Online Battery Impedance Spectrum Measurement Based On Cross-Correlation

With the increasing depletion of global energy and the promotion of environmental protection,electrochemical batteries have been widely used in the energy storage and supply of industry,telecommunications,medical and electric power,especially in the electric vehicle which has received great attention.Accurate performance parameters of the battery are key to manage the battery efficiently,reduce the battery cost and improve the battery safety.The EIS(Electrochemical impedance spectroscopy)is one of the very important parameters,which is a powerful tool to analyze the battery performance and provide references for the battery management.However,there are many shortcomings in the traditional off-line measurement methods,which not only fail to meet the need of real-time measurement,but also take a long time and need expensive equipments.Therefore,this paper presents a new method for online measurement of the battery impedance spectrum based on the cross-correlation principle.In this paper,the rationality and feasibility of the method are proved in detail from three aspects:the theoretical analysis,the simulation verification and the experimental verification.Firstly,this paper introduces the basic theory of the electrochemical impedance spectrum and the basic principle of the impedance spectrum measurement methods,including the basic off-line and on-line methods.The principle and realization scheme of the new measurement method designed in this paper are put forward.Then,the feasibility of the measurement scheme is deduced and demonstrated by mathematical formula.In the process of charging the battery through the DC-DC circuit,the m sequence(Maximum-Length Sequence)is generated and injected into the switch tube of the DC-DC circuit.According to the cross-correlation operation between the m sequence and the battery voltage response or the current response,the impulse response of the system can be obtained.Then the frequency response of the system is obtained by FFT(Fast Fourier Transform)to the impulse response,and the impedance spectrum of the battery can be obtained through operation.Secondly,the equivalent circuit model of the battery is obtained by using the British Solartron workstation and then used to build the simulation circuit for doing the experiment in SIMULINK.The DC-DC circuit is controlled by the constant current.After entering into the steady state,the m sequence is generated and superimposed on the current compensation value and injected into the switch tube together.The impedance spectrum of the battery is obtained by the cross-correlation operation after sampling the battery voltage and the current response values.The results show that this online method can be used to measure the impedance spectrum of batteries with a wide frequency range quickly and accurately.Finally,the experimental platform is built for the experimental verification.The hardware design includes the Buck charging circuit,the driving circuit,the voltage sampling circuit,the current sampling circuit and the digital control circuit.The software design mainly introduces the design of the state machine and the impedance spectrum measurement program including the cross-correlation and FFT algorithm.The experimental results show that this method can be used to measure the impedance spectrum of the battery online quickly and accurately.Comparing with the existing method,it has some advantages.The error analysis of the measurement results is carried out.The improvement and the prospect of this paper are put forward at last.