Optimization And Experimental Study On Compound Heating Strategy Of Lithium-Ion Battery For Electric Vehicle At Low Temperature

Electric vehicles have developed rapidly in recent years due to the characteristics of energy saving,environmental protection and high energy conversion efficiency.At low temperatures,the performance of lithium-ion batteries is severely attenuated,such as discharge power and capacity.When lithium-ion battery is charged at low temperature,the battery can’t be fully charged.Even lithium deposition and lithium dendrites appear on the negative electrode.If the lithium dendrite continues to grow to a certain extent,it will cause a short circuit inside the battery,which is dangerous.All these problems have seriously affected the development and popularization of electric vehicles in the extremely cold areas of north China.Therefore,how to heat lithium-ion battery reliably,efficiently and safely at low temperature is particularly important.Aiming at this scientific problem,this paper proposes a multi-stage constant-current composite heating method,in which the battery discharges power for the external heating film.Then,the electrothermal coupling model considering aging is established,and the balanced heating strategy of single cell is obtained through optimization algorithm.The relationship between battery aging,heating time and capacity gain in single cell is studied.Secondly,the battery pack model is established.And the balanced heating strategy of the battery pack is obtained through the optimization algorithm.Finally,the heating current database of single cell and battery pack in different states are established respectively.The main contents are as follows:(1)The variation of battery parameters at low temperature was studied by the experiments of low temperature discharge characteristics,hybrid pulse power characteristic,low temperature discharge temperature rise and equivalent convective heat transfer coefficient.According to the experimental results,the second-order RC electrical model,the thermal model,and the aging model,a low-temperature electrothermal coupled model considering aging is established.And the accuracy of the model is verified by experiments.(2)A multi-stage constant-current composite heating method is proposed,and a multi-objective nonlinear optimization model of battery aging,heating time and capacity gain is established.The relationship between battery aging,heating time and capacity gain is revealed.On the basis of the low-temperature electrothermal coupled model considering aging,a balanced heating strategy based on NSGA-Ⅱ optimization algorithm is proposed.The single cell heating strategies in different initial states are analyzed: when the initial temperature of the battery is lower,the heating time is longer,the capacity gain is greater,the battery aging is greater,and the peak power gain is greater.When the initial SOC is larger,the capacity gain is larger and the peak power gain is larger.When the initial temperature of the battery is-20 °C,the heating time to 10 °C is253 s,the capacity gain is 4.72 Ah,and the battery aging is 0.482(?).According to the different initial states of the battery,the heating current database of the single cell is established based on the balanced heating strategy.(3)The battery pack model is established and the connection and arrangement of the heating film are optimized.The optimization results in a balanced heating strategy of the battery pack considering the temperature difference between the cells.The heating strategies of battery packs in different initial states are analyzed: when the initial SOC is the same and the initial temperature of the battery pack is lower,the aging of the battery is greater,the heating time is longer,the capacity gain is greater.At the same initial temperature,the capacity gain increases with the increase of initial SOC.When the initial temperature of the battery is-20 °C,the heating time to 10 °C is 217 s,the capacity gain is 5.09 Ah,and the battery aging is 0.595(?).According to the different initial states of battery pack,the battery pack heating current database is established based on the balance heating strategy.