All Amped Up

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Mike Keller of Arlington, Virginia, asks:

"I recently upgraded the two lead-acid batteries on my Catalina to 79Ah Group 24 AGMs, and I’m thinking about wiring them in parallel to double my available power supply at anchor. I’d also add a small PWC battery for use as a starting battery. We don’t have refrigeration, but we do have lots of other gizmos that use power.

My main concern is that this new setup may damage my 55-amp alternator. Two 79Ah batteries in parallel provide a total of 158 Ah, but since I won’t drain the batteries below 50 percent, and I understand that AGMs can draw 75 percent of depleted amp hours while charging, the max draw might be as much as 60 amps. I wonder whether this might be pushing it."

Nigel Calder replies:

If the alternator is well built, which it should be, it will handle this situation with no problem. Let’s say you pull the batteries down to a 50 percent state of charge, at which point their charge acceptance rate is around 40 percent of capacity, or 67 amps. Assuming you run the engine fast enough to get your alternator to maximum output-—nominally 55 amps, but less than 50 in practice, especially once the alternator is warmed up— the voltage on your batteries will climb fairly rapidly. It probably will take 20 minutes to half an hour to get there, but once the voltage reaches the regulator’s set point, probably around 14.2 volts, the alternator’s output will start to taper off. Since it is probably regulated to around 14.2 volts, which is fine for gel cells and AGMs, you won’t be driving it at full power long enough to damage it.

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