Luckily for you BMW's batteries vent the hydrogen gasses outside the vehicle, so no worrys there. That plastic elbow with the vent tube that plugs in the side of the battery puts the gasses outside the vehicle.

And on other vehicles, abtteries sitting idle don't really produce gasses, but when used they do. I've personally seen one blow up in someones face. A master tech at a shop I used to work for was doing a diagnosis on a non starting car so was cranking it checking for spark, fuel pressure, injector pulse etc. so the battery was putting out plenty of fumes... He goes under the the hood and rests his hands on the engine and battery and his left hand went on a battery terminal that was a hair loose and it sparked, blowing the battery in half and covering his whole face in battery acid! The safety glasses did not help much and his eyes were solid bloodshot red for hours.

But as I said, the risk of that when the battery has been just sitting idle, not connected is probably 1% at best and just as low or lower in a BMW that externally vents so the fumes aren't even by the battery.

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2003 4.6is