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Old 07-31-2007
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Agree with Wind Magic -- a topend AGM might live longer by the calendar than a bottom end deep-cycle wet, like the T50 golf cart. But I'd put L16 flooded batts against any gel or AGM in the world. And my batteries -- GNB 3700s, a dozen at 800 lbs each, 1/4" thick plates -- can accept charging current at a higher rate than any AGM. Too big to hurt. (That's conservatively 10,000 amp hours at 24v & 100-hr discharge rate.)

I would agree that AGMs offer rapid charge/deep discharge cycles; they have lots of surface area inside. So they are great for forklifts, floor polishers and such. But the surface area comes at the cost of electrode thickness; the mats will erode with charging & discharging and you may see excellent performance right up until the day they don't work at all. That's not a bad trait, really. At least, if you are in a place you can find new ones. Flooded lead acids have a longer but more insidious taper.

Wet cells remain tops for conversion efficiency and long-term, predictable performance. Cheap wet cells stink; good ones are unmatched for durability. On a cost-per-kWh-lifetime basis, good wets win by a ridiculous margin. That said, on a sailboat I'd plump for sealed AGM. To much pitching, vibration, temperature variation, etc. Not a good environment for flooded cells. (Neither is a car, really.)

The starter battery is an interesting issue. Those offer very rapid shallow-cycling but hate deep-cycling. Your basic car battery. Surprised all boats don't have a dedicated small one lashed to the engine & charge off a small alternator; or at least a diode isolater from the rest of the deep-cycle batts.
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