"Peter253" wrote in
ups.com:

So, the alternator reads what the voltage is on the
batteries, but is not charging them.



How long did you charge the down house batteries and track the voltage? If
the batteries are down a good bit, do not expect to see 14V across the
charging battery terminals. That comes later after the charging current
drops off below the rated output of the alternator. The voltage rises,
slowly, in a curve as the alternator strains to provide the current it was
designed for. You get hardly any voltage rise on a battery that's way down
until the alternator's regulator starts to feel the need to drop the field
current. The other factor, one that is hard to detect in boats, is "belt
tightness and loading". Someone asked me to look at their alternator and
regulator down the dock not long ago. He had his wallet out ready to BOAT
(bring out another thousand) to replace it all, which I suppose makes his
boatyard quite happy to fulfill. At the dock, after the AC charger had
pulled the gravity up to 1.240+, I cranked the engine and she came right up
to over 14V on my DVM on the house batteries. However, as I went through
the cabin turning every load on I could find, the voltage went all to hell
trying to provide 30-40A to my loads. Nothing squealed, like your dry-
bilge car will do. Still loaded, I crawled down to watch the alternator
run and notice it had climbed up its belt from the pull on it. I had him
shut down the engine and found the pulley on the alternator hot to
touch....the belt was slipping. Now shut down, the belt was quite tight,
once again. Cranking it under load and the alternator climbed the belt,
pulling the engine sideways because this alternator had been added for the
house battery alternator mounted to the frame, not the block. The forward
engine mounts were bad. Once replaced, I didn't have enough DC load in the
boat with everything on to make the alternator voltage drop off below 14V,
power to spare. Steak and a fine English ale are such good companions
after a day in the bilge...(c;

Watch the belt as you load the system down and make sure it stays tight. I
didn't use mine on this impromptu project, but I have an old Strobotac
flashing light strobotac I could have used to detect the slippage as the
alternator slowed. You can also hear the alternator really loading down
the little diesels in sailboats. If there's only 17hp, total, and you
don't have it in gear, the RPM drops significantly as you load down the
alternator on a charged-up battery bank.

With this new information about discharged house batteries, I'm leaning
towards poor power transfer...slipping belt. Is this alternator mounted to
the engine block or the mounting, which is notorious for slippage?

--
Larry