Woe to the sysadmin who trusts their UPS to work as expected.  It turns out that some UPSs won't warn you when the battery is dead or low.  You find out it's not functioning when you unplug it. 

So, as a matter of course, it's necessary to test UPSs.  It's not easy.  You have to schedule system downtime, shut down the computer.  Then unplug the UPS and plug in a device, and test how long it stays up, and test if the UPS beeps.  If the battery is dead, then it's time to buy a new set of batteries.