Dear Boat Doctor,
I need some guidance in designing the charging system for my boat. I have reconfigured my house bank to use six volt batteries, and I was able to get enough batteries in the space to give a 1000 amp hour capacity. I am now choosing an alternator and shore power charger but I need some direction.

Rick Rogers
Weston, Massachusetts

Dear Rick,
Let's take a step back and look at the whole problem. Charging is about replacing the energy consumed throughout the day, and the size of the bank should reflect that usage also.

A good guideline is that the battery bank capacity should be about 3.5 times larger than the expected energy consumed per day. In other words, a 100 amp-hour daily draw would require a 350 amp-hour bank capacity. Working that guideline backward, your bank would support a roughly 285 amp-hour daily draw. So, assuming that your bank is sized correctly, you need to find a way to replace 285 amp-hours every day in a practical amount of time.

Keep in mind that a battery will only accept so much charge regardless of the size of the alternator. A battery will accept about 25% of its capacity if it is deeply discharged, as it charges it will accept less. If we do a little loose math here, a theoretical 100 amp-hour battery discharged to 50% will get to 90% in about two hours of charging, and that last 10% will take another two to three hours. In terms of engine run-time this works out to be about four hours per day.

Based on all of this, it is recommended that your alternator be roughly 25% of your bank capacity. In your case, this would be a 250-amp alternator. You can get alternators this big, but they are big and will cost a few thousand dollars. Alternators over about 100 amps require multiple drive belts, so you'll need to set up your engine correctly too.

Shoreside chargers do not need to be as large because the usage pattern is different. When you are underway, you are looking for the minimum engine run time to recharge your batteries, but with a shoreside charger it is assumed that the charger will run for hours if not days at a time. The accepted rule of thumb for charger sizing is 10% of the battery bank, or 100 amps in your case.

I think your battery bank may be a little oversized for your needs. It may be a good idea to size your charging gear for a right-sized theoretical bank. For instance, if you estimate your daily usage requirements at 150 amp-hours, this would dictate a bank size of 525 amp-hours and a 130-amp alternator and 50-amp shoreside charger. This setup would not adequately charge your 1000 amp-hour capacity if it were truly discharged but would handle a partially discharged system.