$20 mini spot welder machine welds thin steel strips on battery packs

When I think about welding machines, I imagine fairly large units, protective gloves and masks, plus sparks coming out of the welding process when two metal parts are melted together.

So when I saw a small “portable DIY mini spot welder machine” going for $25 on Banggood[update: or just under $20 shipped on Aliexpress], I thought to myself: “what is THAT”? It turns out it’s made for welding jobs with thin nickel-plated steel strips such as the ones that can be found attached to 18650 batteries, or full battery packs.

portable mini spot welder

Mini spot welder specifications:

  • Controlled with by unnamed MCU
  • Display – LCD with welding parameters:
    • WELD ENERGY – 01-99 adjustable by up and down keys
    • INPUT – Display input power voltage
    • WELD WAY – Manual / Auto (adjustable with “square label” button)
  • Based on 5x 300A MOSFETs
  • 30cm 10AWG cables for welding pens
  • 25V 10000uF high-frequency low-resistance capacitor to increase welding stability.
  • Misc – Power switch, buzzer
  • Power Supply – 12V via DC jack or car/lithium batteries through 20cm 10AWG input cables

The kit is designed for welding nickel-plated steel strips from 0.1mm to 0.15mm and includes the controller, two welding pens, two power input cables, and a black plug. The best way to understand how it works is to check a demo on YouTube. The one below shows the user welding a metal strip to a 18650 battery.

Note the reviewer above may not have welded the strip in an optimal manner, as one commenter explains:

The two pens must be placed vertically, perpendicular to the plate, on the surface, because then it transfers the strongest energy due to the size of the surface! This is also written by the Chinese manufacturer! Regards from Hungary, spot welder.

It also feels like an electric shock hazard, but I suppose people doing will be extra careful. It’s also sold on Amazon, and some of the user reviews there indicate it can easily be destroyed by short circuit:

After receiving this spot welder i connected it and tried a few spot welds. Device worked correctly at level 20 and level 30. I then set it to level 40 and tried another weld. As soon as i touched the probes to the metal there was a huge explosive spark. Now thats all it does. It appears that the device now provides a direct short circuit to the battery. Apparently,either the mosfets have shorted out or the control circuit is stuck in a fully on state.

Another one reported a similar issue on Banggood,

Worked for 2 spots, third spot, stuck on melted strip. Didn’t shut off and started smoking, disconnected power. Was on auto mode with power set at 20, running off of a 12 volt 7 amp hr battery. Now it pumps full amps continuously, touch 1 prone to metal strip then when you touch the second probe it throws a huge spark and melts / burns through strip / blows a hole right through. In safe to have around in this condition.

but apparently, there are solutions:

The problem is the MOSFETs are not balanced and the power goes through one and overloads, this circuit needs protection,some solutions on YouTube.

This looks to be due to a voltage drop and can be solved with a 470-1000uF 16V capacitor as explained in the video below.

He also explains that if you blow up your MOSFET they can be replaced with Infineon IRL40SC228CT MOSFETs (0.5mOhm 1440A pulsed).

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