Power Battery Impedance Spectrum Test Technology Based on Current Pulse Excitation

and Frequency Spectrum Analysis

Jiang Feng, Xue Manling, Che JinhuiCollege of Computer Science and Technology

Beihua University Jilin 132021, P. R. China

jl_jf001@163.com

Abstract—This paper provides a method and a system based on current pulse excitation and frequency spectrum analysis, which can complete battery impedance spectrum test and analysis. By exerting current excitation onto the battery, synchronously measuring current excitation and terminal voltage response, we count the amplitude spectrum and phase spectrum for current excitation and terminal voltage response under different frequencies. Also, we count the battery impedance under different frequencies according to spectrums of I and U, to get the complete spectrogram for the battery impedance. The technology provides a new solution for calculating and predicting the charge state and health state of the storage battery.

Keywords- Battery Impedance Spectrum; Frequency Spectrum Analysis; Impedance test technologyt; SOC

I. INTRODUCTION

In recent years, it has been widely recognized by the western countries to conduct simple and quick test the SOC State of charge and SOH State of health of the battery with internal resistance/conductivity test technology. The technology has been listed as the test standard for VRLA by IEEE.

According to IEEE standard (1118-1996), measurement on battery conductance is to put the AC voltage with known frequency and amplitude on the two ends of battery and to measure the current. Value of the AC conductance is the ratio between the AC current component (In-phase with the AC voltage) and the AC voltage. Obvious change in conductance (more than 20% of reduction) means the change of battery performance.

Therefore, it’s of great practical value for testing SOC and SOH of the battery to measure the power battery impedance spectrum. There are 2 kinds of battery impedance measurements used in the industry as follows [1][2][3]:

1. DC Discharge Impedance Measurement With test equipment, we force the battery to adopt a

large constant DC current within a short time, usually 2-3s. Then we measure the voltages on both ends of the battery, and calculate the battery impedance at the moment according to the changes of voltage and current. The method is limited to DC impedance.

2. AC Voltage Drop Impedance Measurement

After exerting current with fixed frequency and amplitude, we sample on the voltages and measure them, and calculate the battery impedance with the AC current and voltage value. As battery impedance is the function of frequency, its impedances are different under different frequencies. Therefore, in order to measure the battery impedance spectrum, we should continuously change frequencies of the excitation current. The measurement process above should be repeated. It costs much time with low efficiency.

In recent years, studies on SOC, SOH and impedance of battery both at home and abroad show that SOC and SOH of the battery are intrinsically linked with and closely related to the battery impedance[4][5]. If efficient technologies and measurements are adopted to measure AC impedances under different frequencies, especially quick online measurement, it has a lot of practical significances for studying battery mechanism, improving processing technology and accurately predicting SOC and SOH in use.

II. BASIC PRINCIPLES AND IMPLEMENTATIONS

A. Basic Principles Battery equals to an active network. When a current

excitation I(t)=I0Sin( t) with a certain frequency is exerted on the battery, a response U(t)= U0Sin( t+ ) will occur on the terminal voltage. If we change the frequency of the current excitation I(t), we can calculate the battery impedance Z( ) under different frequencies by measuring the corresponding terminal voltage response U(t).

Based on this basic principle, the paper puts forward a new measurement on battery impedance as follows:

When we exert a square-wave or a square-wave-like current excitation I(t), synchronously measure the current excitation I(t) and terminal voltage response U(t), with FFT or wavelet analysis technology,

F[I(t)] = I(ej ) F[U(t)] = U(ej )we can synchronously calculate the amplitude

spectrum I( k) U( k) and phase spectrum I( k)U( k) of the current excitation I(t) and terminal voltage

response U(t). According to spectrums of I and U, we can calculate

the battery impedance Z0( k) = U( k)/ I( k) under

978-1-61284-722-1/11/$26.00 ©2011 IEEE

2011 International Conference on Mechatronic Science, Electric Engineering and ComputerAugust 19-22, 2011, Jilin, China

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different frequencies and thus get a complete spectrogram. Measurement principles are shown in figure1:

Figure 1. Measurement Principles

B. Implementations Measurement system is composed of controlling and

data-processing CPU unit, high-speed synchronous sampling measurement unit and controlled discharge unit.

CPU unit is used to control the start and stop of high-speed synchronous sampling measurement unit and controlled discharge unit, analyze FFT and wavelet and calculate impedance spectrum according to the current and voltage signals collected by the high-speed synchronous sampling measurement unit. The high-speed synchronous sampling measurement unit synchronously collects and caches the current and voltage signals, sends the signals to the CPU unit to analyze and deal with them. Under the control of CPU unit, the controlled discharge unit discharge for the battery according to the set discharge time and current amplitude.

Detailed measurement is as follows: Firstly, the CPU unit starts the high-speed synchronous sampling measurement unit for data collection. Then, it starts the controlled discharge unit to discharge for the tested battery. When time is over, it stops the discharge unit, and stops the high-speed synchronous sampling measurement unit. According to the basic principles, the CPU unit analyzes and deals with the signals and calculates the battery impedance spectrum.

III. HARDWARE STRUCTURE AND COMPOSITION

A. Major Technical Requirements Range of Voltage Measurement 0 15V

Range of Current Measurement 0 150A Discharge Current: Decided by battery voltage tranche Discharge Time 1ms 500ms Range of Battery Impedance Spectrum 1 5000Hz

B. Structure of the System Structure of the system is shown in figure 2.

Discharge

Controlling Unit

Synchronous High-Speed

Sampling Measurement Unit

Discharge Unit

Controlling and

Data-Processing Unit

Isolation and Adjustment Unit for

voltage and Current Signals

Figure 2. Structure of the System

C. Hardware Composition a) Controlling and Data-Processing CPU Unit:

Basic configurations with embedded PC104CPU module are as follows:

Embedded Pentium (200,300MHz) CPU, 128M Memory, 40GHD, 640X480 Colour LCD. VGA USB RS232 Serial, Parallel, Keyboard Interface and Real-Time Clock. 24-bit Programmable Number I/O.

b) High-Speed Synchronous Sampling Measurement Unit

USB2007data collection module, 14-bit single-ended 4-way synchronous no-difference 200KA/D, ±5V range and 32K FIFO-word memory are used. After divided by RV1 and RV2, the battery voltage is sent to AD620 for adjustment. Then, it is sent to USB2007 data collection module AINO after isolating operational module AD210. The current is measured by 150A shunt, on which, mV-level voltage signal is sent to USB2007 data collection module AIN1 after AD620 amplifier and operational amplifier AD210. Hardware composition is shown in Figure 3.

c) Controlled Discharge Unit After the discharge load composed of 8 N-channel

MOSFET and discharge resistance, and the 8-bit digital output of the CPU unit passes optical isolation, we control the conduction and cut-off of the 8 MOSFETs, through which, we adjust the discharge currents in different levels.

APT10M07JVFR VDSS=100V ID=225ARDS(on)=0.007 in MOSFET;

Respectively, values of the discharge resistances, RL1RL2 RL3 RL4 RL5 RL6 RL7 RL8, are 0.8 0.40.2 0.1 0.08 0.04 0.02 0.01 , with

the same power of 200W.

Pulse Current Excitation Voltage Response

fCurrent Spectrum

I( k)

fVoltage Spectrum

U( k)

SOC=10% Impedance Spectrum f

Z( k)

SOC=20% Impedance Spectrum f

Z( k)

Soc=100% Impedance Spectrum f

Z( k)

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After adjusting the conduction and cut-off of MOSFET, we can adjust the excitation time for the current square wave, that is, the frequency of the basic wave, in order to raise the measuring accuracy on impedance under the frequency of the basic wave.

+

+

P0.0

P0.7

P0.1

AIN0

AIN1

Figure 3. Hardware Composition

IV. DATA COLLECTION AND SOFTWARE PROCESSING PROCEDURE

The system is made up with LabView virtual instrument software platform. Parameter settings, test starting, control on test process, data processing and analysis and other tasks are achieved by main thread of the software; AD convert thread controls USB2007 data collection module and synchronously samples on the current and voltage. Software procedures are shown in Figure 4 and Figure 5.

V. CONCLUSION

a) Measurement and analysis on battery impedance spectrum can be achieved in a short time. Discharge time ranges from several ms to several hundred ms, measuring efficiency greatly improved.

b) Basic wave frequency of current excitation can be adjusted through adjusting discharge time. Impedances of certain frequency bands or frequency points can be more precisely measured and analyzed with signal processing technologies such as FFT, digital filtration, wavelet analysis and so on.

c) Battery impedance can be measured online during shelving intervals of its operation process.

d) Compared with battery impedance under a single fixed frequency, the battery impedance spectrum can more comprehensively reflect SOC and SOH of the battery to provide a new solution for cultivating and predicting SOC and SOH.

Figure 4. Main Thread of the Software

Continuous sampling on the set period and point of the voltage and current

Continuous sampling on the set period and point of the voltage and current

Points enough

Figure 5. AD Convert Thread

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REFERENCES

[1] Wu Yiping, Zhou Guoding, Ge Honghua, etc. Research on AC Impedance of Li/SOCl_2 Battery[J]. Battery, Issue 3, 2006.

[2] Zhang Wenhu, Yang Huabin, Wei Jinping, etc. Research on AC Impedance Spectrum During MH-Ni Battery Discharge Process[J]. Power Technology, 2000 24(1) 25-27

[3] Li Gechen, Gu Yanlei. Prediction of Electrochemical Impedance Spectroscopy on Charge State of Lithium Battery[J]. Power Technology, Issue 09, 2008.

[4] Zhu Yuan, Han Xiaodong, Tian Guangyu. Research on SOC Prediction Technology of Power Battery on Electric Vehicles[J]. Power Technology, Issue 03, 2000

[5] Lin Chengtao, Wang Junping, Chen Quanshi. Principles and Applications of SOC Estimation of Electric Vehicles[J]. Battery, Issue 05, 2004.

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