Soldering Li-ion batteries such as 18650 or 21700 cells can be risky. If overheated, these cells can catch fire, or even explode. That’s why spot welding is always the preferred and safest method for making Li-ion packs. However, if you don’t have access to a spot welder, soldering is still possible with care and the right tools. In this tutorial, I’ll explain how to solder Li-ion cells as safely as possible, step by step.
Disclaimer: Soldering directly on battery can be dangerous, you do this at your own risk. Always work in a fire-safe area, keep a fire extinguisher nearby, and never solder damaged or swollen cells.
If you’re new to soldering, I highly recommend reading my beginner’s soldering guide first: https://oscarliang.com/soldering-guide/
first.
I tested a bunch of Li-ion cells to find the best, and here are the results: https://oscarliang.com/best-18650-li-ion-battery/
Table of Contents
Benefits of Soldering vs. Spot Welding
Soldering and spot welding both have their place when building Li-ion packs. Here’s the trade-off:
Wire Soldering Advantages
- Lower resistance joints: A well-made solder joint with wires has less resistance compared to nickel strip spot welds.
- Less voltage sag: Lower resistance means more efficient power transfer, less sag under load, and potentially longer flight times.
- Stronger current handling: Especially useful in high-performance applications like FPV drones.
Nickel Strip Spot Welding Advantages
- Much safer: Spot welding transfers far less heat into the cell, greatly reducing the risk of overheating or damaging the battery.
- Faster: Once set up, spot welding is quicker for building large packs. This is why nearly all commercial Li-ion packs from big brands use spot welding.
- Cleaner results: Spot welded joints are flat and compact, so packs end up neater.
Tools and Materials You’ll Need
- Soldering iron: At least 60W, ideally temperature-controlled (e.g., TS100, HGLRC RC2, or similar). See my recommendations: https://oscarliang.com/soldering-iron-buyers-guide/#Soldering-Iron-Recommendations
- Solder: High-quality leaded solder (60/40 or 63/37) with rosin flux core. Do not use acid-based flux. See my recommendations: https://oscarliang.com/soldering-iron-buyers-guide/
- Sandpaper or file: To roughen the battery contacts.
- 16AWG Silicone wires
- XT30 or XT60 Connector
- Balance Lead
- Heat shrink tubes
- Multimeter: To check voltages
Step 1: Discharge the Cells First
Before soldering, it’s a good idea to discharge the cells to around 3.0V per cell. Lower voltage = less stored energy = reduced fire risk.
You can do this using a 18650 battery holder, like the one comes with any Radiomater radio.
Step 2: Prepare the Battery Terminals
- Use fine sandpaper or a file to gently roughen the top and bottom contacts of the battery.
- This removes the thin oxide layer and helps solder bond more effectively.
- Wipe away any dust after sanding.
Step 3: Pre-Tin the Terminals
- Set your soldering iron to around 400–450°C. Try to use a thicker soldering iron tip as it transfer heat more effectively.
- Quickly tin both terminals (apply a small blob of solder to each battery terminal).
- The goal is to get in and out fast — ideally in under a couple of seconds. If the iron is in contact with the battery for too long, heat will build up and it becomes risky.
- Let the cell cool down before touching it again.
If the cell gets too hot to touch, you’ve overheated it. Stop and let it cool completely.
Step 4: Attach the Wires or Nickel Strips
- Pre-tin your wires first.
- Place it on the pre-tinned battery terminal and quickly touch it with the iron until the solder flows together.
- Immediately remove the iron and hold the wire/strip steady until the solder solidifies.
- Repeat for the other side.
Pro tip: If making a multi-cell pack (2S, 4S, etc.), solder one terminal at a time and allow enough time for cooling between solder joints.
Step 5: Insulate and Wrap
- Once all connections are complete, cover any exposed solder joints with Kapton tape or electrical tape.
- Wrap the entire pack with heat shrink tubing to protect the cells.
- Always leave space for wires to exit safely without sharp bends.
Example Builds
- 2S Li-Ion pack for my radio transmitter: https://oscarliang.com/taranis-x9d-18650-battery/
- 4S Li-Ion pack for long-range FPV drones: https://oscarliang.com/li-ion-battery-long-range/#Building-Your-Own-Li-ion-Battery-Pack
Build Tips
- Isolation: Always add extra insulation between cells (e.g., electrical tape, Kapton tape, or heat shrink). This prevents cells from rubbing through their protective layer during use.
- Structural Support: If the weight allows, try to add rigid plastic or foam spacers around the pack. This helps absorb impact in crashes and reduces stress on the solder joints.
- Wire Routing: Instead of pulling wires straight out of the pack, route them back over the cells before exiting. This gives the wires a stronger anchor point and reduces strain on solder joints.
- Wire Gauge: For FPV drone packs, 16 AWG silicone wire is usually enough for XT30 or XT60 connectors. Most 18650 cells are rated up to ~30A discharge, and 21700 cells up to ~45A. Using thinner wires keeps the pack lighter and easier to solder without noticeably impacting performance.
Important Safety Tips
- Never solder damaged cells.
- Always work in a well-ventilated area away from flammable items.
- Keep a fire extinguisher or bucket of sand nearby.
- Don’t overheat the battery. If it feels hot, stop immediately and let it cool.
- Store finished packs in a LiPo-safe bag or fireproof container.
Final Thoughts
Soldering Li-ion cells isn’t the safest option, but with the right tools and careful technique, it can be done. The key is speed and preparation: roughen the terminals, pre-tin, and make your solder joints quickly with minimal heat exposure.
Edit History
- Jan 2020 – post published
- Sep 2025 – revised
21 comments
I’m sorry but I have to mention the most often made mistake by people with, well, less than years of soldering exprience. I got to know that a decade or two ago, from an old ’60s … introduction to soldering training video – which surprisingly is still actual, as not a lot in basic physics changed ,)
Of course – as you wrote, the general idea not to blow them up is get in, heat fast, get out. The mistake people often make – is the hardware. They buy pointy soldering iron tips, with 1.5 mm tip or so. The main problem with that is the thermal stability – one don’t have enough heated mass to hold energy when it’s starts to be pulled to the element you heat up. The second is the contact surface.
Few good advices that I feel could improve this guide – or any other soldering guide.
Do not be afraid of bigger tips – I use my 2mm flathead for almost all work – I recently worked on 1608 smds with it, you just need to use the corner of the tip. Especially when you are in places where you need to spot heat piece that has more thermal mass, like 18605
Never apply fluxed solder to your tip. You’ll just burn out flux. Have enough, tiny amount of solder to make initial contact easier, and put fluxed tin onto the element (or wire for that metter) when it’s almost hot enough.
Flux, flux, and you can flux on top of that :) It’s your best friend when soldering. For wires especially, just tip the end in gel version flux, and heat up, touch with solder, and bob’s your uncle.
This post ended up slightly longer than I planned :)
can i buy these 18650 cells with Tabs that will be easier to solder?
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,
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.
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.
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.
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.
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?
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.
Safety warning is never enough when you post things like this on the internet :)
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?
Would you mind including a simple wiring diagram? I’m curious which soldered wires to go to which connectors in your photos.
thanks!
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
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.
I was just wondering what gauge wire you would recommend.
depends on your application, but for the maximum amp you can draw from one single 18650 cell, 20AWG should do.
Hi Oscar, whats that black glue that you are using?
That’s just electrical tape :)
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.
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?
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.