I had the same low level battery light at low rpm's and nothing at high rpm's and the light slightly brighter after I turned the lights off at night. Checked the battery and the ground with no change.
Decided from looking at the alternator that I would take the one off the parts car, have it completely rebuilt and then the "driver" car would not be down as long.
Well the weather has been bad, business has been good and one thing leads to another and I'm still driving with the battery light on slightly when idling over a month (maybe two) after the light first coming on.
Is it possible that is not the problem?
You mentioned 2 hours to replace your alternator. On this 115 240d I have to remove the ac unit to get the alt out and remove the fan and shroud to get to the ac unit and the water pump falls off when you remove the bolts for the ac bracket. I ran out of daylight yesterday so I am not sure what else I will run into.
Is your car alot different from mine or am I doing this the hard way?
Thanks