How-To: ‘USB Battery’ v2
Sunday, December 11th, 2005hack a day has some basic instructions on building a portable USB charger powered by a 9-volt battery. This will allow you to recharge your portable device through its USB port when you don’t have access to a computer or a power adapter.
Update: Posted in one of the comments in the article is a modification of the design to use a switching voltage regulator. Link.
