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wrote:
Tamaroak wrote:

I have a 1997 Monk 36 with the Cummins 6BT5.9M 220 engine. I'm switching
to six golf cart batteries for the house bank and wonder if the stock
alternator is big enough for this application.

Does anyone out there know how many amps that alternator is likely to
crank out?

Capt Jeff
"Adirondack"

Cummins might have rigged that engine with any of several alternators
when it was installed.

You will need to check the info plate on the alternator.

The basic premise for calculating alternator size is 4:1. Your
alternator output in amps should be at least 1/4 the amp hour capacity
of the battery bank it is being asked to charge.

....

Actually, I've heard that it doesn't make sense to go larger the 1/4
of the capacity, since that's the highest charge rate you would want
with regular lead-acid batteries.

My own experience is that 4 Trojan t105s (425-450 Amp-hours) will
accept a little over 100 Amps when down 50%, but are down to 85-90
Amps for much of the charge cycle. For six of these batteries, you
could charge at up to 150 Amps, which I'm guessing is bigger than any
stock alternator. At this size, you would probably want a good mount
with dual belts, etc. (I get by with a simple mount, but I do go
through belts.)

On the other hand, a 100 Amp alternator will "handle" the bank, it
just will take a bit longer to charge. In fact, if your plans were to
power the boat daily for several hours, you might find that even a
60 Amp alternator keeps the bank charged. The only problem comes
when you hang out for a week or so and have to run the engine every
day for 90 minutes to charge up - in this case for the last 30 minutes
of each run you'd be cursing the fact that you didn't get the "full
size" alternator.

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