Have you verified your alternator is functioning? How did you test the battery- while in the car, or did you take it to an auto parts store to have them test it?
With the car running, you should have ~13.5 volts between the positive and negative terminals. With the car off, it should read around 12 volts. You can perform this test with a ~$20 multimeter. There is a way to test your alternator as well, although I am not exactly sure how to do it. One of the wires should read ~13.5 volts when the car is running. I know the alternator is hard to get to on the M60 engines, I have no experience with the M50 engines.
BMWCCA Member# 451235
(Summer) 2001 330Ci - ZSP
(Winter) 1994 530i "Greenie" - LOADED