State-of-Charge Determination in Imbalanced Battery Packs

The state-of-charge (SOC) estimation is of extreme importance for the reliability and safety of battery operation in large scale energy storage systems. How to estimate SOC for an assembly of cells in a battery pack remains a subject of great interest.

We recently proposed a viable method for SOC determination and tracking for multi-cell assemblies¹. We showed that the battery pack attributes, the relationship between SOC and OCV and the relationship between SOC and capacity, could be accurately derived from the attributes of the single cells within the pack. This allows accommodating cell variability and pack configuration in a new way that does not require intensive computation or complicated calibration and that can be easily implemented in a BMS. This approach only requires two distinct measurements of rest cell voltages of all single cells in the pack at a given state of health. Additionally, two parameters were introduced to characterize and track cell imbalance evolution. From this evolution, the capacity loss can be decomposed into two components, the loss associated to the degradation of the single cells and the loss associated by the evolution of the imbalance. From this decomposition, the evolution of the different contributions could be extrapolated independently in order to reach a more accurate prognosis on the battery pack remaining useful life.

In this work we’ll put this method in application to diagnose and prognose the capacity fading of several battery packs operating under different conditions. We will showcase how the OCV=f(SOC) relationship of the battery pack is connected to the one of the single cells and how it could be calculated via simple voltage measurements. The imbalance evolution will allow be discussed and it will be shown how to quantify the different origins of the capacity loss.

References:

¹ “State-of-Charge Determination in Lithium-Ion Battery Packs Based on Two-Point Measurements in Life.” M. Dubarry, C. Truchot, A. Devie and B. Y. Liaw, J. Electrochem. Soc, 162(6), p. A877 (2015). DOI:10.1149/2.0201506jes