I have a couple 48v APCs. I bought a 48v (nominal) LiFePO4 pack with battery management built in. The problem is that it's a 15S pack and not a 16S pack, so the voltage is not quite high enough to meet what the UPS expects (51v vs 55v) and even after adjusting it in the UPS the battery pack goes into protection mode during a calibration or actual power outage, and the UPS never powers back up until it's "jump started" with 54v. Frustrating.
For my 24v APC I got 2x 12v batteries. These are each 4S packs, so the voltage is perfect. Here the problem is that they get unbalanced, and eventually won't hold a load for more than 3 seconds. Then I have to take them out and manually balance them. PITA