There are several types of spot welders, but the best for spot welding batteries is the Capacitive Discharge welder.
The advantages are repeatable energy delivery,and minimum heat effected zone.
A nice production unit is about $6-12K with the weld head.
Since I don't have that kind of money, I have to try building my own.
I did some scrounging in my extensive junk pile, and came up with some possible components.
I found 700,000 Uf of 15V or better capacitors. I mounted them so the lead length would be as short as possible, but did not have the 1"X 1/4" copper bus bars that I would have preferred to use, but the 1/2" X 1/4 that I had should be good enough for the prototype testing.
I got the huge hockey puck SCR to work, and found that it is one of the rare GTO or Gate turnoff types, which means I could pulse the discharge without totally discharging the caps like a regular SCR would.
I spoke with PowerX, but they could not find the part number so I really don't know much about the device except that it is huge, and that it worked when I tested it by firing a smaller cap into a resistor.
I also dug up a small spot welding head.
Not the components I would have bought for the machine, but hopefully it will be close enough to see some welds and get a starting point.
The buss bars are too small, The caps are not big enough, and the weld head is kinda small