I am in the midst of sorting that out here. Had the battery going dead every few days , etc..
I kept checking the voltage and it was showing low 13 vlts + when idling. Thought that was low, but still it was charging some.
Finally I checked it and the voltage was down to 11.9 v when running. That time frame was 3 months. I would guess the alt brushes were wearing down.
I replaced it with a used I had in hand, voltage now at 14.1. Much better, but I will still rebuild the original and put it back in there. This one squeels sometimes and is not charging that well. Still have a slow start some mornings.
I have been told that these really need to put out over 14 v for a robust charge. Best way to check them is with lights, blower, and rear defrost running. Should still show 14.2 - 14.3 volts. Anything less may show that the it is not charging sufficiently.