UPDATE: there is a new article that expands and powers up this idea, you find it here.
For a while an old UPS of mine was abandoned in a corner because its battery was long dead and didn’t sustain the load of a PC for even a few seconds; and for a while I’ve been telling myself I needed to buy a spare battery off ebay or something, but never got around it because the prices were less than inviting; in the meantime, my little home server kept dying at each and every blackout or brownout.
The other day I noticed a car battery lying at home; we actually have a few car batteries here, unneeded but we keep them for the sake of it; none of them is new and they have been taken off old dismissed cars, they still work though.
Going by the theory, it’s still a 12V lead battery, so it should perfectly work with a normal computer UPS, yet I digged for information around several websites, and actually there weren’t any particular warnings (on the contrary, car batteries are made to sustain intense but short drains, followed by frequent charging, and that’s exactly the way an UPS is mostly used), but on the other hand, there could be, maybe, a little risk of production of toxic vapours, due to the acid contained in the battery; in other words, no problem whatsoever, but it’s recommended the setup be put inside an isolated, or well aired room; in the worst of cases you can keep the UPS by the PC, and use cables long enough to place the battery elsewhere; in this case the room I needed it into was quite isolated, so no problem at all.
This setup has been tested to work perfectly, removing the plug activated the switch to battery power seamlessly, and I tested it for about one minute; I don’t know how long it could last, but probably could reach 30 or even 60 minutes, I didn’t test it thoroughly because I don’t really care, blackouts and brownouts last very short over here, and I don’t want to stress the battery for no particular reason, reducing its operative life.
A detail to note: when I wired the battery only, in a test setup, I couldn’t turn on the UPS, and I started thinking I failed somewhere, or even the UPS died for some reason, but it was just a coincidence, because my particular model of UPS is silly enough to refuse turning on if you try doing it without external power, in other words I needed to attach it to the mains plug as well, and I turned it on no problem.