I'm no engineer but: batteries vent hydrogen gas into the atmosphere, and hydrogen accumulations greater then 4% are flammable. Combine this with oxygen and an open flame and.....it can be very flammable. All batteries need to vent to get rid of the hydrogen they produce. If your battery is in the engine compartment it will vent through the cell caps and the hydrogen will dissipate in the atmosphere. The BMW has the battery inside so the gases need to be vented outside the vehicle via the vent tube. If your battery doesn't have a vent tube it's the wrong battery and the gas will damage the inside of your car and could be dangerous to occupants.