Two-Point State-of-Charge Determination in Lithium-Ion 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. Here, a viable method for SOC determination and tracking for multi-cell assemblies is proposed and validated. Using 3S1P strings as an example, an inference of SOC is illustrated in a battery assembly based on an accurate open pack voltage (OPV) versus SOC (i.e. OPV = ¦(SOC)) function. The proposed method only requires the measurement of the rest cell voltages of the single cells on two distinct occasions.

This new method offers great benefits to SOC calibration and tracking, which should improve the accuracy of any SOC tracking method (Figure 1). This method is not calculation intensive and could easily be implemented in any BMS. Additionally, we introduced two parameters which could be used to characterize and follow the imbalance evolution within a pack, and thus, enable remaining useful life determination with improved accuracy.

Coupled with our other diagnostics tools we believe that this approach could lead to a significant improvement of the quality of BMS SOC tracking and SOH diagnostics, for all applications where large battery assemblies are needed. As a matter of fact, and to the best of our knowledge, this is the first time where the pack-level (imbalance) and cell-level (aging) degradation factors of a battery pack could be distinguished and accurately quantified without any disassembly or maintenance downtime required.

Figure 1: Comparison between experimental data and simulation with and without the proposed method.