I think now I should have enough run time to finish my run through halflife2.

That's not a BMS. That's a protection circuit. They're designed to protect against rare, worst-case events by shutting the cell off completely, possibly permanently. Their thresholds are set significantly higher than what may be safe in the long term.

I should also point out that not all cells include a protection circuit - often for cheap devices the protection will be provided by the device itself on its own PCB, and the cell will be unprotected. Dodgy knock-off cells may not even have a protection circuit!

This is still phenomenally unsafe and fundamentally useless - it's not safe for any long term use, or unattended use, which defeats the entire point of a UPS. Less Macgyver, more "I have no concern for my safety or the safety of others".

/r/techsupportmacgyver Thread Parent Link - i.imgur.com