Estimating state of charge of lithium-ion batteries using ultrasonic reflection and transmission coefficients by Legendre orthogonal polynomial method

The state of charge (SOC) of lithium-ion batteries can reflect the usage of batteries. When the battery has problems or aging, the amount of charge must be affected. Knowing the change in battery SOC in advance by using ultrasonic detection will reduce the harm caused by aging. During the charge, the elastic modulus and density of the internal electrode vary. This will affect the propagation of ultrasonic waves inside the battery, especially for the reflection coefficient and transmission coefficient. In this research, the Legendre orthogonal polynomial method is adopted to solve this problem. When SOC varies, the reflection and transmission coefficient curves show regular shift while the amplitude changes accordingly. It proves that both the coefficients are capable of estimating battery SOC. A finite element model of lithium-ion battery was established in the COMSOL simulation software. Both the reflection and transmission coefficients obtained were consistent with the theoretical prediction, which further verified the feasibility of the method.