Anyone know of an external battery powered battery charger for portable devices I can buy online? I've seen similar devices for charging cell phones (ie, put a pair of rechargable batteries into the unit, plug your usb device into it, and it charges your mobile device), and had a 4 cell version for a while (until I killed it), but now I need a new one, and few people have anything except the dedicated ones which have the battery built in.

The idea is that I want to be able to charge my N810 (and other usb enabled devices) on the road, or in an airplane using the charger. I've got a 21hr flight (yes, 21 hours from LA to Tokyo) and the chances of my battery surviving the whole flight is about as good as a snowball surviving in a bonfire.

So anyone who would happen to have a link to one of these, it would be ever so helpful. I have 2500ma AA rechargables I'll be using with it.