James,
The advice given by J.M. from Oklahoma regarding computer UPS systems could be dangerous if followed as cavalierly he/she laid out for charging and using batteries! UPS systems come in several flavors as it relates to how the batteries are used and charged. Using the wrong type or capacity of battery in a UPS system can lead to overcharging either through length of charge or how much power is sent during the charge cycle. This can cause batteries to swell, leak and explode!
J.M. is correct that UPS systems are often disposed of because the batteries have gone dead and the unit is depreciated so a new one is purchased. However you must use the correct type and size batteries for safety! This is not something to fool around with or “come close” in! For larger UPS systems of the kind used by corporations that J.M. is talking about, the “battery” is usually a sled with a number of smaller standard-size batteries wired together to provide the correct voltage and amperage to the system. The batteries can be purchased (relatively) inexpensively if you get the make/model from the batteries themselves and search online for an OEM/parts supplier rather than buy a “battery kit” from the maker of the UPS.
Additionally, corporate data center UPS systems are very intelligent and will protect the equipment plugged into their output at all costs so here again a battery supply used to power the device in a power outage must put out the correct power characteristics. If there’s a under/over volt, amperage drop or other power feed condition that makes the UPS think its supply is bad, it will try to bypass the battery (back to utility feed) or shut off the output jacks completely. If you want to use the UPS as an DC/AC interferer post-SHTF then buy enough of the correct batteries to make multiple sleds and charge the batteries properly using a different system (e.g. a solar charger) and rotate them through the UPS.
I really would not recommend using a UPS system in this way unless you are familiar with electronics, how these UPS devices operate and know how to operate this type of equipment in alternative configurations safely. – X. Vindex