I got the car-kit Garmin do, which also includes a dash-board mount - based around a sandbag, so it doesn't leave sucker marks on the windscreen (which can attract thieves).

However, in practice I run it on Eneloops. I keep some spares in the car, which are usually enough, and battery charger that will run from the ciggy lighter socket for multi-day trips. It saves having power chords in the driver's area. (My car has a cigarette lighter socket in the boot, which I guess shows they aren't really about smoking any more.)