Taken from
www.hackaday.com:
Limor a.k.a. Ladyada has been a long standing staple in the Hack-A-Day community and we were more than happy to hear about her latest project. The Minty Boost is an improvement on the idea of building a battery powered USB charger. Most designs (including ours) use a 7805 linear regulator with a 9V battery. Even in the best case the regulator is only about 60% efficient which means you’re losing at least 1/3 of the power to heat. Limor’s design is a much more efficient boost converter design and uses far more common (and rechargeable) AA batteries. She’s got a thorough write up on how to build the converter in an Altoids gum tin. The end result is 82% efficient and can effectively double the play time of a video iPod.
I found this an incredible concept.
Maybe it could be applied to those recorders who can be powered by USB?
Details on the project
http://www.instructables.com/ex/i/1686FF3E41011029AC23001143E7E506/