Connecting multiple battery banks in parallel

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Table of Contents

Multiple banks with redundancy
Multiple banks sharing one power relay

There are several options to connect multiple battery banks parallel. In all cases, each bank needs a 123\SmartBMS system.

Multiple banks with redundancy #

When the application requires redundancy, each bank will get it’s own (energy efficient) power relay. In case a critical condition in one of the batteries occurs, only that battery bank is disconnected by the 123\SmartBMS. The other battery banks will stay connected to the main bus and will continue to operate.

Important information

When a battery bank is disconnected from the main bus, its voltage may start to diverge from that of the other banks. While this voltage difference might seem minor, it can result in very high inrush currents if the bank is later reconnected. These inrush currents can blow the main fuse or even damage the power relay. To prevent this, the battery bank should not be reconnected to the main bus until its voltage is nearly equal to that of the main bus, for example within 0.3V.
To mitigate this risk, it is highly recommended to set the Charge Restart Delay and Discharge Restart Delay on each 123\SmartBMS to an extended duration, such as 60 seconds x 60 minutes x 24 hours x 7 days. This ensures the battery bank remains disconnected. The operator can then manually reconnect the bank via the 123\SmartBMS app once its voltage is closely aligned with the main bus voltage, which matches the voltage of the other banks that remain connected.

Example of three battery banks in parallel. Each 123\SmartBMS manages it’s own bank and can disconnect the bank from the main bus if required.

Multiple banks sharing one power relay #

When redundancy is not required, it is possible to connect the power relay to each BMS signal relay, in series. This circuit is quite similar to above schematic. However, instead of one power relay per bank, all banks share one power relay. When one of the battery banks has a critical condition, the BMS will switch off and disconnect the supply path to the power relay, making the power relay switch off.

This setup is the most cost-effective and straightforward, but it has the downside that a failure in one bank can cause the entire battery system to shut down.