Research On Power Lithium-ion Battery Modeling And Application

With the rise of energy Internet,the power electronic system designed with battery energy storage capacity has become a research hotspot.One of the key issues pending settlement for the energy storage system is to build up a simple,accurate and practical large/small signal model for the power lithium ion battery.This paper presents the large/small signal charging/discharging model for the power lithium ion battery,and predicts also its steady state and dynamic characteristics;the small signal model of the battery is embedded as a load into the small signal model of the traditional Buck converter,and the small signal model of single battery,battery string and battery pack Buck charger is obtained accordingly;as one Buck type charger corresponds to four control objects,the concept of unified controller and its analytic design method are proposed.The main contents are as follows:(1)Large signal modeling of power lithium ion batteries.Charge and discharge curves and key parameters are tabled based on the user manual.Also,general model and parameter extraction method of power lithium ion battery are tabled to describe electrical characteristics spanning the full life cycle of power lithium ion battery,and predict its steady and dynamic behavior during charging and discharging and special effects as well,including capacity decay,Peukert effect and thermal effect.The new generic model contains circuit models and mathematical models to accommodate different simulation platforms.The new model features the following:1)The innovation of the new model is two new ideal components proposed,that is,charge controlled voltage source and charge controlled diodes,which make the new model feature such advantages as simple structure and small computation.Specifically,the mathematical model is a primitive explicit function,and the amount of computation is small;circuit model is a simple linear controlled source macro model that completely characterizes the battery and simplifies the computation.2)As the new model makes full use of the information provided in the user manual,there is no need to carry out additional experiments and solve nonlinear formulas.As such,the parameters might be acquired easily.3)As the new model can represent the full life cycle electrical performance of the battery under different operating conditions,so it has superior performance.(2)Small signal modeling of power lithium ion batteries.Below are its main contributions and innovations:1)the small signal working principle of the power battery is described in detail on the basis of the large signal model:when a small signal sinusoidal current is injected into the battery,the output voltage of the battery contains a sinusoidal signal of the same frequency;2)That the battery satisfies the conditions of small signal measurement such as stability,causality and linearity during discharge is proved by mathematical method.The effectiveness of the measured frequency characteristics is guaranteed;3)Small signal model of the power battery is proposed based on the internal structure of the battery and the principle of electrochemical operation,and its dynamic characteristics are described;4)Parameter estimation method of small signal model is proposed based on Levy frequency characteristic fitting method.(3)Small signal modeling of power lithium ion battery charger.Based on the proposed high frequency small signal model of power battery,an accurate model of single cell,battery string and battery pack is proposed.The exact model of the battery is a high-order system that challenges the design of the controller.For that reason,a simplified,accurate model for“solving polynomial approximation root”is introduced to obtain a first-order model that has clear physical meaning and can effectively express the main features of the battery results,which model has been made to match effective frequency band of the small signal model of charger.Then,the small signal model of the battery is embedded,as a load,into the small signal model of the traditional Buck converter,and the small signal model of single cell,battery string and battery pack charger is obtained.As Buck charger has two kinds of control strategies and two operating modes,its charger has four small signal models.The paper presents four types of transfer functions.Compared to the traditional Buck converter,Buck charger has the following characteristics:1)An additional pair of high frequency pole zero points caused by the inductance L_b in the first order model of the battery is added.2)The simulation results show that the frequency characteristics of the charger are directly related to the number of batteries.In particular,when the charger carries a long battery string,the charger's transfer function is a second-order system of high Q and low Q value as per two types of standards.(4)Design of charging controller of power lithium ion battery.As one Buck type charger corresponds to four control objects,the design method for unified controller and its analytic method are proposed.The unified controller is a cascade composed of three links,that is,an integrator,lag-lead link and additional high frequency poles.Simulation and experimental results show that the unified controller can make the charger work stably in all operating conditions,thus the charger is saved from the difficult problem of switching controller in different operating conditions.The innovation of the approximate analytical design of the charger lies in:1)In general,the loop gain of the DC converter depends on the input voltage,which brings much inconvenience to the controller design.The height of the jagged input slope of the pulse width modulator is to be controlled by the input voltage to offset the influence of the input voltage fluctuation on the loop gain.The loop gain formula without input voltage influence is deduced;2)Four types of control objects that Buck charger corresponds to are a standard second-order model.In this paper,the frequency band specific-linear phase formula,and the frequency band specific-amplitude formula of the standard-high-Q second-order model are presented,which simplifies the mapping method of Bode plot,and provides an effective tool for analyzing and designing the unified controller.The simulation and experimental results show the effectiveness of the approximate analytical design of the charger.