Research On Thermal Management Of Traction Lithium Ion Battery Systems

Thermal management is essential for lithium ion battery system to ensure safety,extend lifespan,and improve performance.With the leaping development of electrical vehicles in recent years,the capacity and size of lithium ion battery cells are increasing gradually;the number of cells contained in a battery system is also increasing;and more high power operating conditions are included in the drive cycle.These factors brought severe challenges to the design and optimization of battery thermal management system.In order to solve the problems encountered in the development and design of thermal management system for large-scale power lithium-ion batteries,this thesis focuses on the key technologies in research and design of battery thermal management systems,such as lithium ion battery modeling,battery thermal characteristics measuring,and design of cooling structure and thermal control strategies.Firstly,to fulfill different model requirements during the battery thermal management system research and design,a mechanism model,a lumped thermo-electric coupled equivalent circuit model,and a distributed parameters thermo-electric coupled equivalent circuit model were established,respectively.The solving methods of these three models were deduced.The simulation results obtained by these models consist well with the experimental results,which indicates their accuracy.The lithium ion concentration in solid or liquid phase,the exchange current between the solid phase and the liquid phase,the potential of the current collects,the current density,and the temperature inside the battery were analyzed.Secondly,the effects of battery operating temperature on battery internal resistance,capacity utilization rate,energy utilization rate,and capacity degradation rate were studied experimentally.The parameters such as capacity degradation rate,energy utilization rate and full load working time were taken as key factors influencing the temperature control target of the thermal management system.An optimization method was proposed for thermal management system temperature control target using those key factors.Thirdly,the flow-thermal coupled models of air cooling,fin cooling,direct liquid cooling and indirect liquid cooling were established,respectively.The influence of fluid flow rate on each cooling mode was simulated and analyzed.The advantages and disadvantages of each cooling method were analyzed from the aspects of extra weight gain,heat dissipation performance,and energy consumption of thermal management system.The tab cooling method was put forward,and the battery tab cooling test platform was set up.Experiments shown that the tab cooling was not only able to meet the heat dissipation requirements of large format pouch cell under high-power continuous operation conditions,but also could eliminate the influence of the connection resistances of tabs and keep the battery temperature consistency.Finally,different cooling schemes were proposed for a large-format pouch cell lithium ion battery module,and the cooling effect of each scheme was tested by experiments.The battery module temperature control platform including based on Peltiers was built.The control based thermal model of the battery module was obtained by experiments.The dynamic matrix prediction control algorithm for temperature of the battery module is deduced.The effects of sampling period,prediction time domain,control time domain and other dynamic matrix control algorithm parameters on the battery module temperature were analyzed.It is found that the dynamic matrix predictive control could reduce temperature overshoot and cut down maximum cooling power demands,which could solve the control problems caused by slow response time of battery module temperature.