I have the Freedom GPS. I would say that basically, if you want it on your keychain, you'll probably want to get a fix BEFORE you start the car. Once you get your satellite fix there should be no issues with having a signal from that. I actually have a "quick-release" keychain that when you push in the button, the two pieces separate. My GPS and USB flash drive are on one end and keys on the other, for this sort of thing anyway.

I've never let the battery die but I've probably had it on for 8 hours or so with no issues, don't know how long it'll actually last.