Fellow hacker Mike Cowlishaw tweeted me a reminder that he had worked on a design for a spelunker's headlamp that used Luxeon LEDs, and had excellent high performance. I remember that I listened to a talk about this design, but at the time, I wasn't saavy enough in electronics to grasp the details, and it had slipped from my mind. He was designing a headlamp for caving, where battery life was a serious matter of safety, so efficiency was primary concern. The final design they came up with would drive a 350ma Luxeon LED at full power for over 5.5 hours using 4 high capacity 1800maH batteries, and then would automatically step down the current as the battery voltage faded. Their first designs used an LT1512A constant current/constant current battery charge controller, but the version I think most interesting was just a small Atmel AVR chip driving a IF7422 FETKY (combination power MOSFET and Schottky diode. Very neat! Mike references an interesting AVR application note which gives a complete design for a battery charger, which is indeed somewhat similar. I'm printing it out so I can read it later.