Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Reexamination Certificate
1998-12-08
2002-06-25
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
C324S426000, C320S161000
Reexamination Certificate
active
06411098
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to energy device testing and evaluation.
A widely used technique for investigating the behavior of energy devices such as electrochemical cells or batteries is electrochemical impedance spectroscopy, also commonly known as frequency response analysis. See MacDonald, “Impedance Spectroscopy”, Wiley 1987. In general, the technique employs sinusoidal electric stimulation (AC voltage or current of known amplitude, frequency, and phase) of a device under test. Measuring the resultant in-phase and quadrature components of the device's response allows calculation of the real and imaginary components of the device impedance using Ohm's law (E=I*R or R=E/I). By taking a series of measurements over a range of frequencies, the characteristic response of the device under test is obtained. From the impedance parameters, other quantities, such as, for example, phase angle and modulus, may be derived.
Quantitative analysis is regularly achieved by using nonlinear least squares fitting to adjust parameters of a proposed theoretical model or electronic circuit analog. A well chosen model will correspond to the underlying chemical and kinetic processes. Frequency response analysis is very robust, but requires performance of multiple individual tests to obtain a complete frequency response profile. As such, the duration of the test process, especially if high resolution, low frequency response information is required, may be considerable. In addition, the time domain response of the device under test must be derived from a postulated equivalent circuit model, which means that accuracy of the derived response is strongly dependent on the validity of the model and the accuracy of the raw data.
To overcome these concerns, a number of direct time domain measurement techniques have been proposed. Commonly used techniques include voltammetry, polarography, chronoamperometry and chronopotentiometry, among others. The distinguishing characteristic of these time domain methods is that, instead of using a continuous sinusoid as the excitation signal, they use continuous (usually linear or exponentially shaped) segments separated by discontinuities. Typical stimuli include, for example: triangle or square waves; a rectangular pulse or pulse train followed by return to a pre-stimulus condition; a composite signal such as a stepped potential staircase with a smaller signal superimposed on each step; or, finally, a series of alternating charge/discharge events.
For general laboratory applications where theoretical analysis of reaction mechanisms is desired, time domain data may be analyzed using well known mathematical techniques, such as Fourier and Laplace integral transforms. The Fourier method allows derivation of a cell's frequency response, while the Laplace method yields impedance and admittance information. When specifically applied to electrochemical accumulators (energy storage cells and batteries), time domain techniques may be used to assess cell condition and infer a relative state of charge.
SUMMARY
The invention provides a highly accurate technique for measuring and analyzing electric potential changes occurring in a device such as an electric or electrochemical cell as a result of stimulation with a square wave current. An electric cell here is distinguished from an electrochemical cell in that voltage potential changes occurring in the electric cell are indicative primarily of charge storage and energy loss effects due to dielectric behavior (e.g., lossy capacitor), whereas potential changes observed in an electrochemical cell reflect additional processes including physical changes (mass transport, diffusion) and various Faradaic electrochemical reactions.
The technique uses a progressive change of the polarization voltage across a device, which develops over time in response to galvanic stimulation, as an estimator of device condition. Furthermore, suitable quantitative analyses of such changes in polarization, expressed as a joint function of stimulation magnitude, polarity and duration, allow quantitative characterization of various underlying chemical processes, identification of anomalous (fault) conditions, and estimation of state of charge.
The technique may be used to electronically measure devices that exhibit reversible or quasi-reversible reactions in response to a sufficiently small excitation signal symmetrically applied about the instantaneous equilibrium potential. While not all electrochemical systems have this property, a significant number of commercial applications require the precise measurement and characterization of just such devices. The technique can be used to evaluate the time domain response of any system which exhibits the property of electrical impedance.
The invention promises to help meet an ever growing need for reliable electrochemical devices that can deliver electricity on demand, and in many cases, be quickly and easily recharged for further use. Such devices include fuel cells, primary (single use) cells and batteries, and secondary (rechargeable) cells and batteries. There is a commensurate need for a technique for rapidly evaluating the state of charge and overall condition of such a device, regardless of whether the device is static (disconnected) or dynamically operating (charging/discharging).
The ability to rapidly perform a quantitative test of device condition is particularly important when the device is being used to supply power to a critical load, so that an unexpected failure may have serious consequences. Similarly, qualification testing during and immediately after batteries or other electrochemical cells are produced, the charge process would bring new economies to battery manufacturing. The same technique may be used in the field to perform tests prior to and after sale. Finally, the technique may be used in the laboratory to provide immediate information on electrochemical cell behavior under controlled conditions to support evolving battery technologies.
Polarization voltage is operationally defined in this document as the difference between the cell potential just prior to the onset of a step-wise change in current stimulation (i.e., the leading edge of a pulse or square wave) and the value attained at some specific later time during the stimulation. By employing high speed synchronous sampling methods, the actual waveform of the polarization voltage that develops during each half-period of the square wave excitation may be recorded for later analysis.
Specifically, a bipolar square wave current exhibiting a 50% duty cycle (mark-space ratio=1) and an average DC current component of exactly zero is used to stimulate an electrochemical cell or accumulator. The resultant polarization voltage response developed across the cell is repetitively sampled at a plurality of points at corresponding positions during each of the repetitions of the waveform. Piecewise numerical integration is performed by generating the sum of corresponding sample points from consecutive positive half cycles, and the separately the sum for the negative half cycles, respectively. These sums are then each divided by the number of samples (N) yielding an average value exhibiting a “Square Root of N” noise reduction factor. The relative shape and size of these averaged curves may be then analyzed or transformed as required to yield detailed information about the condition and future performance of the electrochemical cell (or battery). Data may be converted to digital format, using either a linear or exponentially spaced sampling algorithms. Linearly sampled data is suitable for processing with integral Laplace and Fourier transforms, while exponentially sampled data is useful for immediate graphical presentation of test results.
For chemical systems embodying reversible or quasireversible redox reactions, it has been determined that, when the galvanic stimulation takes the particular form of an even numbered sequence of square pulses of alternating polarity and of sufficiently small a
Metjahic Safet
Nguyen Vincent Q.
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