Spot Welder Choice
The spot welder used last year for MSXII was a SUNKKO 787, a very underpowered cheap Chinese unit that produces lots of sparks while spot welding. We also acquired a SUNKKO 709A spot welder, capable of 500A for the welds, which is good for up to 0.2mm pure nickel strips.
K-Weld ~$250
https://www.keenlab.de/index.php/product-category/kspot-welder-kit/
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The K-Weld is essentially a commercial spot welder with a hobbyist spot welder price. It has been extensively tested, and is able to produce consistent, solid welds on up to 0.3mm pure nickels strips. Being able to use thicker nickel strips will mean less resistance and thus less heat loss. The Arduino Battery Spot Welder is a reasonable option as well, but for not much of a price jump, the K-Weld provides much better technology to produce consistent welds.
Edit:
I have become aware of a few other spot welders since creating this page, and wanted to link them here for future consideration if we ever need a new spot welder.
BOSS Welder ~ $120
https://endless-sphere.com/forums/viewtopic.php?t=89076
This welder by Aulakaria is super small, though seems to require an external fan and heatsink for the FETs. Powered with XT90 connectors from a 3S LiPo similar to the kWeld. Only does time controlled welding as far as I know. Claims to be able to weld 0.1 and 0.2mm copper with multiple pulses. Apparently someone also used the design to make a super tiny spot welder. Also has an output to power a soldering iron. Great info on the thread as well about spot welding in general and electroplating copper. Because this still uses a LiPo and common FETs, the current would still be roughly what the kWeld delivers.
Ian Hooper Welder ~ $100-300?
https://www.zeva.com.au/Projects/SpotWelderV2/
A spot welder project for personal use that I found recently. Some good discussion on the design in the build log and the comments. Gerber files are available on the site, and parts are not crazy expensive. Per some quick math in the comments, this seems to be capable of around 2000A.
Edit 2:
There's a lot more welders that have popped up recently. One that I am more familiar with is Julien Lemay's spot welder pen. Still the same timed pulse method, but putting it here for completeness.
Its also not too hard to design your own, so that's an option in the future as well (at least for an experienced electrical engineer).
There was another one I saw that used 72 N-Channel TO-220 FETs connected to a 2S3P stack of LiPos that I believe came from a car pack.