METHOD FOR CONTROLLING A BATTERY SYSTEM FOR BALANCING AND DETECTING A DEFECTIVE CELL

Abstract

The present invention relates to a method for controlling a battery system (1) comprising a plurality of energy storage elements (10a, 10b, 10c, 10d) and a control unit (11) configured to implement a balancing function (12) of the storage elements (10a, 10b, 10c, 10d), the method comprising the following successive steps of determining for each storage element (10a) a value (DEQa) of a first parameter (P1) of the difference in the quantity of electricity between the instantaneous state of charge (SOCt) and a target state of charge (SOC100), determining the maximum value of the first parameter (P1) among the plurality of storage elements (10a), determining for each storage element (10a) a value of a second balancing parameter (P2) of the quantity of electricity to be balanced and determining a setpoint balancing each storage element (10a) as a function of the respective value of the second parameter (P2).Figure 1.

Description

Title of the invention: METHOD FOR CONTROLLING A BATTERY SYSTEM FOR BALANCING AND DETECTING A DEFECTIVE CELL

[0001] The field of the invention relates to a method for controlling a battery system, in particular for balancing and detecting a defective storage element.

[0002] Power battery systems, in particular for electrified vehicles, conventionally comprise a set of electrochemical cells connected in series and / or parallel in the system. The term energy storage element refers to an electrochemical cell or an assembly of cells that can be connected in series and / or in parallel. Conventionally, in electromobility applications, the energy storage elements are of the lithium-ion type.

[0003] During the lifetime of a battery, the storage elements tend to age distinctly and cause an imbalance in the state of charge. The imbalance is problematic because it leads to a reduction in the useful capacity of the battery system as a whole. In addition, excessive imbalance of a cell is a sign of electrochemical deterioration caused by lithium deposition and likely to lead to the formation of a short circuit in the structure. To avoid this risk, battery systems implement monitoring and protection functions to warn a user.

[0004] Furthermore, to optimize the useful capacity of a battery, manufacturers also provide for the integration of a balancing function for the storage elements. A balancing function is based on the detection of a difference in state of charge between the storage elements and the control of a balancing circuit to reduce the observed differences. The balancing functions use passive balancing circuits, implementing energy discharge circuits in a resistor, or more sophisticated balancing circuits called active circuits allowing the transfer of energy between the storage elements. The latter make it possible to improve energy efficiency.

[0005] Furthermore, to address these various issues of protection and energy optimization, the state of the art discloses document EP-B1-2171824 describing a solution for deactivating a defective battery element. This solution further provides a method comprising a step of monitoring an electrical parameter of a group of electrochemical cells connected in parallel, which may be a self-discharge rate, a time required to perform an operation resistive balancing or an amount of energy required during voltage balancing. Patent document US-A1-20210396817 is also known, describing a method for detecting abnormal self-discharge of cells in a battery system based on monitoring energy quantities calculated for the needs of a balancing system with respect to a monitoring criterion. Passive and active balancing techniques are described. This method aims to protect a battery system. Document EP-B1-2365352 is also known, describing a method for controlling a battery system implementing a function for detecting a defective cell by comparing the difference between the maximum state of charge among all the cells and the state of charge of each cell with respect to thresholds. When this difference exceeds one of the thresholds, a defective cell detection alert is generated.

[0006] This latter technique is based on measuring the states of charge of each storage element and implementing a balancing of the state of charge parameters of the cells. However, the cells within the same battery system may have capacity differences that may increase with aging. These capacity differences lead to an imbalance of the states of charge as the charge and discharge cycles occur and reduce the useful capacity of a battery. This phenomenon further increases the action of the balancing function and consequently the energy losses. In addition, these capacity differences may cause false alarms to be raised indicating a defective state of the storage elements due to the state of charge differences that appear during the charge and discharge cycle. However, an aging cell does not necessarily present an immediate risk of short circuit.

[0007] There is therefore a need to overcome the aforementioned problems. One objective of the invention is to propose a battery system making it possible to reduce the activation time of a balancing function. Another objective of the invention is to improve the protection function of a battery system against electrochemical cell failures, in particular against false alarms.

[0008] More specifically, the invention relates to a method for controlling a battery system comprising a plurality of energy storage elements and a control unit configured to implement a function for balancing the storage elements. According to the invention, the method comprises the following successive steps: - determining for each storage element a value of a first parameter of difference in quantity of electricity between the instantaneous state of charge and a target state of charge, - determining the maximum value of the first parameter among the plurality storage elements,

[0009] - determining for each storage element a value of a second balancing parameter of quantity of electricity to be balanced so that the value of the first parameter of each storage element is equal to the maximum value, - determining a balancing instruction for each storage element based on the respective value of the second parameter.

[0010] The method according to the invention may include the following additional characteristics, alone or in combination:

[0011] - The target state of charge is the value of a target state of charge setpoint of a battery system recharge control function.

[0012] - The target state of charge is in a range between 50% and 100% of the capacity total of a storage element.

[0013] - The balancing setpoint is a discharge duration in a discharge circuit calculated based on the value of the second parameter.

[0014] - the control unit is further configured for implementing a function detection of a defective storage element comprising the following steps: - determining for each storage element a value of a third parameter representative of a difference between the value of the first parameter of each said storage element and the average value of the values ​​of the first parameter for the plurality of storage elements,

[0015] - the determination for each storage element of a first current cor corresponding to the variation of the value of the third parameter over an observation period, - the determination for each storage element of a value of a fourth parameter representative of the difference between the value of the balancing setpoint of each said storage element and the average value of the balancing setpoints for the plurality of storage elements,

[0016] - the determination for each storage element of a second current cor corresponding to the variation of the value of the fourth parameter over the observation period, - the determination of a fifth parameter representative of the self-discharge current for each storage element over the observation period depending on the first current and the second current,

[0017] - the generation of an alert when the fifth parameter exceeds a first threshold.

[0018] - The detection function further includes the generation of an alert when the derivative of the fifth parameter with respect to time over the observation period exceeds a second threshold.

[0019] - The detection function further includes the generation of an alert when the derivative of the fifth parameter with respect to the quantity of electricity discharged over the observation period exceeds a third threshold.

[0020] Further provided is a battery system comprising a plurality of energy storage elements and a control unit configured to implement a function of balancing the storage elements and a function of detecting a defective storage element, wherein the control unit is configured to implement the control method according to any one of the preceding embodiments.

[0021] Further provided is an electrified motor vehicle comprising such a battery system.

[0022] The invention provides a control unit of a battery system comprising means specifically configured to implement the control method according to any one of the preceding embodiments.

[0023] The invention provides a computer program comprising instructions which, when the program is executed by a control unit of a battery system, cause the latter to implement any one of the embodiments of the control method.

[0024] Further provided is a computer-readable recording medium comprising instructions which, when executed by a computer, cause the computer to implement any one of the embodiments of the control method.

[0025] The invention reduces energy losses of a battery system due to the shorter activation time of the balancing device. The invention further improves the detection of defective storage elements and avoids the generation of false alerts.

[0026] Other characteristics and advantages of the present invention will appear more clearly on reading the detailed description which follows, comprising embodiments of the invention given as non-limiting examples and illustrated by the appended drawings, in which:

[0027] [Fig. 1] schematically represents a battery system configured for implementing the control method according to the invention.

[0028] [Fig.2] represents a block diagram of an embodiment of the control method during the execution of a balancing function.

[0029] [Fig.3] schematically represents a battery system and the parameters calculated during the execution of the method according to the invention for the balancing function.

[0030] [Fig.4] schematically represents a battery system and the parameters calculated during the execution of the method according to the invention for the balancing function.

[0031] [Fig.5] represents a block diagram of an embodiment of the control method during the execution of a function for detecting a defective storage element.

[0032] The invention applies to electrified vehicles, i.e. vehicles comprising an electric motor machine and power electronics, 100% electric or hybrid, preferably motor vehicles, but not only such as aircraft, tractors, bicycles, ships. More generally, the invention applies to any autonomous electrical system such as drones, robots or portable devices, computers, tablets, mobile phones, consoles, cameras, scanners, which are cited as a non-exhaustive list of application examples.

[0033] In [Fig. 1], a battery system 1 intended to implement the control method according to the invention is shown schematically. The battery system 1 comprises a plurality of energy storage elements, at least two, illustrated by way of non-limiting example by four elements 10a, 10b, 10c, 10d, and a control unit 11 configured for implementing the control method comprising a balancing function 12 of the energy storage elements, as well as a detection function 13 of a defective storage element. The storage elements are electrically arranged in series so as to deliver an electrical voltage to connection terminals. Each storage element may comprise at least one electrochemical energy storage cell or may consist of a group of cells connected in series and / or in parallel according to the electrical specifications of the system.

[0034] An electrochemical cell is an electrical energy accumulator having two terminals and presenting a voltage of a few volts, most often between 2.3V and 4.2V, approximately. The cells can be of the Lithium-ion type (a lithium Nickel Manganese Cobalt oxide NMC or a lithium iron phosphate LFP can be cited as examples of positive electrode active materials), Nickel Cadmium (Ni-cd), Nickel-Metal-Hydride (Ni-MH) for example. More precisely, a lithium-ion cell is composed mainly of a porous positive electrode, a porous negative electrode, a separator and an electrolyte (which can be liquid, polymeric or solid). The operating principle of a lithium-ion cell is based on the reversible exchange of lithium ions between the two porous electrodes.

[0035] Furthermore, the control unit 11 (designated by the acronym BMS for “Battery Management System” or TBCU for “Traction Battery Control Unit”) is adapted to supervise the electrical parameters specific to the battery and to each energy storage element 10a, 10b, 10c, 10d by means of current, voltage and temperature sensors, such as the state of charge SOC (“State of Charge”) which designates the level of state of charge of the battery expressed by a ratio between the quantity of electricity stored at a given instant and the maximum quantity of electricity storable at a given instant, the open circuit voltage OCV (“Open Circuit Voltage”) expressed in volts, the charging current expressed in amperes, the state of health SOH (“State of of Health), which designates the battery aging level parameter expressing a ratio between the maximum amount of electricity that can be stored at a given time and the maximum amount of electricity that can be stored in the new state of the battery, or even a quantity of electricity or quantity of energy, expressed in Ah or Wh, stored at a given time by each storage element. A technique known to those skilled in the art for estimating the state of charge is to measure the no-load voltage of a storage element after a relaxation phase, for example when the vehicle wakes up, and to determine an estimate of the state of charge from a map. The stored electricity quantity parameter EQ_10a for an energy storage element 10a can be determined at any time from the product of the values ​​of the state of charge parameters SOCt and the total capacity in Ah

[0036] The balancing function 12 cooperates with a storage element balancing device comprising a balancing circuit 14a for each cell or group of cells of the plurality of storage elements 10a to 10d. The balancing consists of transferring energy in order to reduce the dispersion of the state of charge of the storage elements. The balancing device may be of the passive type and comprises controllable switch and resistor circuits. Alternatively, the balancing device may be of the active type and comprises switch and capacitor circuits arranged so as to control energy transfers between adjacent cells, or groups of cells. The control method according to the invention proposes improved control of the balancing function making it possible to reduce the activation time of the balancing circuit in order to limit electrothermal losses.

[0037] More precisely, for the implementation of the control method according to the invention, the balancing function 12 comprises means for determining and recording in the memory of the control unit, for each storage element 10a to 10d, a first parameter PI having the value of a difference in quantity of electricity DEQa between the instantaneous state of charge SOCt and a target state of charge SOC100, for example the full charge of the storage elements. The parameter PI can be expressed in Ah or Wh. The target state of charge SOC100 can be between 50% and 100% of the state of charge, between 80% and 100%, or preferably be equal to 100% of the state of charge. Unlike known balancing solutions, the target state of charge SOC 100 is a value fixed in calibration of the balancing function.This parameter is the balancing reference for the cells in the state of charge and ensures that balancing is achieved when the battery is charged up to this target value, for example when charging at a terminal.

[0038] The balancing function 12 further comprises means for determining the maximum value or the minimum value recorded in memory of the control unit of the first PI parameter among the plurality of storage elements. maximum value of the PI parameter is used by the control method in order to balance the storage elements in quantity of electricity so as to finalize the balancing during a charge carried out up to the target state of charge. Thus, the balancing is implemented in part during a charge at the terminal which reduces energy losses compared to conventional solutions where the energy is lost entirely in a discharge circuit.

[0039] Furthermore, the balancing function 12 comprises means for determining and recording in the memory of the control unit, for each storage element, a second parameter P2 whose value is a quantity of electricity to be balanced so that the value of the first parameter PI of each storage element is equal to the maximum value DEQmax of the difference in quantity of electricity between the instantaneous state of charge SOCt and a target state of charge SOC100. The parameter P2 is used to determine the balancing setpoint of each storage element. The balancing setpoint is an activation duration of the discharge circuit, for example determined as a function of the quantity of electricity to be balanced.

[0040] Furthermore, the function 13 for detecting a defective storage element has the function of estimating at any time a self-discharge parameter for each storage element in order to generate or not an alert. The detection function is based on the observation in a given period of a first component corresponding to the electricity requirements for a variation in the quantity of electricity stored compared to an average for all of the storage elements, and of a second component corresponding to the needs for activating the balancing function compared to an average for all of the storage elements. An abnormal deviation of self-discharge compared to the average behavior is a suspicion of a defective state of an electrochemical cell, in particular lithium deposition which could cause a short circuit. The detection can implement at least one or more detection criteria compared to at least one self-discharge threshold.

[0041] More precisely, the function 13 is configured to determine for each storage element a value of a third parameter P3 representative of a difference between the value DEQa of the first parameter PI of each said storage element 10a and the average value DEQm for the plurality of storage elements 10a to 10d, and to determine a first current corresponding to the variation of the value of the third parameter P3 over an observation period. The control unit records in memory at least two values, or more, of the parameter P3 and the dates of recording of said values ​​for the purposes of the detection function. Conventionally, a table of sliding values ​​is implemented for recording, for example one hundred values ​​of the parameter P3 are stored in the memory of the control unit. The resolution of the table makes it possible to calibrate different detection thresholds.

[0042] Furthermore, the function 13 is configured to determine for each storage element a value of a fourth parameter P4 representative of the difference between the value of the balancing setpoint of each storage element 10a and the average value of the balancing setpoints for the plurality of storage elements 10a to 10d, and to determine a second current corresponding to the variation of the value of the fourth parameter P4 over the observation period. The control unit records in memory at least two values, or more, of the parameter P4 and the dates of recording of said values ​​for the needs of the detection function. Conventionally a table of sliding values ​​is implemented for recording, for example one hundred values ​​of the parameter P4 are retained.

[0043] Finally, the function 13 is configured to determine a fifth parameter P5 representative of the self-discharge current for each storage element 10a over the chosen observation period. This fifth parameter P5 consists of the contribution of the first current and the second current. The function 13 is configured to generate or not an alert as a function of the fifth parameter P5 with respect to at least a first threshold or as a function of the variation of the parameter P5 in the chosen observation period with respect to a second threshold or a third threshold.

[0044] In [Fig.2], the control method implemented by the battery system control unit is schematically represented by a functional block diagram. The control unit is provided with an integrated circuit computer and electronic memories, the computer and the memories being configured to execute the control method. But this is not obligatory. Indeed, the computer could be external to the control unit, while being coupled to the latter. In the latter case, it can itself be arranged in the form of a dedicated computer comprising a possible dedicated program, for example. Consequently, the control unit, according to the invention, can be produced in the form of software modules (or computer software), or electronic circuits (or hardware), or a combination of electronic circuits and software modules.

[0045] The parameters calculated for implementing the control method are illustrated by Figures 3 and 4 in parallel with the description of the method. In these figures, the storage element 10b has a lower electrical capacity than the other elements 10a, 10c and 10d. Furthermore, the storage elements are in a situation of unbalanced charge states.

[0046] More precisely, the control method implements the balancing function which comprises the following successive steps among which a first step 20 consisting of determining for each storage element 10a a value DEQa of the first parameter PI corresponding to the difference in quantity of electricity between the state of instantaneous charge SOCt and a target state of charge SOC 100, for example 100%. The values ​​of the first parameter PI are calculated from the product of the state of charge, expressed as a ratio to the total capacity, and the value of the total capacity at a given time expressed in Ah, where Pl=SOCt*CTOT, CTOT being the total capacity determinable at each time, either by an on-board estimator, or possibly by a remote estimator connected to the control unit.

[0047] Then, the method comprises a second step 21 consisting of determining the maximum value DEQmax of the first parameter PI among the plurality of storage elements 10a to 10d. In this example, the element 10b has the maximum electricity quantity deviation.

[0048] Then, the method comprises a third step 22 consisting in determining for each storage element 10a a value EBa of the second balancing parameter P2, as illustrated in [Fig.3] by the hatched bands. P2 corresponds to the quantity of electricity to be balanced so that the value of the first parameter PI of each storage element is equal to the maximum value DEQmax, where P2=DEQmax-DEQa, DEQmax being the maximum deviation among the storage elements and DEQa being the deviation of quantity of electricity with respect to SOC 100. During this step, it is determined that the elements 10a, 10c and 10d have values ​​EBa, EBc and EBd respectively, of the parameter P2 of quantity of electricity to be discharged so that the deviation with respect to SOC 100 is identical for the plurality of storage elements of the battery.

[0049] Then, the method comprises a fourth step 23 of determining a balancing setpoint CSBa, CSBc and CSBd of each storage element 10a, 10c, and 10d respectively as a function of the respective value of the second parameter P2. The balancing setpoint is an activation duration of the individual discharge circuit 14a, 14c and 14d. This setpoint is illustrated in [Fig.4] and the controlled discharge is illustrated by a solid arrow for the storage elements 10a, 10c and 10d for which the control unit has determined a quantity of electricity to be discharged. Once the action of the balancing device is executed, all the storage elements of the battery have an identical deviation in quantity of electricity compared to the target state of charge SOC100.

[0050] This strategy makes it possible to reduce the activation time of the discharging balancing device because the discharge in the individual discharge circuit corresponds to only a first phase of the entire balancing process. Balancing will finally be achieved following a second phase during a charging operation from an external charging station. The balancing method therefore comprises the first phase during which the storage elements to be balanced are discharged until the energy quantity deviation from the target state of charge is identical for the plurality of storage elements of the battery, then a second phase during which the storage elements are charged until they reach the target state of charge. The target state of charge may be the target state of charge value triggering the end of a charging operation at a charging station external to the battery system. The target state of charge information may be delivered by an on-board charge controller of the battery system.

[0051] Furthermore, the control method comprises, for the implementation of a function for detecting a defective storage element, the following steps described in [Fig.5].

[0052] The method comprises a fifth step 50, for each storage element of the battery system, of determining a value of the third parameter P3 representative of a difference between the value DEQa of the first parameter PI of each said storage element 10a and the average value DEQm of the values ​​of the first parameter PI for the plurality of storage elements. The values ​​of P3 are calculated according to the following relationship: P3 = DEQm - DEQa, expressed in quantity of electricity. The values ​​of P3 make it possible to identify abnormal self-discharge behavior of a storage element compared to the other elements. The values ​​of P3 are recorded in the memory of the control unit and time-stamped in a table. At least two values, or more, are recorded to delimit a period of observation of the variation of the parameter P3. P3 makes it possible to calculate a first current contribution to evaluate a self-discharge of a storage element.Preferably, a value of P3 is recorded at each start-up of the battery system or is recorded periodically at a predetermined frequency.

[0053] The method further comprises a sixth step 51 consisting of determining for each storage element a first current corresponding to the variation of the value of the third parameter P3 over an observation period, for example the last two values ​​of P3 recorded in the table created for this purpose. The first current is calculated according to the following relation, Ivl = [P3(tl) - P3(t2)] / (tl-t2), Ivl being the first current expressed in Amperes, where tl and t2 are two time-stamped instants of calculation of the parameter P3.

[0054] Furthermore, the method further comprises a seventh step 52 consisting of determining for each storage element a value of a fourth parameter P4 representative of the difference between the value of the balancing setpoint of each storage element and the average value of the balancing setpoints for the plurality of storage elements. The balancing setpoint is for example an activation duration of the discharge circuit and is expressed in seconds.

[0055] The values ​​of P4 are calculated according to the following relation: P4 = CSBa - CSBm, expressed in time units, CSBa being the value of the balancing setpoint for a particular storage element, for example element 10a, and CSBm being the average value of the balancing setpoints for the plurality of storage elements of the battery system. The values ​​of P4 make it possible to identify abnormal balancing behavior of a storage element compared to the other elements. The values ​​of P4 are recorded in the memory of the control unit and time-stamped in a table. At least two values, or more, are recorded to delimit a period of observation of the variation of the parameter P4. P4 makes it possible to calculate a second current contribution for the purposes of balancing. Preferably, a value of P4 is recorded at each start-up of the battery system or is recorded periodically according to a predetermined frequency.

[0056] Then, the control method comprises an eighth step 53 consisting of determining for each storage element a second current corresponding to the variation of the value of the fourth parameter P4 over the observation period. The values ​​of the second current are calculated according to the following relation, Iv2 = [P4(tl) -P4(t2)] * Ibal / (tl-t2), Iv2 being the second current expressed in Amperes, where tl and t2 are two time-stamped instants of calculation of the parameter P4 corresponding to differences in the duration of balancing activation and Ibal being the balancing current value imposed by a discharge circuit of the storage element.

[0057] Finally, the control method comprises a ninth step 54 consisting of determining a fifth parameter P5 representative of the self-discharge current for each storage element over the observation period, the fifth parameter P5 being dependent on the first current and the second current, where P5 = Ivl + Iv2, expressed in amperes.

[0058] Then, the method comprises a tenth step of generating an alert when the fifth parameter P5 exceeds a first threshold. Several strategies for monitoring the self-discharge of a storage element are envisaged based on the fifth parameter P5.

[0059] A first strategy consists of comparing the value of P5 with a first current threshold SI and when P5 is greater than SI, an alert is generated and identifies the storage element at risk.

[0060] A second strategy consists of calculating the derivative of the fifth parameter P5 over an observation period, and when the derivative with respect to time of the fifth parameter P5 over the observation period exceeds a second threshold S2 an alert is generated and identifies the storage element at risk. Indeed, a short circuit can evolve more quickly than the variation of the self-discharge current and it is then preferable to switch off the battery system before the parameter P5 reaches the value of the first threshold SL

[0061] A third strategy consists of calculating the derivative of the fifth parameter P5 on an observation period, and when the derivative with respect to the quantity of electricity discharged over the observation period exceeds a third threshold S3 an alert is generated and identifies the storage element at risk.

[0062] The monitoring method has the advantage that it allows the detection of an abnormal self-discharge during a prolonged relaxation phase of the cells, by monitoring the parameter P3 corresponding to a first self-discharge current, but also during phases of activation of the balancing device by monitoring the parameter P4 corresponding to a second current dependent on the balancing activity.

[0063] The control method applies to the battery system of electrified vehicles, in particular vehicles equipped with means for recharging from an energy source external to the vehicle.

[0064] The invention is described in the above by way of example. It is understood that the person skilled in the art is able to produce different variant embodiments of the invention by associating, for example, the different characteristics above taken alone or in combination, without departing from the scope of the invention.

Claims

Claims

1. Method for controlling a battery system (1) comprising a plurality of energy storage elements (10a, 10b, 10c, 10d) and a control unit (11) configured for implementing a balancing function (12) of the storage elements (10a, 10b, 10c, 10d), the method being characterized in that it comprises the following successive steps: - determining (20) for each storage element (10a) a value (DEQa) of a first parameter (PI) of the difference in the quantity of electricity between the instantaneous state of charge (SOCt) and a target state of charge (SOC100), - determining (21) the maximum value (DEQmax) of the first parameter (PI) among the plurality of storage elements (10a),- determining (22) for each storage element (10a) a value (EBa) of a second balancing parameter (P2) of the quantity of electricity to be balanced so that the value of the first parameter (PI) of each storage element is equal to the maximum value (DEQmax), - determining (23) a balancing setpoint (CSBa) of each storage element (10a) as a function of the respective value of the second parameter (P2).,

2. A control method according to claim 1, wherein the target state of charge (SOC 100) is the value of a target state of charge setpoint of a recharge control function of the battery system (1).

3. A control method according to claim 1 or 2, wherein the target state of charge (SOC100) is within a range between 50% and 100% of the total capacity of a storage element (10a).

4. Control method according to any one of claims 1 to 3, in which the balancing setpoint (CSBa) is a discharge duration in a discharge circuit (14a) calculated as a function of the value (EBa) of the second parameter (P2).

5. Control method according to any one of claims 1 to 4, in which the control unit (11) is further configured for implementing a function (13) for detecting a defective storage element comprising the following steps: - determining (50) for each storage element (10a) a value of a third parameter (P3) representative of a difference between the value (DEQa) of the first parameter (PI) of each said storage element and the average value of the values of the first parameter (PI) for the plurality of storage elements (10a, 10b, 10c, 10d), - the determination (51) for each storage element (10a) of a first current corresponding to the variation of the value of the third parameter (P3) over an observation period, - the determination (52) for each storage element of a value of a fourth parameter (P4) representative of the difference between the value of the balancing setpoint (CSBa) of each said storage element (10a) and the average value of the balancing setpoints for the plurality of storage elements (10a, 10b, 10c, 10d), - the determination (53) for each storage element (10a) of a second current corresponding to the variation of the value of the fourth parameter (P4) over the observation period,- the determination of a fifth parameter (P5) representative of the self-discharge current for each storage element (10a) over the observation period depending on the first current and the second current, - the generation of an alert when the fifth parameter (P5) exceeds a first threshold (SI).,

6. Control method according to claim 5, in which the detection function (13) further comprises the generation of an alert when the derivative of the fifth parameter (P5) with respect to time over the observation period exceeds a second threshold (S2).

7. Control method according to claim 5 or 6, wherein the detection function (13) further comprises the generation of an alert when the derivative of the fifth parameter (P5) with respect to the quantity of electricity discharged over the observation period exceeds a third threshold (S3).

8. Battery system (1) comprising a plurality of energy storage elements (10a, 10b, 10c, 10d) and a control unit (1) configured to implement a balancing function (12) of the storage elements and a detection function (13) of a defective storage element, characterized in that the control unit (11) is configured to implement the control method according to any one of claims 1 to 7.

9. An electrified motor vehicle comprising a battery system (1) according to claim 8.

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