Bulk force in a battery pack and its application to state of charge estimation
There is disclosed an electrical device including a battery pack, a pressure sensor for measuring a volume change of the battery pack, a voltage sensor in electrical communication with a positive terminal and a negative terminal of the battery pack, a temperature sensor positioned in a cell of the battery pack, and a battery management system. The battery management system includes a controller in electrical communication with the pressure sensor, the voltage sensor, and the temperature sensor, the controller being configured to execute a program stored in the controller to determine a state of charge percentage of the battery pack based on a pressure reading from the pressure sensor, a terminal voltage reading from the voltage sensor, and a temperature reading from the temperature sensor.
1. An electrical device comprising:
a battery pack;
a pressure sensor for measuring a volume change of the battery pack;
a voltage sensor in electrical communication with a positive terminal and a negative terminal of the battery pack;
a temperature sensor positioned in a cell of the battery pack; and
a battery management system including a controller in electrical communication with the pressure sensor, the voltage sensor, and the temperature sensor, the controller being configured to execute a program stored in the controller to determine a state of charge percentage of the battery pack based on bulk force measurements from the pressure sensor, a terminal voltage reading from the voltage sensor, and a temperature reading from the temperature sensor.
2. The device of claim 1 further comprising:
a current sensor wired to measure a current flowing from the pack,
wherein the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on the current flowing from the pack.
3. The device of claim 1 further comprising:
a temperature sensor positioned to measure ambient temperature,
wherein the controller executes the program stored in the controller to compensate for the ambient temperature to determine the state of charge percentage of the battery pack.
4. The device of claim 1 wherein:
the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on a charging current and a discharging current from the cell.
5. The device of claim 1 wherein:
the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on a charge/discharge hysteresis state of the cell.
6. The device of claim 1 wherein:
the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on a capacity of the cell.
7. The device of claim 1 wherein:
the controller executes the program stored in the controller to compensate for the ambient temperature to determine the state of charge percentage of the battery pack.
8. The device of claim 1 wherein:
the controller executes the program stored in the controller to compensate for a temperature in a cell of the battery pack to determine the state of charge percentage of the battery pack.
9. The device of claim 1 wherein:
the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on an internal resistance of a cell of the battery pack.
10. The device of claim 1 wherein:
the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on an open circuit voltage of a cell of the battery pack.
11. The device of claim 1 wherein:
the controller chooses one program from a plurality of programs stored in the controller to determine the state of charge percentage of the battery pack.
12. The device of claim 11 wherein:
one of the plurality of programs stored in the controller determines a state of charge percentage of the battery pack based on bulk force measurements of the battery pack, and
another of the plurality of programs stored in the controller determines a state of charge percentage of the battery pack based on a terminal voltage reading from the voltage sensor.
13. The device of claim 1 wherein:
the battery pack includes a plurality of cells.
14. The device of claim 1 wherein:
the battery pack comprises a plurality of cells held in compression between a first plate and an opposed second plate.
15. The device of claim 14 wherein:
the pressure sensor comprises a strain gauge adjacent an outer side of the first plate.
16. The device of claim 14 wherein:
the pressure sensor comprises a load cell adjacent an outer side of the first plate.
17. The device of claim 1 wherein:
the temperature sensor is positioned in contact with a cell of the battery pack.
18. An electrical device comprising:
a battery pack;
a temperature sensor positioned in a cell of the battery pack;
a pressure sensor for measuring a volume change of the battery pack; and
a battery management system including a controller in electrical communication with the temperature sensor, the controller being configured to execute a program stored in the controller to determine a state of charge percentage of the battery pack based on a temperature reading from the temperature sensor,
wherein the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on bulk force measurements from the pressure sensor.
19. The device of claim 18 further comprising:
a current sensor wired to measure a current flowing from the pack,
wherein the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on the current flowing from the pack.
20. The device of claim 19 further comprising:
a temperature sensor positioned to measure ambient temperature,
wherein the controller executes the program stored in the controller to determine the state of charge percentage of the battery pack based on the ambient temperature.
21. The device of claim 19 wherein:
the program stored in the controller contains an algorithm to determine the state of charge percentage of the battery pack combining measurements of terminal voltage and force in proportion to a first and second derivative of their modeled relationship with the state of charge percentage.
22. The device of claim 18 wherein:
the controller executes the program stored in the controller to calculate a prediction of the battery volume change as a non-linear function of the state of charge, a temperature reading from the temperature sensor, and ambient temperature.
23. The device of claim 18 wherein:
the program stored in the controller contains a dynamic relationship between a volume change of each electrode of the battery pack and an overall battery volume change using a hysteretic model based on measured current of the battery pack.
24. The device of claim 18 wherein:
the temperature sensor is positioned in contact with a cell of the battery pack.
25. A method for determining the state of charge percentage of a battery pack, the method comprising:
measuring bulk force changes due to cycling in at least one cell of the battery pack to obtain bulk force measurements; and
calculating in a controller the state of charge percentage of the battery pack based on (i) the bulk force measurements, and at least one of (ii) a current of the battery pack, (iii) a temperature in at least one cell of the battery pack, (iv) a terminal voltage of a cell in the battery pack, (v) an estimate of at least one temperature in at least one cell of the battery pack, and (vi) a temperature of the ambient medium.
26. The method of claim 25 further comprising:
calculating in the controller the state of charge percentage of the battery pack based, at least in part, on a present estimate of the state of charge of the battery pack.