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

by Oscar

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.

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

This is the 18650 batteries I recommend for FPV.

Solder Choice

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.

Heatshrink Wrap

Finally, wrap the battery with heat shrink, like this one: http://bit.ly/2JTOgLy


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Bad Idea 22nd August 2023 - 8:11 pm

DON’T EVER DO THIS! It’s dangerous.

You can get a spot welder from Amazon for under $50. Do it the right way instead,

Thomas 30th April 2023 - 7:18 am

Paweł Spychalski on a YouTube video uses an acid-based flux for tinning the 18650 batteries then rinsed the acid flux off with some water. Says it’s a bad idea to trust sanding the ends of the lithium-ion batteries for soldering. What’s your opinion on this? Thanks.

Oscar 30th April 2023 - 8:25 am

I’ve built several Li-ion packs, never needed to do that. If you followed my advice on solder selection (https://oscarliang.com/soldering-iron-buyers-guide/#Solder-Wire-Recommendations) and get the rosin core solder wires I listed there, they already have a good amount of flux inside them so you probably won’t need to add extra, but of course it doesn’t hurt to add more if you don’t mind the extra cleaning afterwards.

PlaGe 29th April 2022 - 5:39 pm

Is it safe to chuck li ion batteries that only hold 10mAh charge? I got a few at home i want to solder to a 3v dc motor before binning them.

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.

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?

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.

Oscar 7th February 2021 - 9:31 pm

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

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?

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.


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

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.

Taryn 18th March 2020 - 8:30 pm

I was just wondering what gauge wire you would recommend.

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.

Ariuskooo 12th January 2020 - 9:18 pm

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

Oscar 13th January 2020 - 4:46 pm

That’s just electrical tape :)

RICHARD S GIESER 15th May 2022 - 2:24 am

Let me use 10 gauge wire as an example though I am not specifically suggesting you to use 10 gauge with the 18650 battery but it depends. 10 gauge wire has 1 ohm resistance per 1000 feet and is typically rated to 30 amps maximum. If you run 30 amps this wire gets too hot for most applications and therefore wastes power and makes a fire hazard and the voltage loss would be 30 amps x 1 ohm equals 30 volts over the 1000 feet or proportionally less for a shorter length. If you are operating loads with high starting surge currents such as motors or solenoids/ relays, etc., the instantaneous starting current can be 5 or even 10 times the normal continous rated running current so a large instantaneous starting voltage loss in the wire could be significant and may result in sluggish starting which, in turn, would cause prolonged excessive high current draw time which results in more energy loss, more heating of the wire and any transistors or other switching devices in the circuit. So if you do this simple calculation you may see the need for a much heavier gauge wire than you might otherwise not choose even though that higher current capability will be used for less than 1% of the time and the heavier wire will not come anywhere near getting warm. Of course weight and cost for heavier wire must be balanced against these other factors depending on your application. Tradeoffs like these are not always easy to evaluate.

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?

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.