METHOD FOR DETERMINING THE VALUE OF A PARAMETER RELATED TO THE STATE OF HEALTH OF AN ELECTROCHEMICAL CELL IN A BATTERY, SYSTEM FOR ELECTRONIC MANAGEMENT OF A BATTERY, AND CORRESPONDING BATTERY

Abstract

The invention relates to a method for determining the value of one or more parameters relating to the state of health of at least one accumulator of a battery that is intended to provide electrical energy to an external application. A first state-of-health parameter related to the resistance of at least one electrochemical element of the battery is determined according to the method comprising:—a step (1) of determining the value of the state of charge of the electrochemica 1 element, expressed as a percentage of a maximum state of charge; —a step (2) of verifying whether the determined value of the state of charge belongs to a sought range; —a step of repeating, as long as the result of the verification step (2) is not positive, the determination step (1) and then the verification step (2); —calculating the value of the first parameter as a function of at least the value of the state of charge determined during the last determination step (1).

Claims

1. A method for determining a value of a first parameter (R) reflecting state of health which is related to resistance of at least one electrochemical cell of a battery, said electrochemical cell being of the type having a curve for state of charge which represents open circuit voltage (OCV) at the terminals of the electrochemical cell as a function of state of charge (SOC) expressed as a percentage of a maximum state of charge, at least one determined portion (zone 2-3) of said curve being substantially flat or said curve comprising at least one determined portion (zone 2-3) in which there is substantially not a one-to-one relationship between state of charge (SOC) and open circuit voltage (OCV), the method comprising the steps of: determining (1) a value for state of charge (SOC) of the electrochemical cell, expressed as a percentage of a maximum state of charge, or a value of electric charge (Ah) of the electrochemical cell; verifying (2) whether the determined value for state of charge (SOC) falls within a region of interest ([SOC.sub.min, SOC.sub.max]) having a lower bound SOC.sub.min and an upper bound SOC.sub.max, or whether the determined value of electric charge (Ah) falls within an extended region ([Ah.sub.min_et, Ah.sub.max_et]) having a lower bound Ah.sub.min_et and an upper bound Ah.sub.max_et determined from the lower bound SOC.sub.min and the upper bound SOC.sub.max of the region of interest ([SOC.sub.min, SOC.sub.max]), the region of interest ([SOC.sub.min, SOC.sub.max]) corresponding to a portion of the curve representing state of charge that is included in the determined portion (zone 2-3); repeating, as long as a result of said verification step (2) is not positive, said determination step (1) and then said verification step (2); calculating (3) a value of said first parameter (R) as a function of at least a value for state of charge (SOC) or of electric charge (Ah) determined during the last determination step (1).

2. The method according to claim 1, wherein, prior to at least the first determination step (1), a value of electric capacity C of the electrochemical cell is determined (0), and at least one of said determination steps (1) is a step of determining an SOC value for state of charge (SOC) and comprises the steps of: performing the following calculation: SOC=1-Ah/C, where Ah is electric charge of the electrochemical cell expressed in Ampere-hour under discharge of said electrochemical cell; or performing the following calculation: SOC=Ah/C, where Ah is electric charge of the electrochemical cell expressed in Ampere-hour under charge of said electrochemical cell.

3. The method according to claim 2, wherein said first parameter is a resistance (R) of the electrochemical cell, and in that said step (3) of calculating a value R of resistance (R) comprises carrying out the following calculation: R=(U-OCV)/I, where U and I are respective voltage and current values at the terminals of the electrochemical cell, and OCV is a value of open circuit voltage (OCV), OCV being a function of a value for state of charge (SOC) determined in corresponding determination step (1).

4. The method according to claim 3, wherein the OCV value is determined from a table associating values for state of charge (SOC) with values for open circuit voltage (OCV).

5. The method according to claim 3, wherein the OCV value is a constant value of no-load voltage over the region of interest [SOC.sub.min, SOC.sub.max] of the electrochemical cell.

6. The method according to claim 3, wherein the step (3) of calculating a value R of resistance (R) comprises repeating the calculation (U-OCV)/I for several SOC values for state of charge (SOC) within the region of interest ([SOC.sub.min, SOC.sub.max]), and calculating an average of the results of said calculation (U-OCV)/I.

7. The method according to claim 1, wherein the determining step (1) is a step of determining a value Ah of electric charge (Ah), and in that the lower bound Ah.sub.min_et of the extended region ([Ah.sub.min_et, Ah.sub.max_et]) of electric charge values (Ah) is obtained from the region of interest ([SOC.sub.min, SOC.sub.max]) of values for state of charge by carrying out the following calculation: under discharge of the electrochemical cell, Ah.sub.min_et=C×(1-SOC.sub.max); or under charge of the electrochemical cell, Ah.sub.min_et=C×SOC.sub.min, where C is a minimum attainable capacity of the electrochemical cell.

8. The method according to claim 7, wherein the upper bound Ah.sub.max_et of the extended region ([Ah.sub.min_et, Ah.sub.max_et]) of electric charge values (Ah) is obtained from the region of interest ([SOC.sub.min, SOC.sub.max]) of values for state of charge by carrying out the following calculation: under discharge of the electrochemical cell, Ah.sub.max_et=C×(1- SOC.sub.min); or under charge of the electrochemical cell, Ah.sub.max_et=C×SOC.sub.max, where C is the nominal capacity of the electrochemical cell.

9. The method according to claim 7, wherein said first parameter is a resistance (R) of the electrochemical cell, and comprising the steps for each determined value Ah of electric charge (Ah) of: determining a resistance R value (R) using the following calculation: R=(U-OCV)/I, where U and I are respective voltage and current values at the terminals of the electrochemical cell, and OCV is a value for open circuit voltage (OCV), OCV being a function of the determined value Ah of electric charge (Ah), and storing this resistance value (R) in a resistance value table.

10. The method according to claim 9, wherein OCV is determined from a table associating values for state of charge (SOC) with values for open circuit voltage (OCV), each value for state of charge (SOC) being associated with a value for electric charge (Ah).

11. The method according to claim 9, wherein the OCV value is the constant value of no-load voltage over the region of interest ([SOC.sub.min, SOC.sub.max], of the electrochemical cell.

12. The method according to claim 7, wherein a value for electric capacity C of the electrochemical cell is determined, and a value of a first parameter (R) reflecting state of health which is related to resistance is determined from resistance values in the resistance table, for which a value Ah of electric charge in said table falls within a region of interest ([Ah.sub.min, Ah.sub.max]) of electric charge corresponding to the region of interest ([SOC.sub.min, SOC.sub.max]) of values for state of charge.

13. The method according to claim 12, wherein the lower bound Ah.sub.min and the upper bound Ah.sub.max of the region of interest ([Ah.sub.min, Ah.sub.max]) of electric charge values (Ah) are determined from the lower bound SOC.sub.min and the upper bound SOC.sub.max of the region of interest ([SOC.sub.min, SOC.sub.max]) of values for state of charge.

14. The method according to claim 13, wherein the lower bound of the region of interest ([Ah.sub.min, Ah.sub.max]) of electric charge values (Ah) is obtained from the region of interest ([SOC.sub.min, SOC.sub.max]) of values for state of charge by carrying out the following calculation: under discharge of the electrochemical cell, Ah.sub.min=C×(1-SOC.sub.max); or under charge of the electrochemical cell, Ah.sub.min=C×SOC.sub.min, where C is the determined capacity of the electrochemical cell.

15. The method according to claim 13, wherein the upper bound Ah.sub.max of the region of interest ([Ah.sub.min, Ah.sub.max]) of electric charge values (Ah) is obtained from the region of interest ([SOC.sub.min, SOC.sub.max]) of values for state of charge by carrying out the following calculation: under discharge of the electrochemical cell, Ah.sub.max=C×(1-SOC.sub.min); or under charge of the electrochemical cell, Ah.sub.max=C×SOC.sub.max, where C is the determined capacity of the electrochemical cell.

16. The method according to claim 1, wherein state of charge (SOC) is expressed as a percentage of maximum state of charge, and in that the region of interest ([SOC.sub.min, SOC.sub.max]) of values for state of charge (SOC) is comprised in the interval [44%, 60%], preferably [46%, 58%].

17. The method according to claim 1, wherein said first parameter is a resistance (R) of the electrochemical cell, and in that the method comprises a step of determining a value of a second parameter (SOHR) reflecting state of health which is related to resistance of the electrochemical cell as a function of a value R of the determined resistance (R).

18. The method according to claim 17, wherein the step of determining a value of the second parameter (SOHR) comprises a step of determining a first value R.sub.init of resistance (R) as a function of a temperature and/or current value, from a table associating temperature and/or current values, with resistance values.

19. The method according to claim 18, wherein the SOHR value of the second parameter (SOHR) is determined according to the following calculation: SOHR=R/R.sub.init.

20. An electronic battery management system comprising at least one electrochemical cell, the system comprising: means for measuring at least voltage (U) and current (I) at the terminals of said electrochemical cell, under charge or discharge of said electrochemical cell; a microprocessor programmed to carry out the method according to claim 1.

21. A battery comprising at least one electrochemical cell, and at least one electronic management system according to claim 20.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0069] The features and advantages of the invention will become apparent upon reading the description which follows, given solely by way of example, and non-limiting, with reference to the following appended drawings.

[0070] FIG. 1 graphically shows the curve of open circuit voltage as a function of state of charge of an electrochemical cell of the LiFePO.sub.4 type.

[0071] FIG. 2 shows the steps of the method according to the invention diagrammatically.

DETAILED DESCRIPTION OF EMBODIMENTS

[0072] The method of the invention is described below on the basis of two particular embodiments.

[0073] It is a question of determining a value of a first parameter reflecting state of health which is related to the resistance of at least one electrochemical cell of a battery. In the two above-mentioned embodiments, the first parameter reflecting state of health which is related to resistance of the electrochemical cell in question is the resistance itself of this electrochemical cell.

[0074] As can be seen in the example of FIG. 1, the electrochemical cell concerned is of the type having an open circuit voltage curve of which at least one determined portion is substantially flat or which exhibits a determined portion in which there is substantially not a one-to-one relationship between state of charge, SOC, and open circuit voltage, OCV.

[0075] It will be recalled that the open circuit voltage curve of an electrochemical cell graphically represents open circuit voltage OCV at the terminals of the electrochemical cell, depending on state of charge, SOC expressed as a percentage of a maximum state of charge.

[0076] Such a curve is shown in FIG. 1, with in zone 2-3, a substantially flat determined portion in which there is substantially not a one-to-one relationship between state of charge, SOC and open circuit voltage, OCV.

[0077] In this zone 2-3 corresponding to the determined portion of the curve, it is not possible to accurately associate a state of charge with a voltage measurement. Indeed, for one single voltage value on the y-axis (plus or minus a tiny margin) there can be a number of values of state of charge (on the x-axis).

[0078] As shown schematically in FIG. 2, the method essentially comprises a determination step (1) and a verification step (2).

[0079] Step (1) is a step of determining a value of state of charge, SOC of the electrochemical cell, SOC being expressed as a percentage of a maximum state of charge. Alternatively, this step (1) is a step of determining a value of the electric charge Ah of the electrochemical cell.

[0080] Step (2) is a step of verifying whether the determined value falls within a particular interval of values, referred to as the region of interest [SOC.sub.min, SOC.sub.max] when it is a question of determination of state of charge, SOC at determination step (1), or falls within an extended region [Ah.sub.min_et, Ah.sub.max_et] when it is a question of determination of electric charge Ah in determination step (1).

[0081] The region of interest ([SOC.sub.min, SOC.sub.max]) corresponds to a portion of the open circuit voltage curve included in the determined portion.

[0082] Determination step (1) followed by verification step (2) are repeated as long as the result of verification step (2) is not positive, i.e. as long as a value determined in determination step (1) is not included in the corresponding interval ([SOC.sub.min, SOC.sub.max]) or [Ah.sub.min_et, Ah.sub.max_et].

[0083] Then, in a calculation step (3), a value of said first parameter R is calculated as a function of at least a value of the state of charge SOC, or a value of the electric charge Ah, determined in the last determination step (1).

[0084] Prior to the step (1) of determining a value of state of charge SOC or of the electric charge Ah, as will be seen subsequently, a value of the electric capacity C of the electrochemical cell is preferably determined (0).

[0085] In a first embodiment, the successive determination step(s) (1) are steps of determining the SOC value of state of charge (SOC). This SOC value is then calculated as follows: [0086] SOC=1-Ah/C, where Ah is the electric charge of the electrochemical cell expressed in Ampere-hour under discharge of this electrochemical cell;

[0087] Or [0088] SOC=Ah/C, where Ah is the electric charge of the electrochemical cell expressed in Ampere-hour under charge of this electrochemical cell.

[0089] Furthermore, as the first parameter, the value of which is to be determined, is the resistance of the electrochemical cell as indicated above, a value R for resistance calculated in the calculation step (3) is obtained as follows: [0090] R=(U-OCV)/I

[0091] In this calculation, U and I are the respective voltage and current values at the terminals of the electrochemical cell, and OCV is the open circuit voltage value. OCV is a function of the SOC value for state of charge determined in the corresponding determination step (1).

[0092] More specifically, the OCV value is determined from a table that associates SOC values of SOC with open-circuit voltage OCV values. The OCV value may also be the constant value of open circuit voltage over the region of interest [SOC.sub.min, SOC.sub.max] for values of state of charge of the electrochemical cell.

[0093] It is also possible to choose to determine a value of R as being the average of several values of resistances determined by the calculation (U-OCV)/I presented above, for several values of state of charge, SOC comprised in the interval [SOC.sub.min, SOC.sub.max] referred to as the region of interest.

[0094] In a second embodiment, the successive determination step(s) (1) are steps of determining a value Ah of the electric charge.

[0095] In this embodiment, the lower bound Ah.sub.min_et and the upper bound Ah.sub.max_et of the interval [Ah.sub.min_et, Ah.sub.max_et] of values for electric charge Ah, referred to as the extended region, are determined from the lower bound SOC.sub.min_et the upper bound SOC.sub.max of the region of interest [SOC.sub.min, SOC.sub.max] of values of state of charge.

[0096] More specifically, the lower bound Ah.sub.min_et of the extended region [Ah.sub.min_et, Ah.sub.max_et] is obtained as follows: [0097] under discharge of the electrochemical cell, Ah.sub.min_et=C×(1-SOC.sub.max);

[0098] Or [0099] under charge of the electrochemical cell, Ah.sub.min_et=C×SOC.sub.min,

[0100] In this calculation, C is the minimum reachable capacity of the electrochemical cell.

[0101] Furthermore, the upper bound Ah.sub.max_et of m the extended region [Ah.sub.min_et, Ah.sub.max_et] is obtained as follows: [0102] under discharge of the electrochemical cell, Ah.sub.max_et=C×(1-SOC.sub.min);

[0103] Or [0104] under charge of the electrochemical cell, Ah.sub.max_et=C×SOC.sub.max,

[0105] In this calculation, C is the nominal capacity of the electrochemical cell.

[0106] Then, as the first parameter the value of which it is a question of determining is the resistance of the electrochemical cell as indicated above, a value R of the resistance calculated in the calculation step (3) is obtained as explained below.

[0107] For each determined value Ah of electric charge (Ah), a resistance value R is determined and this resistance value R is stored in a resistance value table.

[0108] The calculation of resistance R values is carried out as follows: R=(U-OCV)/I.

[0109] In this calculation, U and I are the respective voltage and current values at the terminals of the electrochemical cell, and OCV is the value of open circuit voltage.

[0110] The OCV value is a function of the determined value Ah of electric charge.

[0111] More specifically, the OCV value is determined from a table associating state of charge, SOC values with open circuit voltage OCV values, each state of charge, SOC value being further associated with a value Ah of state of charge.

[0112] The OCV value may also be the constant value of open circuit voltage over the region of interest [SOC.sub.min, SOC.sub.max] for values of state of charge of the electrochemical cell.

[0113] In this embodiment, it is necessary to predetermine a value of capacity C in order to apply the remainder of the method. This capacity can be determined by any means known to a person skilled in the art (examples: constant current charge or discharge)

[0114] Preferably, for this calculation of the resistance R value, the resistance values stored in the aforementioned resistance table are used, for which it has been verified that the associated electric charge Ah value in said table does fall within a region of interest [Ah.sub.min, Ah.sub.max] of electric charge, this region of interest corresponding to the region of interest [SOC.sub.min, SOC.sub.max] of values of state of charge.

[0115] Thus, the lower bound Ah.sub.min and the upper bound Ah.sub.max of the region of interest [Ah.sub.min, Ah.sub.max] are determined from the lower bound SOC.sub.min and from the upper bound SOC.sub.max of the region of interest [SOC.sub.min, SOC.sub.max].

[0116] More specifically, the lower bound Ah.sub.m of the region of interest [Ah.sub.min, Ah.sub.max] is obtained by the following calculation: [0117] under discharge of the electrochemical cell, Ah.sub.min=C×(1-SOC.sub.max);

[0118] Or [0119] under charge of the electrochemical cell, Ah.sub.min=C×SOC.sub.min,

[0120] Furthermore, the upper bound Ah.sub.max of the region of interest [Ah.sub.min, Ah.sub.max] is obtained by the following calculation: [0121] under discharge of the electrochemical cell, Ah.sub.max=C×(1-SOC.sub.min);

[0122] Or [0123] under charge of the electrochemical cell, Ah.sub.max=C×SOC.sub.max.

[0124] In all embodiments, the applicant has determined that the region of interest for the values of state of charge, SOC, expressed as a percentage of a maximum state of charge, comprised in the interval [44%, 60%], preferably [46%, 58%], gave very good results in terms of accuracy of determination of a value of the parameter reflecting state of health which is related to resistance of the electrochemical cell of the battery, in other words the value of the resistance itself.

[0125] Also, in all embodiments, the value of a second parameter SOHR reflecting state of health which is related to resistance of the electrochemical cell is determined as a function of a value R of the determined resistance.

[0126] To do this, a first value R.sub.init of the resistance is determined as a function of a temperature and/or current value. For this purpose, a table associating temperature and/or current values with resistance values is used.

[0127] Then, the value of SOHR of the second parameter is obtained as follows: SOHR=R/R.sub.init.

[0128] Thus, the impact of temperature or current on the calculation can be decorrelated.

[0129] The method of the invention is particularly suitable for situations in which the electrochemical cell concerned is of the type having an open-circuit voltage curve of which at least one portion is substantially flat or having a portion in which the relationship between state of charge SOC and open circuit voltage OCV is substantially not a one-to-one relationship, as shown in FIG. 1.

[0130] The method of the invention can be implemented by programming a microprocessor of an electronic management system for a battery comprising at least one electrochemical cell. Such a system further comprises means for measuring at least the voltage U and the current I, at the terminals of the electrochemical cell (s), under charging or discharge of the electrochemical cell(s), to allow the microprocessor to carry out the steps of the method.

[0131] A battery comprising at least one electrochemical cell, and at least one electronic management system as presented above, enables efficient and accurate monitoring of its actual ageing, despite notably the modifications of certain internal parameters inherent in ageing, by means of an efficient and accurate determination of resistance R.

[0132] The present description is given by way of example and is not limiting of the invention.

[0133] In particular, the invention is not limited to the two specific embodiments presented by way of example. It is based on the use of an interval of values of state of charge [SOC.sub.min, SOC.sub.max], referred to as the region of interest of state of charge, or of an interval of electric charge values [Ah.sub.min_et, Ah.sub.max_et], whose lower and upper bounds are related, as seen above, to the lower and upper bounds of the state of charge region of interest, the region of interest [SOC.sub.min, SOC.sub.max] being included in the plateau region of an open circuit voltage curve, i.e. the determined portion of this curve which is substantially flat, or a determined portion in which there is substantially not a one-to-one relationship between state of charge, SOC and open circuit voltage, OCV. The two above-mentioned embodiments give two examples of the type of calculation which make it possible to obtain the desired resistance value R.