Two of my UPSes were last re-batteried three years ago, so I decided to update one of them, UPS number 4. Because lead-acid batteries only last 3 or 4 years, in my experience.
I downed the servers attached to it, and put them on a different UPS. I got a set of batteries for £60 (I usually pay over £100 including shipping) and after a few days, they arrived.
I opened up the UPS, there's about a dozen screws, and if it's opened up, I can check that the fan is still working. Fans, being mechanical, have a tendency to fail after several years. Also, if it's open, it's easier to get the batteries out. I removed all eight batteries, and replaced them with the new ones. When I put a voltmeter on the old batteries, they were fine, showing 12 or 13 volts. I screwed the case back together, and replaced the UPS. I'm not sure what to do with those working batteries.
And that, you might think, would be that.
But one of the servers that I had downed, I noticed that when I gave it power again, the PSU fan wasn't spinning. So I took it down again, opened up the PSU and replaced the fan. The old fan was totally gummed up. These 80mm fans only cost £1 or so, so it would be silly to replace the whole PSU.
Then I noticed that another server wasn't responding, and when I power cycled it, it stayed down. I put it on my workbench and opened it up. I took out the PSU, and tested it - dead. So I replaced the PSU, and now that server is working fine.
It's always the case - when you start to mess with servers, some of them fail when you restart them. You have to be ready for this, and I have lots of spare PSUs and 80mm fans.
I'm a glutton for punishment. I've just ordered another set of batteries for the other UPS with three year old batteries.