It’s not only a cost thing, they don’t have the power output for the same form factor. You can technically swap in those 12v LiFePo4 “replacements” into a cheap UPS but they don’t put out nearly as many peak amps as good old SLAs. Lithium Ion and Polymer can but they also run the risk of blowing up. I’ve played with basically all these chemistries doing model airplanes and DIY drones.
I thought the bigger concern was that the charge and duty cycle of the different battery chemistries were incompatible so it was not a good idea to swap lithium based batteries into lead acid ups's.
One of those modding articles discusses how to reprogram the UPS's battery manager with numbers suited to LiFePo4. A bit iffy, that; who's validated those values?
Most of the packaged-up drop-in replacement lead acid to LiFePO4 batteries I am seeing aren't just a group of raw cells, but have a battery management system internally. Even the least expensive should be able to protect the cells from overcharging without modification to the UPS.
From what I read, it's not the overcharging or the charging that is the issue, it's the cutoff voltage at which a lifepo might be harmed being a higher threshold than a lead acid battery, so you have to have a BMS for the lifepo which increases the cost and can cause the ups system to miscalculate runtime as it may cut off when it thinks it has 10% remaining.
But if there is a hack to recalibrate them, then great. I'll do some searching for it now as I have two little bms's that need batteries.
