Tonto,
Like any battery, they have a lifespan. The bigger/more expensive UPS's have hot swappable batteries (no downtime when you replace them as there is redundant back-up battery within) and generally have larger, longer life ones, too.
You can get a lot of back-up for little cash these days, for sure, in buying new.
The CyberPower stuff, sold at Best Buy, seems like great value. It's got UPS AND Voltage Regulation(

). I use one 1500AVR-HO ($169.99) on my computer gear (improved colors on screen and protected now from brownouts, short duration blackouts, line surges, etc)...the better quality of monitor images was an added extra.
I use another one on my (modest) video system - improved the picture dramatically. Combines with a $199.00 Monster Power Bar, I have better line conditioning than I probably have on my 2 channel set up not....and pleasing audio, to boot.