Power unit analysis

Abstract

A method of quantifying contributions to the dynamic resistance of a power unit comprises obtaining an impedance spectrum from the power unit and removing the effects of diffusion impedance from the s

Claims

1. A method of isolating and quantifying individual contributions to dynamic resistance in a power unit, comprising:obtaining an impedance spectrum from the power unit;removing diffusion impedance effects from the impedance spectrum;converting the impedance spectrum into a distribution of relaxation times;isolating individual peaks in the distribution of relaxation times;reconstructing an impedance spectrum for each peak;fitting equivalent circuit elements to the reconstructed impedance spectra to quantify the individual resistance contributions to the dynamic resistance of the power unit; and outputting the quantified individual resistance contributions as dynamic resistance parameters.

2. A method according to claim 1, wherein the impedance spectrum is obtained from the power unit by electrochemical impedance spectroscopy and / or by conversion of pulse relaxation data, obtained following application of a current to the power unit, into an impedance spectrum.

3. A method according to claim 2, wherein the pulse relaxation data is converted into an impedance spectrum by fitting frequency domain elements to the pulse relaxation data using Laplace inversion, and constructing an impedance spectrum from the frequency domain elements.

4. A method according to claim 2 or 3, wherein the pulse relaxation data is obtained by a galvanostatic intermittent titration technique.

5. A method according to claim 2 or 3, wherein the pulse relaxation data is obtained by hybrid pulse power characterisation.

6. A method according to any preceding claim, wherein individual peaks in the distribution of relaxation times are isolated by fitting statistical distributions to the distribution of relaxation times.

7. A method according to claim 6, wherein the statistical distributions include log-normal or Gaussian distributions.

8. A method according to any preceding claim, further comprising displaying the dynamic resistance parameters.

9. A method according to any preceding claim, further comprising comparing the dynamic resistance parameters to known values.

10. A method according to any preceding claim, wherein a fitting based on a pure capacitance term is used to remove capacitance effects from the impedance spectrum.

11. A method according to any preceding claim, wherein a fitting based on at least one of a finite length Warburg term and / or a finite space Warburg term is used to remove the diffusion impedance effects from the impedance spectrum.

12. A method according to any preceding claim, wherein the equivalent circuit elements are RQ elements in which RQ represents a resistor and constant phase element in parallel.

13. A method according to any preceding claim, wherein equivalent circuit elements are fitted based on the equation Z(m) = 1+T(.a))tt where Z(m) is the impedance as a function of angular frequency, 7? is a resistor representing dynamic resistance, t is a characteristic time constant, i is the imaginary number, to is the angular frequency and a is the exponent value.

14. A method according to any preceding claim, further comprising:creating a simulated power unit impedance spectrum using the fitted equivalent circuit elements; andoptimising the values of the fitted equivalent circuit elements to minimise the error between the simulated power unit impedance spectrum and the impedance spectrum obtained from the power unit.

15. A method according to claim 14 wherein the optimisation includes the fitting of an inductor L or resistor inductor parallel circuit element RL.

16. A method according to claim 15 wherein the optimisation includes the fitting of an inductor based on the equation XL(m) = ia>L where XL(m) is the contribution to the impedance spectrum, i is the imaginary number, to is the angular frequency and L is the variable inductance value.

17. A computer-readable medium storing instructions which, when executed by a processor, cause the performance of the method of any preceding claim.

18. A system comprising:an electrochemical impedance spectroscopy (EIS) instrument for obtaining EIS data from a power unit, and / or a galvanostatic intermittent titration technique (GITT) instrument for obtaining GITT data from a power unit and / or a hybrid pulse power characterisation (HPPC) instrument for obtaining HPPC data from a power unit; anda processor programmed to perform the method of any of claims 1-16.

19. The system of claim 18, further comprising a display for displaying the dynamic resistance parameters.

20. An electric vehicle comprising a system according to claim 18 or 19, wherein the dynamic resistance parameters are input as battery health data to a battery management system which manages a battery according to the battery health data.

21. An electric vehicle according to claim 20, wherein the battery management system is configured to output a notification to a user when the battery health data indicates the battery health has degraded below a predetermined threshold.

22. A computer-implemented method of comparing power units comprising:obtaining dynamic resistance parameters for a plurality of power units according to any of claims 1-16;inputting the dynamic resistance parameters into a power unit degradation model;outputting the results of the degradation model;displaying the results of the degradation model; andranking the power units by their expected degradation characteristics.

23. A method according to claim 22 further comprising the step of selecting the highestranked power unit for use in a vehicle.s

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