Welcome fellow slacker. If I had this to do over I would not do it. It cuts well yes, but the battery only last a matter of minutes...

Someone gave me a cordless circular saw with a dead battery and no charger. So I could pay $150 for a new battery and charger, or I could take the battery from my Mikita drill that would work fine and run it off that. You'll need:

wire strippers
soldering iron
screw/drivers of several types
cutters (dikes)
$9 auto-ranging probe I love you Harbor Freight
wire
solder
duct tape
beer or perhaps the kindest kind

Step 1: Will It Work?

Well the battery for the Dewald is 18 volt and 2200 milla amps (1/1000 of an amp) or 2.2 amps. So volts times amps is 36 (or so) watts. The Mikita is 14.4 volts and 2 amps. 28 watts, ya it will probably be a little under powered maybe not spin as fast? I don't care, but I should have. The battery dies after just a few minutes.

I am not happy with this at all. The battery only last a few minutes. I am going to put a cord on it. A 12v 1.5 amp ac adapter barely spins it. The battery charger senses it not hooked up to a battery and does nothing. Maybe I could I could increase amps by putting several smaller transformers in parallel?

Dewalt is no help at all "This saw is designed to run using DEWALT 18v batteries, we do not recommend using any other power source for this saw. We do not have any other motor specifications for this unit." Liar...oops did I say that out loud?

Step 2: Determine Polarity

Get out your meter and see which is positive and which is negative on the battery. Write it on them with a marker.

Step 3: Take the Donor Battery Apart

My Dewalt donor battery has star screws in it. Remove the contact piece/plug inard out of the battery, and remove any wires and/or attaching foil that held the old battery together.

Step 4: Connect the Plug

I used about 6" of the cord off an old AC adapter. Strip and solder it to the donor battery contact plug.

Step 5: Make a Battery Connector

You can see in the drill that the Mikita battery makes contact with metal strips that slide along the side to make contact. I cut some of the thick foil from inside the donor battery to fit. It also has 2 other contact maybe temp sense for charging?

Step 6: Solder the Battery Connectors

I punched a small hole in the contact strip and bent it . An alligator clip also works good. I'm sure with tape and solder you can figure something out. Connect your contact to the battery.

Step 7: Put It Together

Insert your dummy plug/donor battery piece in the tool. I shoved a waded of duct tape behind the contact piece to hold it against the contacts of the saw.

Step 8: Done, Sha-shaw, It Works

Sure I could have used the rest of the donor battery case and made something that would clip in, but that would have made the saw longer and more awkward and not as well balanced. I couldn't insert it, or make a backward clip in because the Mikita battery was fatter than the original Dewalt.

Plenty of duct tape so you don't short it on something.

The saw runs great and cut 3/8 plywood well. It will stall if you try to start the cut too fast but once you're moving it cuts nicely.

I am not happy with this at all. The battery only last a few minutes. I am going to put a cord on it. A 12v 1.5 amp ac adapter barely spins it. The battery charger senses it not hooked up to a battery and does nothing. Maybe I could I could increase amps by putting several smaller transformers in parallel?

Dewalt is no help at all "This saw is designed to run using DEWALT 18v batteries, we do not recommend using any other power source for this saw. We do not have any other motor specifications for this unit."