Wouldn't it be just as easy to carry a spare battery for your phone?

 

It would be cheaper in the long run as well, especially if you add up buying AA batteries.

 

I've got an excellent idea. Buy a car battery, wire up a cigarette lighter outlet to it and use a normal car charger with your phone. Not incredibly portable or nice to look at, but it would work. The car battery should last for quite some time with only a charger taking power from it.

 

i have a portable energizer energi-to-go charger that works great, bad part is that it cost me like 50 bucks

 

http://bestcelldist.com/htc_droid_eris_travel_charger_portable.html

 

This may not be much of a solution, but last year, my phone died after the 1st day and it was actually a good thing. Trying to meet up with people in bonaroo is a real mess. I had much more luck with walkie talkies. Plus, if you happen to get "confused" late night, and lose your belongings, better to be out 30$ than to lose your phone. I'm speaking from experience. I think I lost it in the silent disco.