Perhaps in the soon to nearish future they will come out with something that will enable a human being to use kinetic energy to charge batteries while on the go. The idea is already out there I believe and I think its already in testing.
For now, just carry an extra battery. My primary battery is an 9.6v 5000mah SubC pack and it can last a very very long time, and by that I mean atleast 2 or 3 games where I do alot of firing. I do have a secondary emergency battery but I have personally never had to use it, Ive ended up loaning it out to others but have never had to use it myself.