The battery can give off flammable, invisible gases while being charged. I'm not sure what a battery "maintainer" is but I suspect it is just a trickle charge device.
If I were in this situation I would have try to insure that the area is ventilated while the "maintainer" is hooked up and plugged in.
The only battery I "maintain" is the lawn mower battery and I put it on trickle charge about a couple times through the winter. BTW - the battery is not in the lawnmower during this time.