The batteries are only good for so many years and when they start failing it is usually cheaper to buy an entire new UPS than to replace the batteries.
I ran this cycle for years before I got wise. Buy an expensive UPS, UPS burns out the battery because they can't stick a $0.30 IC in there to intelligently charge the battery (proper float voltage range, -dv detection, proper charge rate based on battery chemistry/Ah), buy expensive battery, then UPS circuitry dies. All the while the thing is only good for a couple of minutes despite the fact that the VA rating is multiple times your expected draw*PF.
Then one day I came across an old UPS for a server rack. No bells and whistles (auto shutoff for computers, etc.), but it would support a large lead acid battery bank and provided a true sign wave output (or at least claimed to).
I bought the four cheapest marine batteries at walmart (system was 48v), threw a desulfator inline with the charge circuit (no idea if they really work, but why not) and the thing has been rocking for 9 years and can support my wimpy gaming rig, monitor, cable modem, and router for longer than I have ever had the opportunity to test (2 day power outage was the worst I believe).
Total cost was maybe $150.
