Hi Mechatroniac,

Having accus on hand with different voltage sounds good.

Almost like Ken, I don`t think soldering destroys the accu as long as you have a large soldering iron, why not building a DIY Spot Welding Machine?
I watched some videos in the past like:

With parts of an old Microwave: https://youtu.be/vStYS6eoscU?t=184

or a bit more risky, just a battery. Risky as it has no on/off switch. https://www.youtube.com/watch?v=yYivIldvS6o

Cheers
Reto