I have a Cyberpower 1000VA which I bought used on Facebook marketplace for $40. The original owner said he thought that the batter was dead and it just needed a new battery so I bought it off him and ordered a new battery for it. After trying the new battery the UPS still does not switch over the battery when I disconnect the power cord from the wall.
If I get an APC unit instead (more of them pop up for sale on the used market) can I use the battery I ordered for a similar sized APC unit?
Yes. The Cyberpower’s tend to use a 9AH battery in the same form factor that’s normally a 7AH, but it’s just a higher capacity. Check what physical size the UPS uses and go from there. Keep in mind you can have different battery sizes for the same capacity UPS. APC used to make a 280, 420 and 650 that all used the same batteries, and a 650 that used the next size up. All appeared identical except for the cutout in the bottom where the battery was loaded in.