Battery Cell State of Charge Estimation

• US 20150321576A1
• Filed: 05/08/2014
• Published: 11/12/2015
• Est. Priority Date: 05/08/2014
• Status: Active Grant

• Current Assignee: Ford Global Technologies, LLC (Ford Motor Company)
• Original Assignee: Ford Global Technologies, LLC (Ford Motor Company)
• Inventors: Lee, Tae-Kyung
• Granted Patent: US 9,472,977 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: B60L 2240/549 Current
• CPC Class Codes: B60L 58/12 responding to state of charge [SoC]
• CPC Class Codes: B60L 58/22 Balancing the charge of battery modules
• CPC Class Codes: H01G 9/00 Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
• CPC Class Codes: H02J 7/00 Circuit arrangements for charging or depolarising batteries or for supplying loads from batteries
• CPC Class Codes: H02J 7/0016 using shunting, discharge or bypass circuits
• CPC Class Codes: H02J 7/0018 using separate charge circuits
• CPC Class Codes: H02J 7/0019 using switched or multiplexed charge circuits
• CPC Class Codes: H02J 7/00716 in response to integrated charge or discharge current
• CPC Class Codes: H02J 7/04 Regulation of charging current or voltage
• CPC Class Codes: H02J 7/14 for charging batteries from dynamo-electric generators driven at varying speed, e.g. on vehicle
• CPC Class Codes: Y02T 10/70 Energy storage systems for electromobility, e.g. batteries

Abstract

Hybrid and electric vehicles include a traction battery including many interconnected cells. Effective battery control, such as cell balancing, may rely on an accurate state of charge value for each of the cells. A method to reduce the computational effort of the state of charge calculation is developed. An accurate pack level state of charge calculation is implemented and represents the average cell state of charge. An average cell voltage is based on a pack voltage measurement. A state of charge difference is calculated for each cell based on a difference between a cell voltage and the average cell voltage. The state of charge difference utilizes the pack state of charge and a characteristic voltage and state of charge relationship for the cell. The cell state of charge is the sum of the pack state of charge and the state of charge difference.

Claims

1. A vehicle comprising:

a traction battery including a plurality of cells; and

at least one controller programmed to, in response to a difference between a cell voltage and an average cell voltage, output a cell state of charge based on a voltage offset derived from the difference and relative to a reference voltage associated with a traction battery state of charge, and balance the cells according to the cell state of charge of the cells.

2. The vehicle of claim 1 wherein the at least one controller is further programmed to, in response to no difference between the cell voltage and the average cell voltage, output the cell state of charge based on the reference voltage associated with the traction battery state of charge.

3. The vehicle of claim 1 wherein the average cell voltage is a quotient of an overall voltage of the traction battery and a total number of cells of the traction battery.

4. The vehicle of claim 1 wherein the voltage offset is further derived from a battery current.

5. The vehicle of claim 1 wherein the cell state of charge is further based on a cell open-circuit voltage difference estimated from a cell model, wherein the difference and a battery current are inputs to the cell model.

6. The vehicle of claim 5 wherein the cell state of charge is further based on an open-circuit voltage and state of charge characteristic such that the cell state of charge is the traction battery state of charge offset by a state of charge difference associated with the cell open-circuit voltage difference.

7. A battery management system comprising:

at least one controller programmed to, operate a traction battery including a plurality of cells according to a cell state of charge of the cells that is based on a voltage difference between an open-circuit cell voltage and an average open-circuit cell voltage and relative to a cell voltage associated with a traction battery state of charge.

8. The battery management system of claim 7 wherein the average open-circuit cell voltage is based on an overall voltage of the traction battery and a total number of cells of the traction battery.

9. The battery management system of claim 7 wherein the open-circuit cell voltage is based on a measured cell terminal voltage.

10. The battery management system of claim 7 wherein the voltage difference is based on a battery current.

11. The battery management system of claim 7 wherein operating the traction battery includes balancing the cells based on the cell state of charge of the cells.

12. The battery management system of claim 7 wherein the voltage difference is based on a cell model output having a difference between a measured cell terminal voltage and an average cell terminal voltage as input.

13. The battery management system of claim 7 wherein the voltage difference is based on a first cell model output having a measured cell voltage as input and a second cell model output having an average cell terminal voltage as input.

14. A battery control method comprising:

operating, by a controller, a traction battery including a plurality of cells according to a cell state of charge of the cells based on a voltage offset derived from a difference between a measured cell voltage and an average cell voltage and relative to a cell reference voltage associated with a state of charge of the traction battery.

15. The method of claim 14 wherein the voltage offset is further derived from a battery current.

16. The method of claim 14 further comprising balancing the cells of the traction battery according to the cell state of charge of the cells.

17. The method of claim 14 wherein the average cell voltage is based on an overall voltage of the traction battery.

18. The method of claim 14 wherein the cell state of charge is further based on an open-circuit voltage and state of charge characteristic such that the cell state of charge is an average state of charge of the traction battery offset by an amount of state of charge associated with an open-circuit cell voltage difference.