Car battery spot welder. How to reduce battery power?
12 volts is WAY TOO HIGH!

I once worked for a firm (RADCO in Toledo, Ohio) that built and refurbished resistance welding equipment. The voltage at the tips was less than 2 volts, but the current was very high. I recall one of the transformer's secondary rating was about 1.6 volts and 100,000 amps!

Forget about using a battery, friend.