I've got an old UPS with a stuffed 12V battery. Is there any reason I
couldn't discard the battery and hook up a cigarette lighter plug to it and
use it as an inverter?
Alot of the cheaper UPSs don't have a high duty cycle, they count on the
fact that the battery will go dead within X minutes and design the UPS to
survive X+5 minutes (for example).
If your UPS has the option of adding battery packs then it should be fine
(and probably also too big to keep in your car). If it has a fan then it
might be OK. If its a good brand or you add a fan then its worth a shot.
Of course all of that only really applies if your putting a fairly high
load on it, if you just want to run a fluro light or an FM radio you
shouldn't be pulling enough power for it to even warm up.
-- Michael Heydon