Drooping cell detection and state of cell health monitoring

1. A method for determining a state of health of an electric battery pack, the battery pack comprising a plurality of battery cells, the method comprising the steps of:

measuring the cell voltage of each individual battery cell of the plurality of battery cells; and

analyzing each measured battery cell voltage to determine the state of health of the corresponding battery cell,

wherein analyzing each measured battery cell voltage comprises:

determining the mean of the individual measured cell voltages in the battery pack;

determining a residual voltage value for each of the individual battery cells, wherein the residual voltage value is calculated as the numerical difference between the measured voltage for the particular individual battery cell and the determined mean;

calculating the standard deviation of the determined residual voltage values at each measurement time;

identifying an outlier battery cell as a battery cell whose residual voltage value exceeds a predetermined multiple of the standard deviation for a predetermined period of time; and

performing a first action upon identifying at least one outlier battery cell.

2. The method of claim 1 , wherein the first action comprises setting a diagnostic indicator.

3. The method of claim 1 , wherein the first action comprises derating maximum power delivered from the battery.

4. The method of claim 1 , wherein the first action comprises calculating a reduced value of remaining energy available from the battery based on an estimate of energy available from the outlier battery cell.

5. The method of claim 1 , wherein the first action comprises reducing an allowable charging rate of the battery compared to a charging rate that is allowed for a battery having no outlier battery cells.

6. The method of claim 1 , wherein the first action comprises:

determining a second mean as the mean of the individual measured voltages excluding voltages associated with all battery cells identified as outlier battery cells;

determining a second residual voltage value for each of the battery cells not identified as outlier battery cells, wherein the second residual voltage value is calculated as the numerical difference between the measured voltage for the particular non-outlier individual battery cell and the determined second mean;

calculating a second standard deviation at each measurement time as the standard deviation of the determined second residual voltage values for the non-outlier battery cells;

identifying a drooping cell as a battery cell whose second residual voltage value exceeds a predetermined multiple of the second standard deviation; and

performing a second action upon identifying at least one drooping cell.

7. The method of claim 6 , wherein the second action comprises setting a diagnostic indicator.

8. The method of claim 6 , wherein the second action comprises derating maximum power delivered from the battery.

9. The method of claim 6 , wherein the second action comprises calculating a reduced value of remaining energy available from the battery based on an estimate of energy available from the drooping cell.

10. The method of claim 6 , wherein the second action comprises reducing an allowable charging rate of the battery compared to a charging rate that is allowed for a battery having no drooping cells.

11. The method of claim 1 , wherein analyzing each measured battery cell voltage comprises:

estimating a battery cell resistance value during constant charging or discharging conditions using a rolling average algorithm, a Kalman filter, or a recursive least square algorithm, and

performing a second action if the estimated battery cell resistance value exceeds a predetermined threshold.

12. The method of claim 11 , wherein the second action comprises setting a diagnostic indicator.

13. The method of claim 11 , wherein the second action comprises derating maximum power delivered from the battery.

14. The method of claim 11 , wherein the second action comprises calculating a reduced value of remaining energy available from the battery.

15. The method of claim 11 , wherein the second action comprises reducing an allowable charging rate of the battery compared to a charging rate that is allowed for a battery having no cells with an estimated battery cell resistance value exceeding the predetermined threshold.

16. A method for determining a state of health of an electric battery pack, the battery pack comprising a plurality of battery cells, the method comprising the steps of:

measuring the cell voltage of each individual battery cell of the plurality of battery cells; and

analyzing each measured battery cell voltage to determine the state of health of the corresponding battery cell, wherein analyzing each measured battery cell voltage comprises:

identifying a battery cell corresponding to the lowest measured battery cell voltage of the plurality of battery cells;

determining whether the battery cell corresponding to the lowest measured battery cell voltage has an output voltage that remains below a predetermined threshold voltage for a time duration greater than a predetermined threshold time duration; and

performing a first action when the battery cell corresponding to the lowest measured battery cell voltage has an output voltage that remains below the predetermined threshold voltage for a time duration greater than the predetermined threshold time duration;

wherein the predetermined threshold voltage is a calibration value that is indicative of a short-circuited battery cell.

17. The method of claim 16 , wherein the first action comprises analyzing the battery cell voltage of a battery cell adjacent to the battery cell corresponding to the lowest measured battery cell voltage to determine if the adjacent battery cell is showing indications of cell droop.

18. The method of claim 16 , wherein the first action comprises comparing a battery pack temperature, a rate of rise of the battery pack temperature, a battery cell temperature, a rate of rise of the battery cell temperature, a battery pack pressure, and/or a rate of rise of the battery pack pressure to a predetermined threshold value.

19. The method of claim 16 , wherein the first action comprises activating a warning signal.

20. The method of claim 16 , wherein the first action comprises activating a diagnostic indicator.