YouTuber Robert Dunn (known as Under Dunn) had just received a large box of loose cylindrical battery cells and therefore wanted to join them together to create a large battery pack. Ordinarily, this is accomplished by utilizing a specialized type of tool called a battery spot welder that is used to tack nickel strips onto the terminals. But Dunn didn’t want to spend the money on a new one, so he built his own DIY version using only an old microwave, an offcut of copper, some wire, and an Arduino Uno with a solid-state relay attached to control the current discharge timings.

Dunn began by extracting the transformer from the microwave and cutting it open to expose the coils within. As a quick refresher, transformers step voltage up or down by using a pair of opposing coils inside of a magnetic field, which also causes the current to increase or decrease in an inversely proportional manner. Because Dunn wanted to maximize the current, he replaced the smaller coil with an even smaller one made from six-guage wire that could handle the extremely high current. 

An Arduino was used in conjunction with a solid-state relay module to pulse the current going into the transformer. Its circuit also contained a seven-segment LED module that showed the number of milliseconds the transformer would be on, as well as a red button for activating it. Timing could be adjusted with a rotary potentiometer. 

Once everything was wired up and placed into a wooden enclosure, Dunn tested out his DIY battery spot welder by pressing the button, and it worked as expected.