How dangerous? On a scale of 1 to 10, I'd say "eleven". Not the Sulfur Dioxide, which I guess is what you smell...that's just stinky. But, when a battery is recharged, it releases hydrogen gas--remember the Hindenburg? Yeah, same stuff. The hydrogen gas is combining with sulfur deposits in the battery to cause the stink, but it's the gas that can cause an explosion. Not likely, but entirely possible. If the equipment MUST be battery powered, then put the battery outdoors and run a wire in.