Understanding and Mitigating Inter-Cluster Circulation in Battery Energy Storage Systems (BESS)

Inter-cluster circulation is a critical issue in Battery Energy Storage Systems (BESS) that can significantly impact the lifespan and efficiency of batteries. It refers to the flow of current between battery clusters, which can cause imbalance and degradation over time. Understanding the causes and implementing preventive measures is crucial to maintaining the optimal performance of BESS.

1. What is Inter-Cluster Circulation?

Inter-cluster circulation occurs when there is an uneven flow of current between different battery clusters in a BESS. In a series-connected battery system, each pack within a cluster can have slight differences in internal resistance. These variations lead to imbalance during charging and discharging, resulting in unequal current flow between clusters, known as inter-cluster circulation.

This phenomenon disrupts the balance within the battery system, accelerating battery aging and potentially leading to system malfunctions or even damage. Therefore, addressing inter-cluster circulation is vital for extending battery life and maintaining system efficiency.

2. Causes of Inter-Cluster Circulation

The primary cause of inter-cluster circulation is the inconsistency among individual battery cells. Differences in internal resistance among cells lead to uneven charging and discharging rates, creating current imbalances between clusters.

For instance, in a 1500V battery system, a single cluster typically comprises 416 cells connected in series, with a prefabricated battery cabin consisting of 9 to 12 clusters. Given the large number of cells (often configured as 5289 or 52812), maintaining cell consistency during the manufacturing process is crucial to minimizing inter-cluster circulation.

3. Effective Measures to Mitigate Inter-Cluster Circulation

To minimize the effects of inter-cluster circulation, several strategies can be implemented:

3.1 Cell Matching

Ensuring consistency during the manufacturing process is the first line of defense against inter-cluster circulation. This involves carefully selecting and grouping cells with similar characteristics, such as internal resistance, to create balanced clusters.

3.2 Active Balancing through BMS

The Battery Management System (BMS) plays a key role in balancing the battery packs by actively monitoring and adjusting the voltage differences between cells. Although the BMS can reduce voltage differences, its effect on inter-cluster circulation is often limited, making it more of a supplementary measure.

3.3 Circuit Adjustment Techniques

    •    DC/DC Converter Integration: A more effective approach involves using a DC/DC converter for each battery cluster, ensuring that all clusters are at a consistent voltage level before connecting to the DC side. This method prevents voltage discrepancies, reducing the chances of inter-cluster circulation. The downside is the additional energy loss and increased costs due to the inclusion of DC/DC components.
    •    Optimizing Power-On Logic: Adjusting the power-on sequence and logic can significantly mitigate inter-cluster circulation. By employing a pre-charging resistor within the high-voltage box of the battery cluster, inter-cluster circulation is balanced during the power-on process.

The typical power-on logic for inter-cluster circulation balancing involves:

    1.    Voltage Difference Check:
    •    If the voltage difference is less than 10V, the pre-charging contactor closes, and after a short delay, the main positive contactor closes, completing the power-on process.
    •    If the voltage difference is between 10V and 20V, the pre-charging contactor engages, balancing the voltage until it drops below 10V, at which point the system proceeds to the previous step.
    •    If the voltage difference exceeds 20V, the BMS triggers an alarm, indicating power-on failure, and prohibits grid connection until manual intervention and maintenance are performed.

Conclusion
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Inter-cluster circulation is a significant concern in BESS, impacting system efficiency and battery lifespan. By implementing measures such as cell matching, active balancing through BMS, and circuit adjustments with DC/DC converters, the adverse effects of inter-cluster circulation can be minimized. Understanding and addressing these challenges are key to ensuring the long-term stability and efficiency of Battery Energy Storage Systems.

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