I have tried several chargers, and many were simply not up to the job. To big, to heavy, no power, etc etc. But foremost, that many phones, stop charging, when a cloud comes by, shadow of a branch etc.
Since last year i used the Suntactics sCharger5. (Suntactics.com) 1,5 hours from 5% to a full iphone 4. While charging you can use the phone.
the sCharger uses 2 big cells, has a usb port. It weighs 236 grams. and is folded, 15 x 15 x 1 cm, except te usb port is a bit thicker. unfolded 30 x 15 x 0,5 cm.
The charger needs sun. So a high capacity battery of 5000 tot 10.000 mAh is usefull, when the phone is full, and the sun is still there, for the days that there no sun at all.
For as i know, this charger, keeps every few minutes restarting loading, and if a cloud comes by or somebody walks by, it is no problem at all.
The pouch in which the charger comes, is not very usefull, and i made one from some foamsheet, tyvek, and patches of velcro.
My whole set, exept the high capacity, with: small charging cable, small bamboo sticks to keep it at an angle, a small 3,7 volt usb charger for the camera battery, a piece of rope, and cable for capacity battery, weighs 323 gram. With this i stayed 4 weeks, of grid for my camera and iphone.
The owner Dean Sala, is always happy to answer any questions. As he did mine, for drilling holes to hang the cells from. In the corners i drilled a 3 mm hole for the small rope.
Because the material can be stiff, i made on the back two holders from ductape, so i van put a small bamboo stick in.
He advices not to use the charger on a backpack. unless you are walking in the dessert, but to empoy it as resting. I tried this, and he is right. half an hour statically in full sun at the right angle does more than a day, on a backpack at the wrong angle.
For me, its lightweight, waterproof, and actually works dependable.