Study On Electro-thermal Models And Warm-up Control Of Lithium-ion Batteries At Low Temperatures

To counteract the energy crisis and global climate change,transportation electrification is regarded as an effective energy-saving and emission-reducing countermeasure.Electric vehicles,as an important part of transportation electrification,have received widespread attention.At present,lithium-ion batteries are widely used as energy storage components in electric vehicles.However,their charge/discharge performance is greatly undermined at low temperatures,bring great challenges to the use and popularization of electric vehicles in cold geological regions.To this end,it is essential to develop battery warm-up strategies to restore the performance of lithium-ion batteries at low temperatures in a quick and efficient way.This thesis aims to develop a battery-powered warm-up strategy that can restore battery performance as much as possible based on the electro-thermal model of lithium-ion batteries when externalpowered heating is not accessible.To begin with,a series of experiments on the studied lithium-ion battery have been carried out under different ambient temperatures,including discharge capacity test,pulse discharge test,and open-circuit voltage test.Then,based on the experimental data,an electro-thermal model is established.In terms of the battery model,a novel SOC model considering the battery capacity variation at low temperatures has been proposed and this model has been explained from the perspective of electrochemistry.For parameter identification,since the battery has different voltage responses to pulse discharge under different SOCs and temperatures,an adaptive identification approach has been proposed to improve the accuracy of the identified model.Secondly,due to the existence of two-way coupling relationship in the electrothermal model,the battery electrical model and the thermal model would affect and interact with each other.To this end,this paper studies the influence of different submodel combinations on the accuracy of the electro-thermal model,based on the commonly used control-oriented electrical and thermal models.Meanwhile,the accuracy of different electro-thermal models under various ambient temperatures and current conditions has been systematically compared and analyzed in order to provide knowledge for selecting an appropriate battery model at low temperatures.Finally,for a typical operating scenario where electric vehicles must use their stored energy for warm-up and start-up at low temperatures,the effectiveness of warm-up strategy has been defined in this thesis from the perspectives of available energy and power of lithium-ion batteries.Based on these two objectives and the electro-thermal model,a multi-objective optimal control problem during the battery-powered warm-up process has been formulated.Radau pseudospectral method with adaptive multi-mesh interval collocation is leveraged to solve the infinite-dimensional nonlinear optimal control problem.By adjusting the relative importance of the two warm-up objectives,three typical preheating strategies,namely maximum-energy oriented warm-up,maximum-power oriented warm-up,and balanced warm-up,are obtained and analyzed by using the pre-defined principle of effective warm-up.In addition,sensitivities to initial SOC,equivalent heat transfer coefficient,and ambient temperature have also been investigated and analyzed.