The Effect of Different Open-Circuit Voltage Curves on the Accuracy of Model-Based State Estimation in Lithium-Ion Batteries

Abstract

Due to their unique characteristics, the condition monitoring of lithium-ion batteries in the applications where they are used is inevitable. Most of the condition monitoring approaches employed today are based on electrical equivalent circuit models (ECM). Currently, there are many different structures of ECM in use. However, the common component among all of them is the voltage source that provides the open circuit voltage (OCV), and its accuracy directly affects the results of the analyses. The voltage of this source depends on the state of charge (SoC) of the battery. This dependency varies based on the structure of the battery. Therefore, the relationship between the OCV and the SoC of a battery is determined using data obtained from experiments conducted for this purpose. These experiments involve complex processes, including charging, discharging, and/or operation at low current values. Due to the structure of lithium-ion batteries, the OCV curve obtained for charging differs from the one obtained for discharging. Therefore, in condition monitoring applications, OCV curves derived from these two processes are used in models based on different approaches. These approaches generally include four types of OCV curves derived from the data of charging and discharging experiments conducted at one-wentieth of the nominal current level: the charge curve alone, the discharge curve alone, the curve obtained by averaging the charge and discharge curves, and a curve that aligns with the discharge curve at high states of charge and the charge curve at low states of charge. In addition, OCV curves obtained directly from current and voltage data in Hybrid Pulse Power Characterization (HPPC) tests are also used. In this study, a different approach is examined, where the OCV during charging follows the charge curve and the OCV during discharging follows the discharge curve, based on the direction of the current. The results obtained from this approach are compared with those obtained using other approaches. In the comparisons, the mean absolute error and the root mean square error calculated during the estimation of the terminal voltage are used as criteria. The comparison shows that the lowest error values are achieved when the OCV and SoC curve obtained from low-current discharge experiments is used.