Research On The Method Of Internal Heating For Lithium-ion Battery At Low Temperatures

With the rapid development of the automobile industry in the world,environmental pollution problems and the global energy crisis are aggravating.The electric vehicles,a more environmental friendly and efficient means of transport,have attracted more and more attention for their research and development.Because of less pollution,long life,high energy density and good power performance,Lithium-ion batteries have become the main source of energy for electric vehicles.However,the performance of lithium-ion battery is vulnerable to ambient temperature.The battery life mileage decreased significantly at low temperatures.Battery charging at low temperatures can’t be charged fully,but also causes permanent damage for battery and reduces the cycle life.These problems caused a huge obstacle for the promotion and development of electric cars in the northern cold areas.The current affects both the capacity decay of battery and heating time when lithium-ion batteries are heated by the way of internal discharging at low temperatures.Aiming at the contradiction,both the battery capacity decay and heating time are proposed as the optimization goal,and the battery discharging for heating at low temperatures is researched by Simulation and Experiment.Firstly,the characteristics for lithium iron phosphate batteries were tested at different temperatures,such as capacity test,open circuit voltage test and hybrid pulse power characterization test.Parameters of the battery equivalent circuit model were identified by least square method.The influence of temperature on the battery performance and internal parameters is analyzed from qualitative and quantitative point.Secondly,the temperature rise model is established according to the battery thermal equilibrium equation.And the model parameters such as entropy coefficient and equivalent thermal transfer coefficient in the temperature-rise model are obtained.The experiment of constant-current discharging for heating battery at low temperatures was carried out.The error between the simulation results of the battery temperature rise model and the actual temperature during the battery experiment is so small that it can verify the validity of the battery temperature rise model.Finally,an objective function is established to optimize the heating time and the capacity fade in the battery heating process,which is based on the battery temperature-rise model and the cycle-life model.The needs of different heating effects are met by setting the weight factors for heating time and capacity fade in the objective function.The dynamic programming algorithm is used to solve the objective function to obtain the optimal discharging current for heating at different weighting factors.And the optimal discharging current was applied to the experimental subjects in order to compare the multi-objective optimization heating method with constant current discharging for heating method from the battery heating time,the capacity fade and the power consumption caused by the heating process.Simulation and experimental results show that the multi-objective optimized heating method for battery at low temperatures can not only reduce the battery capacity fade and heating time,but also decrease the power consumption in the progress of heating battery.