I'm already using a 3600mAh external cellphone battery that outputs 5V USB power to run my Zoom H4n audio recorder for extended recording times.
Recently I found a 10'000mAh battery that not only has 2 USB outputs but also a third output that can be switched to 12V or....9V.
Perfect to run the GH2 camera!
It turns out that even the proper adapters are already included and have the polarity required.
Here is what I used:
This is about 5.6x the capacity of a Panasonic battery for this camera.
Of cause it will be significatly less of this device also powers the Zoom H4n and 1-2 phantom powered microphones through it.
...or multiple GH2 cameras from one power bank.
...or something else that runs on 5V USB power from the second USB port. (Arduino? Video light? ...)