I would plug in your obd2 Bluetooth adapter and monitor it on one of the many android apps out there. My e46 started showing 12.5 volts and I kept an eye on how much it was charging and saw it slowly go down. After a few months it was only charging at 11.5 volts.

I emailed a guy that rebuilds alternators and sells the parts, and he said it is probably the brushes in the voltage regulator going out. Either I can replace just the brushes myself for about 5.00 or I can replace the entire VR. I found a good deal on a new Valeo open box VR and went that route for half price. If you change the VR inspect the rotor and make sure it is not showing any plastic. If it shows plastic the rotor is done.

__________________
2004 E60 545i Sport 6MT. Black/Black Build Date 6-2004 102k miles