I like battery packs, they are handy, rechargeable and have better output characteristics but there is one small problem: It, s not the best idea to solder them. You will shorten their lifespan or even kill the battery packs completely!
The alternative is spot welding. There, you apply a short but high power pulse to a condictive metal like silver. At the two points, where you press the anode and cathode on will be so much heat, that the points will melt a little bit and produce a strong connection. The best thing is, that the terminal of the battery will not heat up.
CD Spot Welding
CD spot welding is nearly the same as the normal spot welding but with one difference: The power source is not the mains power grid, but capacitors in my case, I used 4 supercaps with each a capacaty of 50 farad in parallel. This equals around 1000W of power when fully charged.
My Design of a CD Spot Welder
I use an Arduino micro as the brain and a Mosfet as a high power switch. The LCD is for showing the pulse time. I also programmed a lockphase, so that the caps and the Mosfet can cool down after every pulse.