Step 1: Exterior Comparison

The Chicago Electric battery from Harbor Freight is similar in size and shape to the Porter Cable battery. The connectors are completely different, of course.

Step 2: Disassembly

The Porter Cable case is held together with six T10 screws. The Chicago Electric case uses four #1 phillips screws. Inside we find that each battery contains three rows of five sub-C NiCad cells wired to a connector on top. The Porter Cable cells are shrink wrapped, while the Chicago Electric cells are wrapped with a strip of cellophane tape.

Step 3: Temperature Sensors

The Chicago Electric battery has a thermal switch taped to one of the rear cells. The Porter Cable battery has a thermistor under the connector that sticks down into the middle of the pack through a hole in the insulator. Although it wouldn't be used, I left the Chicago Electric pack's thermal switch in place, disconnected, since I had plenty of room for it in my Porter Cable battery case.

Step 4: Cell Configuration

Both packs are 15 cells in series, but the Porter Cable in the foreground taps the ends of the series string in the middle of the pack, while the Chicago Electric battery's connections are on the last two cells in back. The packs have their positive and negative connections on opposite sides, so the wiring will have to cross sides for the Chicago pack to work in the Porter Cable case.

Step 5: Plenty of Room in Here

Fortunately Porter Cable was generous with space in their battery case. The cells are suspended above the bottom of the case by a knob of plastic under each one, and that gives us the room we need to run our wires underneath. There's also room around the perimeter of the case which I used to bring the wires around to the connection points on the pack, as you'll see in the next step.

Step 6: Wiring the Replacement Pack

Desolder the connector and the wires from the Chicago Electric pack, and poke a pair of holes in the insulators to pass the wires through from bottom to top to match the Porter Cable pack's wire location. Route the wires around the edges of the cells on the bottom so they won't get pinched between the cells and the little knobs that are molded into the lower case. Wrap the wires around the front of the pack and over to the opposite sides where you'll solder them to the tabs on the two end cells. Put the pack in the lower case and pack the wires into the empty space so that the top will go on. A test fit at this stage is probably a good idea. On my battery, I used brown for positive and blue for negative, and I stuck the silver insulator from the Porter Cable pack on top.

Step 7: Attach the Connector

The wires are welded to the connector terminals on the Porter Cable battery, so you'll need to cut those free. Slide the positive terminal out of the connector toward the rear and solder it to the positive wire from the new battery. Go easy on the solder so you don't make the wire too stiff to allow the connector block to seat. Slide the negative terminal up and out of the block and solder it to your negative wire, being careful with the other connections on this one. Put the block back together and don't forget to make a hole for the thermistor under the connector block. Put the top of the case on, fiddling with the wires as necessary to get everything to go into place, and secure it with the six screws.