DETERMINATION OF NOMINAL CELL RESISTANCE FOR REAL-TIME ESTIMATION OF STATE-OF-CHARGE IN LITHIUM BATTERIES

• US 20150226807A1
• Filed: 02/10/2015
• Published: 08/13/2015
• Est. Priority Date: 02/12/2014
• Status: Abandoned Application

• Current Assignee: Seeo, Inc. (Robert Bosch GmbH)
• Original Assignee: Seeo, Inc. (Robert Bosch GmbH)
• Inventors: Aumentado, Albert, Mullin, Scott Allen, Singh, Mohit
• Application Number:
  US14/618,902
• Publication Number:
  US 20150226807A1
• 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 31/3842 combining voltage and current measurements
• CPC Class Codes: G01R 31/389 Measuring internal impedance, internal conductance or related variables
• CPC Class Codes: H01M 10/052 Li-accumulators
• CPC Class Codes: H01M 10/0565 Polymeric materials, e.g. gel-type or solid-type
• CPC Class Codes: H01M 10/48 Accumulators combined with arrangements for measuring, testing or indicating the condition of cells, e.g. the level or density of the electrolyte constructional details of current conducting connections for detecting conditions inside cells or batteries, e.g. details of voltage sensing terminals, H01M50/569
• CPC Class Codes: H01M 2010/4271 Battery management systems including electronic circuits, e.g. control of current or voltage to keep battery in healthy state, cell balancing
• CPC Class Codes: H01M 2220/20 Batteries in motive systems, e.g. vehicle, ship, plane
• CPC Class Codes: Y02E 60/10 Energy storage using batteries

Abstract

A functional relation relationship has been established between SOC, nominal resistance (R_nom) and average applied load (P_avg), such that a function f(R_nom, P_avg)=SOC can be determined empirically. Load can be described using either average power or average current. The cell is tested initially to determine the relationships among these values prior to operation to create a look-up table. During operation, R_nom and P_avg can be sampled with no cell down time and can be used as input parameters with the look-up table to determine SOC accurately.

Claims

1. A method of determining the state of charge for a battery cell in real time comprising the steps of:

a) measuring voltage and current values as a function of time while the battery cell is operating;

b) recording in a memory location the voltage and the current values over various time periods

c) determining a nominal resistance associated with each time period from recorded the voltage and current values in the memory location using a computer processor;

d) recording in a memory location data pairs of the nominal resistance with the associated time period;

e) determining the state of charge of the battery cell by comparing, using a computer processor, the data pairs to a previously-populated look-up table in a memory location, wherein the look-up table correlates the data pairs to state-of-charge values.

2. The method of claim 1 wherein the battery cell comprises lithium metal as an anode, lithium iron phosphate as a cathode, and a polymer electrolyte as a separator.

3. The method of claim 1 wherein, in step (b), the time periods are from about 0.5 to 60 seconds long.

4. The method of claim 1 wherein, in step (c), determining a nominal resistance comprises using the following expression to calculate the nominal resistance (R_nom):

5. The method of claim 4 wherein V_avg is given by:

6. The method of claim 1 wherein, in step (e), the previously-populated look-up table is created by:

determining a total capacity of the cell in total Coulombs;

using a battery tester to count Coulombs moving into and out of a representative cell as the tester pulls and pushes current according to a predetermined program over an entire capacity range of the cell to generate a look-up table that comprises associated values of voltage as a function of time and of Coulombs as a percentage of total Coulombs in the cell (SOC);

storing the look-up table in a memory location; and

using a microprocessor to calculate nominal cell resistances for various time periods in the curve and storing the nominal cell resistances in the look-up table.

7. The method of claim 1 wherein, in step (e), the microprocessor is programmed to perform a linear interpolation between values in the look up table as needed.

8. A computer-readable medium comprising code which, upon execution by a computer processor implements a method, the method comprising:

a) measuring voltage and current values as a function of time while the battery cell is operating;

b) recording in a memory location the voltage and the current values over various time periods

c) determining a nominal resistance associated with each time period from the voltage and current values recorded in the memory location using a computer processor;

d) recording in a memory location data pairs of the nominal resistance with the associated time period;

e) determining the state of charge of the battery cell by comparing, using a computer processor, the data pairs to a previously-populated look-up table in a memory location, wherein the look-up table correlates the data pairs to state-of-charge values.

9. The computer-readable medium of claim 8 wherein the battery cell comprises lithium metal as an anode, lithium iron phosphate as a cathode, and a polymer electrolyte as a separator.