Research On Energy Equalization Strategy For Lithium-ion Battery Pack Of Electric Vehicles

Lithium iron phosphate(LiFePO4) battery is the power source of electric vehicles(EVs). There is inconsistency among every single cell of the battery pack due to the impact of production process and external environment, which leads to the reduction of available capacity, mileage and battery life. So battery pack equalization technology has become a key issue in the development of EVs.Battery management is a hot spot in the research field, including battery monitoring, SOC estimation(state of charge), equalization strategy, etc. SOC consistency can most effectively improve the consistency of battery and the accuracy of SOC estimation restricts the research progress of equalization strategy. The passive equalization strategy has been so far well established, but there are still many problems in the active equalization strategy.Battery active equalization system, aiming at the problems existing in the equalization strategy of EVs, is studied based on Extended Kalman Filter(EKF) algorithm and bi-directional fly-back transformation equalization circuit(LTC3300). The following work has been carried out:1. To study the electrical characteristics of Lithium-ion battery and analyse the causes of inconsistency. In this paper, the causes and phases of inconsistency are studied in detail in the process of the manufacture, application and storage of Lithium-ion battery. Then the experiment object is selected to carry out the experiment and the data is recorded. The present form in external battery of the internal inconsistencies is analysed according to the data and the reference can be given to the selection of the equalization criterion.2. To study and analyse the advantages and disadvantages of several existing SOC estimation methods and equalization strategies. SOC estimation algorithm and Lithium-ion battery equalization circuit are designed. Bi-directional fly-back transformation equalization circuit is selected as the dominant direction of circuit design in this paper. Open circuit voltage and SOC are determined to be the equalization criterion. The difference between SOC of single cell and the average SOC is made to measure the consistency of the battery. SOC estimation algorithm and equalization criterion in several states based on EKF algorithm are presented. Then the simulation verification is carried out.3. To handle on the hardware circuit and software design of the battery equalization system. The overall program is proposed according to the actual situation of EVs. Then the details of the circuit are considered and finally the whole circuit welding and PC software programming are finished.4. To test the equalization system and the PC software and the equalization effect is observed.At last, the EV equalization strategy designed in this paper is tested. And the result shows that there is great improvement in available capacity, mileage and battery life of Lithium-ion battery pack with equalization strategy proposed in this paper. The equalization effect is consistent with the design requirements and it has certain practical value.