according to WDS, the battery light comes on when the balance on the rectifying diode bridge is disturbed. there are 4 diodes, connected in a specific way (google rectifying bridge), so if one burns, the balance is off... it has been a while, so i can't recall now the details of repairs, do a search for M62 alternator bridge repair (here is one -
http://www.bimmerforums.com/forum/sh...led-alternator), so you might be able to repair it.