I thought that tube on the side of the battery is to divert Hydrogen rather then acid fluids.
It is common knowleage that car batteries produce a fair amount of hydrogen when being charged.
Since BMW batteries are stored in a relatively air tight place, the trunk, in order to prevent hydrogen build-up, a tube was installed to divert hydrogen gas to the exterior of the trunk.
This was done aimed to prevent explosions might be caused by static electricity or a collision, etc.
Other manufactures do the same for trunk install batteries. The 2nd gen Miata is another example...