I'm planning to make a power injector based on a simple DC/DC converter. It'll take a pair of AA batteries - supply your own. It will have a mini USB cable coming out one end, which will plug in to your 770. The unit will have a USB type A female socket.

I'm guessing that it'll end up selling for around $20. It's possible that the price may go up slightly depending on the costs of parts, as well as assembly - I'm trying to find power supply chips that are easy to solder, but most of the efficient switched mode ones are microscopic, and require both careful soldering and a good circuit board.

I'm also thinking about whether to try to hack a power supply into a USB hub. That's a bit more of a challenge since you want the _smallest_ hub you can - but you want batteries in it too. And you want to charge them. The power injector is a bit simpler, since you can just pop out the AA's and put new ones in. Also, USB hubs vary in price a lot. I don't want to buy a couple dozen hubs to find one that is the right size - so if anybody has any hubs that would fit batteries, and would work nicely on the 770, let me know please

Depending on power measurements, I may be able to do this using smaller or fewer batteries - depends on how long you want to run the thing for, etc.