This is where I expected to be at, when LC first tried to solder, which is why I said I was impressed even with the first job that didn't wet properly. To get solder to stick to 18650s, you need to sand, apply flux evenly and leave for long enough, large tip with thermal mass, very short and precise contact.
LC, you CANNOT melt solder onto the soldering iron and then try to transfer the solder onto the target with anything larger than about an 8AWG cable. It's too big a heat sink. Solder will only stick to a hot surface. If you've had success on cables smaller than 8AWG, it's because your iron has been able to transfer enough heat to the small, exposed part of the cable before your tip ran out of thermal momentum.
I can't believe I am taking the time to do this right now, when I'm supposed to be writing management reports on a Sunday...
You cannot dispute any of these facts. I'm sure DA and WTurber will agree with me:
1. Solder will only properly stick to a surface that is above its melting point.
2. Doing things poorly, you *can* sometimes get away with heating the solder above melting point, then transferring it to a small item like a thin wire. The reason for this is because solder melts at 190*C roughly, and most soldering irons heat to 300*C+. What is actually happening, is that the solder has enough energy to heat SMALL items to above 190*C, the solder sticks, then solidifies. On a large item, this will not happen. The energy has to spread out further and faster, never letting the item get to required 190*C+
3. For a large item, there are two ways to get the surface hot enough - You heat it for a long time, or you heat fast. Transferring heat to an item to be soldered, is like trying to fill a bucket with a small hole in it. As you fill the "bucket", some of it is going to leak out to the air, or worse, in your case, to the actual chemicals inside the cell, shortening its life.
If you use an iron that has a low wattage, it's like having poor water pressure. It doesn't matter how big the tip or the hose is, the bucket will leak faster than you can fill.
If you use an iron that has a high wattage, but has a small tip, it's like using a fire engine with a drinking straw. It doesn't matter that you can pump thousands of litres a minute (60 watts of heat), you can't push it through that tiny small tip.
That's the end of the facts. The rest is my opinion:
1. Sand the ends of the caps until you can see criss-crossed scratches on them.
2. Clean the ends of the cells with alcohol, and don't touch them with your fingers. Any oil from your fingers will stop you soldering, and even if your hands are clean, as long as you're alive, you have very small amounts of oil on them.
3. Apply flux to the end of the cell. Leave for a couple minutes to allow it to etch the surface a bit
4. Make sure you get the biggest tip on the highest power rating iron you have. The tip HAS to have a flat surface. The edge is round, and you cannot use pointed tips. big surface area = fire hose. Fine surface area = drinking straw, remember? Ensure the tip is entirely clean, has been "tinned" with solder (The tip is silver, not black/brown).
5. Press the tip against the cell, and say "One thousand and one". If you're like most people, that's about a second give or take 10%.
6. Touch the solder to the CELL not the soldering iron, though you should be as close as possible without touching. If you have done this right, the solder should melt on the CELL and properly adhere forming a dome, instead of balling. Take the iron away, and blow until the solder is solid.
7. If you're willing to take the risk, very quickly dip the first 1-2mm of the cell end into a plate of water to cool it instantly, reducing the damage to the cell.
That should give you a very small dome of solder on the end of the cell, with no wire attached.
Now, tin each wire or nickel strip away from the battery. When you're ready to connect, simply melt the solder on the wire and the solder on the battery together. As the solder is already properly wetted to the battery and wire, you don't need such high heats and big tips. You can use a very fine tip small iron and be very precise.
I had better get back to my management reports. Good luck, let me know how you go, but I will be on a management retreat for the next week and probably won't get to respond. I *may* get to read your responses though (Can't watch videos though)