I would be checking out the belt for a polished or loose one.  I test
for a polished belt by taking a cold off engine and seeing if I can hand
slip the alternator pulley.  If I can, the belt is polished up and won't
grab under load so the battery only gets a surface charge.

If the belt is loose, the crank pulley needs a washer moved from the
center to the outside I believe.

A test with a meter is in order if that isn't it.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's
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