[JimD]

Wikipedia describes certain car batteries as being Valve Regulated Lead Acid (VRLA). They normally recombine the hydrogen and oxygen generated as the battery is discharged back into water. But sometimes the battery is charged or discharged too fast and the valve opens to prevent the pressure inside the battery from getting too high. This battery would normally be described as "sealed" but it isn't totally since the valve can open.

I don't know that the VRLA type is what bimmers have but it may be. But that would not eliminate the chance of there being some hydrogen around when you are working at the battery. But it is never very likely and is even less likely with the VRLA battery.

As a practical matter, opening your trunk as you gather tools to do the job would probably give any hydrogen a chance to be diluted to the point it would not burn or explode.

I do not worry about it.

Jim