An electric-thermal coupling modeling method for lithium-ion battery using the state of charge normalization calculation method

The improved equivalent circuit model for Lithium-ion battery (LIB) has gained popularity in engineering due to its practical and straightforward structure, encompassing core parameters of interest such as open-circuit voltage, internal resistance, temperature, and state of charge (SOC). However, the nonlinear and coupled nature of electrical and thermal parameters makes it challenging to solve these parameters online in a battery management system. Thus, this paper proposes a battery model to decouple the electric-thermal coupling relationship, comprising analytical sub-models for open-circuit voltage, internal resistance and thermal parameters. The proposed method involves designing sets of off-line experiments under constant-current battery conditions and normalizing SOC calculation by utilizing a coulomb efficiency coefficient. The electrical and thermal parameters are then decoupled using off-line parameter identification to obtain open-circuit voltage, internal resistance, and temperature data under specific conditions. An open circuit voltage modeling method is proposed based on the Nernst equation and fitting method. Then, an internal resistance model of the battery is also modeled based on the Arrhenius equation and fitting method. In addition, according to the principles of heat generation and heat transfer, a thermal model is constructed and its model parameters are identified. Through the verification of internal resistance and terminal voltage under different ambient temperature and current conditions, the results illustrate that the modeling method proposed in this paper is able to accurately estimate the open-circuit voltage, internal resistance, and temperature based on online operational data of the battery.