I have a 250ft spool of this and I love it! It solders to copper and nickel like a dream. It's very easy to work with but not too soft. The picture in the link isn't 12GA wire but I can confirm that what they send actually is. I made 4 of the packs in the attached picture, soldered 2 - 12GA stranded (pre tinned also) on each side, connected 4 together in series with xt90 connectors, and successfully pulled 200amps for 15 seconds with a carbon pile load tester. The connectors, wiring, copper and nickel weren't even warm. the batteries were warm and they stayed warm for about 15 minutes or so. I'm very happy with this setup so far and I just wanted to share this awesome material.
https://www.amazon.com/gp/product/B0051X507O/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B0051X507O/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1