The essence of soldering is:
1) clean surfaces
2) all parts heated above the melting point of the solder (regardless if it's by flame or electric soldering iron) but not hot enough to cause flash-corrosion (heat-discoloration)
3) the correct solder & flux for the surfaces involved (rosin-core for Copper/electrical work)
Also, given the size of the cable involved, proper position is important. The solder may wick up into the stranded cable & solidify, making it impossible to bend later. So it has been pre-bent & the terminal oriented to the bend so that it will be easy to install later.
In addition to the main battery cable being soldered, I'm also connecting a smaller body ground wire, and a few extra wires for future circuits.
The torch flame is set VERY low, and is aimed at a heavy area of the terminal to maximize the heat transferred from the flame into the metal. It's also worth considering where the flame's heat column will go so that the wire's insulation & surrounding parts aren't unnecessarily scorched.
Initially, only the terminal is being heated. As it reaches the solder's melting temperature, the solder liquefies & flows around the wires inside the socket. Eventually, this brings the wires up to the melting temperature, and everything wets with solder making an exceptionally strong & conductive joint. If the solder appears slow to melt or the solder strand sticks to the terminal, it indicates insufficient heat in the joint. Cold solder joints are poor conductors, and are far more likely to fail mechanically (wires dropping out). But if the solder remains liquid for a few seconds after the heat source is removed, it indicates the temperature was appropriate.
For this job, I used ~12" from a spool of Oatey brand Lead-Free (95% Tin; 5% Antimony) Rosin-Core Electrical Solder. But wherever solder-on battery terminals are sold, solder pellets are usually available. Pellets are typically NOT cost-effective compared to bulk solder spools, and they rarely fill the terminal socket.
See also:
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Motorcraft F2TZ-14301-B Negative Battery Cable with body, frame, & block grounds
