Research On Equalization System For Lithium Battery Pack In Electric Vehicle

With the development of industry,the problem of resources and environment are becoming more and more prominent,the energy structure urgently needs to be changed,and the global wave of new energy is set off.Electric vehicles have become the focus of the research.Lithium battery has a lager nominal voltage,high energy density,long cycle life,and many other advantages,often used as a driving force of electric vehicles.However,due to the influence of the manufacturing process and environment,the lithium battery string often presents an unbalanced state,restricting the available capacity of the entire battery pack and seriously affecting the mileage of the electric vehicle.In this paper,SANYO NCR18650 B ternary lithium battery is used as the researched object,the existing active equalization techniques are summarized;the equalization circuit is decomposed into two parts: equalization structure and equalizer,the theoretical analysis of equalization structure is carried out;the equalization circuit established with the appropriate equalizer selected achieves the equalization of battery.This paper summarizes the existing active equalization techniques from the perspective of the main equalization elements.Based on the summary,the equalization structure,one of the two parts of the active equalization technique,is classified.Under the MATLAB environment,the theoretical analysis based on the mathematical models of Series-based,Layer-based,Module-based and Global-based equalization structures established respectively is carried out.The method and the conclusion formula of equalization time,the battery SOC inconsistency after the equalization and the battery SOC average derivation are shown.Due to the existence of a various equilibrium strategies based on Global-based balanced structure,because space cause,this article focuses on Highest-Lowest equilibrium strategy and Highest-Others equilibrium strategy based on Global-based equalization structure.The optimal modular scheme of Global-based equalization structure is given,and the results are analyzed from the perspective of probability.Mathematical model simulation results are the same.In order to avoid the misleading caused by the special case experiment,the general conclusion is obtained through the simulation under a large amount of data.When the number of battery strings is small,the Layer-based equalization structure has the best equalization characteristics,and as the number of strings increases,the conventional Series-based equalization structure and its derived Module-based equalization structure have better equalization characteristics,and due to the limit of equalization path,Global-based equalization structure haver the lowest speed of equalization.With the conclusion of the equalization structure and the existing problems in the actual circuit design,the equalization structure is selected from four aspects: equalization time,SOC inconsistency after equalization,fault tolerance and energy flow.With the equalization structure selected,parallel architecture for equalization circuit based on carrier phase-shifting is established.In the following,the principle analysis of the equalization circuit under normal and fault conditions,the determination of equalization criteria and equalization strategy are carried out.Taking the consistency of SOC as the equilibrium goal,an equilibrium strategy is designed.Finally,the hardware experiment platform such as main control board,voltage and current sampling board,equalization circuit and host computer software based on LabVIEW programming environment were set up.The equalization experiment of parallel structure for equalization circuit based on carrier phase-shifting is carried out based on the hardware experiment platform environment,which verify the validity of the proposed equalization circuit.