Convert a Perfectly Good Cordless Drill to a Corded One

Convert a cordless drill to plug into a 12 volt battery pack when your battery packs die.

About twenty years ago, I sprang for a state of the art, best that money could buy, a Porter Cable 12 volt cordless drill. (the highest voltage then). It came with a charger, steel carrying case and two nicad battery packs. I got a lot of good use out of it over the years. About eight years ago, the battery packs started to die. I bought one new one for $50 or so, which has since died. At that point, I ended up with three dead batteries and a really nice drill that I couldn't use.
I intend to rebuild two of the batteries with new NiMH cells eventually, but for now, I decided that as long as it is a 12 volt drill, it can run from a 12 volt battery pack, of which I have several. This battery case was fairly easy to convert. Some of them may be harder or easier than mine.
You will have to open it up and engineer it to hold the contact plates firmly in place with the wires attached. If you want to do this with a drill that is less than 12 volts, you will have to determine a way to drop the voltage to a safe level. Putting a resistor in line is the simplest way. If your drill is a higher voltage, I don't know of any simple way to provide a higher voltage.
At every step of the wiring, it is a good idea to check the wiring with a continuity tester or ohmmeter to determine that the connections are complete and that there are no opens or shorts.
Before chopping, cutting, drilling, examine the components to determine the best path for the wires. It is a good idea to mark the ends of the base and top of the case to prevent confusion.
If you have problems with my instructions, drill a hole through your hand, set your hair on fire or burn down your house, all correspondence regarding these issues should be made to my attorney. You will be promptly awarded 16.4 million dollars after sending the appropriate handling fees to his PO Box in Nigeria.)

Step 1: Parts and Tools

Parts List
Main Components:
- One old battery pack
- One cigarette lighter plug and cord, the heavier the better. Mine looks like about 14 gauge and holds a replaceable fuse in the plug. The cord

is about seven feet long. Try to find a cord that is long enough to comfortable reach the battery pack from where you will be working.
- Short length of rubber hose for strain relief that the cord will fit snugly through, about four inches.
- Fuse - fuse size is not critical. It is mainly to prevent a short circuit from causing a fire or meltdown. Probably a 5 amp would work. If it blows,

go larger.
- inline fuse holder (if your cord does not have one built in.)
- 2 or 3 tie wraps to secure the cord and strain relief


Tools:
- electrical tester for checking polarity, voltage, continuity.
- Soldering iron
- Screwdriver
- Diagonal cutters
- Drill

Step 2: Prepare Battery Case

Determine the polarity of the contact plates. Fortunately mine was marked on the outside of the case. If your battery pack has any capacity at all, you can read the polarity with an electrical tester. If you get it wrong, it will probably still work but the direction switch will work backward. Worse case is if there is a diode that prevents it from working at all. It is a good idea to mark the polarity with tape or a sharpie to prevent confusion. (I am an expert on confusion and can supply references)
Open up one of the batteries and extract the cell assembly. Clip off the lower cells that fill up the main body. Throw them away. No, don't throw them away. Rechargeable and button batteries should be disposed at authorized battery collection sites. Take them down to your local Radio Shack and make them throw them away.
Remove the thermal sensor, the small metal thing tucked between the cells that is wired to the third contact plate(or leave it in if it isn't in the way). This leaves two dead cells, that hopefully are not too nasty and corroded, that fit into the "tower" that have the contact plates attached. If your cells are not usable, or the case is a different shape, you might be able to carve a block of wood that fits the case and can hold the contact plates. Silicone caulk, Friendly Plastic or Fimo might also work.

Drill a hole that the hose will fit through in the base of the case. Thread the wire into the hose and thread both through the hole. A knot can be tied in the cord, or tie wraps added to be tightened later to prevent the cord from pulling out and straining the soldered connections.

Step 3: Wiring

Solder the wires to the remaining cell assembly, either to the connection wires, contact plates, or the body of the cells (Not really desirable if you can avoid it. If you do this, be advised that applying high heat to a battery cell may have unpredictable results. At least wear protective glasses and clothes. Performing work using electrical tools naked is generally not a good idea. Same with cooking bacon.)

Step 4: Preparing Case

Make a spacer block that will fit snugly in the space left by the removed cells to hold the remaining cell/contact assembly firmly in the tower. I made mine from assorted blocks of wood with a tunnel carved in one to allow passage of one wire to the other side. This can be made from a single block of wood, friendly plastic, paper mache, etc. Just be sure it is hard enough to resist the pressure of inserting the case.

Step 5: Final Assembly

Pull the wire out through the hose and tighten tie wraps. Assemble parts, carefully juggling the block, wires and screws. Be sure screws don't poke into the wires. As you put all the parts together , Close case and screw together.

(note: There is no next step. Do not click on that button. Don't say I didn't warn you.)