Ultrasonic reflection/transmission characteristics for state of charge of li-ion battery

Lithium-ion batteries are widely used in various fields, such as electronic products, energy storage and transportation, etc. Accurate estimation of the state of charge (SOC) of lithium-ion batteries is essential for maintaining the safe operation of batteries. In this work, considering the multilayer porous properties of lithium-ion batteries, a theoretical model of ultrasonic reflection and transmission characteristics is established, by combining the state vector and Legendre polynomials. Spectrums of reflection and transmission coefficients of the lithium-ion battery under charged and/or discharged conditions are obtained, which are calculated theoretically and simulated by finite element analysis. Meanwhile, in order to extract the shift properties of the ultrasonic reflection/transmission spectrums versus SOC, the sensitive points representing the local extreme values in the frequency and angular spectrums during charging or discharging were curve fitted. It is found that with the increase of SOC, the local extreme value points in frequency spectrums intend to shift to the high frequency side, and angular spectrums will shift to the small angle side. Moreover, an ultrasonic reflection experimental system was built to measure the reflection coefficient spectrum of two kinds of batteries with different specifications. By comparing with the theoretical calculation results, it is found that the experimental and theoretical results are in good agreement. And the results also show that the SOC is closely related to the ultrasonic reflection characteristics. Therefore, this research provides a new way to evaluation the SOC of the lithium-ion battery.