"A GPS without Batteries is as useful as a Gun without Bullets"

Therefore I always carry two sets of spare batteries.

On longer trips I now take 8 Rechargeable AA. I have also been experimenting with the use of a Solar Charger to charge my batteries when I am on longer trips. This way I can have a permanent supply of AA/AAA Batteries. This is no good on a short trip because of the weight of the panel and charger, but on longer trips it does help.