Just had a scary moment building my monster 2.5KW pack for my Stealth Fighter.
I had turned the pot up pretty high and getting some decent strong welds, probably a bit on the strong side, but not too extreme. I was on my first layer of nickel which is possibly where I went wrong turning it up this high.
Anyway, I welded the negative terminal of an 18650 and it instantly sizzled (but only for a fraction of a second) with some clear liquid coming out of where I had just welded. At first I thought it could just be some left over alcohol from the cleaning I do with isopropyl alcohol wipes just prior to welding, but after wiping it away, more reappeared and I could see it very slowly seeping out from underneath the point I had just welded!
After wiping it away a few more times, and smelling it, I realised it was the batteries electrolyte leaking out
The cell wasn't getting hot and it was one row in from the edge, so I carefully, but quickly cut the already connected nickel and pulled apart just enough cells to pull that one out. Replaced it now and re-welded in some more nickel and everything seems ok.
Anyway, I hope that serves as a warning to all, not to crank up the power too high with your spot welder just cause you can. I think especially when welding the first nickel layer that directly contacts the cell and especially on the negative side, you want to only give it the power it needs...nothing more.