Research On Lithium-ion Battery Life With Direct Current Impedance Spectroscopy

With the gradual upgrading of environmental protection concepts and the support of government policies in recent years,the electric vehicle industry has developed vigorously.Shipments of lithium-ion power batteries,the core of electric vehicle power systems,have also grown rapidly.In order to ensure that the performance and safety of lithium-ion power batteries can meet the daily needs of customers,car manufacturers need to detect the status of lithium-ion power batteries,which promotes the need for real-time diagnosis of on-board lithium-ion batteries.The battery real-time diagnosis technology can quickly obtain information on the aging state of the battery（such as maximum capacity,charging voltage,internal resistance,etc.）,and can effectively make early warnings before some safety problems（such as sudden battery failure,battery explosion,etc.）occur.The capacity of a lithium-ion battery is primarily controlled by its internal resistance,which varies with the state of the battery,such as temperature,state of charge,and state of health.Therefore,characterizing resistance is essential for evaluating battery performance and tracking its state of health.Currently,a number of techniques have been used to estimate the resistance of batteries,including detection methods using DC pulsed signals,such as pulsed power testing or hybrid pulse power characterization（HPPC）testing;detection methods using AC pulsed signals,such as electrochemical impedance spectrum（EIS）and multi-sine pulse method,etc.Among them,electrochemical impedance spectroscopy（EIS）is regarded as the most promising diagnostic technology for automotive batteries because of its high accuracy and recognition.However,electrochemical impedance spectroscopy also faces problems such as complex test environment,high test system cost,and long test cycle.The feasibility of being put into use on a large scale is low.In response to the above problems,this article takes 18650 batteries as the research object,and proposes a low-cost,high-precision electric vehicle power battery diagnostic method—DC impedance spectroscopy（DCIS）.This method detects the battery electrolyte resistance（R_Ω）,solid electrolyte film resistance(R_SEI)and charge transfer resistance(R_ct)by controlling the current pulse width of the measured resistance according to the resistance-capacitance（RC）structural time constant model.These characteristic parameters can provide theoretical parameter basis for battery capacity,health,safety,and other aspects.Unlike the AC impedance,DCIS does not rely on frequency domain impedance to obtain battery internal parameters.It is a method of measuring the time domain impedance of internal resistance through a time function.We first use the same batteries to test the characteristic parameters in EIS and DCIS,and compare and verify the effectiveness of the DCIS method in battery diagnosis.Then,according to the time domain detection characteristics of DCIS,combined with the particularity of electric vehicle battery diagnosis,we propose a fast DCIS method that only measures resistance parameters to meet the market demand for electric vehicle power battery testing.Based on the above research on battery cells and DCIS,we collected and analyzed the internal characteristic parameters of 18650 lithium-ion batteries with different maximum battery capacities,and obtained the changes of internal parameters of 18650 lithium-ion batteries with battery aging.In addition,we also used DCIS to test the working conditions,and the results show that this method can be used for real-time diagnosis of battery life,safety and has good accuracy.