Research On Decoupling Modularized Active Equalization System For Electric Vehicle Battery Pack

With the emergence of global warming,oil crisis and environmental degradation,people are increasingly concerned about the environment and energy problems caused by traditional internal combustion vehicles.In recent years,electric vehicles have seen rapid development.However,limited by the small voltage and capacity of cell,hundreds or thousands of cells are usually required to form a battery pack through series and parallel mode to meet the power and range requirements of electric vehicles.However,the energy density,cycle life and safety of the battery pack will be significantly reduced due to the inconsistency of cells in the battery pack.Therefore,an active equalization control system should be added to the Battery Management System(BMS)to control the inconsistency of battery pack in real time.Based on the active equalization control system as the research object,in order to improve the equalization efficiency and power,the main research on the equalization topology,equalization hardware circuit design and equalization strategy under the conditions of different imbalances,and combined with the simulation and experiment on the design of equalization system verifies the effect of equalization.The main research contents include the following parts:1.Equalization overlap of different equalization topologies and structures are found and proposed.The equalization overlap phenomenon is shown as follows: in the process of equalization,some cells are charged and discharged repeatedly without meaning to make their voltage or SOC approach to the equalization voltage or SOC in a zigzaggy way.On the one hand,the existence of equalization overlap will reduce the equalization efficiency and equalization rate of the equalization system.On the other hand,it will consume the cycle life of the cell and have a negative impact on the SOH(State of Health)of the battery.For the nonmodular active equalization topology,only the single-step cell-to-cell equalization topology can completely avoid the occurrence of equalization overlap.However,for large-scale series battery packs,modular equalization is usually needed to improve the equalization rate.However,due to the coupling relationship between intramodule equalization and intermodule equalization,the direct modularized active equalization structure will still inevitably have equalization overlap.2.A decoupled modularized active equalization structure is proposed to avoid the occurrence of equalization overlap.The structure connects the equalization switch array between modules and the equalization bus of the switch array within each module.When balancing between modules,not only each module as a whole can be balanced,but also any cell or adjacent cells in each module can be balanced,which removes the coupling relationship between intramodule equalization and intermodule equalization and avoids the occurrence of equalization overlap.3.The equalization circuit is designed,and the equalization circuit adopts the equalization topology of cell-to-cell based on Buck-Boost converter.According to the dead time existing in the equalization period,the inductive continuation circuit is designed.In order to prevent the cell from charging the inductor in reverse at the end of energy release,a current detection circuit based on current sensor is designed.After the design of the equalization circuit is completed,the switching timing sequence is precisely arranged according to the transition between modes in the equalization cycle.Finally,the values of equalization inductance and the switching frequencies of different working modes of intermodule equalization and intramodule equalization were determined by the method of simulation and experiment.4.The equalization strategy is designed.The core design idea of the equalization strategy is to ensure that each cell gets close to the average SOC of the series battery pack in a monotonous way,and try to achieve a higher equalization power when balancing between modules.For this reason,the equalization effect is guaranteed through the following four points: 1)Selecting the modules that only need to be balanced within the module to reach the final equalization state.2)In order to prevent equalization overlap,only intermodule equalization is carried out for modules in which the SOC of each cell is higher than the average SOC of the battery pack or the SOC of each cell is lower than the average SOC of the battery pack.3)When balancing between modules,the modules with a large number of energy cells adjacent to each other and the modules with a large number of energy rich cells adjacent to each other are selected for balancing.4)Stepped-type intramodule equalization is used for intramodule equalization.5.Equalization simulation and experimental verification are carried out for the proposed decoupled modularized active equalization system.The experimental results show that,compared with the direct modularized active equalization structure,the amount of charge transferred by equalization is reduced by 13.0%,and the amount of charge transferred by equalization overlap is reduced by 90.7%,which effectively avoids the occurrence of equalization overlap.The final equalization rate and equalization efficiency are increased by9.7% and 2.0%,respectively.In addition,since the decoupled modular active equalization structure does not need to balance the modules as a whole,the disturbance brought by equalization is smaller,and the maximum SOC difference and SOC standard deviation of the battery pack can be controlled to be smaller at the end of the equalization.