Joining high current wires
You will need some crimp-on connector like these.

Use the right crimping tool! I insist on that. Hexagonal crimp is the way to go. As you can see in the image below, the result is basically a solid mass of copper, all cold-welded together. Really tough, low-resistance, this is what you want.



More info here on other crimping styles which are much easier to screw up.