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!