METHOD FOR DETECTING A DEFECTIVE STORAGE ELEMENT IN A BATTERY

Abstract

The present invention relates to a method for detecting a defective energy storage element among a set of storage elements in a battery system comprising a balancing device for said set. The method comprises controlling a first diagnostic mode (41) consisting of detecting a defective storage element when a first parameter (P1) is greater than a first threshold (S1), determining an inactivity period for the balancing device (3, 8), and, if the inactivity period is detected to be longer than a predetermined inactivity period, controlling a second diagnostic mode (42) consisting of determining a second self-discharge parameter (P2) for each storage element of said set during the inactivity period and detecting a defective storage element when said second parameter (P2) for the inactivity period is greater than a second self-discharge threshold (S2). Figure 1.

Description

Title of the invention: METHOD FOR DETECTING A DEFECTIVE STORAGE ELEMENT IN A BATTERY

[0001] The field of the invention relates to a method for detecting a defective energy storage element for a battery system, in particular for systems equipped with a balancing device.

[0002] Power battery systems, particularly for an electrified vehicle, typically comprise a set of storage elements. 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.

[0003] As a safety measure, these systems are equipped with electrical protection functions (e.g., current control and electrical contactors) as well as diagnostic functions designed to prevent the risk of degradation of the storage elements. For electromobility applications, vehicle manufacturers generally choose to store energy in lithium-ion electrochemical cells due to their higher energy density. It is well known that this type of cell is particularly thermally unstable, which is why protection and diagnostic functions are essential. Therefore, it is necessary to periodically monitor the health of the cells.

[0004] A criterion of a cell's good health is its ability to retain its charge. Healthy cells, therefore without risk of thermal runaway, exhibit low self-discharge, generally less than 2% of their total charge capacity per month, expressed in ampere-hours or points of charge. Conversely, cells considered defective will generally discharge more rapidly.

[0005] In this description, an electrochemical cell is said to be defective when it is in an electrochemical state affecting its self-discharge behavior. Indeed, a defective cell, due to degradation of the insulation between the anode and the cathode—degradation linked to an impurity—will be characterized by an additional self-discharge resistance compared to the self-discharge resistance of a healthy cell, thus generating a faster discharge. A normal cell is considered to discharge by approximately 2% per month, which can be represented by a normal self-discharge resistance of approximately 1000 ohms. A cell is considered defective when its resistance is less than 500 ohms, with a risk of thermal runaway when it falls below approximately 100 ohms. also that the phenomenon is progressive, the resistance will gradually decrease over several weeks, as the battery is used.

[0006] Furthermore, to optimize the usable capacity of a battery system, balancing devices exist. Prior art document KR101610927B1 describes a technique for balancing the cells of a battery which consists of determining a voltage difference between a maximum voltage and an average voltage of a set of cells, and triggering either a first passive balancing mode when the difference is small, between 100 mV and 1 volt, or an active balancing mode when the difference is large, greater than 1 volt, and the vehicle is parked or stopped for a prolonged period.

[0007] Balancing techniques reduce voltage differences between cells. While necessary to optimize the payload capacity of a battery system, balancing reduces the likelihood of detecting defective cells during self-discharge. This requires finely calibrating the detection function to enable detection while avoiding false alarms.

[0008] There is therefore a need to address the aforementioned problems. One objective of the invention is to improve the defective cell detection functions of a battery equipped with a balancing device.

[0009] More specifically, the invention relates to a method for detecting a defective energy storage element among a set of storage elements of a battery system comprising a balancing device for said set, the detection method being implemented by a control unit of the battery system and comprising the control of a first diagnostic mode consisting of detecting a defective storage element when a first parameter representing the difference in charge between the most charged storage element and the least charged storage element is greater than a first threshold.According to the invention, the method further comprises the following successive steps of determining an inactivity period of the balancing device, and in the event of detection that the inactivity period has a duration greater than a predetermined inactivity period, of commanding a second diagnostic mode consisting of determining a second self-discharge parameter for each storage element of said assembly during the inactivity period and detecting a defective storage element when said second parameter of said storage element for the inactivity period is greater than a second self-discharge threshold.

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

[0011] - The balancing device is configured so as to be deactivated during a battery system shutdown and in which the determination of the inactivity period of the balancing device consists of measuring a shutdown period of the battery system.

[0012] - To measure the battery system's downtime, a recording step of a first instant of detection of a battery system stop command, and a recording step of a second instant of detection of a battery system start command subsequent to the stop command.

[0013] - The second self-discharge parameter of each storage element is determined from a first value of the voltage of each storage element at the first instant, and a second value of said voltage at the second instant, and the calculation of the voltage difference between the first and second values.

[0014] - The second threshold has a value dependent on the duration of the period of inaction.

[0015] - The second threshold has a value dependent on an average value of the parameter self-discharge for all storage elements measured during the period of inactivity.

[0016] - The first parameter is determined from the measurement of the open-circuit voltage maximum and minimum open-circuit voltage among all storage elements during battery system startup.

[0017] - The predetermined period of inaction is between 15 days and 30 days.

[0018] A battery system is further envisaged comprising a set of energy storage elements, a balancing device for said set and a control unit, in which the control unit is configured to implement the detection method according to any one of the preceding embodiments.

[0019] An electrified vehicle comprising such a battery system is also planned.

[0020] A control unit comprising specifically configured to implement the detection method according to the invention.

[0021] A computer program is also provided 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 detection method according to the invention.

[0022] It is further provided a computer-readable recording medium comprising instructions which, when executed by a computer, lead the computer to implement any one of the embodiments of the detection method according to the invention.

[0023] The detection method has the advantage of being a fully software-based detection solution and does not require the implementation of a specific sensor. The invention takes advantage of a prolonged period of vehicle inactivity to implement a specific diagnostic mode that prevents false detections. The invention improves the safety and protection of battery systems.

[0024] Other features and advantages of the present invention will become more apparent upon reading the following detailed description, which includes methods of implementation of the invention given by way of non-limiting examples and illustrated by the attached drawings, in which:

[0025] [Fig. 1] schematically represents a battery system intended to implement the process according to the invention.

[0026] [Fig.2] is a graph illustrating the detection strategy of the first diagnostic mode according to the invention.

[0027] [Fig.3] schematically represents by a sequence of functional algorithm an embodiment of the process according to the invention implementing the second diagnostic mode.

[0028] The invention applies to electrified vehicles, that is, vehicles comprising an electric motor and power electronics, whether 100% electric or hybrid, preferably motor vehicles, but not exclusively, such as trucks, aircraft, tractors, bicycles, and ships. More generally, the invention applies to any autonomous electrical system such as drones, robots, or portable devices, computers, tablets, mobile phones, consoles, cameras, and scanners, which are cited as a non-exhaustive list of application examples.

[0029] The invention provides a method for detecting a defective energy storage element in a battery system. The method aims to improve detection for systems equipped with an energy storage element balancing device.

[0030] In [Fig. 1], a battery system 1 configured to implement the detection method is schematically represented. The system comprises a set of energy storage elements Cl, C2 to Cn arranged electrically in series so as to deliver an electrical voltage at connection terminals. Each storage element Cl to Cn 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.

[0031] An electrochemical cell is an electrical energy storage device with two terminals and a voltage of a few volts, most often between approximately 2.3V and 4.2V. Cells can be of the Lithium-ion type (lithiumized Nickel Manganese Cobalt Oxide (NMC) or lithium iron phosphate (LFP) can be cited as examples of active materials for the positive electrode), Nickel Cadmium (Ni-Cd), or Nickel Metal Hydride (Ni-MH), for example. More precisely, a Lithium-ion cell is mainly composed 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.

[0032] Furthermore, the battery system includes a control unit 2 (designated by the acronym BMS for "Battery Management System" or TBCU for " Traction Battery Control Unit”) adapted to monitor the electrical parameters 9 specific to the battery and each energy storage element by means of current and voltage sensors, such as the state of charge SOC (“State of Charge”) which designates the level of charge state of the battery expressed by a ratio between the amount of energy stored at a given time and the maximum amount of energy that can be stored at a given time, the open circuit voltage OCV (“Open Circuit Voltage”) expressed in Volts, the charging current expressed in Amperes, the state of health SOH (“State of Health”), which designates the aging level parameter of the battery 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.

[0033] Furthermore, the battery system 1 includes a cell balancing device comprising a control means 3 and a balancing circuit 8 for each cell or group of cells of the storage elements Cl to Cn in the assembly. Balancing consists of transferring energy to reduce the dispersion of the state of charge of the storage elements. The balancing device may be passive and include controllable switch and resistor circuits to discharge the most charged cells in order to homogenize the charge of the cells. Alternatively, the balancing device may be active and include switch and capacitor circuits arranged to control energy transfers between adjacent cells, or groups of cells.

[0034] The control unit 2 is capable of detecting a balancing activation / inactivity period for each storage cell. Preferably, the balancing device is activated only when the battery system is in a started state and when the battery system control unit's computer is running. Balancing is operational during the charging, discharging, relaxation, and rest phases of the cells. When the battery system is stopped, the balancing device is therefore in an inactive state.

[0035] However, in one embodiment, the balancing of a Cl to Cn storage cell is possible even when the battery system 1 is off. It is envisaged that each storage cell could have an individual, self-powered control circuit allowing discharge of the electrochemical cell even when the battery system control unit's computer is switched off. By default, this balancing mode is disabled when the battery system is off.

[0036] Among the electrical protection and safety functions, the battery system further includes a detection device 4 for identifying a defective storage cell within the set Cl to Cn. The detection device 4 is capable of identifying a storage cell within the set containing a defective cell based on the real-time measurement of electrical parameters 9. More specifically, the detection device 4 is configured to implement at least one diagnostic mode. Due to the action of the balancing device, the detection device 4 implements two distinct diagnostic modes in accordance with the method according to the invention.

[0037] A first diagnostic mode 41 is executed when the battery system is started, for example, when the vehicle is started. This first diagnostic mode 41 consists of measuring a first parameter PI representing the difference in state of charge between the most charged and least charged storage element in the set of storage elements Cl to Cn. If this difference exceeds a first threshold SI, then at least one storage element is reported as defective, namely the least charged one. The fault is reported to the battery system control unit and the vehicle supervisor to indicate it via an indicator light and / or to prevent the vehicle from starting.

[0038] In [Fig. 2], a graph illustrates the detection strategy of the first diagnostic mode, in which the first parameter PI is determined from the measurement of the maximum voltage Vmax and the minimum voltage Vmin among the voltages of the storage elements C1 to C100 distributed along the x-axis. The voltages are, for example, the open-circuit voltages at the start of the battery system. The set comprises 100 storage elements in this non-limiting example. On the y-axis, the individual voltage values ​​of each storage element are represented by points. In this example, the storage element Ck is defective because its voltage deviates significantly from the other cells. This indicates a higher self-discharge than the other cells. The first threshold SI, represented by a double arrow, is a voltage deviation value, for example, between 0.2 volts and 1 volt.The first threshold S1 is determined experimentally and is stored in the memory of control unit 2. SI is calibrated according to the storage element balancing strategy implemented by control unit 2.

[0039] In one embodiment, the first PI parameter representing the state of charge deviation can be estimated from the difference between the average voltage value and the minimum measured voltage, said voltages being included among the measured voltages of all the storage elements Cl to Cn. In another embodiment, the first diagnostic mode can consist of comparing a maximum state of charge deviation with a first threshold SL. The detection strategy then relies on state of charge estimation models based on measurements of the exchanged current. The first mode is therefore operable at any time during system use, for example, while driving or charging.

[0040] According to the invention, a second diagnostic mode 42 is activated only when a period of inactivity of the balancing device is detected and if this period of inactivity is greater than a predetermined duration of inactivity of between fifteen days and several weeks, for example about four weeks. This second Diagnostic mode 42 is designed to be run only occasionally during the vehicle's service life, for example, just once or twice a year, unlike the first mode, which runs at each start-up or periodically while driving or under load, for example. This second diagnostic mode 42 is capable of determining a second self-discharge parameter P2 for each storage cell Cl to Cn during the inactivity period and of detecting a defective state of a storage cell when said second parameter P2 of said storage cell during the inactivity period exceeds a second self-discharge threshold S2. The second threshold S2 is expressed in volts or points of charge variation. The second threshold is, for example, between 0.1 volt and 0.3 volt for a corresponding state of charge deviation of between 3% and 5% of SOC in self-discharge for a cell rest period of approximately four weeks.The detection strategy of the second mode 42 aims to detect a cell exhibiting a higher self-discharge rate than a healthy cell. For example, S2 is provided by a table that takes as input the inactivity period DR and outputs a self-discharge voltage deviation value specific to the inactivity period DR. The second threshold S2 is determined experimentally and stored in the memory of the control unit 2. In a variant, the second mode 42 estimates an average self-discharge value for a group of storage elements or for the entire Cl to Cn assembly and compares the second self-discharge parameter P2 of each cell against this average. The second threshold S2 is then a value corresponding to the average plus an allowed deviation.

[0041] The first threshold SI and the second self-discharge threshold S2 have different values. For example, SI can be a fixed value or a value calibrated according to the activation rate of the balancing device over an observed period, and S2 is a value that is a function of the duration of the inaction period DR.

[0042] This second self-discharge threshold S2, distinct from the first threshold SI, prevents false detections because the detection strategy is performed individually for each storage cell and is not affected by measurement scatter that would result from the dispersion of measurements between the battery's storage cells. Furthermore, this strategy allows for more precise calibration of the detection because it occurs only after a period of inactivity of the balancing device and individually for each cell.

[0043] The second parameter P2 representing self-discharge can be an estimate of the change in state of charge, expressed in volts, ampere-hours, or total capacity points. The state of charge or voltage can be determined from the measurement of the cell voltage, for example, using a sensor specific to the cell or group of cells.

[0044] Furthermore, the battery system 1 is intended to communicate with a communication bus 7, for example of the CAN ("Controller Area Network") type, in order to exchange data with external systems, for example a supervisor 5. In one embodiment, the inactivity period can be determined from time parameters delivered by a clock 6 of the supervisor 5. In addition, the battery system 1 is capable of signaling the fault status of a storage element, or an alert and diagnostic message to the supervisor 5, or even commanding the temporary shutdown of a system powered by the battery system 1.

[0045] Some or all of the functions of the battery system 1 are controlled by the control unit 2. The control unit 2 may include at least one processor that executes instructions stored in a computer-readable medium such as non-volatile memory. The control unit 2 may also consist of multiple computing devices that control individual components or subsystems of the battery system 1 in a distributed manner. The processor may be any conventional processor, such as a commercially available central processing unit. Alternatively, the processor may be a dedicated device such as an application-specific integrated circuit (ASIC) or another hardware processor.

[0046] In [Fig. 3], an embodiment of the detection method is schematically represented by a sequence of a functional algorithm. In a first step E1, the battery system is in operation. The battery system control unit operates the first diagnostic mode aimed at detecting a defective cell. In this first diagnostic mode, the control unit measures, at the start-up of the battery system, the charge (or voltage) difference between the most charged and least charged storage cells of all the storage cells in the battery system, and compares the difference with the first threshold S1. More precisely, this first mode consists of measuring the maximum voltage Vmax and the minimum voltage Vmin among the measured voltages of all the storage cells. The voltage values ​​are easily obtained by the battery system at startup. The first threshold S1 is, for example, between 0.2 volts and 1 volt.When the deviation exceeds SI, an alert is triggered. Alternatively, the charge deviation can be estimated from battery system models and estimators.

[0047] In parallel, during the El step, the control unit periodically performs a balancing of the electrical storage elements based on checks carried out periodically during its operation, for example at each start of the battery system.

[0048] In a second step E2, the control unit detects a request to stop the battery system. The battery system is configured so that balancing is not active during a battery shutdown.

[0049] In a third step E3, the control unit implements steps to detect a period of inactivity DR of the balancing system during the shutdown of the battery system, and the duration of this inactivity. To this end, the control unit records a first instant tl of the battery system shutdown command, for example, the date or any time information provided by a clock or a vehicle navigation system. In addition, the control unit records a first value of an electrical parameter representative of the state of charge for each storage cell at this first instant tl. The electrical parameter is, for example, the cell voltage value at the time the vehicle was stopped, or an estimate of the state of charge.

[0050] The battery system is controlled when the vehicle is off. The balancing device is configured to be inactive during the shutdown period.

[0051] At a fourth stage E4, following the system shutdown period, a start command is detected.

[0052] Then, in a fifth step E5, the control unit records a second instant t2 of the battery system start command after the stop command. It also measures a second value representing the state of charge of each storage cell at the second start instant. The electrical parameter is, for example, the cell voltage at the vehicle start instant or an estimate of the state of charge.

[0053] In a sixth step E6, the control unit checks whether the system downtime DR, and therefore the inactivity period of the balancing device, is greater than a predetermined inactivity period DI. The DI duration is between fifteen days and several weeks, for example, thirty days. This duration is also sufficient to distinguish a cell with a self-discharge anomaly.

[0054] If the inactivity time DR is greater than Dl, then the control unit operates, in a seventh step E7, the second diagnostic mode, which consists of determining the second self-discharge parameter P2 for each storage cell of the battery system assembly for the inactivity period DR and detecting a defective state of a storage cell when said second parameter P2 is greater than a second threshold S2. The second self-discharge parameter P2 for each cell is the voltage difference between the measured voltage V1 at the inactivity time tl and the measured voltage V2 at the start-up time t2. The second self-discharge threshold S2 is a voltage difference equivalent to twice the voltage difference resulting from the self-discharge of a healthy cell. For example, S2 is provided by a table that takes as input the inactivity time DR and provides a self-discharge voltage difference value specific to the inactivity time DR.In case of . Upon detection of a defective cell, the battery control unit emits an alert signal, for example, the activation of an indicator light and / or the shutdown of the system.

[0055] The second diagnostic mode takes advantage of detecting a period of inactivity in the balancing device for a sufficiently long time to observe the self-discharge of each storage cell individually and to compare the observed self-discharge with the second threshold. It also allows for the detection of multiple defective cells because the diagnosis is performed individually for each cell. Furthermore, it has the advantage of using a more precise detection calibration than detection based on the difference between the maximum and minimum voltages of all the cells. Preferably, the second threshold S2 has a value lower than the first threshold SI. The second mode is activated only when a sufficiently long period of inactivity is detected, for example, approximately four weeks.The second diagnostic mode is operated once for each storage element at startup time, then the first diagnostic mode is reactivated in nominal operating mode.

[0056] In addition, if the inaction period DR is less than the predetermined inaction time Dl, then the first diagnostic mode is executed at startup.

[0057] The detection method applies to a battery system comprising a device for balancing the state of charge of the storage elements, and is preferably implemented for a battery system of an electrified vehicle, in particular a motor vehicle. The battery system is the traction battery system powering at least one electric drive machine.

[0058] The invention is described above by way of example. It is understood that a person skilled in the art is able to carry out different variant embodiments of the invention by combining, for example, the different features above taken alone or in combination, without departing from the scope of the invention.

Claims

Demands

1. A method for detecting a defective energy storage element among a set of storage elements (Cl-Cn) in a battery system (1) comprising a balancing device (3, 8) of said set, the detection method being implemented by a control unit (2) of the battery system (1) and comprising the command (E1) during a start-up of the battery system (1) of a first diagnostic mode (41) consisting of detecting a defective storage element when a first parameter (PI) representative of the charge difference between the most charged storage element and the least charged storage element is greater than a first threshold (SI), the method being characterized in that it further comprises the following successive steps: - the determination (E6) of an inactivity period (DR) of the balancing device (3, 8), - in the event of detection that the inactivity period (DR) has a duration greater than a predetermined inactivity period (D1),the command (E7) of a second diagnostic mode (42) consisting of determining a second self-discharge parameter (P2) for each storage element of said assembly during the inactivity period (DR) and detecting a defective storage element when said second parameter (P2) of said storage element for the inactivity period (DR) is greater than a second self-discharge threshold (S2).

2. A detection method according to claim 1, wherein the balancing device (3, 8) is configured to be deactivated during a shutdown of the battery system (1) and wherein the determination (E6) of the inactivity period (DR) of the balancing device (3, 8) consists of measuring a shutdown period of the battery system (1).

3. A detection method according to claim 2, comprising, for measuring the shutdown period of the battery system (1), a recording step (E3) of a first instant (t1) of detection (E2) of a command to stop the battery system (1), and a recording step (E5) of a second instant (t2) of detection (E4) of a command to start the battery system (1) subsequent to the shutdown command.

4. A detection method according to claim 3, wherein the second self-discharge parameter (P2) of each storage element (Cl-Cn) is determined from a first value (VI) of the voltage of each storage element at the first instant (t1), and a second value (V2) of said voltage at the second instant (t2), and from the calculation of the voltage difference between the first (VI) and the second value (V2).

5. A detection method according to any one of claims 1 to 4, wherein the second threshold (S2) has a value dependent on the duration of the inaction period (DR).

6. A detection method according to any one of claims 1 to 4, wherein the second threshold (S2) has a value dependent on an average value of the self-discharge parameter for all storage elements measured during the inactivity period (DR).

7. A detection method according to any one of claims 1 to 6, wherein the first parameter (PI) is determined from the measurement of the maximum open-circuit voltage (Vmax) and the minimum open-circuit voltage (Vmin) among the set of storage elements during a start-up of the battery system (1).

8. A detection method according to any one of claims 1 to 7, wherein the predetermined inaction time (Dl) is between 15 days and 30 days.

9. Battery system (1) comprising a set of energy storage elements (Cl, Cn), a balancing device (3, 8) for the storage elements (Cl-Cn) of said set and a control unit (2), characterized in that the control unit (2) is configured to implement the detection method according to any one of claims 1 to 8 during a start-up of the battery system (1).

10. Electrified vehicle comprising a battery system according to claim 9.

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