I found this awesome $6 flashlight (charger and battery included) on eBay which uses a 18650 Li-ion battery and a tiny Cree white LED. Search "18650 Cree Q5 LED flashlight". It has a glow in the dark button cap and o-ring on the lens, and 5 modes including 3 power levels, strobe, and SOS(I think that one is dumb). The reason I bought it is because I found some really cool 3 watt 430nm pink blacklight LEDs "pink violet 3W 420-430nm UV LED chip" designed for use in fish tanks. It's a white LED, and the color it gives off is an eerie blue pink. It made the flashlight way cooler.
This instructable is documented after the fact, but I have to realign the LED and maybe apply some thermal adhesive.
You will need:
1x 3W or 1W LED (your choice of color)
1x Cree flashlight with battery and charger
Screwdriver or snap ring pliers
Step 1: Disassembly
Unscrew the tailcap and take out the battery. Then unscrew the head and it will reveal a retaining ring around the gold positive spring of the LED driver. This is the hardest part of the build, and that's unscrewing that sticky ring. I had to order a second flashlight because the first one wouldn't come undone no matter what I tried and I didn't want to ruin it. To get it going, you need a very rigid pointy object. I used a screwdriver, but you should try to find something softer than hardened steel to turn the ring so you don't mess up the threads on accident. Lefty-loosy. After you get it going, you can use a pen to wheel the ring around, but use a cheap pen because you still need to apply a lot of force.
Instructables member ledspeakermaster commented that it is much easier to use a pair of snap ring pliers, which I didn't even know existed before. Follow his advice.
Step 2: LED Removal
Tiny LEDs are very stubborn to remove from circuit boards. I was informed to use a solder vacuum or wick to remove the solder, and if you look very close, you can see some very small half-circles on the contacts of the small LED which are filled with the solder that you can remove. You just have to look reeeaaally close, which I didn't do, so I just destroyed the LED. You could use a dremel if you want. There's no wrong way to remove it (except with your teeth), so do whatever's easiest
I tried to push the LED off of the board with a hot heavy duty soldering iron, but got nowhere. I tried sanding the LED off with a medium grit sandpaper but that damaged the LED board a little. Then I figured I'd just chisel it off. I used a hammer with a nail, and holding the module in a vise I cut the LED clean off the board into a hundred little pieces. Careful not to warp the module too much with the force of the vise.
Step 3: LED Replacement
Unsolder one of your new LEDs from its star heatsink and apply thermal paste to its metal backside (if it's not cheap, it may already have some paste on it). I'm not sure if the thermal paste really helps or if the LED board is just copper coated. I found the most likely LED driver to be in this flashlight on eBay, and the seller says that it operates at a maximum of 1.2 amps. I measured the voltage across the LED on a full battery(4.1v) and got 3.25v. 3.25volts(1.2amps)=3.9watts which is a moderate amount of heat for the size of the LED. You might want to consider thermal paste to prolong the life of the LED.
Anyway, glue your LED to the board so that the LED contact pins aren't on top of the leads from the driver making sure the LED is as centered as possible (eyeball the distance from the LED to the 4 edges of the board as a reference for how centered you are). There is a small " - " indentation in the LED's cathode so you don't get it backwards. Then solder the pins to the leads with a firm but gentle grip on the LED to make sure it will shine straight as possible.
Step 4: Reassembly
If you're using an LED the same shape as mine, you will have to cut the positive gold spring from the driver down half a centimeter to be able to fit an 18650 battery in without destroying it. Use hardened steel sheers, maybe the little scissors kids use in school so you don't mess up an expensive tool. You will also have to remove the small plastic retainer in the reflector for the old LED (not if you're installing another tiny LED). I already threw mine out, so sorry for not having a picture of this step. It's not glued in, so it's easy to carefully put the point of the nail on it from the inside of the reflector to press it out the back of the reflector in your hands.
Clean the finger prints off of the lens and put it in the head of the flashlight. Then the reflector and the LED module. Throw out that flat spring (it's what made it difficult to unscrew the ring). Through trial and error, screw in the retaining ring while centering the LED. Don't let the gold spring rest touching the retaining ring or it will short the battery, which might make it explode if it's a really cheap battery. Then screw the ring in tight with your screw driver, but not too tight or you will damage the reflector. The ring is tight enough when you can flick the flashlight head and it doesn't rattle. Then screw the head back onto the battery tube on the side with the round channel. The battery goes in positive first.
Step 5: Dazzle Your Friends
It won't be as well focused as it was before the operation (it's not a bad focus at all), but if you choose to use just another little tiny flat LED, it will be fine. Replacing the 1 watt LED with a 3 watt LED didn't make the flashlight much brighter, but the color is beautiful. If you replace your LED with a green LED, it will improve the brightness. It's because the peak sensitivity of the human eye comes from green receptive cone cells with a peak absorption of 555nm. I can't recommend that you swap out the old driver for a new one unless you have a metal lathe because removing the driver may leave the module wider and looser than it was. I ruined an aluminum host that way once before.
My flashlight is awesome. Compared to the original light, it makes everything look eerie is if you're walking in a giant fish tank. It's just like those annoying xenon headlights people have that make it look like they're flashing their brights at you. It's perfect for general use in the dark outside. But be wary about shining it at others' faces. It is an eye hazard, and from a yard away it's pretty much as bright as the sun a half hour before sunset.