I just built a pack with a copper nickel sanwich. Maximum copper thickness I was able to spotweld with kWeld was 0.15mm copper but this needs a lot of energy with kweld (around 150J). I used 0.1mm copper with 0.1mm Nickel on top. Welds with 80J with kweld. After 30 welds the electrodes got too hot and I had to make a break.
The pack works well so far.