Batteries? Pfft!
Upscale these plans http://theepicenter.com/tow082099.html with this http://www.americanarmature.com/military%20brushless.htm alternator. 20HP engine and you are good-to-go.
I'm planning on carrying my newly acquired Inova Radiant-AA in my messenger bag. The obvious problem is the light getting switched on in my bag. The tail cap could be backed off about 1-3/4 turns, but I am looking for something more elegant. Bonus points for using a common household / office...