The battery needs to ventilate up to disperse the hydrogen that is produced while being charged. Propane needs a bottom vent as vent any leaked propane which is heavier than air. Hydrogen has a much wider flammable/explosive range of mixture than propane even though they both fall in the same NFPA class. The good thing about hydrogen is it will most likely disperse through lid looseness. Propane will collect in a bottom sealed box though. I would not place propane in a bottom sealed box. Have a look at the propane cylinder compartments found on some trailers and you will see they have open bottoms.
If the box is open bottomed I would not have much worry about sparks.
flammabilities