Alternative to soldering - How is this done?

Ive seen this method in some batterypacks. Its not solder. It looks almost glued together, it does not react to the solering iron/heat. What is this method of attaching wire to nickel called?

I believe it was likely spot-welded with a large MOT. 2V and 700A

That one sort of has the look of ultrasonic welding. Like this one...

Thank you voltron, you seem to be right on point.

I now found this video on the topic

[youtube]yLGiY6Ehofo[/youtube]

This is something that might work in large factories but i think i'll stick to soldering for now.

Before soldering I like to punch a few holes in the strips. Simply use an awl to punch a small hole in your strips using some soft wood underneath, flip it over repeat on the back side flatten your strip out with a hammer or whatever tool.
Now solder both sides and make sure it the solder sucks up the into the holes. Creates a very strong connection and more solder is in contact with the surface of the strip. I like to pool some solder on the backside. If the strips are doubled up which I usually do with the terminating ones, this is also a way to join them up if they extend beyond the cells.
I make the strips and the soldered leads up first then weld them down.