Method and apparatus for measuring complex impedance of cells and batteries
First Claim
1. Apparatus for evaluating a component of the complex impedance of an electrochemical cell or battery at a discrete frequency comprising:
- current excitation circuitry coupled to said cell or battery and adapted to pass a periodic excitation current through said cell or battery, said periodic excitation current characterized by a smallest period equal to the reciprocal of said discrete frequency;
current sensing and processing circuitry coupled to said current excitation circuitry and adapted to provide a current signal in response to said periodic excitation current;
voltage sensing and processing circuitry coupled to said cell or battery and adapted to provide a voltage signal in response to a periodic voltage across said cell or battery;
current-signal sampling circuitry coupled to said current sensing and processing circuitry and adapted to provide sampled values of said current signal, said sampled values acquired at discrete current-signal sampling times synchronized with said periodic excitation current and uniformly distributed in time over half-period or full-period intervals of said smallest period of said periodic excitation current;
voltage-signal sampling circuitry coupled to said voltage sensing and processing circuitry and adapted to provide sampled values of said voltage signal, said sampled values acquired at discrete voltage-signal sampling times synchronized with said periodic excitation current and uniformly distributed in time over half-period or full-period intervals of said smallest period of said periodic excitation current; and
, computation and control circuitry coupled to said current-signal sampling circuitry and to said voltage-signal sampling circuitry and adapted to combine said sampled values of said current signal and said sampled values of said voltage signal to evaluate said component of said complex impedance at said discrete frequency.
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Abstract
A periodic time-varying current with smallest period 1/f1 excites a cell/battery and provides a timing reference. Linear circuitry produces two signals, one proportional to the excitation current, the other proportional to the responding time-varying voltage. These signals are processed with identical frequency-limiting filters to attenuate higher-order harmonics and noise. Using the timing reference for synchronization, a microprocessor/microcontroller commands analog to digital converters to sample the frequency-limited current and voltage signals at equally-spaced times over a period and accepts the digitized samples as inputs. The digital samples are averaged over multiple periods and employed to calculate averaged Fourier coefficients of in-phase and quadrature components of frequency-limited current and voltage at frequency f1. By numerically combining these Fourier coefficients, the microprocessor/microcontroller determines real and imaginary parts of the cell/battery'"'"'s complex impedance at frequency f1.
392 Citations
52 Claims
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1. Apparatus for evaluating a component of the complex impedance of an electrochemical cell or battery at a discrete frequency comprising:
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current excitation circuitry coupled to said cell or battery and adapted to pass a periodic excitation current through said cell or battery, said periodic excitation current characterized by a smallest period equal to the reciprocal of said discrete frequency;
current sensing and processing circuitry coupled to said current excitation circuitry and adapted to provide a current signal in response to said periodic excitation current;
voltage sensing and processing circuitry coupled to said cell or battery and adapted to provide a voltage signal in response to a periodic voltage across said cell or battery;
current-signal sampling circuitry coupled to said current sensing and processing circuitry and adapted to provide sampled values of said current signal, said sampled values acquired at discrete current-signal sampling times synchronized with said periodic excitation current and uniformly distributed in time over half-period or full-period intervals of said smallest period of said periodic excitation current;
voltage-signal sampling circuitry coupled to said voltage sensing and processing circuitry and adapted to provide sampled values of said voltage signal, said sampled values acquired at discrete voltage-signal sampling times synchronized with said periodic excitation current and uniformly distributed in time over half-period or full-period intervals of said smallest period of said periodic excitation current; and
,computation and control circuitry coupled to said current-signal sampling circuitry and to said voltage-signal sampling circuitry and adapted to combine said sampled values of said current signal and said sampled values of said voltage signal to evaluate said component of said complex impedance at said discrete frequency. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18)
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19. Apparatus for measuring at least one component of the complex impedance of an electrochemical cell or battery at a discrete frequency comprising:
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current excitation circuitry coupled to said cell or battery and adapted to pass a periodic excitation current through said cell or battery, said periodic excitation current characterized by a smallest period equal to the reciprocal of said discrete frequency;
current sensing circuitry coupled to said current excitation circuitry and adapted to produce a current signal proportional to said periodic excitation current;
voltage sensing circuitry coupled to said cell or battery and adapted to produce a voltage signal proportional to a periodic voltage developed across said cell or battery;
filtering circuitry coupled to said current sensing circuitry and to said voltage sensing circuitry, said filtering circuitry characterized by a complex transfer function and adapted to provide a frequency-limited current signal in accordance with said complex transfer function and a frequency-limited voltage signal in accordance with the same said complex transfer function; and
,evaluating circuitry coupled to said filtering circuitry and to said current excitation circuitry and adapted to evaluate at least one component of said complex impedance of said electrochemical cell or battery in accordance with said frequency-limited current signal, said frequency-limited voltage signal, and said periodic excitation current. - View Dependent Claims (20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36)
sampling and converting circuitry coupled to said filtering circuitry and adapted to provide digital representations of sampled values of said frequency-limited current signal and of said frequency-limited voltage signal, said sampled values acquired at periodically repeating sampling times synchronized with said periodic excitation current and uniformly distributed over half-period or full-period intervals of said smallest period of said periodic excitation current; and
,computation and control circuitry coupled to said current excitation circuitry and to said sampling and converting circuitry, said computation and control circuitry adapted to initiate said sampling times and to compute at least one component of said complex impedance from said digital representations of said sampled values of said frequency-limited current signal and said frequency-limited voltage signal.
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27. Apparatus as in claim 26 wherein the number of current-signal samples acquired per period and the number of voltage-signal samples acquired per period are equal.
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28. Apparatus as in claim 27 wherein said current-signal samples and said voltage-signal samples are acquired coincidentally.
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29. Apparatus as in claim 26 wherein said current-signal samples and said voltage-signal samples are acquired sequentially.
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30. Apparatus as in claim 26 wherein said computation and control circuitry is further adapted to initiate the timing of said periodic excitation current and to identify said sampling times by measuring from a periodically repeating timing point of said periodic excitation current.
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31. Apparatus as in claim 30 wherein said current excitation circuitry includes a controlled switch, and said computation and control circuitry initiates said timing of said periodic excitation current by periodically turning said controlled switch on and off thereby periodically interrupting a current through said cell or battery.
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32. Apparatus as in claim 30 wherein said current excitation circuitry includes digital-to-analog circuitry and said computation and control circuitry initiates said timing of said periodic excitation current by periodically outputting appropriate digital data to said digital-to-analog circuitry.
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33. Apparatus as in claim 26 wherein said current excitation circuitry includes function generating and timing circuitry adapted to spontaneously generate said periodic excitation current and to communicate periodic timing pulses to said computation and control circuitry in synchronism with said periodic excitation current, said computation and control circuitry being adapted to identify said sampling times by measuring from said periodic timing pulses.
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34. Apparatus as in claim 33 wherein said function generating and timing circuitry includes an alternator for charging said cell or battery.
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35. Apparatus as in claim 33 wherein said function generating and timing circuitry includes a transformer and rectifier for charging said cell or battery.
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36. Apparatus as in claim 26 wherein said computation and control circuitry is further adapted to average said digital representations of said sampled values of said frequency-limited current signal and to average said digital representations of said sampled values of said frequency-limited voltage signal to obtain averaged current signal samples and averaged voltage signal samples, to numerically combine said averaged current signal samples and averaged voltage signal samples to evaluate averaged Fourier coefficients, and to numerically combine said averaged Fourier coefficients to evaluate at least one component of said complex impedance of said electrochemical cell or battery at said discrete frequency.
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37. A method for measuring a component of complex impedance of an electrochemical cell or battery at a discrete frequency comprising:
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exciting said cell or battery with a periodic time-varying current characterized by a smallest period equal to the reciprocal of said discrete frequency;
forming a current signal proportional to said periodic time-varying current and a voltage signal proportional to a time-varying response voltage across said cell or battery;
processing said current signal and said voltage signal with the same complex transfer function to obtain a frequency-limited current signal and a frequency-limited voltage signal; and
,combining said frequency-limited current signal and said frequency-limited voltage signal to determine said component of complex impedance of said electrochemical cell or battery at said discrete frequency. - View Dependent Claims (38, 39, 40, 41, 42, 43, 44, 45, 51)
sampling said frequency-limited current signal and said frequency-limited voltage signal at uniformly spaced sampling times synchronized to said periodic time-varying current to acquire data samples and converting said data samples to digital format; and
,computing said component of complex impedance at said discrete frequency from said data samples converted to said digital format.
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39. A method according to claim 38 wherein said step of computing further comprises:
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averaging said samples converted to digital format over multiple periods to obtain averaged digital samples;
evaluating averaged Fourier coefficients of in-phase and quadrature components of said frequency-limited current signal and said frequency-limited voltage signal from said averaged digital samples; and
,combining said averaged Fourier coefficients numerically to determine said component of said complex impedance at said discrete frequency.
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40. A method as in claim 37 wherein said step of exciting said cell or battery with a periodic time-varying current comprises exciting said cell or battery with a periodic square-wave current.
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41. A method as in claim 37 wherein said step of exciting said cell or battery with a periodic time-varying current comprises exciting said cell or battery with a periodic sinusoidal current.
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42. A method as in claim 37 wherein said step of processing said current signal and said voltage signal comprises processing said current signal and said voltage signal simultaneously, and said step of sampling said frequency-limited current signal and said frequency-limited voltage signal comprises sampling said frequency-limited current signal and said frequency-limited voltage signal simultaneously.
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43. A method as in claim 37 wherein said step of processing said current signal and said voltage signal comprises processing said current signal and said voltage signal sequentially, and said step of sampling said frequency-limited current signal and said frequency-limited voltage signal comprises sampling said frequency-limited current signal and said frequency-limited voltage signal sequentially.
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44. A method according to claim 37 wherein said step of processing said current signal and said voltage signal with the same complex transfer function comprises processing said current signal and said voltage signal with the same complex low-pass transfer function.
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45. A method according to claim 37 wherein said step of processing said current signal and said voltage signal with the same complex transfer function comprises processing said current signal and said voltage signal with the same complex band-pass transfer function.
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51. An apparatus for measuring a component of complex impedance of an electrochemical cell or battery at a discrete frequency adapted for performing the steps according to claim 37.
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46. A method for measuring at least one component of complex impedance of an electrochemical cell or battery at a discrete frequency comprising:
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exciting said cell or battery with a periodic time-varying current characterized by a smallest period equal to the reciprocal of said discrete frequency;
forming a current signal in accordance with said periodic time-varying current and a voltage signal in accordance with a time-varying response voltage across said cell or battery;
sampling said current signal and said voltage signal at equally spaced times over a half-period or full-period interval of said periodic time-varying current and converting sampled values of said current signal and sampled values of said voltage signal to digital format;
averaging said sampled values over multiple periods to obtain averaged sampled values;
evaluating Fourier coefficients from said averaged sample values; and
,numerically combining said Fourier coefficients to determine at least one component of said complex impedance of said electrochemical cell or battery at said discrete frequency. - View Dependent Claims (47, 48, 49, 50, 52)
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Specification