For anyone thinking of doing this, please please don't. A car battery is probably never a sealed deep cycle battery, and the UPS's charging circuitry is not intended to charge a battery of this size (this is assuming you're using a lead based battery, and not something even more crazy and dangerous like Li-Po or LiFePO4). God forbid you have a cell fail on a car battery and that charger starts cooking the battery. I've had actual car lead acid batteries explode because of poor choices someone else made trying to do something like this, and man when they go, they're dangerous and scary. You really need to pick hardware that's all properly specced and sized for the job...there's a reason Eaton and APC charge what they do.
The better approach (if you have EE skills) is to build your own UPS with LiFePO4 batteries, a proper BMS, and a bunch of USB-C PD ports. Skip the lossy inverter entirely and pick equipment that runs on USB-C PD directly.
I don't know why nobody sells these as COTS yet.
Or buy equipment targeting alirack compatibility, i.e., with 240V PSUs that are minimal effort designed to also specifically work on the appropriate DC voltage so that Alibaba, the alirack standard's originator, could delete the inverter from their UPS and feed PV MPPTs directly into the batteries.
I agree entirely, and wouldn't do it again.
To each their own. I'd personally sleep far more soundly with even a car battery UPS under my bed than with one of those consumer ready lithium ion portable power station batteries they sell on Amazon.
But if you can't explain the difference between voltage and current, or know what "short circuit" means, then this isn't something to poke at.