Breathing powers USB charger

How can you actually utilize something as simple as breathing to power a USB charger? This idea was made possible by the folks at MAKE, but you’ll need a reasonable amount of D.I.Y. knowledge before getting started. The entire premise involves using a rechargeable AA battery to charge any USB-compatible device, where your breath in turn keeps the AA battery recharged. The generator currently produces around 50mW of power, which means you take an entire day just to charge a single AA battery. I’m sure Darth Vader could have done that in half the amount of time with his heavy breathing and all.