Research On Active Balancing Strategy And System Implementation For Battery Pack Of Pure Electric Vehicle

The battery pack of pure electric vehicles is composed of a series of single cell batteries in series,in the process of charging and discharging,due to the inconsistency between the single cells,a single cell may reach the charge-discharge cut-off voltage in advance and stop charging and discharging.Therefore,the inconsistency between single cells reduces the capacity utilization of the battery pack,reduces the driving range and increase the cost.Active balancing control module is an important part of the BMS,which monitors battery pack status in real time and equilibrates the battery pack according to the corresponding control strategy.So it can reduce the impact of inconsistencies between the cells in the battery pack,increase the utilization of battery capacity and the driving mileage of vehicles.The research object is the series lithium-ion battery pack,based on the analysis of the mechanism of battery inconsistency,determin the control scheme of active equilibrium system,Formulate the balanced control strategy and complete system hardware and software design,finally,do the bench test and verify the feasibility of the system scheme and the effectiveness of the control strategy.The specific research work is as follows:1,Analyze the causes of the inconsistency and its expansion between the cells from the three aspects of production process,use process and storage process,and propose measures to improve battery pack inconsistency.Obtain the characteristic parameters of the battery and the manifestation of different characteristic parameters on battery pack inconsistency through battery model and battery characteristic tests.2,Compare the advantages and disadvantages of the common equilibrium topology and select the active equilibrium topology based on fly-back transformer.According to its characteristics,two modes of work have been established.Calculate the transformer parameters and complete the design of the peripheral circuit.Build models and optimize parameters through simulation.3,Compare stability and measurement accuracy of open circuit voltage,working voltage and SOC,finally,select the open circuit voltage as the equilibrium variable and the maximum voltage difference as the control variable.Lithium battery voltage has a small difference between the cells in the platform phase and a large difference in the non-platform phase,so,different opening thresholds are formulated at different stages.“Charging Balancing” and “Parking Balancing” control strategies are formulated,and the model is built,the effectiveness of the balanced control strategy is verified by the simulation results.4,Balanced system adopts "master-slave" distributed architecture to improve the system adaptability and reliability.The components selection and circuit design of power supply module,voltage acquisition module,current acquisition module,equalization module and communication module are completed,and complete the system software design.Builds bench test platform and the effectiveness of the balanced control strategy is verified by the bench test,the system improves battery pack inconsistency and battery pack energy utilization.