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While for DIY it’s a good idea, you need to consider the fact that computer UPS are designed to supply small amounts of power over a short period of time, enough to allow you to work for, say 10–15 minutes, save all your work and then shut down properly.
You can simply connect a car battery to the battery-correct terminals of the home UPS and you’re good to go.
But when it comes to charging, or even backup, the internal circuitry of the computer’s UPS might not withstand the extra power requirements and supply your car battery can need/provide. A car battery could be, say 12v 35Ah SMF lead-acid battery. APC UPS usually has one or two(connected in series) 12v 7.5 Ah batteries. So a UPS’s charging circuitry might be configured to charge a 12v 7.5Ah battery. When you hook in a 35Ah battery, the UPS’s charging circuitry will need to work extra hours to charge the battery because its capacity is way more than the original battery’s and thus will have extra work to do which might cause unwanted load and more chances of components failing.