One of my big UPSes has been showing a red "Battery dead" light for a while. And every now and then it has a fit of beeping. So I finally decided to do something about it.
First, I needed a UPS to put in while I worked on it. I have several old UPSes from BPC, that haven't worked for a long time. So I decided to put new batteries on those. I got eight batteries so that I could revive two of them, each set of four was £74, which is pretty cheap compared to a new UPS - on Ebay, 3000 AV UPSes with new batteries go for about £300. The batteries for the APC 3000 cost £95 for a set of eight, I've done this a few times.
The first one rebatteried easily, although it was a struggle to get the old batteries out. When batteries get really old, they swell slightly. But my faithful crowbar helped, and I got them out. Replacing was easy, except when I connected up the last battery connection, there was a big fat spark. Quite alarming if you aren't expecting it. That's the capacitors charging up.
Then I re-batteried the second one. But when I powered it up, it dodn't work. So I took the battery tray out and put it in a third one, which worked fine.
The big UPSes I use are 3000 AV, which is about 2200 watts - enough for a dozen or so computers. But I only put half that many on each one, it means that in the event of a power failure, I get twice the run time. They have 8 batteries, each 12 volts, 7ah, so that's 672 watt-hours. The smaller BPCs are also 3000 AV, but they use four batteries at 12 AH, giving 576 watt-hours. So they'll handle the same load, but will last 15% less time, This isn't a problem. And the BPCs feel somewhat lighter; I can actually walk while lifting one, which I can't with the APCs.
For comparison - when I go out on my electric bike, I take three sets of batteries; each set is three 4S batteries at 5 AH. So each set is 288 watt-hours, and my saddlebag total is 864. And I can lift it with one hand, whereas the big APC UPSes I can barely lift with both, it's 60 kilograms. My three sets of bike batteries are about 5 kg. That's partly the difference between LiPo and lead-acid, and partly the great weight of the transformer inside the UPS.
So I disconnected all the computers from the UPS I was worried about, and connected them up to one of the newly-batteried ones. The batteries on the old APC are vintage 2011 (when I put batteries in a UPS, I stick a label on with the date). So it's not too surprising that four year old batteries aren't good. But then I powered the UPS off, and back on again, and whatever error condition was worrying it, seems to have cleared. No red light!
So maybe I should have done that first.