The final assembly

Step 1: Tools and Materials

1. Soldering iron

2. Solder

3. Flux

4. #1 Philips screwdriver

5. Side cutter

6. Drill and bit

7. Small screw

8. 18650 li-on battery salvaged from laptop battery pack

9. 18650 charger ($1 on Ebay)

10. 18650 battery holder ($.25 on Ebay)

11. Harbor Freight free flashlight

Step 2: Free With Purchase

Everyone loves these free little buggers from Harbor Freight

Step 3: Flip on Backside

And remove the 3 philips screws to open case.

Step 4: Batteries

Remove the 3 batteries that came with the light

Step 5: Remove

Lift and remove the center spring terminal

Step 6: Drill

Drill a mounting hole through the body into the battery section on the side that formerly housed the center battery terminal. You will attach the battery holder here.

Step 7: Flux

Add flux to this spring and this terminal

Step 8: Solder

Solder the battery holder. The red wire to the terminal and the black wire to the spring. Then mount the battery holder to the flashlight with the small screw in the hole you drilled previously.

Step 9: Notch

Notch the back cover with the side cutters so your wires can go into the battery area without being pinched and shorted. Then reattach rear cover tucking wires inside.

Step 10: Test and Enjoy

The electronics behind it all is this. An 18650 battery is comfortable discharging up to 500ma. The flashlight on high beam uses 380 ma. I personally have had it running on high for 2 hours before battery was drained. Free batteries for life I guess.

Step 11: Battery Stuff

18650 rechargeable batteries are real common among laptop battery packs. There are a lot of websites showing how to salvage these. I took some Dell battery packs from the recycling center, disassembled, kept the good batteries and recycled back the bad. You will need a battery holder for the build and a charger to recharge these batteries. These are dirt cheap on Ebay.