Soldering Li-ion batteries, such as 18650 cells, can be dangerous. Overheating may cause the battery to catch fire and explode. If you decide to solder a battery, you do so at your own risk.
If you’re new to soldering, check out my beginner tutorial on the subject first.
Use high-quality solder with a flux core and avoid using additional acid-based flux (solder paste), as it can corrode the connection or battery over time. See my solder recommendation here.
Discharge Battery First
Before soldering, it’s best to discharge the Li-Ion battery down to 3V. The more energy stored in the battery, the more dangerous when things go wrong. 3V is the minimal safe voltage for 18650 to be discharged to. Even slightly lower voltage is okay but might be bad for the life span of the battery in the long run.
Roughen up Battery Terminals
Before soldering, use sandpaper to scratch the top and bottom sides of the cell, removing the oxide layer. This will help the solder adhere better.
Do It Quick
“Tin” both sides of the batteries with a small amount of solder, allowing it to cool down before soldering the wires.
Keep the time your soldering iron touches the battery terminals to a minimum. The longer the iron is in contact with the battery, the more heat will build up. To accomplish this, use a powerful, temperature-controlled soldering iron.
A less powerful iron won’t maintain its temperature as effectively since the heat will be absorbed while soldering large pieces of metal. I personally use the TS100 iron, which works exceptionally well.
Finally, wrap the battery with heat shrink, like this one: http://bit.ly/2JTOgLy