Research On Equalization Control Of Electric Bus Power Battery System

In the context of the energy crisis and the deteriorating living environment,new energy vehicles,electric buses have gradually become the target of China’s green transportation development with its characteristics of energy saving,environmental protection and easy promotion.However,due to the constraints of the current power battery technology,the inconsistency of the power battery pack,resulting in the mileage of the electric bus and the service life of the battery is greatly reduced,which has become an important factor restricting the promotion of electric buses.Therefore,the development of an effective power battery equalization control system is of great significance for improving the inconsistency of the power battery pack.In order to select a more reasonable equalization reference variable,the open circuit voltage and the charge and discharge characteristics under different charge and discharge rates and ambient temperature of lithium iron phosphate battery were studied experimentally.Combined with the reasons for the inconsistency of the battery pack and the experimental results,the battery operating voltage,open circuit voltage and SOC were compared as the advantages and disadvantages of the equalization reference variable.It was found that the SOC can reflect the inconsistency of the battery pack more comprehensively and intuitively.Established the Thevenin equivalent circuit model of the power battery,and initially identify the battery model parameters.The Kalman filter algorithm was used to realize the online identification of battery model parameters.Based on this,the extended Kalman filter algorithm was used to estimate the battery SOC.The simulation model of SOC estimation was established on Simulink platform.The simulation results showed that the extended Kalman filter SOC estimation algorithm based on online identification has better robustness and estimation accuracy than offline identification,and provides an effective equalization variable for equalization control.By comparing and analyzing the typical balanced topology circuit structure,a bidirectional flyback transformer equalization topology circuit structure based on flyback DC/DC converter was designed.The structure can not only realize the bidirectional flow between the battery pack and the battery unit,but also ensure the isolation of the input and output under the action of the MOS tube.According to the parameters of lithium iron phosphate battery,the parameters of the equalization circuit were designed and calculated to ensure the performance of the transformer.In order to effectively improve the inconsistency of the power battery pack,an equalization control strategy was established in which the SOC value difference and the average value of each battery cell were collectively used as an equalization threshold.For the different requirements of the three working states of charging,discharging and standing for the equalization effect,the equalization method of “peak clipping”,“filling the valley”and according to the actual state of the battery to the SOC average value were adopted respectively.The simulation model was built on the Simscape platform to analyze the effectiveness of the balanced topology circuit and control strategy.The simulation results showed that the equalization control system studied in this paper can effectively improve the inconsistency of the battery pack,achieve balanced efficiency and ensure the battery capacity,it can be used most effectively during the charging and discharging process.