GBM wrote:
We have a 55A Hitachi alternator on our 2 cyl 13HP Yanmar. It is charging 2
banks of batteries - 200AH in house and 100AH in Starting.

Today, I had an alarm sound when I started the engine (turned out to be low
oil level!), but it caused me to check the engine belts etc.

The alternator belt was loose, so I adjusted it - it had been slipping. I
noticed that despite only having run for about 5-10 minutes at low rpm
(~1200), the alternator was quite hot to the touch - I would guess in the
50-60 deg C range (120-140F), so I would keep my hand in contact with it.
The engine block was not nearly as hot - I could put my hand on it
anywhere - just warm.

There is a combiner, so both banks MAY have been connected. Refrigeration
was running at time. Batteries were not fully charged - maybe just below
12v. I don't have an ammeter, so don't know what amps are being put out.

Should the alternator run that hot?

Would low speed caused by slipping belt cause overheating?

If not, what could cause be? Or is it normal?

GBM

They get hot - easily over 120-140F. How hot is "normal" for your
setup you'll have to figure out. My guess is that the slipping itself
adds to the heat. Certainly I usually notice slipping on mine when I
smell the burnt rubber.

BTW,your alternator and batteries are not too large, but still they
can drain 2-3 HP from from your engine, so if your batteries are very
low, don't be surprised if you engine overheats if pushed hard. My 18
HP 2GM with a 100 Amp alternator will run 15 degrees hotter and even
refuse to rev up fully when charging. Fortunately, I can turn down
the output to 30 Amps if that suits my pans better.