SOC-dependent voltage relaxation and dual time-constant behavior in lithium-ion batteries: A time-domain analysis

Voltage relaxation plays a critical role in open-circuit voltage (OCV) characterization and state of charge (SOC) estimation in lithium-ion batteries. In practice, a fixed rest duration is commonly applied across the SOC range, implicitly assuming uniform relaxation behavior. This study experimentally investigates SOC-dependent voltage relaxation dynamics in large-format NMC811 cells using controlled C/20 step-discharge tests at −10 °C, 10 °C, 25 °C, and 40 °C. The main contributions of this work are as follows: (1) a derivative-based segmentation method is introduced to identify fast and slow relaxation regimes without predefined time windows; (2) fast and slow relaxation time constants are systematically extracted across the full SOC range; (3) the results reveal a pronounced SOC-dependent U-shaped variation of the slow time constant across all temperatures; and (4) SOC-dependent rest-time guidelines are proposed, demonstrating that fixed rest durations can lead to systematic equilibrium voltage errors. These findings establish a direct link between time-domain relaxation behavior and practical battery characterization, providing improved strategies for OCV calibration and BMS-oriented SOC estimation.