I agree with the all the troubleshooting steps mentioned above by other owners.
For my case, it was my orig alternator bearings. replaced mine with Genuine RFMD BMW (by valeo) 140amp alternator (retail at ~$600+ no tax, I got for only ~$400 including tax ) and kept the old one for autopsy/fun experiments.
Some will recommend buy RMFD or new OEM alternator for even less money. Keep in your orig alternator specs 120amp or 140amp or xxxx amp, air cooled versus water cooled .
Some will recommend DIY or go to an automotive alternator rebuild shop - defective replaceable parts like the bearings, VR and etc - most economical way to get you back on the road...
Just adding my .2 cents:
- Alternator isn't charging the battery (most cases -> the alternator is intermittently failing)
* Check terminal connections / ground cables
* Check voltage while car is OFF - should be approximately 12 volts
* Check voltage while car is idling - should be approximately 14 volts
* Check voltage at battery terminals while car is revved up a little (maybe 3k-4k RPMs) - should still be approximately 14 volts.
* Turn on everything Radio, AC/Heater and etc - the works
- Broken or lose accessory belt
* Check belt tension and position
* Check idler pulley and tensioner pulley
- Faulty ignition switch (can cause flickering or solid red battery light)