My main concern would be if you were using a battery type that off-gassed hydrogen. I would do anything in your power to avoid that situation, select some battery type that won't give you that problem.
My next concern would be that sooner or later, you'll need to replace the battery. That will mean repetetive opening and closing of your enclosure, which can often lead to poorly sealed enclosures, missing bolts, etc. Also, you would want to make absolutely sure that there were no hazardous fumes around when you changed the battery.
Is there any chance to relocate the battery to a GP location, and avoid the issue entirely?
Also -- the intent of Article 501 is generally to protect against sparking equipment only in Div 2 areas (that's not entirely true -- surface temperatures are an issue too) -- maybe a GP enclosure would make more sense than a NEMA 1 (Div 1) enclosure?
As a side note -- there's certainly plenty of small battery operated portable equipment available, including flashlights, pagers, clocks, etc. . . Generally, they seem to do little or nothing to seal the small batteries (double-A, C, D, etc) used in those devices. Are you talking a battery like a disposable double-A, or something more like a lead-acid battery?