Condition-Dependent Open Circuit Voltage Behavior in Vanadium Redox Flow Batteries and Implications for State-of-Charge Estimation

Accurate state-of-charge (SOC) estimation is essential for reliable operation of vanadium redox flow batteries (VRFBs), yet many model-based methods treat the open-circuit-voltage (OCV)-SOC relationship as a fixed calibration curve. This study experimentally investigates condition-dependent SOC-OCV behavior using a laboratory-scale VRFB equipped with a bypass OCV cell. The bypass OCV method was validated against an intermittent discharge–rest method, with OCV differences below 5 mV. SOC-OCV characteristics were then examined under different electrolyte flow rates, cycling histories, electrolyte/component refreshing conditions, and a dynamic stress test profile. The results show that the SOC-OCV curve varies with cycling history and flow rate, while electrolyte/component refreshing and dynamic operation further modify the measured OCV response. Fixed-curve-based SOC inversion confirms that calibration mismatch can introduce substantial SOC estimation errors, with a case-specific maximum error of 11.36% observed when the initial post-preconditioning constant-current curve was applied to the cell after 50 cycles under the tested 96 mL min−1 DST condition. These findings highlight the need for adaptive OCV correction and condition-dependent SOC-OCV mapping in practical VRFB SOC estimation.