You do have to be a bit careful, however there are vents in the back of the boot and it seems that they do a good enough job, but not enough that I would really trust it alone.
I will say however, batteries vent hydrogen and oxygen as they charge, also in a perfect combination for nice combustion. They can go bang, I have seen it happen (not in a car) and I can honestly say the result was serious with not allot left of the battery and one hell of a mess around it, this was in an open space, I hate to think what would the result would be in the back of a car.