How to Connect Two 12V Batteries in Series for 24V

Connecting two 12V batteries in series is a simple way to increase your system voltage from12V to 24V while keeping the same amp-hour capacity.

This type of battery configuration is commonly used for 24V equipment, trolling motors, marine systems, off-grid applications and other equipment designed to operate from a higher-voltage battery bank.

For example:

2 x 12V 120Ah batteries connected in series = 24V 120Ah

Unlike connecting batteries in parallel, a series connection increases the voltage, not the amp-hour capacity.

What Does Connecting Batteries in Series Mean?

When batteries are connected in series, the batteries are linkedpositive to negative.

With two 12V batteries:

• The positive terminal of Battery 1 connects to the negative terminal of Battery 2
• The remaining negative and positive terminals become the main connection points for the 24V system

The voltage of the two batteries is then combined.

Example

Battery 1: 12V 120Ah
Battery 2: 12V 120Ah

Series battery bank: 24V 120Ah

The voltage doubles from 12V to 24V, but the battery bank remains rated at120Ah.

How to Connect Two 12V Batteries in Series

To create a 24V battery bank using two 12V batteries:

1. Position both batteries securely next to each other.
2. Connect thepositive (+) terminal of Battery 1 to the negative (-) terminal of Battery 2 using a suitable battery linking cable.
3. The unused negative (-) terminal of Battery 1 becomes the negative connection for the 24V system.
4. The unused positive (+) terminal of Battery 2 becomes the positive connection for the 24V system.
5. Connect your 24V equipment or electrical system across these two remaining terminals.

The result is:2 x 12V 120Ah = 24V 120Ah

24V Series Wiring Diagram

Example: Two 12V 120Ah batteries connected in series to create a 24V 120Ah battery bank.

The diagram shows the link between the positive terminal of the first battery and the negative terminal of the second battery, with the system output taken from the two remaining terminals.

Series vs Parallel Battery Connections

Series and parallel battery connections serve two very different purposes.

Batteries Connected in Series

A series connection increases voltage while keeping the amp-hour capacity the same.

For example:

2 x 12V 120Ah = 24V 120Ah

The connection is:

Positive → Negative

Series is used when the equipment requires a higher operating voltage.

Batteries Connected in Parallel

Aparallel connection keeps the voltage the same but increases the total amp-hour capacity.

For example:

2 x 12V 120Ah = 12V 240Ah

The connections are:

Positive → Positive
Negative → Negative

Parallel is commonly used when you want longer operating time from a 12V system without increasing its voltage.

Quick Comparison

Configuration	Two 12V 120Ah Batteries Produce	What Increases?
Series	24V 120Ah	Voltage
Parallel	12V 240Ah	Amp-hour capacity

The correct configuration therefore depends on what your equipment requires.

If your equipment is designed for12V, do not connect two 12V batteries in series and supply it with 24V.

Why Use a 24V Battery System?

A 24V battery bank can be useful when the equipment being powered is specifically designed for 24V operation.

Common applications can include:

• 24V trolling motors
• Marine equipment
• Pumps
• Electric motors
• Off-grid power systems
• Inverters
• Industrial equipment
• Specialist vehicle and recreational applications

Higher-voltage systems can also reduce the amount of current required to deliver the same amount of power compared with a lower-voltage system.

However, all connected equipment, chargers and accessories must be suitable for the system voltage.

Important: Check That Your Lithium Batteries Support Series Connection

Not every lithium battery can be connected in series.

Lithium batteries contain aBattery Management System (BMS) which controls and protects the battery. The BMS must be designed to operate as part of a series-connected battery bank.

Before connecting Trolltek™ lithium batteries in series, always check the specifications for the particular battery model and confirm the permitted series configuration.

Do not assume that a battery is suitable for series connection simply because it is rated at 12V.

Use Matching Batteries

When creating a series battery bank, use batteries that are as closely matched as possible.

Ideally, the batteries should be:

• The same brand
• The same model
• The same voltage
• The same amp-hour capacity
• The same battery chemistry
• A similar age
• At a similar state of charge

Using matching batteries helps the battery bank charge and discharge more evenly.

Bring the Batteries to a Similar State of Charge

Before connecting batteries together, ensure they are at a similar state of charge.

Large voltage differences between batteries can contribute to uneven operation once they are connected into the same battery bank.

Following the battery manufacturer's charging and installation instructions is always recommended.

Use Suitable Battery Linking Cables

The cable connecting the batteries must be appropriately sized for the expected current of the system.

Battery cables that are too small can result in:

• Excessive voltage drop
• Heat generation
• Reduced performance
• Potential damage to wiring or equipment

Use quality battery linking cables and ensure the cable lugs are suitable for the battery terminals.

Connections should be clean, secure and properly tightened.

Use the Correct 24V Charger

Once two 12V batteries are connected in series to form a24V battery bank, the charging arrangement also needs to be appropriate for the complete system.

A charger connected across the 24V battery bank must be designed for:

• 24V battery systems
• The correct battery chemistry
• The required charging profile

For lithium installations, always use charging equipment suitable for LiFePO4 batteries and the specific battery system configuration.

Make Sure Your Equipment Is Designed for 24V

This is particularly important when converting an existing 12V installation.

Once two 12V batteries are connected in series, the output of the battery bank is approximately24V nominal.

Do not connect normal 12V accessories directly across a 24V battery bank unless an appropriate voltage converter or other correctly designed solution is being used.

Supplying 24V to equipment designed only for 12V may permanently damage the equipment.

Common Series Connection Mistakes

Some common mistakes to avoid include:

• Connecting positive to positive instead of positive to negative
• Accidentally creating a parallel configuration
• Connecting 12V equipment directly to the 24V bank
• Using batteries that are different capacities or specifications
• Connecting batteries with significantly different charge levels
• Using undersized battery cables
• Using a 12V charger across the complete 24V battery bank
• Assuming all lithium batteries support series connection
• Exceeding the battery manufacturer's maximum permitted series configuration

Always check the battery and equipment specifications before completing the installation.

Can You Use Two 12V 120Ah Batteries to Make 24V?

Yes, provided the batteries are designed and approved for series connection.

When two suitable12V 120Ah batteries are connected in series, the result is: 24V 120Ah

The voltages are added together, while the amp-hour rating stays the same.

If you instead connected those same two batteries in parallel, you would have: 12V 240Ah

This is why understanding the difference between series and parallel is important when designing a battery system.

Series or Parallel – Which Do You Need?

The answer depends on the voltage required by your equipment.

Choose a series connection when you need to increase voltage:

12V + 12V → 24V

Choose a parallel connection when you want to increase capacity while retaining 12V:

12V 120Ah + 12V 120Ah → 12V 240Ah

Always determine your required system voltage before selecting a battery configuration.

Final Thoughts

Connecting two compatible 12V batteries in series is an effective way to create a 24V battery system.

Using two 12V 120Ah batteries as an example:

2 x 12V 120Ah connected in series = 24V 120Ah

Remember that a series connection increases voltage, whereas a parallel connection increases amp-hour capacity.

Always confirm that your lithium batteries support series operation, use matching batteries and suitable battery cables, and ensure your chargers and connected equipment are designed for the resulting 24V system.

If you're unsure about your installation, consult a suitably qualified auto electrician, marine electrician or electrical installer.

Explore Trolltek™ Lithium Battery Solutions

Trolltek™ offers a range ofLiFePO4 lithium batteries, battery chargers, cables, connectors and electrical accessories for 4WD, marine, caravan, camping and off-grid applications.

Explore the Trolltek™ range and our growing collection of How-To Guides for more battery installation information, wiring diagrams and practical advice.