What is the right AGM battery discharge level?

I very much enjoyed Steve D’Antonio’s excellent article The ABCs of AGMs (Issue 142, Nov./Dec. 2004). He makes the point on several occasions that AGM batteries should be recharged to 100 percent capacity after each discharge. In the real world of cruising, this just isn’t practicable, and I would appreciate a little real world advice.

My boat has four 220-amp-hour (AH) AGM batteries for a total capacity of 880 AH. I also have a Link 2000 system. When running high-draw systems like the refrigeration or the watermaker, I try to run either the engine or the genset at the same time so that there is no drain on the batteries. My alternator is a 100-amp Balmar that actually puts out about 80 amps or so. (There was no room for a larger one.) My Trace inverter/charger puts out about the same — 70 to 80 amps. Even if I run both of them together, that’s only 160 amps.

In general, I never let the batteries go to minus 200 amps. At that point, and usually well before that point, I will charge the batteries until the Link goes to its float setting (about minus 50 AH), at which point I stop charging. If I’m weekend cruising, I know the boat will be back on the battery charger within two or three days, so I don’t worry much about the batteries. On an extended trip, I usually wind up doing extended motoring (so why do I have a sailboat?) every three or four days, so the batteries get fully charged at that time.

Am I babying my batteries too much? Should I let them discharge more than I do? Or am I being too hard on them? And finally, given my pattern of use, what sort of battery life expectancy is realistic?

Peter Ellison sails a Passport 456 out of Mamaroneck, N.Y.

Contributing Editor Steve D’Antonio responds:

I agree, 100 percent recharge isn’t practical every time, particularly for voyagers. In the article, I recommended that the AGM user try for a complete recharge as often as possible (every 25 discharges or so, certainly whenever you have access to shore power). Every four or five days, as you state you are able to do, is certainly within that realm.

With sufficient charge capacity, you should be able to discharge your batteries by 40 percent; i.e., use 350 AH or so. The rule of thumb is your charge source should be a minimum of 25 percent of your amp-hour capacity, or in your case, 220 amps. I don’t believe you’ll do any harm to your batteries with the shallower discharge. However, overall, the number of amp-hours you get out of the batteries over their lifetime will not be as high.

It’s difficult to project battery life, but, based on the way my customers use their boats, I frequently see Lifeline AGM batteries lasting between four and six years.

By Ocean Navigator