Well, I don't know a ton about uninterruptable power supplies, but I do know that any device that sparks and smells like it's burning is probably malfunctioning in some way. I'm willing to bet it has some nasty capacitors in it (along with the battery), so I would avoid disassembling it, because those can discharge with enough amps to kill.
If it's still within warranty, contact manufacturer's customer service. Otherwise, it would probably be best to get a new one.