To highlight Steve's point with some examples of simple and cheap circuits, with efficiency depending on the power source. Each of them use pretty much the same parts - the leds are
red Cree XP-E (US$2.01), the driver is a
simple linear driver with a constant current of 700 mA (US$1.49).
Version 1 is simple & cheap - 1 li-ion battery, linear driver, led. Not very efficient though as at full charge, the driver has to burn off (4.2-2.3) x 700 mA = 1.33W and efficiency is going to be ~55%. Even when almost discharged, the driver has to burn off (3.0-2.3) x 700 mA = 0.49W and efficiency is going to be ~77%. Run time is over 4.5 hours
Version 2 is same led, same driver, but 2 x AA NiMh. Efficiency is greater, the driver has to burn off (2.4-2.3) x 700 mA = 0.49W, ~96%. Note I haven't personally built this circuit & 2.4v is lower than the spec for the
7135 regulator but IIRC it reported to work.
Version 3 uses 3 leds, a
similar driver with 350 mA and 2 li-ion batteries in series. At full charge the driver has to burn off (4.2-2.2) x 350 mA = 0.63W, efficiency ~79%. 8.4v is too high for the driver (max 6V). An
lflex would be a much better linear driver to use, but at US $25 , it isn't cheap. Instead my light uses a simple trick to get around this. 2 leds drop the voltage to be within spec, while the linear driver means it is still a constant current. The only downside is that the light output will start to drop at ~6.6V.
