My UPS number 1 has been showing a "battery fault" light. The batteries are four years old, so that's not too surprising.
I ordered a new set of batteries (£95, from Ebay) and they arrived yesterday while I was out caching. Today, I took down all the servers being powered from that UPS, and put them on another one temporarily. Then I shut down the UPS, dragged it out to where I could get good access to it (wow, those things are HEAVY, more than 50 kilos), took off the top plate, and the batteries slid out easily. When I checked them with a voltmeter, nearly all of them were fine.
I put the wiring harness on the new batteries, slid them into the UPS, heaved it back into place, powered up, and it's working fine, so I put the servers I'd moved, back onto that UPS.
I also added an external cooling fan onto UPS 1 and 2 (I already have one on the other three).
I should be OK for a few years now!