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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.)
*or
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.    
I know it's been about a year, but I just wanted to note that I was wrong in calculating this, it is in fact 3v@3a, and therefore only 9w. A 10w resistor could theoretically be used, though it would get exceedingly hot.
Nice. Gonna make me one of these!
Cool, but I just found a new product today that I'd recommend you use. the new-to-DX.com XM-L L2 is 20% more efficient, and 1150 lumen, instead of the normal 975. They're avaliable here (round) : http://dx.com/p/16mm-cree-xm-l2-10w-1150lm-white-bulb-board-for-flashlight-black-199057 <br>as well as here (star board); http://dx.com/p/20mm-cree-xm-l2-1150lm-cool-white-bulb-board-for-flashlight-black-grey-199058 <br> <br>They both get free shipping, to boot.
The reason I over-specified the resistors was because despite the voltage drop on the LEDs, the resistors are still taking 36W, because there is still a 12V differential, and you're running 3A through them. Even if it is 2.25W each, the 10W resistors I used at the beginning ran at over 200C, which is WAY more than they ever should in an enclosed space. Hence, I recommended (and later used) a greater total wattage of resistors, to keep the ammount that each resistor takes (and thus the overall temperature) down. Though the same ammount of heat is created, it is better dissipated, and thus does not reach the same temperature with more resistors. <br> <br>Also, the LEDs do not get very hot for a while, but will heat up the heatsink I had (which was not exactly a high-quality one), but a good computer heatsink could last quite a while, and much longer if it's moving or there's a breeze.
Hi, nice design. These look like interesting LEDs. <br> <br>The resistors are however over-specified in terms of the total wattage needed. The LEDs are specified at 3-3.5v@3A. The LEDs therefore are taking 9v@3A. Thus the resistors must drop about 3V@3A or 9W in total. Because you are using four resistors of the same value (is 9 x 1ohm above in the parts list a typo?) , this load is spread across all four, so each resistor only has to take 9/4 or 2.25W watts each. This means you can use either 3W or 5W resistors. 5W resistors would probably be the ones to use as running the 3W ones at 2.25W means they would get fairly hot. <br>Alternatively you could use 4x4.7ohm 3W or above in parallel. Although slightly over 1ohm at 1.2ohms, this would give a bit of leeway on the battery voltage being over 12v when fully charged (typically 14v). <br> <br>Also, how hot does the heatsink get when the fan isn't running?
Hi, nice design. These look like interesting LEDs. <br> <br>The resistors are however over-specified in terms of the total wattage needed. The LEDs are specified at 3-3.5v@3A. The LEDs therefore are taking 9v@3A. Thus the resistors must drop about 3V@3A or 9W in total. Because you are using four resistors of the same value (is 9 x 1ohm above in the parts list a typo?) , this load is spread across all four, so each resistor only has to take 9/4 or 2.25W watts each. This means you can use either 3W or 5W resistors. 5W resistors would probably be the ones to use as running the 3W ones at 2.25W means they would get fairly hot. <br>Alternatively you could use 4x4.7ohm 3W or above in parallel. Although slightly over 1ohm at 1.2ohms, this would give a bit of leeway on the battery voltage being over 12v when fully charged (typically 14v). <br> <br>Also, how hot does the heatsink get when the fan isn't running?

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