saw one thing about a 60w dc type supply you apparently mounted in a pci slot for powering a desktop from an external source. But only 60w? I would be skeptical with something like that working out well.
Spmething like a car or tractor battery would be obviously oiut since you can't carry the rest of an auto along with you for the alternator in order to recharge the car battery!
There are some ac to dc adapters that see an auto type 12v plug for using something like a car stereo on ac. But those would also serious lack the needed amperage for a desktop. Something like the APC product line for batter backups is still the best idea to look into there. Those are designed for pc application when ac power is out. Those are simply recharged later after the short term use they offer.