Tutorial: Soldering Li-Ion Battery Cells (18650, 18500 21600)

by Oscar
Published: Last Updated on

Soldering Li-Ion batteries such as 18650 can be dangerous. Overheat can cause the battery to catch fire and explode. If you decide to solder battery, you are doing so at your own risk.

If you are new to soldering, you should check out my beginner tutorial on the subject first.

This is the 18650 battery I recommend for low current application.

Solder Choice

Use good quality solder with flux core, avoid using additional acid based flux (solder paste) as it will corrode the connection/battery in the long run. 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, scratch the top and bottom sides of the cell with sand paper to remove the oxide layer which will help solder to stick to.

Do It Quick

“Tin” both both sides of the batteries with a small amount of solder, let it cool down before soldering the wires to it.

You want to keep the time your soldering iron touching the battery terminals to a minimum (e.g. less than a second). The longer you leave your iron on the battery, the more heat will build up. For this, you want to use a powerful, temperature-controlled soldering iron.

A less powerful iron will not hold the temperature as the heat will simply get sucked out when soldering a big piece of metal. I use the TS100 iron personally, and it works really well.

Heatshrink Wrap

Finally you want to wrap it with heatshrink, such as this http://bit.ly/2JTOgLy

Example

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14 comments

Doubs 10th April 2021 - 2:20 am

hey oscar I am planning on soldering some rechargeable batterys into some oculus rift cv1 controllers, any tips on soldering batteries into a spring tension design? I know it sounds like a really weird application but it’s a flaw with vr controllers that have spring tentioned batteries because if you swing hard enough they disconnect witch is frustrating. so, any thoughts on things to keep in mind before jumping right in with it.

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Oscar 10th April 2021 - 10:00 am

but do you have to? why not just put something in between the spring to stop it from compressing, and when you need to change battery you can remove that something?

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Chris Barth 3rd February 2021 - 5:00 am

I’ve been into electric flight before lithium polymers were introduced. Brushless motors and lipos started trickling in around the same time. The performance improvements were incredible and it was clear the hobby was going to change but I had no idea that it would lead to what we have today.

Anyway, I never really considered lion cells worthy for use in flight until recently. I was flush in thousands of barely used Panasonic ICR18650BD cells used in Bird scooters. Each scooter had 60 cells, harvesting the individual cells was difficult and potentially dangerous but I’m pretty good with this stuff. I was surprised at how these cells performed and quickly soldered together a 4s pack for a quad that usually uses a 1500mah 4s lipo.

I too have the Ts100 and find it sufficient to solder cells but I also have a large cheap, 200W iron that has massive thermal capacity. I’ve found it takes literally takes only a second to make a quality solder joint with this iron. A bit of rosin flux makes it even faster. Pretinning the wire and cell is a must regardless. Lion cells can easily and safely be soldered with a dumb, cheap, large, hot iron.

Just my 2¢s on this topic.

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Oscar 7th February 2021 - 9:31 pm

Safety warning is never enough when you post things like this on the internet :)

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Rob 17th September 2020 - 2:51 pm

Great article. You should mention that Li Ion applies to any quad. You could use it to make your freestyler go longer if you just want to cruise around. Would maxing out the freestyle quad strain the battery? Or would the quad just not perform as well as Lipo?

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Chris S 5th July 2020 - 6:21 pm

Would you mind including a simple wiring diagram? I’m curious which soldered wires to go to which connectors in your photos.

thanks!

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Dominik 30th June 2020 - 12:14 pm

Hi Oscar,

great article but I wonder why everyone is building the 18650 LiI-on-Packs parallel to the long side and not along the short side? I am thinking of a “old flasshlight design” where you put several AA batteries in series.

Is there any problem with that?

Regards Dominik

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Oscar 4th July 2020 - 3:38 pm

If you connect them in parallel, voltage doesn’t change, but the capacity doubles.
If you connect them in series, capacity doesn’t change but the voltage doubles.
It depends on what output voltage you want out of the pack.

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Taryn 18th March 2020 - 8:30 pm

I was just wondering what gauge wire you would recommend.

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Oscar 30th April 2020 - 4:53 pm

depends on your application, but for the maximum amp you can draw from one single 18650 cell, 20AWG should do.

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Ariuskooo 12th January 2020 - 9:18 pm

Hi Oscar, whats that black glue that you are using?

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Oscar 13th January 2020 - 4:46 pm

That’s just electrical tape :)

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Ahmadalbab 11th January 2020 - 2:58 am

Hi Oscar. Can you show how do you charge your 18650 using the balance charger? I have a 3500mAh x 2 (7.4V) 18650 battery, how do I charge them? Should I put it at 3.5A 7.4V or do I need to use lower amp for 18650 batteries?

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Oscar 13th January 2020 - 4:40 pm

I documented how to charge them in this post (scroll down): https://oscarliang.com/best-18650-li-ion-battery/
Yea charge a 2S 18650 under 3.0A should be fine, check the temperature regularly during charging, to make sure it doesn’t get warm. If they get warm then you should lower the current.

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