I have a PowerBag backpack which is pretty great:
http://www.amazon.com/Powerbag-Charg.../dp/B005JLQL26
The battery itself weighs about 7 ounces, recharges from flat in about 1 hour and a full charge is enough to fill my iPad twice from flat. My only complaint is that there are not enough pockets and compartments. It is basically a backpack with two massive compartments. I can fit a lot of stuff; it's just all dumped together.