The internal battery in my GoPro 3 only powers the camera and LCD for about an hour. I need longer shoots than that, so I built a lightweight (38 gram / 1.3oz) adaptor to allow me to use my spare FZ200 / GH2 batteries to power the camera via its USB power input. A single BLC12 battery runs the GoPro/ Touch-LCD combo at 2.7K-cine protune for about 2 hrs before the internal battery kicks in for another hour. Together the GoPro 3 will now run at 2.7K for 3 hrs, long enough to fill the 64GB SDXC cards I am using :)

I used an LM3485 3Amp switching regulator evaluation board because 1) I had one lying around, and 2) they are nearly 100% efficient, and don't get warm. The battery holder is a tray from a BLC12 charger I bought off Ebay, and the LED voltage display is also off Ebay (it draws a negligible 10ma and allows you to guesstimate how much battery charge is left). Photos of the rig below.

It would be much easier to just have bought an external USB battery pack, but they are heavy and bulky. This way I get double use out of the batteries I already carry around with my rig.