Methods and systems for detecting variations in minor total-impedance contributors in electrochemical cells
Described herein are methods and systems for detecting variation in minor total-impedance contributors in sets of electrochemical cells. For example, a method comprises maintaining a substantially constant current through the set of electrochemical cells and obtaining multiple voltage readings of the cells while the substantially constant current is maintained. The method then proceeds with determining multiple differential capacity values from the multiple voltage readings, characterizing one or more peaks in the multiple differential capacity values, and determining the variation in the minor total-impedance contributor based on one or more peaks. More specifically, partial capacitance values can be assigned to different impedance channels based on these peaks or, more specifically, based on the separation of adjacent peaks. The variation in the minor total-impedance contributor can be attributed to one or more of a tap-weld quality, electrolyte wetting, tape damage, active material activation energy variations, and diffusion variation of the ion-conducting material.
1. A method of detecting a variation in a minor total-impedance contributor of a total impedance in a set of electrochemical cells, the method comprising:
maintaining a substantially constant current through the set of electrochemical cells at a first temperature;
obtaining multiple voltage readings from the set of electrochemical cells while the substantially constant current is maintained through the set of electrochemical cells;
determining multiple differential capacity values from the multiple voltage readings; and
determining the variation in the minor total-impedance contributor based on the multiple differential capacity values;
heating or cooling the set of electrochemical cells to a second temperature, different from the first temperature;
maintaining the substantially constant current through the set of electrochemical cells while the set of electrochemical cells is at the second temperature;
obtaining additional multiple voltage readings from the set of electrochemical cells while the substantially constant current is maintained through the set of electrochemical cells;
determining additional multiple differential capacity values from the additional multiple voltage readings; and
determining the variation in the minor total-impedance contributor based on the additional multiple differential capacity values.
2. The method of claim 1 , further comprising characterizing one or more peaks in the multiple differential capacity values, wherein determining the variation in the minor total-impedance contributor is performed based on the one or more peaks, associated with different impedance channels.
3. The method of claim 2 , wherein characterizing the one or more peaks in the multiple differential capacity values comprises one of:
determining second-order differential capacity values from the multiple differential capacity values, or
comparing a plot of the multiple differential capacity values to a reference plot.
4. The method of claim 1 , wherein:
the total impedance is determined by the minor total-impedance contributor and a major total-impedance contributor,
the minor total-impedance contributor is attributed to a first portion of the set of electrochemical cells, and
the major total-impedance contributor is attributed to a second portion of the set of electrochemical cells, connected in series with the first portion.
5. The method of claim 1 , wherein the set of electrochemical cells comprises multiple electrochemical cells connected in parallel.
6. The method of claim 1 , wherein the variation in the minor total-impedance contributor is attributed to one or more characteristics selected from the group consisting of tab-weld quality, electrolyte wetting, tape damage, active-material activation energy variations material, and diffusion variations of an electrolyte of the electrochemical cells.
7. The method of claim 6 , wherein the variation in the minor total-impedance contributor is used to differentiate one of the one or more characteristics.
8. The method of claim 1 , further comprising associating the variation in the minor total-impedance contributor with one or more battery defects in the set of electrochemical cells.
9. The method of claim 1 , wherein the multiple voltage readings are obtained from the set of electrochemical cells when the electrochemical cells are at a state of charge (SOC) selected based on and away from phase transition peaks of active materials of the electrochemical cells.
10. The method of claim 1 , further comprising, after maintaining the substantially constant current and obtaining the multiple voltage readings repeating operations using an additional substantially constant current, thereby:
maintaining the additional substantially constant current through the set of electrochemical cells, wherein the additional substantially constant current is different from the constant current;
obtaining additional multiple voltage readings from the set of electrochemical cells while the additional substantially constant current is maintained through the set of electrochemical cells;
determining additional multiple differential capacity values from the additional multiple voltage readings;
characterizing one or more additional peaks in the multiple additional differential capacity values; and
determining the variation in the minor total-impedance contributor based on the one or more additional peaks.
11. The method of claim 10 , wherein the additional substantially constant current is selected such that the one or more additional peaks, in the additional multiple differential capacity values, are more detectable than one or more peaks in the multiple differential capacity values determined while the set of electrochemical cells is subjected to the substantially constant current.
12. The method of claim 1 , wherein a difference between the first temperature and the second temperature is at least about 10° C.
13. The method of claim 1 , the second temperature is selected such that one or more additional peaks, in the additional multiple differential capacity values, are more detectable than one or more peaks in the multiple differential capacity values determined while the set of electrochemical cells is at the first temperature.
14. An apparatus for detecting a variation in a minor total-impedance contributor of a total impedance in a set of electrochemical cells, the apparatus comprising:
a current source configured to flow a substantially constant current through the set of electrochemical cells;
a voltmeter configured to obtain multiple voltage readings from each electrochemical cell in the set of electrochemical cells while the substantially constant current is applied to the set of electrochemical cells and while the set of electrochemical cells is at a first temperature and also to obtain additional multiple voltage readings from each electrochemical cell in the set of electrochemical cells while the substantially constant current is applied to the set of electrochemical cells and while the set of electrochemical cells is at a second temperature, different from the first temperature; and
a processing element configured to
determine multiple differential capacity values from the multiple voltage readings and also determine additional multiple differential capacity values from the additional multiple voltage readings, and
determine the variation in the minor total-impedance contributor based on the multiple differential capacity values and also based on the additional multiple differential capacity values.
15. The apparatus of claim 14 , wherein the processing element is further configured to screen the set of electrochemical cells and associate the variation in the minor total-impedance contributor with one or more battery defects in the set of electrochemical cells.
16. The apparatus of claim 14 , wherein the processing element is further configured to identify one or more electrochemical cells in the set of electrochemical cells when the variation in the minor total-impedance contributor associated with each of the one or more electrochemical cells is above a threshold.
17. The apparatus of claim 16 , wherein the threshold is one of an expected-value threshold and a mean-of-population threshold.
18. The apparatus of claim 17 , wherein:
the threshold is the mean-of-population threshold, and
the one or more electrochemical cells are identified in the set of electrochemical cells when the variation in the minor total-impedance contributor associated with each of the one or more electrochemical cells is away from the mean-of-population threshold by at least a set Z-score.
19. The apparatus of claim 14 , further comprising a temperature monitoring device configured to measure the first temperature and the second temperature.
20. An apparatus for in-situ diagnostics of a set of electrochemical cells based on variation in a minor total-impedance contributor of a total impedance in the set of electrochemical cells, the apparatus comprising:
a battery charger configured to flow a substantially constant current through the set of electrochemical cells;
a battery management system configured to obtain multiple voltage readings from each electrochemical cell in the set of electrochemical cells while the substantially constant current is applied to the set of electrochemical cells and while the set of electrochemical cells is at a first temperature and also to obtain additional multiple voltage readings from each electrochemical cell in the set of electrochemical cells while the substantially constant current is applied to the set of electrochemical cells and while the set of electrochemical cells is at a second temperature, different from the first temperature; and
a processing element configured to
determine multiple differential capacity values from the multiple voltage readings and also determine additional multiple differential capacity values from the additional multiple voltage readings, and
determine the variation in the minor total-impedance contributor based on the multiple differential capacity values and also based on the additional multiple differential capacity values.