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Yet another spot welding thread
I've watched tons of videos, and made a few attempts at a spot welder for my 18650s.  So far, I have failed.  What I have also failed at is getting what I believe is good information since everyone who makes a video is an expert who claims to know all the things about all the things and their way is the best way, and that other guy is a crackpot who is going to kill someone...  but I digress

So, this guy made a spot welder with three 9V batteries in series, and one 4700µF 25V capacitor.  Look at him go, it's amazing!  Well, it's amazing at spot welding tin foil to other tin foil, and sometimes burning holes in the tin foil, but, from this I could figure out some basic information.

He made a thing that sparked and sorta fused things with much less Joules than other videos claimed were required.  Essentially, if I have figured this out correctly:

  • Power in = 27V
  • Max voltage of capacitor = 25V
  • Total Farad of capacitor = .0047F
  • Max charge of capacitor at 25V = 117.5m Coulombs
  • Max energy of capacitor at 25V = 1.469 Joules

One discussion I saw included the phrase "“good spot welder must be 200 Joules”, so naturally, now I figured it really didn't need to be quite that high.  I looked around and pieced together what I could to get some real world information.

What did I have?
  • 3 - 50V 4700µF DC Capacitors
  • 12 - 50V 220µF DC Capacitors
  • 1 - old AC\DC power thing from some old thing (output: DC 30V 400mA 12W)
  • 1 - DPDT (on-off-on) switch
  • 1 - awesome button that was red and looked dangerous (momentary)
  • 8 AWG wire (3 conductor romex)
  • a "can do" attitude
  • Not enough information to know if this was a good idea or not
So I gave it a shot...  I put all 15 capacitors in parallel.  I cut about 4 inches of the romex, removed the center conductor, exposed about a cm of the remaining two conductors, and some other mad science to end up with what I thought would be a good pokie\weldie thing.  Did some crazy things with the switch so I could charge the system in one position, set it to OFF in the center position, and enable the red button in the other last position.  Once the button was pressed in, the capacitors could discharge through the "probe" thing.
  • Power in = 30V
  • Max voltage of capacitors = 50V
  • Total Farad of all 15 capacitors in parallel = .01674F
  • Max charge of capacitor at 30V = 502.2m Coulombs
  • Max energy of capacitor at 30V = 7.533 Joules
This thing was theoretically able to deliver 5 times the energy of that tin foil burning thing.  Surely it would do a reasonable job with nickle strips on 18650s?!  It seemed like it at first, but then I noticed how weak the welds were.  I tried with both probes on the strip, pressing lightly down (parallel weld).  I tried with one probe on the battery and the other probe pressing on the strip (step weld).  I tried a whole bunch of things and just couldn't make it work better.

Then I stumbled upon an AC capacitor, specifically a 97F8053 (240VAC 120µF).  Did some math and figured this thing alone should be reasonably able to deliver about .864 Joules at 120V.  So I did the most logical thing I could, put spade connectors on my probe thing, put spade connectors on an AC power cord, plugged them both into this capacitor and plugged it in and let it charge.  I was smart enough to unplug it before attempting to weld with it, but it was still terrifying.  Why was this thing better than the other one I made, and tons better than the tin foil welder?  At less than 1 Joule, it should have been less effective, but nope.

So, essentially, I haven't learned anything concrete yet.  Here are my questions that I hope others can help clear up a bit:

How many Joules are required to spot weld a nickle strip to an 18650?

Should the weld be a parallel weld or a step weld?

How hard should I be pressing down with the probe?

If I use a group of capacitors, they should be wired in parallel, correct?

About how long do you think it should take to charge 15 capacitors in parallel with a 30V power supply?

As always, thank you for your feedback
My opinion is like @$$holes...everybody has one, and they all stink.

That being said, my experience is in research concerning ebike battery packs. They are small-size and high-amps per cell. That being said, the packs around here are typically large size and low-amps per cell.

When it comes to spot-welding, the materials concerned only care about melting together to make a physical connection by heat. The volts and amps are mere details. The volts used are not a trivial detail, is the amps that create most of the heat. The total watts/joules can be debated, but my research indicates that 8V (approximately) is ideal. Adjusting the amps is the key to success.

Who read this thread?
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