http://www8.garmin.com/manuals/Dakot...nersManual.pdf
The battery life in the nuvi is shorter but if your in the car that doesnt factor in. Battery life in the gps can be managed, like haveing the screen power off sooner, less back lite etc. I havent need an extra set of batteries but a couple of extra AA could fit in your pocket or pouch. I would say the regualar AA's last about 6hrs of continous use?
@ Tony, you can find more info in the garmin instruction mannual link above, sorry I can't be more specfic because I dont use it in the car, but I got that from instructions and device. See page 23.