You don't need any extra batteries for a 3-night trip. Just put fresh batteries in before you start. For a week-long trip, especially with the short days and long nights we have this time of year, that's a different story!

I do carry one of those tiny Photon microlights, along with whistle and compass, on a cord around my neck, just in case. So far the only times I've used the little microlight were when I couldn't find my regular headlamp in the dark, or when the headlamp came apart and the batteries fell out, but it's good to have a backup for the regular light. The tiny microlight weighs far less than extra batteries!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey