We do lots of custom electrical work in the fab shop, and one job that seems to come up all the time is making custom battery cables. Trying to find ready-made cables that are exactly the right length is difficult at best, and damn near impossible for those extra-long cables when your battery has been moved to the trunk. Given the weight and cost of heavy gage copper cables we make our own, using SAE-rated battery cable, and heavy-duty cable lugs to get exactly the right length.
A failing battery cable is a big problem, so we both crimp and solder the lugs to the cable. The soldering is done after crimping so we drill a hole to feed in solder. We drill first because it’s easier to manage the lug without the heavy cable already crimped in place.
After stripping the insulation, and coating the exposed wire with flux, we insert the cable into the lug so that the insulation goes in about an 1/8″. I use these crimping dies in a vice with a 3 ft cheater bar. It’s a four handed operation to get everything lined up and the vice snug to begin the crimping operation, so a helper is important.
After crimping, it’s solder time. We use a butane mini-torch (an electric solder gun just can’t generate enough energy to heat that much mass), and heat the lug until we can push the solder into the wire inside the lug, making sure we get solid solder bond. It’s tempting to feed too much solder into the joint, but don’t: too much solder will wick up the wire past the joint and make the wire stiff, which can eventually cause failure. 4 or 5 inches of solder is about right.
The soldering operation partially melts the insulation at the edge of the lug. This causes no harm, and can be covered with some large diameter shrink wrap, trimmed with a sharp box cutter, or left as is. The metal discoloration is caused by the flux, and wipes up easily with some acetone or similar solvent.