This is a 2U 750VA APC UPS. I've just bought a new Lithium Ion version as the battery on this one died again so this is up for sale. It's a model that I got second hand but I've had it for 5+ years and it's been fine. The battery pack is the RBC123 and can be had for £100 from Comms Express or other retailers. It lasts around 3 years.
eBay