State of charge indicator

• US 6,127,806 A
• Filed: 05/14/1999
• Issued: 10/03/2000
• Est. Priority Date: 05/14/1998
• Status: Active Grant

• Current Assignee: Nissan Motor Co., Ltd.
• Original Assignee: Nissan Motor Co., Ltd.
• Inventors: Iwai, Ken, Tanjo, Yuji, Nakagawa, Toyoaki, Horie, Hideaki, Abe, Takaaki, Kawai, Mikio
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): TOATLEY, GREGORY J
• Application Number:
  US09/311,884
• Time in Patent Office: 508 Days
• Field of Search: 320/132, 324/432, 324/433
• US Class Current: 320/132
• CPC Class Codes: G01R 31/374 with means for correcting the measurement for temperature or ageing
• CPC Class Codes: G01R 31/3842 combining voltage and current measurements

Abstract

A state of charge indicator of a lithium ion battery computes a state of charge (SOC) corresponding to a cell open circuit voltage from a SOC-cell open circuit voltage characteristic map, and displays the SOC on a display device. Herein, the SOC-cell open circuit voltage characteristics are obtained by defining the battery charge amount when the open circuit voltage of the cell is 3.9 V as SOC=100%, and defining the battery charge amount when the open circuit voltage of the cell is 3.5 V as SOC=100%. By defining SOC=100% and SOC=0% in this way, the SOC can be correctly computed and displayed even if the battery has deteriorated.

Claims

1. A state of charge indicator for a battery comprising plural cells connected in series, the indicator comprising:

a display device;

a sensor which detects an open circuit voltage of the battery;

a sensor which detects a charging/discharging current of the battery; and

a microprocessor programmed to;

compute an open circuit voltage of a cell based on the detected battery open circuit voltage;

compute an SOC (state of charge) corresponding to the computed cell open circuit voltage from an SOC-cell open circuit voltage characteristic map obtained by defining the battery charge amount when the open circuit voltage of the cell is a first predetermined voltage as SOC 100%, and defining the battery charge amount when the open circuit voltage of the cell is a second predetermined voltage as SOC 0%, wherein the open circuit voltage of the cell is in a range between the second predetermined voltage and the first predetermined voltage, the second predetermined voltage being set to be lower than the first predetermined voltage;

compute the SOC by integrating the detected charging/discharging current when the open circuit voltage of the cell is greater than the first predetermined voltage; and

display the computed SOC in the display device.

2. A state of charge indicator as defined in claim 1, wherein the battery comprises a lithium ion battery and the first predetermined voltage is 3.9 V.

3. A state of charge indicator as defined in claim 1, wherein the battery comprises a lithium ion battery and the second predetermined voltage is 3.5 V.

4. A state of charge indicator for a battery, the indicator comprising:

a display device,

a sensor which detects an open circuit voltage of the battery, and

a microprocessor programmed to;

computer an SOC (state of charge) corresponding to the detected battery open circuit voltage from an SOC-battery open circuit voltage characteristic map obtained by defining the battery charge amount, when the decrease rate of maximum output power of the battery exceeds a predetermined value, as SOC=100%, and defining the battery charge amount, when the maximum input power and maximum output power of the battery are equal, as SOC=0%, and

display the computed SOC on the display device.

5. A state of charge indicator as defined in claim 4, comprising a sensor which detects a charging/discharging current of the battery, wherein the microprocessor is further programmed to compute the SOC by integrating the detected charging/discharging current when the SOC exceeds 100%.

6. A state of charge indicator as defined in claim 4, wherein the microprocessor compares the SOC-battery open circuit voltage characteristics at an SOC higher than 100% with the SOC-battery open circuit voltage characteristics when the battery has deteriorated, and determines that the battery has deteriorated when the two characteristics coincide.

7. A state of charge indicator for a battery comprising plural cells connected in series, the indicator comprising:

a display device;

a sensor which detects an open circuit voltage of the battery; and

a microprocessor programmed to;

compute an open circuit voltage of a cell based on the detected battery open circuit voltage;

compute an SOC (state of charge) corresponding to the computed cell upon circuit voltage from an SOC-cell open circuit voltage characteristic map obtained by defining the battery charge amount when the open circuit voltage of the cell is a first predetermined voltage as SOC 100%, and defining the battery charge amount when the open circuit voltage of the cell is a second predetermined voltage as SOC 0%, with the second predetermined voltage being set to be lower than the first predetermined voltage;

display the computed SOC on the display device;

compare the SOC-cell open circuit voltage characteristics at an SOC higher than 100% with the SOC-cell open circuit voltage characteristics in a deterioration state; and

determine that the battery has deteriorated when the two characteristics coincide.

8. A state of charge indicator as defined in claim 7, wherein the battery comprises a lithium ion battery and the first predetermined voltage is 3.9 V.

9. A state of charge indicator as defined in claim 7, wherein the battery comprises a lithium ion battery and the second predetermined voltage is 3.5 V.