Prevent a power bank welder from short circuiting
If you'll take a look at page 20 of this guide by a welder manufacture you'll see that 1/8 welding rods are recommended for 16 ga material, and recommended currents run from 70 to 115 amps.

This is for a welder with current control. In your case, current will be whatever you get given your lead resistances, and may well be either higher or inadequately low.

I really don't recommend doing what you want to do. You are almost certain to blow up your batteries.