Batteries on my desktop UPS bit the dust, but after checking the maths I realized I could replace them with a 36V LiFePO₄ pack - bulk float charge voltage for 36V SLA pack (13.8×3) sits right below the charge voltage for a LiFePO₄ pack.

Beware! the battery leads of your UPS may at mains voltage!

@rechner make sure the battery pack has its own charge control stuff, a lot say they're SLA drop-in compatible but are just yoloing it and abusing the batteries