UPS battery backup as car inverter?

I hope this isn't a dumb question, but if I were to take an old UPS unit, remove the battery, wire a 12V lighter socket plug to the battery cables in the UPS, and plug it in to the car, would it convert the 12V DC to 120V AC while the car is running? And if it worked would it draw too much power or damage anything? I wouldn't use this to run anything with a motor, just a laptop charger and maybe a light. Thanks! :)

Yeah. I think that would work. In fact, I think I have all the ingredients for this recipe myself.

I mean, I've got a car that will start, and I have a small pile old UPSes. I've got wire, and solder.

I should try this, and report back.

AFAIK an UPS needs mains power to work.
Once mains is gone it switches to battery without interruption.
But without mains power and only 12V input it should just beep and complain.