A Study On Electro-Thermal Coupling Model And Temperature Control Of An Air-cooled Battery Pack For Electric Vehicles

The global energy shortage is becoming more and more serious.As a clean and energy efficient travel tool,electric vehicles are widely promoted.Batteries provide power for electric vehicles.Lithium batteries are widely used in electric vehicles because of high energy density and low self-discharge rate.During the operation of electric vehicles,batteries release a large amount of heat energy,resulting in the increase of battery temperature.The suitable working temperature of lithium batteries is 15 °C-35 °C.When working in high temperature environment,the life and capacity of lithium batteries decrease rapidly,and a large amount of heat accumulates inside the batteries,which may lead to thermal runaway or even explosion.Therefore,it is necessary to effectively estimate and control the battery temperature.Due to the inconsistency between the battery core temperature and the surface temperature,the core temperature reaches the thermal runaway threshold before the surface temperature under extreme conditions.And the core temperature cannot be directly measured by the sensor.Therefore,the thermal model of battery is established to estimate the battery surface temperature and the battery core temperature in real time.And by controlling the cooling air temperature at the inlet,adjusting the heat dissipation of the battery,the battery core temperature is maintained near the target temperature.It is beneficial to improve battery performance and ensure battery safety.On the basis of choosing air-cooled heat dissipation,the research contents of this paper are as follows:An electro-thermal coupling model of the battery is established to estimate the battery core temperature.The battery electro-thermal coupling model consists of an equivalent circuit model and a two-state thermal model.Firstly,the battery is equivalent to the battery shell and the core.The heat generation characteristics and heat dissipation characteristics of the battery shell and core are respectively analyzed.The heat balance equation of the battery shell and core is established to form the thermal model of battery.The parameters of thermal model are identified by least square method.In the equivalent circuit model part,the second-order equivalent circuit is used to simulate the battery electrical characteristics.The parameters of equivalent circuit model are identified offline by charge-discharge experiments.Independent identification of equivalent circuit model and thermal model parameters greatly reduces the complexity of model parameter identification.The battery heat generation is calculated by an equivalent circuit model and transmitted to the heat model.After obtaining the heat generation,the thermal model estimates the battery surface temperature and the battery core temperature.And the battery temperature is transmited to the equivalent circuit model to determine the temperature-related parameters in the equivalent circuit model.The electro-thermal coupling model of battery is composed of equivalent circuit model and thermal model through the heat generation and the temperature transfer.Based on the electro-thermal coupling model of cell,the heat transfer between cells in battery pack module is analyzed,and the electro-thermal coupling model of battery pack is established.In ANSYS,a computational fluid dynamics(CFD)model is established to simulate the structure and heat dissipation conditions of the actual battery system.The reliability of CFD model is verified by experimental correlation.It is shown that the temperature data obtained by CFD model simulation can replace the experimental data and be used to verify the accuracy of the thermal model of battery pack.Under the new European Driving Cycle(NEDC)condition and High Speed/Load Transient Control Cycle(US06)condition,the accuracy of electro-thermal coupling model of battery pack is verified.The results show that the proposed electro-thermal coupling model can accurately estimate the temperature change of battery core.Based on the linear,time-varying and constrained characteristics of electro-thermal coupling model of battery pack,a temperature controller based on model predictive control(MPC)is designed.By controlling the temperature of cooling air at the inlet,the heat dissipation of the battery is changed,and the battery core temperature is maintained near the target temperature.The effectiveness of MPC temperature controller is verified by the joint simulation of ANSYS and MATLAB.ANSYS is used to simulate the temperature changes of the cell surface and core at a certain cooling air temperature,and transfers the temperature data to MATLAB.Based on the obtained battery temperature data,the temperature controller in MATLAB calculates the cooling air temperature and transmits it to ANSYS.ANSYS simulation is used to get the battery temperature at this cooling air temperature.Under NEDC condition and US06 condition,the effect of constant cooling air temperature,PID temperature controller and MPC temperature controller on battery temperature control are compared,and the validity of MPC temperature controller is verified.In this paper,an electro-thermal coupling model consisting of an equivalent circuit model and a two-state thermal model is established to estimate the core temperature.The equivalent circuit model and thermal model parameters are identified by charge-discharge experiment and least square method,respectively,to reduce the complexity of model parameter identification.The MPC temperature controller is designed to maintain the battery core temperature near target temperature by controlling the temperature of cooling air at the inlet.Finally,the validity of MPC temperature controller is verified by the joint simulation of ANSYS-MATLAB.