Are 100% sure the odor is a result of the battery? I know for a fact that a lot of 3-series, mine included, carry an odor around in the trunk that is a result of the inner lining of the trunk inself; anyhow that is what I've been told.
Some people say that it is the spare tire causing the odor and some say, like you, that it is the battery causing the odor.
Do you find it that offensive? Because if you do, maybe it isn't the same odor then, because to me, the odor isn't that bad. In fact, now this may sound weird, my wife kind of likes it - - yea, yea, I know, but no comments here, please.
The odor also slightly permeates into the cabin of the vehicle, but not badly.
Describing the odor is tough. I would say that it has kind of a smokey odor, not tobacco smoke, but a woody smoke type odor.
In the end, the issue of an odor in the truck is a real one - many people have complained