Lithium Ion batteries on a UPS

if i build a battery pack of 18650's can i replace the 48V Battery in my UPS?

The Ups is an APC 2200VA ups with SLA batteries

Many many many threads on this topic. Just search for the UPS in the thread title.

Yes you can replace them. Watch AveRageJoe on Youtube to get a full breakdown. He has a full system running on a SU3000 (or is it SUA).

If you are going to be using the UPS as a UPS, please keep in mind that they keep their batteries fully charged which is good for Lead Acids, but not as great for Lithium Ion. Also, some have built-in non-customizeable equalization charges every 28 days or so. My suggestion would be to add one extra cell in series to lower the voltage. I have a 5000VA TripLite UPS that charges at 54.3V, which would be perfect for 13S (4.18V per cell), but I would run 14S to keep the cells at 80% charge. Also, 2200VA is almost 50 Amps at 48V. Make sure that you use a BMS that can handle it Most Chinese BMS's are way overrated and will tell you (buried in their details somewhere) that a 60A BMS can only handle 26 Amps!