A study of the open circuit voltage characterization technique and hysteresis assessment of lithium-ion cells

Among lithium-ion battery applications, the relationship between state of charge (SoC) and open circuit voltage (OCV) is used for battery management system operation. The path dependence of OCV is a distinctive characteristic of lithium-ion batteries which is termed as OCV hysteresis. Accurate estimation of OCV hysteresis is essential for correct SoC identification. OCV hysteresis test procedures used previously do not consider the coupling of variables that show an apparent increase in hysteresis. To study true OCV hysteresis, this paper proposes a new test methodology. Using the proposed methodology, OCV hysteresis has been quantified for different lithium-ion cells. The test results show that a battery's OCV is directly related to the discharge capacity. Measured battery capacity can vary up to 5.0% depending on the test procedure and cell chemistry. The maximum hysteresis was found in a LiFePO4 (LFP) cell (38 mV) and lowest in the LTO cell (16 mV). A dynamic hysteresis model is used to show how better prediction accuracy can be achieved when hysteresis voltage is a function of SoC instead of assuming as a constant. The results highlight the importance of the testing procedure for OCV characterisation and that hysteresis is present in other Li-ion batteries in addition to LFP.