I strap mine to the top of my back pack. It'll keep my phone / IPOD and four AA's charged. I use my phone for emergencies, last ditch GPS and most importantly so I can have music in my head on the trail. I've hiked all day and had my IPOD on for much of that time and when I get to camp, my phone is still fully charged. It seems to work pretty well even on partly sunny and slightly overcast days. Don't count on it charging on a cloudy, rainy day. Save your battery on those days.
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