SoC Imbalances: What are they and what can you do about them?

What are cell imbalances?

Cell imbalances can occur because battery energy storage systems comprise of hundreds of thousands of individual battery cells, and while these cells are part of the same system, they vary in quality and aging. The weakest cell among them dictates the performance. Thus, when the BESS is charged, not every cell will charge to the same targeted value (e.g., 100% SoC). At the same time, when discharged, not every cell will be discharged to the same planned value (e.g., 0% SoC).

This has profound implications for the BESS. On the one hand, this can cause considerable stress for the battery and drive systems out of the pre-defined and safe operation windows, leading to safety issues, downtime and shorter lifetime. Additionally, this leads to wasted energy, meaning that BESS are less likely to be able to fulfil their market obligations.

The figure below indicates the amount of wasted energy (red) that can occur due to imbalances. On the left, you see the SoC of various cells that should be fully discharged. However, they do not have an equal amount of charge. Therefore, when they are charging, the cell (second to the left) reaches 100% charge before the others, meaning the other cells do not reach 100%, causing considerable wasted energy.

What can you do about cell imbalances?

Address state of charge imbalances in your energy storage system proactively. Use the Performance Manager to pinpoint nodes with high imbalances or leverage predictive insights. Share these findings with your O&M team to plan corrective actions, like balancing during off-peak hours when electricity prices are low.

Once the balancing process is complete, verify its effectiveness using the Performance Manager. If imbalances persist, consider replacing highlighted components in consultation with your O&M team. Additionally, leverage the Performance Manager to ensure proper thermal management to maintain uniform temperatures across the system, further promoting healthy battery life.

Root Causes of Imbalances

While SoC imbalances can occur naturally due to quality differences among the individual battery cells, they can also occur due to the way in which the battery management system (BMS) calculates state of charge. The BMS usually calculates the state of charge based on the Coulomb Counting method or voltage. The Coulomb Counting Method measures the current of the battery, but errors accumulate when the battery is used for a long time without being fully charged or discharged, making it difficult to accurately estimate SoC.

The limitation of using the voltage method to calculate the SoC is that the open circuit voltage (OCV) changes with temperature and aging. This is particularly problematic for LFP storage systems because these have a flat OCV, therefore small errors in voltage cause huge distortions for the SoC estimation.

When several cells or modules are aggregated, the inaccurate SoC provided by the BMS means that some cells are overcharged and some cells are undercharged, leading to even stronger SoC imbalances – thus more wasted energy, and higher safety risks.

Summary

Cell imbalances are a key indicator that can lead to low availability, revenue loss, safety risks and shorter battery lifetimes. Cell imbalances cause low availability as they lead to wasted energy and over time, lead to increased stress on the battery.

While it is helpful to know when and where cell imbalances are occurring, it is even more helpful to understand the root cause of the problem. This could be a problem that occurred during manufacturing or construction, it might have to do with HVAC components such as the cooling system, or it might result directly from the way that the storage system is operated.