DIY Spot Welding machine for battery packs

Introduction

NE555

Transformer

Theory: NE555 based

Idea

The 555 timer circuit controls MOSFETs that deliver short, high-current pulses for welding nickel strips. The 555 shall work in monostable mode.

The MOSFETs (IRLB4132 or IRF3205) handle the high welding current.

Kits

99 Gears 12V Spot Welder Kit

• Problem: Blowing up.
  • Not enough amperes from the battery. If not enough current for the mosfets, those will blow.
  • Medium (20–40Ah) is the sweet spot
  • "The destruction of the MOSFETs is likely to be caused by DROP of the Gate Drive, if the the battery voltage drops. That is where the Cranking Amperes come to play. If the weld location resistance is very low, with near zero drop there, more of the drop happens inside the battery and in general before the point where the gate drive voltage is picked."
  • Nickel thickness of 0.1 - 0.2 mm: 120 - 180 Amps.
• Problem: Welding takes a lot of practice.
  • Resistance is the key. Push just hard enough to get a contact, and not with perpendicular probes.

Connect battery → almost short circuit → let physics decide the current”. The current is roughly

$I={\frac {U}{R_{\text{internal}}+R_{\text{wires}}+R_{\text{contact}}}}$

Thus

• Lower resistance: massive current spike
• Higher resistance: weak weld
• High CCA (cranking current): low internal resistance → higher weld current

and bigger batteries usually have lower internal resistance.

A big car battery has very low internal resistance (~milliohms) and can deliver 1000+ amps instantly. My welder's cheap electronics and limited current handling causes that current spike exceeds MOSFET limits and they fail explosively.

References

Some references related to 555 based

https://hackaday.io/project/204777-diy-battery-spot-welder-build-your-own-power-pack/details

https://www.instructables.com/DIY-Battery-Spot-Welder-Build-Your-Own-Power-Pack-/

pcbway.com/project/shareproject/