PLEASE read the whole description!
This video helps prove that there is something (some) CFL bulbs are good for. ;-)
In light of the blizzard and the fact that my electricity has been going up and down around the time I made this video, here is a presentation on how to repurpose an old uninterruptible or standby power supply with an external battery. Toward the end of this video, you also get to see a demonstration of how it works.
First: consider your safety. Don't do this if you don't understand what, why and how to do this. Be honest with yourself and your skills. It's not worth being hurt or burned. Disassembling a UPS presents a risk of electrical shock and some of the parts inside can remain energized! A big UPS has its own hazards. Always use fuses and appropriately sized wiring. Never charge a lead acid battery in an enclosed space or in a heavily populated part of your home. Explosive gases can build up and be ignited.
Second: consider that not all UPS units will properly charge or operate from any other battery than the one they were sold with. You will have to experiment to see what works with what you have.
Third: yes, there are other ways of doing this--a large enough charger and inverter can perform the same function if you supply the batteries. It's a "good idea" to use a battery intended for the purpose, such as a deep cycle type, but cheap car or lawn tractor batteries will usually work better than you'd expect and live long enough to be a more economical choice, especially if your electrical service is reliable.
UPS units with dead batteries are available almost anywhere. The plastic box came from a Dollar General store and was drilled and cut appropriately. I bought the toggle switch at Wal-Mart, in the automotive department. You can get a wide variety of panel meters from eBay. There are scores of sellers in Hong Kong, China and Taiwan that carry them. Such meters usually cost less than $20 (US).
Most UPS units have a "modified sinewave" or "stepped approximation to a sinewave" output. This can cause problems with some equipment. Really old and/or cheap UPS units will have a squarewave output. If you get lucky and find a UPS with a true sinewave output circuit (usually marked as such--also try downloading the manual for the unit you have), it will safely operate almost any electrical device you can connect, so long as the output rating of the inverter is not exceeded.
Recycle any dead batteries appropriately and do not put them in the trash.