See How One Maker Combined an Old Microwave and Some Extra Wood to Create a DIY Spot Welder
This cool (and potentially dangerous) spot welder uses parts from an old microwave oven and an Arduino to attach batteries together.
Assembling battery packs
Rechargeable battery packs almost never consists of a single, high-voltage cell, but rather many cells that are arranged in a particular series/parallel configuration that yields the desired voltage and current rating. But the process of creating one is tough since many nickel strips have to be tacked in place, which is usually accomplished by using battery spot welders that consume a large amount of current to melt the bonding material and adhere the strips. Robert Dunn, who runs the Under Dunn YouTube channel, wanted to make his own battery pack while also saving money by creating his own DIY battery spot welder with just a few components.
Gathering the hardware
The material for this project were quite cheap and common, including an old microwave oven, a piece of copper, some 6-guage wire, and an Arduino Uno with a solid-state relay attached. The Uno controls the alternating current flowing to the microwave's transformer which then steps down the voltage dramatically and sends it to a pair of copper leads that create a very hot arc.
A few modifications
Perhaps the most apparent issue with the microwave's transformer is that it normally takes in 120V AC and steps it up to around 1kV, and due to energy conversion laws, the current decreases in an inversely proportional manner. So because the spot welder need more current, Dunn swapped the usual input and output sides so that incoming 120V gets stepped down to around 2V, thus increasing the current dramatically to nearly 600 amps. This also meant he had to replace the smaller coil with a few turns of 6-guage wire which would also have the copper electrodes attached to their ends.
Creating some circuitry
Dunn was not simply content with pushing a button every time he wanted to weld something due to a need for precise timing. Therefore, he used an Arduino Uno that waits for a red pushbutton switch to be depressed which then activates the solid-state relay for a certain amount of time. The delay can be adjusted by rotating a potentiometer, and the value in milliseconds is displayed on a seven-segment LED module.
What can it do?
After taking this circuit from its breadboard orientation and soldering it together, Dunn cut a few pieces of wood paneling and glued them to form a small enclosure that has openings for the potentiometer, 120V AC input, 2.2V welding output, the seven-segment module, and the button, he plugged in the device and was able to generate a 300ms-long arc.
In the near future he plans to go through the numerous battery cells he bought and tack them into a much larger battery pack for a fun project.