Its better to solder. Use 60/40 on stuff like that, and there is a noticeable quality difference between the cheap stuff and decent stuff, so stay away from Harbor Freight for the solder and wire. I personally like the insulator on the painless wiring kit for battery cables.
You have a lot more to worry about than the connection coming loose if the cable gets hot enough to melt solder, I couldn't give you an exact temperature, but it would be more than hot enough to burn even a good insulator.
|