WARNING Long post. Skip to the end and see the pretty pictures.Well... I did it. Not sure what I did exactly, but I did it.
Just updating this as some may be curious if I went up in a ball of flames
There was absolutely no question that this was a dangerous endeavor. Spot welding on 18650s is no joke, and you best be on your toes at all times. (more on this at the end)
As I stated earlier in this thread, I would not be attempting to use this setup unless it proved worthy/reliable. While it was not 100% reliable, after performing over 250 test welds on food can lids, I was fairly confident in my ability to make this work. Even so, I took extra precautions, just in case.
To me, the welds seemed pretty decent. As you can see in the photo, there is a slight bulge inside the base on the 18650 where the weld was a little too hot. After a lot of testing, I came to the conclusion that the "weld pressure" was more critical than the power of the weld alone. Pressing too hard while welding made the metal cave in, and made for a weak weld. Too little pressure also produced a weak weld, while also having the chance of "blow out".
Also, I found the the length of the wire (all of it.. primary, secondary, incoming power) had a dramatic affect on the welders output if changed at all. I am talking about inches of length difference, not feet. I must have been EXTREMELY lucky with the initial build of this welder, as it was nearly perfect.
What I ended up doing is going back to the original wiring and design, after attempting to improve it and failing every time.
I tried welding wire, was just a bit longer than my original wire, so I cut it to the same length. Even then, the fine strand welding wire did not perform as well as the large stranded wire I had used initially. Had to go back to my old wire to get enough power for a decent weld.
Then I built a foot switch, so I could use both hands, instead of having to hold both welding leads with one had while pressing the switch with the other.
Foot switch was nice, but I had a bad loss of power. And all of that loss through about 4ft of 10awg going to the MicroSwitch mounted on my foot switch, which was on the primary side of the welder. I was not expecting that big of a power loss from slightly longer high voltage primary wiring. If the foot switch was controlling power on the secondary side, sure I would have expected the losses.
During my testing of that foot switch (about 100 welds in) it decided to stick, and burned a hole in the can lid I was testing on. I was quick to remove the stuck welding lead to the material, but it had already made a large hole. All of this happened in under 1sec (something near 70-80ms I would guess). Check the last photo.
Dangerous? Very much YES. Imagine if that had been on one of the 18650s. Would have made for a very unpleasant evening.
Anyways, had to return to my original design to get decent welds again.
In the photo that shows the battery pack welded up, that is only the parallel ties. Each parallel string was also welded in series to the next string. That photo is about 20% completion on ONE pack. I made 3 packs in total.
During my welding, I only had one event where a weld got stuck and could have blown a hole in my pack. Still have no idea how that happened, but I was quick to break the lead loose when it stuck. Beyond that, no other issues came up besides some of the welds not being consistent, which I just went back over and redid after letting them cool a bit.
Overall, I accomplished my goal. Nearly 2200 welds, and each one manually controlled with a press of my finger. Sketchy is putting it lightly...
Packs are all finished up now and work just fine. No damage to any of the cells, and my recharge of each pack is actually yielding more capacity than what the spec sheet for these cells show. Which is amazing considering all I put the poor things through.