How to Solder 18650 Batteries Together: Complete Guide 2025

Want to build your own DIY battery pack? You’re not alone.

In 2025, more makers than ever are looking into how to solder 18650 batteries together for custom power solutions. But here’s the thing: while it’s possible, it’s also something you need to approach with extreme caution.

As a professional 18650 battery pack manufacturer, I’ve spent countless hours researching and testing different methods, and I’m going to share everything I’ve learned about this process – including some important warnings you absolutely need to know before starting.

Important Safety Warning

Before we dive in, I need to be super clear about something:

Major battery manufacturers explicitly warn against direct soldering to 18650 cells. This includes:

• LG: “Do not solder on battery directly”
• Samsung: “Don’t heat partial area of the battery with heated objects such as soldering iron”
• Panasonic/Sanyo: “Do not directly solder the battery… the battery may catch fire, smoke, heat generation or explode”
• Sony: “Do not apply solder directly to cells”

Why such strong warnings? Direct heat can damage the battery’s internal components, create gas pressure, and potentially cause catastrophic failure.

The safest approach is spot welding. But if you’re determined to solder, let’s cover how to do it as safely as possible.

What You’ll Need

If you’re proceeding despite manufacturer warnings, gather these materials:

• Quality 80W soldering iron
• Soldering flux/acid
• Stranded copper wire
• Leaded solder
• Sandpaper or steel wool
• Cleaning solvent
• Wooden block for applying pressure
• Safety equipment (goggles, gloves, fire extinguisher)
• Well-ventilated workspace

How to Solder 18650 Batteries Together

Preparation: The Critical First Step

Proper preparation makes all the difference between a dangerous mess and a successful connection:

1. Work in a safe environment

   • Well-ventilated area
   • Fire-resistant workspace
   • No flammable materials nearby
   • Fire extinguisher within reach

2. Battery preparation

   • Lightly abrade the terminals with sandpaper or steel wool
   • Clean terminals thoroughly with solvent
   • Apply flux to the terminals
   • Allow batteries to reach room temperature if stored in cold

3. Equipment setup

   • Set soldering iron to high heat (400-450°C)
   • Properly tin the iron tip
   • Ensure tip and solder are oxide-free

The Fastest Soldering Technique

After testing numerous methods, I’ve found this technique minimizes heat exposure while creating strong connections:

1. Pre-tin your wire

   • Strip insulation from copper wire
   • Apply flux to exposed wire
   • Tin the wire generously with solder
   • Trim to appropriate length

2. The one-second technique

   • Place pre-tinned wire on fluxed battery terminal
   • Apply hot soldering iron to the wire (not directly to battery)
   • Hold for NO MORE than 1 second
   • Remove iron immediately
   • Use wooden block to apply pressure as it cools

3. Testing your connection

   • Allow to cool completely
   • Gently test connection strength
   • Connection should be solid, not “balled” on surface
   • Should require significant force to separate

This technique minimizes direct heat to the battery while creating a solid electrical connection. Watch the joint closely – if solder doesn’t flow within 1 second, stop, clean everything, and try again.

Alternative Connection Methods

Soldering isn’t your only option for connecting 18650 cells:

Spot Welding (Safest Method)

Spot welding uses brief electrical pulses to fuse nickel strips to battery terminals. It’s:

• Recommended by manufacturers
• Creates minimal heat transfer
• Requires special equipment
• Results in stronger, safer connections

Battery Holders

Pre-made battery holders:

• Require no permanent connection
• Allow for easy battery replacement
• Add bulk to your design
• May have higher resistance

Spring Connections

Found in many commercial products:

• No heat applied to batteries
• Simple mechanical connection
• Less reliable over time
• Higher resistance than welded connections

Common Mistakes to Avoid

When working with lithium batteries, mistakes can be dangerous:

1. Overheating the battery

   • Never exceed 1 second of contact time
   • If you can’t make the connection quickly, stop and reconsider

2. Using excessive force

   • Batteries contain sensitive internal components
   • Rough handling can cause internal shorts

3. Ignoring manufacturer warnings

   • There’s a reason every major manufacturer warns against this
   • Understand you’re accepting significant risk

4. Inadequate ventilation

   • Soldering produces fumes
   • Batteries can release gases when heated

5. Working with damaged cells

   • Never solder dented, corroded or otherwise damaged cells
   • Increased risk of catastrophic failure

Tips for Better Battery Connections

Want to improve your results? Here are some pro tips:

1. Use flux generously

   • Quality flux makes all the difference
   • Helps solder flow quickly, reducing heat exposure

2. Keep batteries cool

   • Work on one terminal at a time
   • Allow cooling between operations
   • Consider heat sinking with alligator clips

3. Practice your technique

   • Test on dead cells before working with good ones
   • Perfect your timing and pressure

4. Invest in quality materials

   • High-quality solder flows better
   • Better flux reduces heat requirements
   • A quality iron maintains consistent temperature

5. Consider battery tabs

   • Some 18650s come with solder tabs pre-attached
   • Much safer than direct soldering

When to Choose Each Method

Different projects have different requirements:

• Permanent installations: Spot welding is safest
• Prototyping: Use holders or spring contacts
• Low-current applications: Holders work fine
• High-current needs: Welding provides lowest resistance
• DIY with limited tools: Soldering may be your only option, but proceed with extreme caution

Final Safety Considerations

Working with lithium batteries is inherently risky. Always:

• Test voltage before and after connections
• Incorporate appropriate protection circuits
• Use insulation between cells
• Store partially-completed packs safely
• Never leave charging batteries unattended
• Dispose of any damaged cells properly

Sonuç

Building your own battery packs can be rewarding but comes with serious risks. While I’ve shown you how to solder 18650 batteries together using the safest possible technique, remember that manufacturers explicitly warn against this practice.

If you decide to proceed, do so with extreme caution, proper preparation, and a healthy respect for the energy stored in these powerful cells. For most applications, spot welding remains the safer, manufacturer-approved method.

Whatever approach you choose, prioritize safety over convenience. Your DIY power project isn’t worth a fire or injury.