Prismatic Li-ion battery cells are increasingly employed in electric transportation, where accurate state of charge (SoC) estimation is critical for reliable and efficient operation. However, conventional SoC estimation methods, primarily based on voltage measurements, face challenges with LiFePO cells due to their characteristically flat voltage-SoC profile. To address this limitation, ultrasonic diagnosis offers a valuable alternative by leveraging the coupling between SoC and the mechanical properties of the cells. In this study, we propose a novel ultrasonic feature — the wave velocity along internal layers — as a robust indicator of SoC. Its sensitivity to SoC is validated experimentally, and the correlation is further confirmed through numerical simulations. A full-scale finite difference time domain (FDTD) model is developed to simulate ultrasonic wave propagation, capturing microscale fluid-solid interactions within the porous electrode and separator layers, as well as the coupling between them. Furthermore, multiscale modeling based on finite element model (FEM) and transfer matrix method (TMM) is introduced. Both simulation methods reveal that the observed correlation primarily arises from internal pressure variations, which are directly related to SoC and modulate the interfacial stiffness between the electrode and separator layers. As a result, ultrasonic wave velocity serves as a non-destructive proxy for internal pressure measurements and SoC estimation. With this theoretical insight, the repeatability and transferability of this ultrasonic velocity-based approach are demonstrated, positioning it as a promising complement to existing electrical and mechanical methods.
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