0.6 amps, or about what you would expect from a 10-watt panel. That will feed about 20 amp-hours into your batteries per week—less when it is cloudy. If that doesn't meet your demands, you can go up to a 20-watt panel (or two 10s) without significant risk of overcharging.
The alternator on your engine will not keep your batteries up with 30 minutes of running time a week, no matter what its rating, and putting a bigger alternator on an 11-horsepower engine is probably not a good idea. Either the panels or the charger will be the least costly, least complicated, and most effective approach for your boat-use pattern. Good luck to you.