This is one mother of a flashlight, and at about thrice the power of a good car headlight, its got quite an impact. The whole build cost me about $50 total, but buying everything new would increase the price a bit. Its got about a 2 hour battery life on a 12V, 7.2Ah lead acid battery. It uses CREE XM-L LEDs for intense light and longevity. The build requires minimal knowledge, but it is pretty easy to destory parts instantly if something is assembled wrong. And unlike commercial flashlights, this will not heat up at all, and will actually pump out all 3450lm for a long time.
However, I must make some notes on safety.
First, this may just be a flashlight, but it WILL cause damage if you look too close. Don't point it in someone elses eyes, or look at the spot on a bright surface too closely.
Wear welding goggles when first testing it, or any other time you have a chance of accidentally looking directly at the LEDs. Wear sunglasses if you test it inside. It may seem like petty over-safety, but this can and will cause damage if you use it wrong.
Point is, don't be stupid or complain that I didn't warn you.
Step 1: Parts
To build anything, you need the parts. You will need:
3 x XM-L L2 1000 lumen LEDs avaliable at http://dx.com/p/20mm-cree-xm-l2-1150lm-cool-white-bulb-board-for-flashlight-black-grey-199058
1 x 12V lead acid battery, at least 2Ah (> 5Ah recommended), (8Ah and 5Ah versions avaliabe at sciplus.com)
1 x CPU heatsink and fan (avaliable at dealextreme.com, and most computer stores)
1 x High current switch, at least 3A@125V (avaliable at sciplus.com)
1 x enclosure (avaliable amost anywhere, I got mine from a UPS backup battery)
9 x 1ohm MINIMUM 10W resistor, higher wattage is better (avaliable at radioshack, Jameco.com, alliedelectronics.com, etc.)
any number of resistors totalling >60W-70W at 1 ohm
2 x quickconnect terminals that fit the battery (avaliable at radioshack for most batteries)
1 x 4A-6A fuse (avaliable at radioshack)
1 x tube of heatsink compound (avaliable at dealextreme.com or radioshack)
wire (avaliable at radioshack or just about anywhere else)
Step 2: Heat
The LEDs will produce a lot of heat that must be managed. To do so, use a liberal ammount of heatsink paste, and attach all three to the center of the heatsink. Clamp them down until it is dry, and don't worry if some of the heatsink paste squirts out the sides, as it will not affect the performance of the light, unless it gets on the actual dome of the LED. You may also need to heatsink the resistors, as they will run fairly hot. This shouldn't be a problem for them, more a problem for the LEDs, fan, battery, or enclosure, as they may get hot enough to melt any of the above.
For my enclosure, I had to remove some of the plastic bits and trim the edge to fit the fan. This should not be very difficult, but make sure the fan is secure and that none of the wiring will get caught in it.
Although a 3A fuse would probably work, the biggest danger is a short circuit, which will draw MUCH more than 4A. If the LEDs were to be shorted directly to 12V, a fuse wouldn't be fast enough to save them.
The circuit is not compleatley accurate. It should show 9 resistors, hooked up in a 3x3 block. This will give a high power capability with the same one ohm of resistance.
Step 3: Closing It Up
That's it! Just connect the battery and switch, and close up the enclosure and you have the most powerful flashlight in town. Congratulations! Don't forget to vote for me on the lamps and lighting and UP! Contests! Final safety notice: DO NOT LOOK INTO IT (duh), also, DON'T LET SOMEONE ELSE LOOK INTO IT! (also duh). This light packs a serious wallop, and WILL cause damage to unprotected eyes, especially at night, after being in the dark for a while. Also, don't point this at motor vehicles or at low-passing helicopers or UFOs. Do not use while operating heavy machin-oops wrong warning. Sorry. The point here is: Don't be stupid, it is not dangerous if you take basic safety into account, but can very quickly become so if you don't think first.