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Old 10-30-2019, 01:58 PM   #6
rslifkin
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City: Rochester, NY
Vessel Name: Hour Glass
Vessel Model: Chris Craft 381 Catalina
Join Date: Aug 2019
Posts: 7,550
I'd want to test what your alternators do in practice. Knowing Mercruiser, I'd expect those alternators are some kind of Motorola or Mando unit. My older 55A Mandos on my Mercruiser 454s are internally regulated, but output varies a bit with engine room temperature. Usually they're around 14.3 - 14.4 volts at cold start up to as high as 14.9 volts in cold weather from what I've seen. By the time the engines are good and warmed up, voltage settles down to between 13.9 and 14 volts and stays there.

I figure 14 volts isn't exactly optimal for the engine start batteries that get warm in the engine room (they'd probably be happier at lower voltage after a couple hours of running), but my house batteries are outside the engine room and stay cooler. 14 volts is above their recommended float voltage (13.8 volts for my AGMs), but below absorption voltage. So I figure it's not enough to hurt anything, even after a few hours of running.

Of course, 14 volts is also lower than ideal for topping the batteries off, but even at 14 volts these batteries accept a decent bit of current, so they'll get to full (or at least very close) eventually. Of course, I don't really expect 100% charge from engine runtime anyway, that'll normally happen during a generator run or on shore power.
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