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Acid (plumbing) Solder
Just found out I made a rookie mistake while soldering together a battery pack a few months ago.  While doing a few test runs I discovered that using the plumbers flux made the job of soldering to the battery terminals (18650) very easy.  My process was:
  • Pry off any spot welded pieces with snips
  • dip solder pen in the plumbers flux, wipe off any excess
  • spread a thin coat on the battery terminal where the solder will go
  • apply solder to the iron tip
  • quickly touch iron tip to the battery terminal where the flux is
  • place the pre soldered nickel strip over the battery terminal
  • quickly touch iron tip to nickel strip to adhere to battery terminal
  • once done spray a little brake cleaner on a shop towel & wipe solder joints
Now that I've read how bad acid based flux is for electronics soldering, I'm a little worried that the whole battery is a write off.  I guess my question is, how harmful is acid flux really?  If used properly, can it work out just fine?  The plumbers flux works so much better than the standard rosin based flux for this purpose, as the adhesion is practically instant, which keeps the heating of the battery to a minimum when soldering, plus the solder flows very easily and leaves a strong joint.  It is probably ok, or should I just put down the soldering iron and back away slowly?
The battery may very well be a write off. In a worst case scenario it is very harmful because it will lead to corrsion of the cells, you have to watch out for that. If you think that plumbers flux works so much better then you probably using the wrong tools, you are just doing it wrong or you a way too scared by the potential heat transfer into the cells. Or maybe a combination of these. If done properly there is almost no heat transfer into the cells whatsoever. And if you are really worried about it then you should change to spot welding cells, for that you need smooth surfaces though so it is much more work on reclaimed cells.
Korishan likes this post
If it hasn't been very long since you finished the job, I believe it would be worth your time to clean off any flux residue. Although acid-based flux is known to be very damaging to electronics, I believe that particular warning was more focused at circuit-boards. The traces on electronic circuit boards are quite thin (and often narrow), and they are created by masking the shapes you want to save and soaking the board in acid.

Plumbers flux is specifically formulated to be aggressive to raw copper oxide films. The ends of 18650 cells are nickel-plated steel.

If you properly clean every place that you used the acid-based flux, and you do not see any corrosion, there is a good chance that that pack may be fine. The majority of any flux is evaporated during the soldering operation, but of course there are varying amounts of residue left behind...
completelycharged and Korishan like this post

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