How to solder 18650 Li-Ion batteries

Acid flux paste https://amzn.to/2BwXxX2
All 18650 Li-Ion batteries are hard to solder. Well, not only 18650 but all nickel-plated metal elements can be very hard to solder. But only if you do not have the proper tools. In this case, proper flux: soldering acid. Forget about sanding, scratching and other voodoo. All you really need is a small drop of soldering acid. I'm using AG TermoPasty Soldering Acid with phosphoric acid. A small drop of it on each of 18650 and soldering is super easy. You only have to be careful and not inhale too much of that.

The next step might be a spot welder, but with soldering acid, soldering of 18650 Li-Ion batteries is simple enough to be done on small scale without the need for more advanced tools.