Advancing LiFePO4 battery SOC estimation: Electrochemical impedance spectroscopy with short-period sine-wave pulses

State-of-charge (SOC) estimation for LiFePO4 (LFP) batteries presents challenges due to their flat open-circuit voltage. Recent studies suggest that electrochemical impedance spectroscopy (EIS) offers a promising approach for SOC estimation in LFP cells. This work investigates a practical SOC estimation method based on EIS data obtained from short-duration sinusoidal current pulses. First, the EIS of LFP cells is characterized across a broad frequency range [0.01 Hz, 1,000 Hz] and SOC range [0, 1]. The EIS magnitude and phase at 0.01 Hz exhibit the highest signal-to-noise ratio and are thus selected as features for SOC estimation. An EIS identification algorithm is then developed and validated to reconstruct EIS at 0.01 Hz. This method utilizes Fourier series expansion to approximate the voltage response to small sine-wave current perturbations. SOC estimation is subsequently performed by mapping the reconstructed EIS to experimental EIS data. Finally, the proposed SOC estimation approach is validated using sine-wave currents of varying amplitudes (0.05 A and 0.1 A) and different cell operation modes (discharge and charge). The results demonstrate rapid and accurate initialization of LFP cell SOC using this estimation algorithm.