Starting off with the recommendation for a type (not necessarily the seller, I didn't buy from this one):
6.5V-16V 99 Gears Adjustable Spot Welder
This has been the best I've tested so far. I am using with a 16AH SPIM08HP 3S pack. With that pack I have successfully welded .2 quality nickel strips and offset .15 strips for 21700 cells. It can get hot after a large number of welds. I plan to upgrade the weld pens / leads and add a better heatsink / fan. My voltage sag on that pack is low enough that I haven't done it, but you can easily provide controller power to it via a separate power supply to ensure proper MOSFET operation in the event of excessive voltage sag. The UI is a bit odd, but it will provide 3 separate pulses and you can adjust the timing between them and the power level of each pulse.
This particular type has a small OLED display, not the segmented LED display. Note that the unit I purchased is one of the newer board revisions which has 6 MOSFETs on each side of the PCB. The older style had all 12 on the front side of the PCB, which would make cooling easier for sure. There is also another style that looks similar, but it has a much larger color screen and a dial interface. I'm interested in testing one of those, but I really don't have any reason to spend the money on it right now. Plus, the only listings I've seen only have 5 MOSFETs and likely can't handle as much current. Funny thing is the pads are on the back of the board- it should be possible to add them if you can get the right ones.
Why am I qualified to recommend this? Well, I'm not, but I have tried lots of different spot welders.
I went through a couple of different cheap spot welders, first an AC triac based unit, because of my own stupidity it died. I then tried one like A-DamW linked to- it still powers on, but some of the FETs have blown. I also purchased a MiniSpotA from Lee a few years ago, it works but not as strong as I'd like, it gets hot quick and my Lipo battey started to swell. Plus, the form factor isn't great to be honest, pretty awkward to use.
I then bought a SeeSii 11000mAh from Amazon, it seemed great with my initial testing. I let it sit in the box for a few months and it self-discharged. Since then, after fully charging, it will power on, but has no-where near the spot-welding power I need, it will barely weld .10 nickel.
Yes, I've spent a fair amount over several years this way, but at the start I never could have justifed the expense of something like KWeld and I've learned a lot along the way. I understand kWeld is the "spend good money on good product" solution. But claiming that is the one and only valid choice is anti-thetical to this very forum. Imagine if anytime someone wanted to build their own e-bike the answer was always "go to the Bike shop and buy a Trek or Juiced". No shade against kWeld, at some point if I had more serious spot-welding needs, I might purchase one, but seems they've been resting on their (expensive) laurels for quite some time now.
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