Returning to the safety aspect again for a moment, and I'm sorry to harp on about this, but I want to make sure I put my prior warning in perspective.....
UPSs deserve a lot more respect than most other electronic appliances. This (now retired) colleague who would never touch UPSs spent his whole working life in the back of TV sets, monitors and so on. 24kv EHT supplies never fazed him one bit.....

Ok, that's me done wittering on about safety

I think pictures of the circuit might be helpful although I'm unsure exactly where that'll lead, maybe nowhere. Right now I'm trying to visualise whether the circuit has flying leads that connect to the battery or whether it's the other way around. And, which one (+'ve or -'ve) was connected while the other shorted against the case?