Safely soldering Li-Ion cells without causing any thermal damage – [En]


Soldering Li-Ion is a big problem even for the more experienced techs out there.
Many try to solder Li-Ion cells or phone batteries. It is not advised by the battery manufacturers to heat them up regardless the reason.
By using heat on Li-Ion cells, you can damage the plastic separator, pressure protections or even the main internal lithium layers.
There are ways to actually solder them with no heat, but those require special tools that usual people don’t have.
Therefore, I will explain you how to actually do it with standard tools.
Yes, it can be done, but you must have some experience with soldering and know some physics too.
Speaking of physics…heat requires time to transfer. Yes, thermal damage amount depends on the materials quite a lot, but the key to not overheating components or, in this case, Li-Ion batteries is the limited heat exposure time.
I’m not going to show you complex graphics of thermal transfer curves, I’ll get to the practical point.
Here’s what you need:
– rosin and solder or rosin core solder
– alcohol (acetylene ethyl or anything equivalent)
– high power soldering iron (100W+ recommended but not mandatory)
– metal wires or whatever you want to solder to the cell

Note: if your soldering iron takes quite a lot to heat up (30 sec +), remove it’s tip in favor of a thinner one. A 1mm wire tip will heat up significantly faster than a 2mm wire tip. Do not exaggerate though. If it’s too thin, it will melt.

Now here’s the procedure:
1. Clean up the Li-Ion cell contacts with alcohol. Same for the clamp or wire.

2. Heat up the soldering iron and melt some rosin. Put a little rosin on the cell. Don’t have contact with it more than a second. Do the same for the wires or clamp you want to attach.

3. After you got a little rosin on both the clamp/wire and cell, it’s time to test the soldering. For that, we’ll use the clamp. Note: if you only have rosin solder, you can melt a little of it so rosin comes out.

4. Heat-up the iron very well. If it does not get to the temperature needed to melt solder in a few seconds, it means it’s not powerful enough, but it still can be used. You’ll just have to determine the time it need to get to solder melting point. After you know that, continue heat it up 1/3 of that time. so if it melts solder in 10 seconds, heat it up for 15 before using it.
Now melt some solder (it should melt instantly) and touch the clamp/wire for less than a second. If the clamp or wire was cleaned correctly, the solder will stick to it and envelop the wire or spread on the surface that has rosin on it.
The clamp or wire does not need to benefit from this procedure, but it’s better to test this on it. After you’ve put solder on the clamp or wire, don’t let anything get too cold. Keep the iron hot.

5. Take the clamp or wire and melt again the solder on it with the soldering iron. You can add more solder to the mix if needed. When it all melts down, keep the iron on it and put everything on the cell’s contact.
Remove the iron as fast as you can do it (half a second should do). It needs way less than a second to connect to the cell’s surface, so the cell will not take thermal damage. After you remove the iron, keep the contact (clamp/wire) there for a few seconds so the solder solidifies.
It actually solidifies in around a second or so if it’s of a good quality (tip: less lead in the solder, higher quality).

One more thing you can do to have even less heat transfer: immediately after the first second passes, put some big piece of metal (aluminum recommended) over the soldered contact so it instantly transfers most of the heat into the big metal piece, instead of slowly cooling and allowing some of the heat to get to the cell. You can use the flat side of an aluminum CPU cooler for example. Frying pan can do too, as long as it’s now too waxed.

If you’re not skilled enough with this, it will require some testing. You can test on dead cells or anything that resembles them in size of contact surface.

By using this procedure cell portion near the contacts will never get even near recommended safety limits.

So this is the most important aspect to remember here: you must obtain the least possible contact time between the cell and the heated parts.

May you have an efficient soldering !

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2 Responses to Safely soldering Li-Ion cells without causing any thermal damage – [En]

  1. I’ve been trying to solder cells, and I’ve been applying the iron for over 10 seconds. Have I wrecked the 50 cells I’ve soldered?

    • xaeus says:

      It depends a lot of the internal construction of the cell, its manufacturer. some take more damage than others. Anyway, high temperature certainly affects Li-Ion in a negative way.
      If the cells still work, they probably have diminished capacity. That is the main aftereffect of overheating them.

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