The batteries in most UPSes are Sealed, Valve-Regulated  
Calcium Lead Acid.  They tend to last about as long as a 
car Lead Acid battery: about 3 to 5 years or so.  Battery 
replacement cost should be a small fraction (less than 
30 to 40%) of the overall cost of the UPS.

They do need to be replaced after a few years.  The UPSes
generally do a periodic self test by themselves and will 
tell you when the battery goes bad, but of course you must 
check the UPS occasionally to notice that.  If you let the 
battery go bad without replacing it, the UPS is nearly 
useless in that it won't provide backup power during a 
power outage.  (Still does power line filtering, etc.)

So a very little maintenance and care is required in 
exchange for peace of mind about power glitches, brown 
outs, black outs, lightning, etc.

Radio Shack can get replacement batteries, or you can
support your local battery or electronics shops.  Be
sure to check the date code on any replacement batteries
and get only reasonably fresh ones.  Generic replacements 
like those from PowerSonic, etc. save money and are 
essentially identical to the factory ones.  Any battery 
vendor can help choose the correct replacement type.  
Be sure to stick with the same battery type, which again 
will generally be Calcium Lead Acid.

For example, here is a cross reference to PowerSonic 
replacement batteries for APC UPSes:

  http://www.batteryweb.com/apc.cfm 

It's best to get batteries locally to save on shipping 
costs.  Most dedicated battery stores also move their 
inventory quickly so they're fresh.  Don't be discouraged
if they don't keep a lot of inventory on hand; they do
that deliberately so the ones they have are fresh.

Jeff C.

2006 Update: Most new UPSes probably use Absorptive Glass 
Mat batteries now.  These are Lead Acid batteries that have 
the Lead plates immobilized and wrapped with fiberglass 
cloth which absorbs all the acid.  AGMs are mechanically 
and chemically much more stable than regular wet Lead Acid.
They also perform better mechanically, chemically and 
electrically.  Since the acid is fully absorbed in the 
fiberglass, spills are nearly impossible even if the battery 
case is cracked open.  AGMs are truly zero maintenance.  
There's no off-gassing or watering or other maintenance as 
long as they're not grossly overcharged, overheated, etc.

Older Calcium Lead Acid cells may not be fully compatible 
with AGMs.  Check with your UPS manufacturer and battery 
vendor.  Definitely DO NOT mix battery types.  If changing
one type to another, replace all the batteries in the UPS 
at the same time.  Given aging, that's probably appropriate 
anyway.  If one battery is getting old, the others may be 
wearing out also, especially if they're all the original 
ones.