This is a way to seriously extend the battery life of anything you can charge off of USB. It's sort of like a giant minty boost, minus the mint.
After I built this, I discovered that Milwaukee already makes something that does just that, called a Power Port for their 12v lineup of power tools. Why not save $20 and build it into a tool you've already got? The 12v -> 5v USB adapter I used can handle up to 24v of input, so you could do this with your 18v tools as well. It also has a handy status LED, so you know when the power is switched to it.

Materials:

12-24v battery operated power tool
Car cigarette USB adapter
Switch

Tools:

Solodering iron
Dremel
Glue gun
Screwdriver

Step 1: Disassembly

The first thing to do is take your tool apart and make sure there's enough room in there for the guts of the car adapter. I was surprised by how little space I found on the inside of the Milwaukee light, but with a little rearrangement, everything fit snugly.

Also take apart the car charger, mine had a sneaky screw under the label.

Step 2: Soldering and Testing.

You'll want to desolder the prongs on your car adapter PCB, and replace them with short lengths of wire. The two big prongs on the sides are negative, and the center one is positive.
These wires will need to be attached to the battery contacts, but first put your switch if you're using one, on the positive wire.
As you can see, the wires will have to be pretty short to fit in the confined space, use heat shrink and hot glue where necessary.

Also, if your design requires your USB port or your switch to be installed from the outside of the tool casing, make sure to cut the hole and install it before finishing the soldering. My hole lined up with the seam line of the two case halves.

Once everything is together, plug in a battery and switch it on, you should see your LED status light come on. Test the output with a multimeter, you should be getting 5v across the outside two pins of the USB port.

Step 3: Finishing

The last step before reassembly is to cut some holes for the new parts to stick out. I used a drill bit and then a high speed burr in my Dremel tool, and was really amazed at the resilience of the plastic, it was like cutting through sheet metal.
Once everything fits the way you like it, hot glue the parts in place. I also covered the exposed PCB contacts to keep the 2 boards from shorting against each other. Screw the case back together and you're done!

I use this a lot for geocaching, because my GPS has a terrible battery life, and quite often a flashlight can come in handy for finding well hidden caches, even during the daytime.