How to Charge 18650 Batteries in Series (The Complete Guide)

Charging 18650 batteries can be tricky, especially when you need to charge multiple cells in series. Connecting lithium-ion cells incorrectly can lead to fires or explosions, so safety should always be the number one priority.

In this complete guide, as a professional 18650 battery pack manufacturer, I’ll show you the safest and most effective ways to charge 18650 batteries in series, whether you’re working with 2, 4, 6 or even more cells. We’ll cover battery management systems, voltage requirements, wiring configurations, and charger options to help you get set up properly.

Overview of Charging 18650 Batteries in Series

When cells are wired in series, their voltages add up, but their capacity does not. For example, two 3.7V 2500mAh 18650s connected in series equals 7.4V 2500mAh. This higher voltage output makes series arrangements ideal for powering devices and tools.

However, there are a few important safety considerations when charging in series:

• All cells must have the same state of charge – If one cell is significantly lower, it can become overdischarged or charged backwards during the process, resulting in damage or fire. Always balance cells before charging.
• Use a charger designed for the specific series configuration – Charger voltage and charge rate capabilities need to match the total voltage and mAh rating of the pack.
• Include cell balancing and monitoring – Small differences in self-discharge and internal resistance can develop over time and cycles between cells in series. Cell balancing and continuous monitoring are key to safety.

Now let’s look at how to correctly set up charging for 2, 4, 6 and more 18650 cells in series.

How to Charge 2 x 18650 Batteries in Series

When charging just two li-ion cells in series, the simplest option is to use a lithium battery charger designed for the 3.7V (nominal voltage) of one cell and add your own balancing adapter cable.

This allows the charger’s circuitry to supply sufficient voltage while giving you tapped access to monitor and manually balance each cell as needed.

Here is what you will need:

• 2 x matching 18650 batteries
• 2S lithium battery balancer adapter
• 3.7V li-ion battery charger

Step 1) Connect the batteries in series using the balancer adapter cable. This gives you access to the positive terminal of Battery 1, the negative terminal of Battery 1 (which is the positive terminal of Battery 2), and the negative terminal of Battery 2.

Step 2) Connect the charger leads to the fully exposed positive and negative terminals of the whole series connection. For two cells, this will be 7.4V.

Step 3) Turn on the charger and monitor cell voltages during charging. Balance as needed by periodically applying a low current load across any cell that reaches 4.2V before the other.

Step 4) Stop charging when both cells show a full charge of 4.2V.

The key things to remember are using a lithium-ion compatible charger, manually monitoring cell voltages, and balancing when one cell reaches full capacity before the other during charging.

How to Charge 4 x 18650 Batteries in Series

When wired in series, four 18650 cells have a fully charged voltage of 16.8V (4.2V x 4 cells). This arrangement requires a more advanced charger designed for the specific voltage and battery chemistry.

Here is an overview of a proper 4-cell 18650 charging setup:

Equipment

• 4 x high-quality matched 18650 batteries
• 4S lithium battery balance charger / BMS
• 16.8V compatible charging source

Matching high-drain 18650 cells designed for longevity as well as a Battery Management System (BMS) are essential for safety.

The BMS actively balances cell voltages during charging, prevents overcharging or excessive discharging of cells, and protects from short circuit and over-current damage that can occur when cells have gone out of balance from use or wear.

A basic charger may rely on manual cell monitoring and balancing. But an advanced 16.8V lithium balance charger with built-in BMS will automatically handle everything as long as you connect matched cells properly.

High-quality 18650 battery packs designed for large power tool or EV applications also have integrated battery management and may be an easier charging solution.

Wiring Configuration

Connect cells in series according to the above diagram making sure polarity of cell connections follows proper positive-to-negative contact sequence.

Follow manufacturer specifications for correctly attaching balance leads from BMS wiring harness which tap into the full voltage exposure points between cells.

Charging

The BMS will continuously monitor all cells and matched charger will shut off automatically when pack is fully charged. This avoids individual cells experiencing over-voltage and ensures even charging of all four cells given slight differences in capacities.

Advanced lithium chargers often indicate charging status and may include options to set desired final target voltage anywhere from storage voltage (3.6 or 3.7V) up to a full 4.2V charge on all cells in series.

How to Charge 6 x 18650 Batteries in Series

As the number of cells rises in series configurations, it becomes even more critical to utilize proper lithium battery management systems to actively provide charging control, monitoring, and protection functionalities.

Let’s look closer at best practices for 6-cell arrangements.

Equipment Needed

• 6 x high-quality matched 18650 cells
• 6S lithium battery BMS with balance charger
• 25.2V charging source

With six 18650 cells in series, total fully charged voltage is 25.2V (6 x 4.2V/cell). It’s essential that you use a lithium balance charger designed for charging up to this higher voltage that also includes continuous cell monitoring and balancing.

You will damage batteries and risk dangerous failures without these protections in place since small differences between the cells can develop over continued usage cycles.

High-quality lithium battery packs with integrated BMS often allow for plug-and-play charging without worrying about individual cell monitoring. Just ensure pack voltage matches charger specifications.

For DIY charging of individual cells, choose an advanced programmable lithium BMS that actively balances during charging up to 25.2V maximum. You may also need to set target voltages based on cell specifications.

Wiring Configuration

As before, properly connect cell positive to negative terminals in sequence. The BMS typically has wiring taps that connect between cells to monitor voltages and facilitate balancing on all six cells throughout charging.

Charger Compatibility

You need a high-quality lithium ion battery charger designed for the 25.2 volt 6S configuration that supports up to 5A fast charging if using high capacity cells.

Integrated battery management handles shutting off power when fully charged to protect from overcharging damage.

How to Charge 8+ 18650 Batteries in Series

The overall goals and equipment needs for charging higher series arrangements of 8 or more 18650 batteries remain the same.

Let’s summarize key requirements:

• Match cells capacities and quality as close as possible
• Use lithium battery management system (BMS) designed specifically for your pack voltage
• Appropriate high-volt charger with sufficient current capability
• Actively balance cells during charging routine
• Allow BMS safety protections to automatically stop charging at full capacity

With very large DIY battery packs, adding external safety monitoring and controls as an extra layer of protection is also a good idea to prevent damage or dangerous failures as the overall stored electrical energy rises substantially.

Integrated lithium battery packs with BMS may still be easier to implement for large banks needing high capacity runtimes. Consult manufacturers to determine optimal charging specifications for each series configuration.

FAQs – Charging 18650 Batteries in Series

Can you charge 18650 batteries in series without a BMS?

It is not advisable or safe to charge any significant number of li-ion cells in series without utilizing proper battery management and monitoring. Minor differences between cells will become more pronounced without active balancing leading to premature failure or dangerous operating conditions over repetitive cycles left unchecked.

What charger do I need for charging 18650 batteries in series?

You need a lithium ion battery charger designed specifically for the total series voltage and cell configuration you are working with. For DIY setups, look for programmable lithium chargers with matching specs that also facilitate necessary battery monitoring and balancing through added BMS equipment.

How do you balance charge 18650 batteries in series?

Actively balancing cells during charging is handled automatically in integrated battery packs with BMS while DIY configurations rely on lithium battery management systems that tap into the full voltage monitor points between cells to continually address imbalances as needed. Advanced chargers may perform this balancing function as well.

Can I leave batteries in series plugged into the charger after fully charged?

No. Continuing to apply a charge to already full lithium ion batteries will eventually lead to damage from over-voltage exposure. Properly designed BMS devices actively prevent overcharging already balanced cells by automatically shutting off current flow into a fully charged series battery pack whether or not it remains plugged in.

Conclusion

Charging multiple high-capacity 18650 batteries connected in series is completely safe when following basic guidelines:

• Match cell capacities and quality
• Incorporate lithium battery management systems
• Use series-configuration compatible chargers
• Actively balance cells throughout charging process
• Stop applying charge current once cells reach full capacity

Following these essential safety steps and equipping your pack with advanced monitoring and protection helps maintain batteries for greatly extended lifespans.

Implementing proper 18650 charging practices for series arrangements allows building battery banks with sufficient capabilities to reliably power almost any application while avoiding dangerous failures down the line.

Hopefully this guide has provided you with everything needed to safely setup and charge any series configuration of 18650 lithium ion batteries for your projects! Let me know in the comments if you have any other questions.