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

In recent years,energy crisis and air pollution problems become more serious.Therefore,electric vehicle has been rapidly developed as a clean and efficient energy vehicle.The performance of the power battery pack decides on power,stability and safety of the electric vehicle.Power battery packs continuously charge and discharge with large current when the vehicle is moving.A series of chemical reactions take place in the batteries,which releases a lot of heat and results in the increase of temperature leading to the decrease of battery lifetime and capacity,even combustion and explosion.Therefore,the establishment of battery pack thermal model,and the design of the effective battery temperature control algorithm are essential for the security and stability of the electric vehicles.Based on air-cooled method for the battery pack temperature control,the main contributions of this paper are as follows:The thermal model of the battery pack is established.First,the heat generation mechanism of the lithium ion battery and the heat transfer characteristics between battery and ambient air are analyzed.Based on Bernardi heat generation model and Newton cooling rate,the thermal model of battery with time-varying internal resistance and heat transfer coefficient is established.In consideration of the characteristics of the battery pack temperature distribution increasingly from inlet to outlet based on the air-cooled method,the thermal models of the two single cells with the highest and the lowest temperatures are established as the thermal model of the battery pack.But the air temperature is immeasurability.Based on the assumption of equal temperature increase of batteries and air from inlet to outlet,to predict the changes of battery temperature,air temperature variables are eliminated.The accuracy of the battery pack thermal model is verified by ANSYS/FLUENT software.First,the geometric model is built.The ANSYS software simulation process is set for air cooled condition and it is verified by experimental correlation.Then,based on the simulation results of experiments adopting constant current,New European Driving Cycle and US06,the thermal model is verified to be high accuracy.A battery pack temperature controller based on nonlinear model predictive control and particle swarm optimization algorithm is designed.The controller adjusts the wind speed to control battery pack temperature.Nonlinear model predictive control not only has a feedforward-feedback structure,but also can deal with constraints explicitly and actively,so it is suitable for non-linear,multi-constrained battery pack temperature control system.The particle swarm optimization method is adopted to get wind speed.Because the iterative logic of particle swarm method is simple,the particle swarm method can quickly solve the problem.The co-simulation of ANSYS and MATLAB is achieved and used to verify the effectiveness of the battery pack temperature controller.ANSYS simulates the temperature changes of the battery pack cooled by air,and it passes on the temperature data to MATLAB.The temperature control program calculates the control variable,and returns the control variable to ANSYS to change the simulated wind speed.In this way,the effectiveness of the battery pack temperature controller is verified.Respectively,in the NEDC condition and US06 condition to verify the effectiveness of the algorithm,the results show that the nonlinear model predictive control can keep the maximum temperature of the battery in the vicinity of the control target,and it can keep the overall temperature difference of the battery pack small.This paper mainly builds the thermal model of the battery pack based on the air-cooled method,and it designs the battery pack temperature controller based on the model.Firstly,the heat generation equation and heat dissipation equation of the battery are analyzed and deduced.The thermal model of battery pack with time-varying internal resistance and heat transfer coefficient is established,and the model accuracy is verified by ANSYS.Then,the nonlinear model predictive controller is designed and the optimization problem of the predictive controller is solved by using the particle swarm optimization algorithm.Finally,the co-simulation of ANSYS and MATLAB is realized,and the effectiveness of the battery pack temperature controller is verified.