Research On Whole Process Oriented Lithium-ion Battery Pack Equalization Control Strategy

The Lithium-ion battery cell voltage is low and capacity is small. In order to meet the performance requirements of electric vehicles, numerous cells are connected into packs in series and parallel. With the research and promotion of electric vehicles, single inconsistency problem is getting wide attention. The capacity utilization and cycle life of batteries after grouping is far less than a single cell because of the single inconsistency. The equalization is one of the effective ways to solve the problem and has important practical significance. Nevertheless the research on equalization control method is at beginning. The control strategy is rough and many problems need to be solved. This thesis takes a deep study on equalization control strategy.Firstly, this thesis introduces the performance parameters of the battery and the charge and discharge characteristics under different conditions, then comparatively analyses the battery model and the state of charge estimation method. The thesis discusses the cause of inconsistency, the impact of inconsistency and the way to solve this problem. The thesis explores advantages and disadvantages of equalization with different control targets and establishes models to analyze the valid capacity and the state of charge of battery packs under different equalization energy transfer methods so as to research the performance of battery packs.Secondly, this thesis establishes a microscopic model of battery discharge process and explores the ideal equalization state and accessibility based on this model. Then this thesis set up a realistic equalization target, which controls the uniformity of terminal voltages at the end of charge or discharge. While in other period this method combines the current with the ability of equalization circuit to predict the variation of remain capacity and limit the difference into a scale that the equalization circuit is able to handle. Then design a whole process oriented equalization control strategy according to the target and analyze the performance of the strategy.Finally, this thesis validates the equalization strategy through simulation and experiment and evaluates the strategy by the discharge capacity and the mean equalization capacity and compare with terminal voltage center equalization strategy which is widely applied. The results of simulation and experiment show the strategy designed by the thesis can decrease the equalization capacity and promote the discharge capacity effectively.