Method for determining state of charge of a battery by measuring its open circuit voltage

• US 6,366,054 B1
• Filed: 05/02/2001
• Issued: 04/02/2002
• Est. Priority Date: 05/02/2001
• Status: Expired due to Term

• Current Assignee: Samsung Electronics Co. Ltd.
• Original Assignee: Honeywell International Inc.
• Inventors: Palanisamy, Thirumalai G., Hoenig, Steven, Singh, Harmohan
• Primary Examiner(s): Wong, Peter S.
• Assistant Examiner(s): LUK, LAWRENCE W
• Application Number:
  US09/847,801
• Time in Patent Office: 335 Days
• Field of Search: 320/132, 320/130, 320/107, 324/425, 324/427, 324/430, 324/432
• 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 31/379 for lead-acid batteries
• CPC Class Codes: G01R 31/386 using test-loads
• CPC Class Codes: G01R 31/388 involving voltage measurements
• CPC Class Codes: H02J 7/0048 Detection of remaining charge capacity or state of charge [SOC]

Abstract

A method for determining the state of charge (SOC) of a battery by measuring its open circuit voltage (OCV) either with the battery in a fully rested state of chemical and electrical equilibrium or an active state during a period in which the battery settles after charge or discharge is stopped. A first type algorithm is developed to correlate the OCV in a fully rested condition (OCV_REST) to the state of charge at which that measurement is taken. A second type algorithm is developed that predicts a final settling OCV of a battery (OCV_PRED), based on the set of parameters of OCV, rate of change of OCV, and battery case temperature, acquired during the settling period of a battery not at rest. To determine the SOC of a battery being tested that is in the fully settled state the measured OCV_REST is applied to the first type algorithm. To determine the SOC of a battery that has not fully settled, the data of the OCV, rate of change of OCV and battery temperature is applied to a second type algorithm to determine OCV_PRED and the OCV_PRED value is used in the first type algorithm to determine SOC.

Claims

1. A method to determine the state of charge (SOC) of a battery comprising the steps of:

at least one of charging or discharging the battery to reach different levels of SOC;

measuring the battery open circuit voltage (OCV) at each of the different levels of SOC at a time after the charging or discharging has stopped and the battery has reached an equilibrium settled state; and

developing a first type algorithm of battery OCV in the settled state (OCV_REST) for different values of SOC.

2. The method of claim 1 further comprising the steps of:

measuring the OCV_REST of a battery being tested; and

applying the first type algorithm to the measured OCV_REST of the battery being tested to said first type algorithm to determine the battery SOC.

3. The method of claim 1 wherein said first type algorithm is

4. The method of claim 3 wherein for a lead-acid battery said first type algorithm is

5. The method of claim 1 further comprising the step of acquiring data parameters of battery OCV, rate of change of OCV and battery temperature at various times during a settling period from when charge or discharge of the battery is stopped at a given level SOC and until it reaches its settled state;

and

developing from the acquired data at least one second type algorithm that computes a predicted OCV value (OCV_PRED) related to the data parameters acquired during said settling period.

6. The method of claim 5 wherein said at least one second type algorithm is of the form

7. The method of claim 6 wherein two second type algorithms are developed each having different values for the terms a, b, c and d for each of respective different ranges of battery SOC.

8. The method of claim 7 wherein for a lead-acid storage battery the weighting of the terms are:

Below (2.08V)*(# of cells)

9. The method of claim 5 further comprising the steps of:

measuring the current OCV, rate of change of OCV and temperature of a battery being tested at any time during a settling period;

applying the measuring data of the battery being tested to a second type algorithm to determine the OCV_PRED; and

applying the first type algorithm equation to the OCV_PRED value to obtain a battery SOC value corresponding to the battery being tested being in the rested state.

10. The method of claim 7 further comprising the steps of:

measuring the current OCV, rate of change of OCV and temperature of a battery being tested at any time during the settling period;

applying the measured data of the battery being tested to one of said second type algorithms selected based on the battery SOC range to determine the OCV_PRED; and

applying the first type algorithm equation to the OCV_PRED value to obtain a battery SOC value corresponding to the battery being tested being in the rested state.

11. The method of claim 10 wherein for a multi-cell lead-acid battery the second type algorithms are:

Below (2.08V)*(# of cells)