System and method for measuring fuel cell voltage and high frequency resistance

• US 20020196025A1
• Filed: 03/29/2002
• Published: 12/26/2002
• Est. Priority Date: 09/29/2000
• Status: Active Grant

• Current Assignee: Hydrogenics Corporation (Cummins Incorporated)
• Original Assignee: Hydrogenics Corporation (Cummins Incorporated)
• Inventors: Masse, Stephane, Gopal, Ravi B., Freeman, Norman A.
• Granted Patent: US 6,816,797 B2
• Time in Patent Office: Days
• US Class Current: 324/426
• CPC Class Codes: G01R 19/0084 measuring voltage only all subgroups of G01R19/00 take precedence
• CPC Class Codes: G01R 27/02 Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant by measuring phase angle only G01R25/00
• CPC Class Codes: G01R 31/3648 comprising digital calculation means, e.g. for performing an algorithm
• CPC Class Codes: G01R 31/3835 involving only voltage measurements
• CPC Class Codes: G01R 31/386 using test-loads
• CPC Class Codes: G01R 31/389 Measuring internal impedance, internal conductance or related variables
• CPC Class Codes: G01R 31/396 Acquisition or processing of data for testing or for monitoring individual cells or groups of cells within a battery
• CPC Class Codes: Y02E 60/50 Fuel cells

Abstract

A method and apparatus are provided for measuring impedance and voltage characteristics of cells of multi-cell electrochemical devices, for example a battery or a fuel cell stack. Voltages are measured across individual cells, or groups of cells. At the same time, a load is connected in series with the electrochemical device. Both the device for measuring the voltages and the load are controlled together, preferably by means of a controller, which can include some form of microprocessor. This enables the load to be controlled to provide a desired combination of DC and AC current characteristics. By setting appropriate DC and AC current characteristics, desired characteristics of the impedance of individual cells can be measured.

Claims

1. An apparatus for measuring impedance and voltage characteristics of cells of a multi-cell electrochemical device, the apparatus comprising:

a voltage measuring means including a plurality of inputs for connection across the a plurality of measuring points between cells of the electrochemical device, to generate voltage signals indicative of the measured voltages;

a load, connectable in series with the electrochemical device; and

a controller connected to and controlling the voltage measuring means and the load, for controlling the characteristics of the load and for receiving the voltage signals from the voltage measuring means.

2. An apparatus as claimed in claim 1, wherein the load is adapted to draw a DC current with a superimposed AC perturbation current, and wherein the controller is adapted to control the load to provide desired DC and AC load current characteristics.

3. An apparatus as claimed in claim 2, wherein the voltage measuring means provides a plurality of primary channels for the voltage signals, there being one channel for the voltage across adjacent measuring points, and wherein the voltage measuring means includes a channel splitter for separating out at least the DC components of the voltages across adjacent measuring points from the primary channels, the channel splitter having first channels as outputs for the DC components.

4. An apparatus as claimed in claim 3, wherein the channel splitter includes second channels as outputs for the AC components of the voltages across adjacent measuring points.

5. An apparatus as claimed in claim 3, wherein the voltage measuring means includes a plurality of instrumentation amplifiers connected to the inputs of the voltage measuring means and having outputs providing the plurality of the primary channels and an analog multiplexer connected to at least the first channels from the channel splitter, wherein a multiplexer control line is connected between the controller and the analog multiplexer for controlling the analog multiplexer to switch sequentially between at least the first channels.

6. An apparatus as claimed in claim 5, which includes a first analog to digital converter connected to the output of the analog multiplexer, a voltage data bus connected between the first analog to digital converter and the controller and an analog to digital control line connected between the controller and the first analog to digital converter for control thereof.

7. An apparatus as claimed in claim 6, which includes a digital to analog converter having an analog output connected to the load, a load control line, a load data bus both connected between the controller and the digital to analog converter.

8. An apparatus as claimed in claim 7, wherein a shunt is provided connected in series with the load for measuring the current, wherein the shunt is connected to the controller.

9. An apparatus as claimed in claim 8, wherein outputs of the shunt are connected to a current amplifier and wherein the current amplifier has an output for a current measurement signal connected to the controller.

10. An apparatus as claimed in claim 9, wherein a current analog to digital converter is provided having an input connected to the output of the current amplifier and having a current output and a control input, and wherein a data bus connects the current output to the controller and an analog to digital control line is provided between the controller and the control input of the current analog to digital converter.

11. An apparatus as claimed in claim 1, 3, 4, 8 or 10, wherein the controller includes an input, connectable to another computing device for supply of control signals for controlling the controller.

13. A method as claimed in claim 12, which includes controlling the load to provide a desired combination of a DC current and a superimposed AC current perturbation.

14. A method as claimed in claim 13, which includes, between steps (i) and (ii), controlling the load to provide a series of set load conditions, and recording, for each load condition, at least some of the voltage characteristics of the electrochemical device.

15. A method as claimed in claim 14, which includes, between steps (i) and (ii), varying the frequency of the superimposed AC current perturbation, recording the voltages at selected frequencies for the superimposed AC current perturbation, and determining from the recorded voltages real and imaginary components of the impedance of the cells.

16. A method as claimed in claim 13, which includes connecting inputs of a plurality of differential amplifiers across adjacent measuring points between cells of the electrochemical device, measuring the voltages across adjacent measuring points between the cells with the plurality of differential amplifiers to generate a plurality of voltage signals, supplying the voltage signals to a multiplexer and operating the multiplexer to sequentially supply the voltage signals to the controller.

17. A method as claimed in claim 16, which includes converting each voltage signal selected by the analog multiplexer to a digital signal in a voltage analog to digital converter.

18. A method as claimed in claim 17, which includes providing a shunt connected in series with the load for measuring the current through the load, measuring the voltage across the shunt to determine the current through the load and thereby generating a current measurement signal and supplying the current measurement signal to the controller.

19. A method as claimed in claim 18, which includes converting the current measurement signal to a digital current measurement signal, and supplying the digital current measurement signal to the controller.

20. A method as claimed in claim 19, which includes supplying from the controller a digital load signal to a digital to analog converter, controlling the digital to analog converter with the controller and generating with the digital to analog converter an analog load control signal and supplying the analog load control signal to the load.

21. A method as claimed in claim 17, 18, 19 or 20, wherein each cell of the electrochemical device is calibrated, each voltage is measured across an individual cell of the electrochemical device, and the voltage for each cell is calculated in accordance with the following equation to give a calibrated measured cell voltage V_R:

V_R=V_A*V_A/D[V_A/D(V_A)−

V_A/D(V₀)]−

V_OFF 

 

(1)

Where;

V_R is the calibrated measured cell voltage;

V_A/D is the output value of the voltage digital to analog converter for the respective cell during voltage measurement;

V_A is a voltage applied differentially to the inputs of the differential amplifier for the respective cell, during calibration;

V_A/D (V_A) is the output value of the voltage analog to digital converter when the voltage V_A is applied to the inputs of the differential amplifier for the respective cell during calibration;

V_A/D (V₀) is the output value of the voltage analog to digital converter when the inputs of the differential amplifier for the respective cell are tied to ground during calibration; and

V_OFF is the voltage output of the differential amplifier for the respective cell, when the inputs thereof are tied to ground during calibration.

12. A method of monitoring the voltage and impedance characteristics of cells of a multi-cell electrochemical device, the method comprising:

(i) providing a load connected in series with the electrochemical device;

(ii) controlling the load to provide desired current characteristics;

(iii) measuring the voltage across a plurality of measuring points between cells of the electrochemical device; and

(iv) recording at least some of the measured voltages.