I believe the battery light encompasses the whole charging system. I got it recently in my 300SD. It turned out that the alternator was only producing 13v when it should be producing 14.5v. Replaced it and all was normal. Bought one from Autozone for $90 for the 55amp. It came with a lifetime warranty. Can't beat that. If you find the belt is properly tensioned and you know your battery is good, that leaves the alternator and it's built in regulator. I personally find that replacing the whole alternator is easier and saves more time.

Nolan