Methods to Determine Battery Cell Voltage Relaxation Time Based on Cell Usage History and Temperature

• US 20160011269A1
• Filed: 07/14/2014
• Published: 01/14/2016
• Est. Priority Date: 07/14/2014
• Status: Active Grant

• Current Assignee: Ford Global Technologies, LLC (Ford Motor Company)
• Original Assignee: Ford Global Technologies, LLC (Ford Motor Company)
• Inventors: Wang, Xu, Chang, Xiaoguang, He, Chuan
• Granted Patent: US 10,481,210 B2
• Time in Patent Office: Days
• US Class Current: 1/1
• CPC Class Codes: G01R 31/367 Software therefor, e.g. for battery testing using modelling or look-up tables
• CPC Class Codes: G01R 31/3835 involving only voltage measurements

Abstract

Lithium ion battery state of charge (SOC) is a function of open circuit voltage (OCV). Battery internal diffusion process needs to be nearly completed to be able to measure battery open circuit voltage. The length of the minimum settling time depends on the battery type, usage and temperature. Described are methods to determine electric vehicle battery voltage relaxation time based on battery temperature and usage history.

Claims

1. A method of estimating battery state of charge (SOC) in an electric vehicle, comprising:

measuring a battery temperature;

measuring battery currents between a first and second time;

calculating an RMS current based the measured battery currents between the first and second time;

determining an estimated battery relaxation time based on the RMS current and temperature; and

measuring battery open circuit voltage after the relaxation time for calculating the SOC.

2. The method of claim 1, wherein the RMS current is determined over an interval greater than a minimum predetermined calibrated value, wherein the minimum predetermined calibrated value is one second.

3. The method of claim 2, wherein the RMS current is determined over an interval less than a maximum predetermined calibrated value, wherein the maximum predetermined calibrated value is five seconds.

4. The method of claim 1 further comprising:

recording a value of the estimated battery relaxation time at power down;

initiating power up;

comparing the recorded estimated battery relaxation time to an actual time elapsed between power down and power up; and

immediately measuring the open circuit voltage for an updated SOC estimate if the elapsed time is equal to or greater than the recorded value.

5. The method of claim 1, wherein the battery is a lithium ion battery.

6. A Battery Energy Control Module (BECM) comprising:

a relax time look up table (LUT) of battery relaxation time based on root mean square (RMS) current and temperature, wherein the LUT provides a set of relaxation time values corresponding to RMS current and temperature,

wherein the BECM monitors power up and power down events and estimates battery capacity in accordance with a state of charge estimation based on an open circuit voltage, the BECM being further configured to generate an output based on the estimated battery capacity at power up if an elapsed time is greater than the relaxation time provided in the relax time LUT for a measured RMS and temperature.

7. The BECM of claim 6, wherein high-end-value is used as the relaxation time for actual RMS current and temperature values intermediate to RMS current and temperature values listed in the relax time LUT.

8. The BECM of claim 6, wherein linear interpolation is used to estimate a relaxation time for actual RMS current and temperature values intermediate to RMS current and temperature values listed in the relax time LUT.

9. The BECM of claim 6, wherein nonlinear interpolation is used to estimate a relaxation time for actual RMS current and temperature values intermediate to RMS current and temperature values listed in the relax time LUT.

10. The BECM of claim 6, further comprising:

non-volatile memory, wherein the memory stores at least one value selected from;

a measured battery temperature, a measured battery terminal voltage, a current RMS value, and an estimated battery cell relaxation time at power down; and

a SOC-OCV LUT of temperature, SOC, and cell voltage.

11. A method of producing a look up table (LUT) for battery relax time, comprising:

a) soaking a battery to a target temperature;

b) applying a charge/discharge current to the battery; and

c) recording a relax time to a LUT for a certain temperature and current when the battery relaxes to within a predetermined amount of a steady state value.

12. The method of claim 11 wherein the predetermined amount is 3 mV.

13. The method of claim 11 including the step of repeating steps a-c for additional target temperatures and currents.

14. A Battery Energy Control Module (BECM) comprising:

a battery relaxation time model as a function of the root mean square (RMS) current and temperature, wherein the battery relaxation time model provides a relaxation time value corresponding to RMS current and temperature, and

wherein the BECM monitors power up and power down events and estimates battery capacity in accordance with a state of charge estimation based on an open circuit voltage, the BECM being further configured to generate an output based on the estimated battery capacity at power up if an elapsed time is greater than the relaxation time provided in the relax time LUT for a measured RMS and temperature.

15. The BECM of claim 14, wherein a neural network is used to describe the relationship between the relaxation time and RMS current and temperature values, wherein the neural network determines a relaxation time, from RMS current and temperature values.

16. The BECM of claim 14, further comprising:

non-volatile memory, wherein the memory stores at least one value selected from;

a measured battery temperature, a measured battery terminal voltage, a current RMS value, and an estimated battery cell relaxation time at power down; and

a SOC-OCV LUT of temperature, SOC, and cell voltage; and

a neural network of temperature, RMS current, and relaxation time.

17. A method of producing data for a learning data based model for battery relax time by temperature and usage, comprising:

a) soaking a battery to a target temperature;

b) applying a charge/discharge current to the battery;

c) measuring a relax time, wherein the battery relaxes to within 3 mV of its steady value;

d) recording the relax time for a certain temperature and current; and

e) repeating steps a-c for additional target temperatures and currents.