When I moved into the Smart Apartment I was one of the first residents. The power would go off occasionally as building work was done. So I installed a small Uninterruptable Power Supply (UPS). This is a SOLA UPS 305 520. It looks like an oversize power board, with three sockets for equipment protected by the UPS and two ordinary power sockets. It has a small gel lead acid battery (Panasonic UP-RW12200CH1) which can supply power to a typical desk top computer for a few minutes while it shuts down in an orderly fashion (there is a data port on the UPS to tell the computer it is on battery power and should shut down). In practice the UPS would run for about half an hour with my modest computer set-up, longer than most power outages.
Recently the power went off and the UPS failed to operate for more than a few seconds. The battery needed replacing (they last only a few years). There is a separate battery compartment with a cover held on by one screw, so they are reasonably easy to replace. Care should be taken with a UPS, as it can generate dangerous high voltages, even when disconnected.
Gel lead acid batteries are sold by electronics shops, so I thought it would be reasonably easy to get a replacement. Unfortunately the commonly available batteries are the same dimensions as the Panasonic, but 10 mm longer. As a result the replacement would not fit in the case.
Rather than buy the battery 10mm larger, I decided to buy an even bigger one. The Panasonic unit has no capacity marking on it but appears to be about 4 AH (Ampere Hours). I noted that there was a sweet spot at 7.5 AH: the smaller batteries did not seem to be much cheaper and the larger batteries got significantly more expensive from this point on. The 12 volt 7.5 AH batteries are the size used by many UPS, home burglar alarms and the like.
So I bought a battery from Adelong in Sydney for AU$22.85. This has the same connectors as the smaller battery. I then just needed to cut a hole in the side of the battery compartment to fit the larger battery. It will not be elegant, but should work longer than the original.
By the way, you should, in general, not replace a rechargeable battery for one of a different voltage or chemistry (don't replace a 6 volt battery with a 12 volt one, or a lead-acid battery with a NiCad one). Also you need to take care you connect the battery terminals the right way around. The gel batteries have no polarised connectors: you can connect it the wrong way around. If you use the wrong battery, or connect it the wrong way, the result could be an explosion and fire. However, for those with the skills and confidence, replacing the battery in the UPS can save a lot of money.