With the motor running, check your battery voltage with a DMM, you should be seeing 13.8-14.1 volts. With the battery light coming on until the RPMs are up I'm leaning towards a new voltage regulator. Should be able to replace the regulator, or brush pack, without having to remove the alternator.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.