Research On The Equilibrium Of Series Lithium Battery Packs For Electric Vehicles

With the concept of energy conservation and environmental protection strongly advocated in various countries,oil-burning cars are gradually being phased out,and the development of electric vehicles powered by clean energy has become a new research direction of various countries.The performance of electric vehicles depends on the performance of the battery.Due to its high voltage,high energy density,high safety and low self-discharge rate,lithium batteries are now the most widely used power battery in the electric vehicle field.In order to meet the requirements of the output voltage of the electric vehicle,a plurality of lithium battery cells are generally connected in series,and a plurality of packages are connected in series.After multiple cycles of charge and discharge,even if the battery cells produced by the same manufacturer,the same batch,and the same model have different power levels between the cells,the inconsistency of the series battery will lead to the whole.The capacity is reduced,and it is prone to overcharging and over-discharging.In severe cases,the battery will explode.The equalization technology is born to overcome this practical problem.In this paper,the inconsistency and balance of charge and discharge in series lithium battery packs are studied.The main research contents are as follows:(1)The battery characteristics and working principle of lithium-ion battery are briefly introduced.The causes,manifestations,solutions and research status of the performance of each single cell in the series lithium battery pack from the factory to the use are analyzed.By comparing the advantages and disadvantages of the state of charge,open circuit voltage and operating voltage,the operating voltage is selected as the equalization object.(2)Several commonly used single-cell lithium battery models are introduced,and the advantages and disadvantages of each model are compared.A second-order RC equivalent circuit model is proposed from the perspective of nonlinear models,and the popular PNGV(the Partnership for a New)Generation of Vehicles models are compared,and their respective circuit structures,mathematical equations,and parameter identifications are established,and MATLAB simulation software is used for simulation comparison.(3)A two-way equalization method for lithium battery pack based on double-ended switching arm of single-ended flyback circuit is proposed.The improved single-ended flyback circuit is used as the main circuit,and the double-layer switching arm is used as the channel for strobing the single battery.When the battery pack is charged,the highest single-cellenergy can be transferred,and the battery can be supplemented with the lowest voltage when the battery pack is discharged.The monomer energy finally achieves a free balance of charge and discharge of the entire battery pack.The equilibrium topology and principle are analyzed.The voltage relative range is used as the condition for the equalization control strategy to be turned on and off.The corresponding equilibrium threshold is set and the simulink simulation experiment is built.(4)The charging and discharging equilibrium experimental platform of the series lithium-ion battery pack was built.The online discharge equalization and online charging equalization were performed on the three series lithium battery packs,and the working voltage of each single cell was collected in real time to verify the feasibility of the equalization scheme.