MOT Spot Welder for 18650 Li-ION batteries

Recently I followed an instructables on building my own MOT spot welder, and the build went great. I am getting power from a normal home outlet, which is 120V and 15A, and I am getting out around 2 volts, so around 900A for the output of my second winding. I used a large gauged stranded wire for the second winding, and at the end of it, tied so smaller gauge solid wire that is around 2 mm thick as the soldering tips. I started out using it and it was going alright with decent and clean welds, and an occasional spark, with the nickel plate burned with a hole through by one of the copper tips. Now, everytime I try and weld with it at least one of the tips makes a spark, the surrounding weld has a black burn mark, and the nickel strip is burned straight through making a hole, so nothing ends up sticking to the battery well. I have tried reshaping the tips I am using and re-positioning them in so many different ways, but cant get a clean weld anymore. I have also tried changing the pulse time from 10-30 ms with the arduino im using as a timer, but shorter or longer pulses does not prevent the sparks and burns from happening. Any one have any idea why this could be happening and if I just got lucky with the weld I did on roughly 30 batteries?

Without current control its very difficult to get good welds. Try to decrease time or get some control module.

My understanding is, the Kweld is the best, for that exact feature: https://endless-sphere.com/forums/viewtopic.php?t=89039

I have tried decreasing the time and when I do, sometimes the nickel plates end up just sticking to the copper welding tips but dont adhere to the battery terminals. I have seen simple builds that do not require more controllers and would prefer to just use the arduino time and relay system to control the pulses, so do you have any suggestions for what I can change about the wiring or tips or timing that may help? Or is there anything I could do to lower the current coming out of the welder? Thanks

Try to use thicker electrodes. Also increase pressure. You can buy cheap chinese controller for MOT spot welder.
I use this and works great on 230V input. https://www.aliexpress.com/item/32873812813.html?spm=a2g0s.9042311.0.0.27424c4dzNOrsE

I also recommend thicker gauge wire for the probes. I use 6 gauge pure copper welding cables. The thicker wire allows to lower the power setting and get better welds. Right now you probably have the setting too high and its just not giving you good welds. On some of the pictures of MOT welders I see thick gauge on the probes.

When I was using thing 8 gauge copper clad cable I was getting bad welds even at the higher setting I had to use. I was also using solid wire copper as probe tips but they aren't rigid enough.

6 gauge pure copper welding cables, and using copper nails for the probe tips, this work excellent at lower power settings.


If you can put a picture of your welder tips.

I just tested my bigger transformer (1750VA) and its working just fine.
I tested it on pure nickel 0.1 and 0.15 and welds are very good. I dont have thicker nickel right now but will test in the future.
This transformer should be enough for 0.25 nickel IMO - maybe 0.3.
Welds are clear and strong. I managed to weld fuse copper wire without problems. All this welds are made on 30-40% power settings with 20ms welding time (cant go lower with this chinese control board).

BTW can someone explain how to add picture here? I only managed to do it with bbc code from upload website.