I kinda enjoy failure analysis so here’s my take:
Everyone setup is going to be different, but the kweld manual recommends 50J and I weld at 58J for .2 thickness nickel. So I’m going to call that insufficient.
PVC shrink is very brittle, it will tear pretty easily if there’s and cut or hole to start which may have been small and unnoticed then resulting in the full failure on this jump.
This is why having multiple spotwelds is important, because all the current is flowing through those dots. In this case it’s likely you had a single weld point, maybe two, still holding on to the cell carrying that entire load.
The nickel popping off in such a way proves to me that you didn’t have sufficient penetration on the spot welds. They did not vaporize, they were not actually there to begin with.
The cell has broken free, the higher voltage one that is. So it was not draining. You were pulling full current and rapidly draining the single cell that was still attached. You stopped just in time before it could have been a big issue.
None of these welds are sufficient, not even close really. The tiny dots left behind are only slightly better than the nothingness. You should have to tear the nickel off destroying the strip in a way that looks like wolverine removed it, they should never pop off by prying.
I have faced this issue myself when I was first experimenting with .2 nickel and with P42A specifically. Whatever alloy comp they use on the cans is more difficult to weld to, especially the negative side. This is why testing you welds is so important. I would assume you can remove every piece of nickel on that pack that easily and retire the packs until further repair or remediation can be done.