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The resistor in the circuit does use up some power and thus reduces the amount of time a battery will last. But the resistor is necessary to keep from blowing up the LED (or at least reducing its lifetime). The most efficient method is to match the voltage with the number of LEDs used. For example, if you used 2 batteries at 9 volts each for 18 volts, you could use 5 LEDs (5*3.6 = 18) without a resistor.

Another approach is to make the white light from a combination of red, green, and blue LEDs. Colored LEDs typically take 1.8 volts, so five in series would work with a 9-volt battery. In the same way you could use two white LEDs (which are slightly bluish) and one green LED (2*3.6 + 1.8 = 9).

Here's one more thing to try, but be careful because over-driving an LED can make it explode! Even though LED manufacturers tell you the maximum current for the white LEDs most commonly available is 20 ma (0.02 A), they can be run with a lot higher current (which makes them brighter), however this does reduce the lifetime of the LED. But since they can last for 100,000 hours, this may be a good compromise. OtherPower.com is experimenting with this and has run one LED at 98ma for over 100 hours.

If you try some of these experiments, please share your results.

Ready for more fun with LEDs? Try these out:

• NEW and IMPROVED LED flashlight Junior
• One LED Flashlight
• One white or one red LED
• Two white LEDs
• Four white LEDs
• One or four white LEDs
• LED Headlamp
• Improved One or Six White LED Headlamp
• Simple One or Two White LED Headlamp
• Converting Flashlights to use LEDs
• Black Light LED Flashlight