There's nothing so frustrating to me as your battery dying when you are in the backcountry. Cold weather drains the juice even more than normal. I found a solution to this issue with the Mophie Powerstation XL. I love the size and weight and speed with which it recharges my iPhone. The smaller Powerstation is plenty of extra power for a day hike and the larger XL version holds up to 8 recharges for longer trips. The cold doesn't seem to affect the unit like it does the iPhone. You'll always find this in my pack.