The first test of the welder on the separated cells was disappointing to say the least.
Some testing with my storage scope clearly showed that the big SCR was dissipating most of the energy. The SCR was designed to switch about 800VDC at 1200 A,which is nearly a megawatt, but the on resistance is fairly high compared to the nearly dead short of the weld zone.
I did some internet research, and found the Sunstone Engineering site where they sell some nice dual pulse spotwelders.
Sunstone Engineering
I called them and spoke to the owner/ chief engineer who offered to do some test welds for me.
I referred him to this blog so he could see where I was heading, and what had been done to date.

The test welds which were done with 630WS and a 1/8" electrode looked as good as the factory welds, so I was seriously considering buying one of their machines so I could get back to building the rest of the cell testing and weld fixturing.
Unfortunately when I asked for a quote, they asked me to sign a noncompete and non disclosure agreement, which I would do if I got one of their welders, but they never got me a quote, so it looks like I will have to go back to plan A, and build my own.