Replacement of NiCd Battery With External Power Supply

I paid $3 for this Black-Decker Firestorm drill on the local garage sale. The owner told me that the drill is OK, but the battery is dead (as expected for NiCd power source). I decided to bring it back to life with external power. It is not easy to find the exact 14.4 V supply, but 12V supplies are ubiquitous. I found one powerful enough AC-DC adapter (Belkin, 12 V, 6 A output) in e-waste bin at the office where I work. Would drill work with it? Make a sense to try. Apart from drill and adapter, I needed 2.1 mm barrel jack, which I bought online.

Updated: 8/18/2019. Later I have added to the project Toroid Core Inductor (Toroid Core Inductor) At the last step there will be an explanation.

First I detached the battery. Disassembling of battery unit was easy. I unscrewed four long crews, took off the battery housing top, and removed the battery itself.

The battery has no value and must be recycled, but electrical terminals I saved to use later in the project.

I made plywood insert to fit battery unit top. On the picture above you can see it with electrical terminals connected by wire to a barrel jack (soldering worked for both ends: terminals and jack contacts). In the unit top I made the hole to allow inserting of adapter terminal into the jack.

Putting the top with insert back on the unit and screwing it was a trivial job.

And here is the test. The drill works OK but not without the glitch. I have to turn it on slow enough, otherwise, it will not start properly. That acceptable tradeoff: for less than five dollars I got the workable and useful tool.

Still, I wanted to improve the project. First, there is a need to understand why the fast start is not possible. So I did the test measuring voltage during start. As you can see in the second video during the fast start voltage drops to zero. The only possible explanation is that while drill motor is not rotated yet current through the winding is really high and that forces power supply protection to kick in. In the case of slow start trigger (which actually does not just switch but the variable regulator) limits current till the motor rotates fast enough. After that back EMF limits current through the motor winding.

To fix the fast start problem I have added into the circuit small toroid inductor. After that, disregarding of the fact how fast I pull the trigger (as you can see on the video) voltage does not drop to zero and the motor does not stop. As the last step, I disassembled the terminal unit, soldered inductor between one of the barrel jack terminal and the terminal unit, and assembled the unit back. Now drill is fully ready for the job.

Some rumination regarding this topic you can find on my personal blog