PPc's are a great idea but the problem is they tend to fill up with liquid and make a mess or worse short out your batteries. This is from my experience at least. What I would recommend is a portable power charger. Basically a 2900 mAh battery that can be used to charge your batteries with a USB charger!
Awesome idea! I have a few iPhone battery backups I bought from China. If I can find a "female iPhone end" to mini USB cord, I'm in business!