Can I connect Lithium batteries in series?

A common question we get asked about in relation to Lithium batteries is how to best connect multiple batteries together to achieve the correct voltage or capacity for a specific system.

At Valen, we don’t recommend series connecting Lithium batteries. However, Lithium batteries can be placed in parallel if required and done correctly.

To explain why it is best to first understand how Lithium battery technology works.

Lithium batteries are constructed utilising smaller cells that look in physical appearance much the same as standard AA size battery cells. These are generally around 3.2V3200mAh.

Lithium cells are arranged in configuration to create the larger battery cell. A BMS (Battery Management System) is wired into the battery to equalise the charge and discharge of the individual battery cells. This ensures that each smaller cell is equally maintained and that there is no breach of safety.

So why can’t Lithium batteries be connected in series?

It’s not recommended to install Lithium batteries in series because when a Lithium battery is charging, the BMS is actively controlling the charge to the cells within the battery.

The BMS prevents the cells from over-charging, balances the cells and performs other tasks to ensure that the batteries are maintained correctly.

Once the cells are fully charged, the BMS will go open the circuit to prevent further charge from going into the battery. This stops the battery from becoming over-charged. When Lithium batteries are connected in series and this occurs, the charging circuit is broken and the charger will stop charging the batteries.

The battery that first gets to this state will be fully charged while the other batteries in the series string will not be. The remaining batteries will be unable to continue charging due to the single battery having gone open circuit.

If you are going to series connect Lithium batteries, we recommend putting one BMS over all the Lithium batteries in a series which will control the charge to each battery and ensure all the BMS’s go open circuit at the same time, preventing any single BMS from completely stopping the charging circuit.

However, we strongly recommend avoiding series connecting Lithium batteries. We instead recommend using Lithium batteries that are designed and manufactured to the required system voltage.

For example, a 24V 100Ah caravan battery system could be built by utilising 2 x 24V 50Ah batteries in parallel or a single 24V 100Ah battery. However, we do not recommend using 2 x 12V 100Ah batteries connected in series. Once these cells have been built, they can be safely connected in parallel to add up to the required capacity. This way, you’ll always have full control over the voltage.

Which battery is right for my application?

How should I build my system voltage for longevity?

It is best to talk to a manufacturer who knows Lithium technology for advice on connecting your batteries.

The Valen Team are happy to help with customised Lithium battery solutions, including the BMS and applications with physical space constraints. To get in touch, simply click here to contact us.

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