On a different car, I had a slow coolant leak from the top of the radiator. I think that while driving, the leaking coolant got blown back towards the engine, including down onto the alternator. Around the same time I found the leak I started having intermittent charging problems, thinking it was bad luck to have two unrelated problems at the same time.

After I figured things out, and with a new radiator in place, I realized that the residue from the dried coolant (also seen throughout the lower engine bay) was probably inside the alternator, perhaps causing conductivity problems with the brushes.

So at the point when intermittent turned into constant and dead, I figured I did not have much to lose, so with the engine warm but off, I sprayed the alternator with a garden hose, rinsing it as thoroughly as I could. Then I blew it dry with the leaf blower, then compressed air, then a little later I ran it, and voltage was as good as ever. Still is. Totally fixed.

__________________
2001 X5 3.0i, 203k miles, AT, owned since 2014