The main battery cables for outboard motors are a constant source of problems. If the battery and cables are not secured in the boat properly they move around and the constant bending leads to a failure of the insulation where the cable enters the lug. Once this insulation cracks the moisture gets into the wire and starts to corrode it. As you can see in this photo, the end of this cable broke right off when I bent it.
To repair this cable I cut several inches off of the end to get back to a less corroded portion of the wire. I then used a wire brush to clean the wire up in preperation for soldering.
I use a large punch with a rounded end to crimp the new ends on the cables. You must use a large hammer and a solid surface to pound on. A properly made crimp will produce enough pressure to bond the metal parts together (like a cold weld). To do it properly requires a rather expensive tool with dies made specifically for each size fitting. A hammer and punch comes close.
To ensure good electrical contact I solder the terminals after crimping them. Remember to put some flux on the wire before you start. It takes a lot of heat to make a good solder joint this size. I use a propane torch for this.
To make a lasting connection you need to keep the moisture out. I use liquid electrical tape to seal all the connections. Simply brush it on and wait a few minutes for it to dry.
The last step is to put a few wraps of tape around each fitting. This protects the seal underneath and also makes a simple strain relief to prevent the wire from bending right at the terminal.