Do you have to worry about fumes from the car battery, or do you only use
the "sealed" type of car battery?
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The 'fumes ' you refer to are hydrogen and oxygen ,with hydrogen potentially
dangerous when not vented.
That's why I keep non-sealed lead acid batteries on a well ventilated loft.

BTW : Hydrogen has such a low density that , as tests (not by me ) have
proved , it is impossible to ignite hydrogen released from vertically
positioned tubing by a naked flame located at a distance of more than 100
mm ( 4 inches) from the hydrogen exhaust.

Providing this info does not mean I wish to encourage anyone to 'mess about
with hydrogen'.
It is only to show that hydrogen doesn't spread throughout an enclosed space
but ,upon release, moves upwards very rapidly .

Frank GM0CSZ / KN6WH