BATTERY MONITOR WITH CORRECTION FOR INTERNAL OHMIC MEASUREMENTS OF BATTERY CELLS IN PARALLEL CONNECTED BATTERY STRINGS

• US 20120119745A1
• Filed: 05/12/2011
• Published: 05/17/2012
• Est. Priority Date: 05/14/2010
• Status: Active Grant

• Current Assignee: Vertiv Corporation (Vertiv Holdings Co.)
• Original Assignee: Liebert Corporation (Vertiv Holdings Co.)
• Inventors: Deveau, Edward W., Bie, Xiaoquan
• Granted Patent: US 8,933,702 B2
• Time in Patent Office: Days
• US Class Current: 324/429
• CPC Class Codes: G01R 31/386 using test-loads
• CPC Class Codes: G01R 31/389 Measuring internal impedance, internal conductance or related variables
• CPC Class Codes: G01R 31/396 Acquisition or processing of data for testing or for monitoring individual cells or groups of cells within a battery

Abstract

A battery monitor determines an internal resistance of a battery cell of a battery having parallel connected battery strings which accounts for errors introduced by the parallel connected battery strings. When determining the internal resistance of a battery cell, the battery monitor determines a baseline intercell resistance of an intercell connecting the negative terminal of the battery cell to the positive terminal of an adjacent battery. The battery monitor then applies a momentary load across the battery cell and immediately prior to releasing the load, measures the voltage across the battery cell (loaded voltage) and the voltage across the intercell. The battery monitor then calculates the current flowing through the intercell (intercell current) by dividing the measured voltage across the intercell by the determined intercell resistance. Immediately after releasing the load, the battery monitor measures the voltage across the battery cell (recovered voltage) and determines the battery cell internal resistance by dividing the difference between the recovered voltage and the loaded voltage by the intercell current.

Claims

1. A system for determining resistance of a battery cell in a battery string, comprising:

a voltage sense circuit operable to measure a voltage drop across a first battery cell and to measure a voltage drop across an intercell connection between the first battery cell and a second adjacent battery cell, where a negative terminal of the first battery cell coupled by the intercell connection to a positive terminal of the second battery cell;

a test load circuit that operates to apply a load across the first battery cell and the intercell connection; and

a controller in data communication with the voltage sense circuit to receive a voltage drop measure across the first battery cell while the load is applied and immediately after the load is released, the controller determines a voltage difference between the voltage drop across the first battery cell while the load is applied and the voltage drop across the first battery cell promptly after releasing the load, determines a current through the intercell connection, and determines an internal resistance of the first battery cell from the voltage difference and the current through the intercell connection.

2. The system of claim 1 wherein the controller determines a baseline resistance of the intercell connection and determines the current through the intercell connection from a voltage drop measure across the intercell connection and the resistance of the intercell connection using Ohms law.

3. The system of claim 2 wherein the test load circuit operates to measure current through the applied load when the battery string is isolated electrically from other battery string parallel connected thereto and the controller determines resistance of the intercell connection from the voltage drop across the intercell connection and the current through the applied load using Ohms law.

4. The system of claim 1 wherein the voltage sense circuit has voltage sense inputs that are electrically coupled across the first battery cell and across the intercell connection.

5. The system of claim 1 wherein the controller determines a float current in the first battery cell and adjusts the current through the intercell connection prior to determining the internal resistance of the first battery cell.

6. The system of claim 2 wherein the controller determines a float current in two or more intercell connections in the battery string and validates baseline resistances for each of the intercell connections by comparing the float currents of the intercell connections to each other.

7. The system of claim 1 is incorporated into a battery monitor device.

8. A battery monitor device, comprising:

a voltage sense circuit electrically coupled across a first battery cell and to a positive terminal of a second battery cell, where a negative terminal of the first battery cell coupled by the intercell connection to a positive terminal of the second battery cell, the voltage sense circuit operates to measure a voltage drop across the first battery cell and to measure a voltage drop across the intercell connection between the first battery cell and the second battery cell;

a test load circuit electrically coupled across the first battery cell and the intercell connection and operates to apply a load across the first battery cell and the intercell connection; and

a controller in data communication with the first voltage sense circuit to receive a voltage drop measure across the first battery cell while a load is applied and promptly after the load is released and to determine a voltage difference between the voltage drop across the first battery cell while the load is applied and the voltage drop across the first battery cell promptly after releasing the load, the controller further determines a current through the intercell connection from a voltage drop measure from the second voltage sense circuit and determines an internal resistance of the first battery cell from the voltage difference and the current through the intercell connection.

9. The battery monitor of claim 8 wherein the controller determines baseline resistance of the intercell connection and determines the current through the intercell connection from a voltage drop measure across the intercell connection and the resistance of the intercell connection using Ohms law.

10. The battery monitor of claim 9 wherein the test load circuit operates to measure current through the applied load and the controller determines resistance of the intercell connection from the voltage drop across the intercell connection and the current through the applied load using Ohms law.

11. The battery monitor of claim 8 wherein the controller determines a float current in the first battery cell and adjusts the current through the intercell connection prior to determining the internal resistance of the first battery cell.

12. A method for determining resistance of a battery cell in a battery string of a battery system, comprising:

measuring a voltage drop across a first battery cell in a first battery string while a load is applied across the first battery cell, a negative terminal of a first battery cell coupled by an intercell connection to a positive terminal of an adjacent second battery cell;

measuring a voltage drop across the first battery cell promptly after releasing the load across the first battery cell;

determining a voltage difference between the voltage drop across the first battery cell while the load is applied and the voltage drop across the first battery cell promptly after releasing the load;

determining current through the intercell connection; and

determining an internal resistance of the first battery cell from the voltage difference and the current through the intercell connection.

13. The method of claim 12 further comprises determining baseline resistance of the intercell connection while the first battery string is electrically uncoupled from other battery strings in the battery system;

and measuring a voltage drop across the intercell connection while the load is applied.

14. The method of claim 13 further comprises determining the current through the intercell connection from the voltage drop across the intercell connection and the resistance of the intercell connection using Ohms law.

15. The method of claim 13 further comprises determining resistance of the intercell connection from the voltage drop across the intercell connection and the current through the intercell connection using Ohms law.

16. The method of claim 12 further comprises determining a float current in the first battery cell and adjusting the current through the intercell connection prior to determining the internal resistance of the first battery cell.

17. The method of claim 12 further comprises determining a float current in two or more intercell connections in the battery string and validating baseline resistances for each of the intercell connections by comparing the float currents of the intercell connections to each other.