electrical on 1990 BMW 535i

I had a electrical diagnostic done at a BMW dealer they said alternator was bad I replaced had old alternator benched tested was working fine battery is on 4 months old, vehicle running of battery only, what else could be the problem?

Asked by for the 1990 BMW 535i
So you have a new Alternator on your vehicle that was diagnosed and replaced by the BMW Dealer? And the original problem still exists?
Sounds like the original diagnosis was incorrect, but since you had another shop install the alternator, it will be difficult to address that issue. I would go to the dealer and see what they say, see if they will work with you to get this resolved since you did pay for their expert diagnosis, and apparently it was incorrect, leading you to spend money on a part that may not have been needed.
Good Luck
I had a mechanic install new alternator on car I then took old one and had it tested was told ther wa nothing wrong with it, after installing new alternator same problem is occuring the battery is not being charged by alternator.