I have to admit to being inspired to do this from this Instructable. I had already made a mains adapter for my battery powered tools and figured that this would be simple enough that even though I'm not sure when I'm going to need this, I may as well make it.

I figured that even though a car battery only runs at 12.5 volts and the alternator runs at 14.6 volts, it should still operate a tool intended for 18v, if not just a little slower.

Since I'd already taken apart packs before, I knew exactly what I would do. The only question was, would it work?

Lets find out!

Step 1: What You Will Need

Materials

A battery pack

A 12v plug with cord that does not have a fuse. A lot of plugs have an in line fuse that runs in the 3 amp range. At 12-14v there's not any way the power tool would be damaged by the power. There's already a fuse in the car in case of a short so that's all sorted.

Cyanoacrylate glue

Rosin core solder

Tools

A long, thin philips screwdriver

Wire cutters (I used my leatherman)

Something to strip wires with (used the knife on my leatherman)

A Dremel tool or something else able to cut a circular hole of the proper size in plastic.

A soldering iron

Step 2: Crack the Pack

Take apart the battery pack by removing the six screws from the case. Take out the side clips and the springs. Keep them in a safe place because you'll be putting them back later.

Remove the ni-cd pack and take the positive battery terminal off the battery pack. It's soldered on so you could desolder it, I just bent it back and forth until the metal stressed off.

You could use the existing negative wire terminal and solder the cord to it, but I opted to solder directly to the terminal so I removed it.

The thermocouple attached to the third terminal in the pack is useless at this point but for some reason I left mine on (nostalgia?).

Step 3: Cut a Hole in the Case

The next step is to cut a hole large enough for the 12v plug to fit through. Our case is almost totally empty so it may as well store the cord and the plug.

I cut mine just big enough that the negative terminal clips on the side of the plug hold it in the case.

Step 4: Attach the Plug

Separate the plug wires and strip back the insulation on them about a quarter inch (half cm?). Just enough to solder the wires to the terminals.

On my plug the wire connected to the center terminal (positive) has a white stripe on it, very handy for figuring out which wire goes to what terminal.

Solder them in, center terminal wire to positive and side terminal wires to negative.

Step 5: Glue in the Terminal Clip

Use the cyanoacrylate glue to glue the terminal clip in place. Without glue, the terminal clip has nothing to hold it in place. It used to be held in by the ni-cd cell that went up the stalk but we got rid of that.

You'll have to move quickly and press the terminal clip in firmly so that the terminals seat properly in the stalk. I didn't push mine in all the way the first time and I had to pull it back out and try again. There's a small window for this.

Something to keep in mind, cyanoacrylate glue is an insulator. More on that later.

Step 6: Put It All Together

The battery case is now the cord and plug holder. Run the tip of the plug through the hole in the case, stuff the wire in, reattach the side clips and screw the case back together. When you want to use it, pull on the plug and let out however much wire you need. When you're done, stuff the wire back in.

The case is really very spacious. I could have fit a lot more wire into it but I didn't have any immediately available. I may take it apart (the glue might make that difficult) and use a long length of speaker wire to increase the cord length. As it is, the cord is only about eight feet long. That's not very long when I'll probably want to use the pack on the outside of a vehicle.

Step 7: Does It Work?

I got everything together, went out to my car, plugged in and clipped the pack into a drill. Nothing.

I thought, that maybe 12.5v wasn't enough to run the drill but if I turned the car on, the alternator would give me 14.6v. So I started the car up but still got nothing. I tried an older drill, still nothing.

I noticed that the terminals had some glue on them so I scratched it off with my key. Still nothing. I started to think this wouldn't work at all. This surprised me, I expected them to run slowly, maybe even poorly. I didn't expect not at all.

I tried a reciprocating saw and all of a sudden, that worked. In essence the glue was just starting to wear off enough that the power would get through. I cleaned the terminals more thoroughly and now the drills started working.

There is a noticeable change in the speed of the tools, but because the car battery (or alternator) has far more total amperage available, they seem to perform well enough. I can't get the drills to bog down completely and I was actually surprised that the reciprocating saw worked as well as it did.

No overheating observed, no problems with the build other than I want a longer cord now. I wonder if a longer cord would get hot because of running the high current through it? Only one way to know for sure.