METHOD FOR LIMITING BATTERY BALANCE FOR DIAGNOSIS OF DETECTION OF A DEFECTIVE ENERGY STORAGE ELEMENT

Abstract

The present invention relates to a method for limiting the balancing of an energy storage element among a set of storage elements (C1-Cn) in a battery system (1) for performing a diagnostic test to detect a defective storage element. The battery system (1) comprises a balancing device (3, 8) and a detection device (4) capable of controlling a diagnostic mode (41) for the storage elements. The method comprises the following steps: activating a limitation on the balancing time of at least one storage element from the set during a limitation period, and controlling the diagnostic mode at the end of the limitation period. The invention is applicable, for example, to electrified vehicles. Figure 1.

Description

Title of the invention: METHOD FOR LIMITING A BALANCING OF A BATTERY FOR A DIAGNOSTIC OF DETECTION OF A DEFECTIVE ENERGY STORAGE ELEMENT

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

[0002] Power battery systems, particularly for an electrified vehicle, conventionally comprise a set of energy 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 (for example current control and electrical contactors) as well as diagnostic functions aimed at preventing a risk of damage to the storage elements. For electromobility applications, vehicle manufacturers generally choose to store energy in lithium-ion type electrochemical cells for their better energy density. It is well known that this type of cell is particularly thermally unstable, which is why protection and diagnostic functions are essential. It is therefore necessary to periodically check the health of the cells.

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

[0005] In the present description, an electrochemical cell is said to be defective when it is in an electrochemical state affecting its self-discharge behavior. Indeed, a cell defective by degradation of the insulator 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. It is considered that a normal cell discharges by approximately 2% per month, which can be represented by a normal self-discharge resistance of approximately 1000 ohms. A cell is considered to be defective when its resistance is less than 500 ohms, with a risk thermal runaway when it falls below approximately 100 ohms. The phenomenon is also considered to be progressive, the resistance will gradually decrease over several weeks, as the battery is used.

[0006] Furthermore, to optimize the useful capacity of a battery system, there are balancing devices. Document KR101610927B1 is known from the prior art, which 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 have the effect of reducing cell voltage differences. Although necessary to optimize the payload capacity of a battery system, balancing reduces the chances of detecting defective cells in self-discharge. This requires fine calibration of the detection function to allow their detection while avoiding false alarms, particularly in systems where the balancing function is permanently activated.

[0008] There is therefore a need to overcome the aforementioned problems. One objective of the invention is to facilitate the operation of a device for detecting a defective storage element for a battery system equipped with a balancing device.

[0009] More specifically, the invention relates to a method for limiting a balancing of an energy storage element among a set of storage elements of a battery system for carrying out a diagnosis for detecting a defective storage element, said battery system comprising a balancing device and a detection device capable of controlling a diagnostic mode of the storage elements.

[0010] The method according to the invention is implemented by a control unit of the battery system and comprises the following steps of activating a limitation of a balancing duration of at least one storage element among the set during a limitation period, and controlling the diagnostic mode at the end of the limitation period.

[0011] The method according to the invention may include the following additional characteristics, alone or in combination:

[0012] - The activation of the balancing time limitation is programmed so as to be activated only once annually or only twice annually.

[0013] - The limitation period has a duration of between three weeks and six weeks.

[0014] - Limiting the balancing time consists of disabling balancing during the limitation period.

[0015] - Disabling balancing is only allowed when the gap between the maximum voltage and minimum voltage of the storage elements of the set is lower than a balancing threshold.

[0016] - Limiting the balancing duration consists of limiting the balancing action by a maximum duration during the limitation period.

[0017] - The maximum balancing time is between ten hours and thirty hours. during the limitation period.

[0018] There is further provided according to the invention a battery system comprising a set of energy storage elements, a device for balancing the storage elements of said set, a detection device capable of controlling a diagnostic mode of the storage elements and a control unit, in which the control unit is configured to implement the balancing limitation method according to any one of the preceding embodiments for the execution of a diagnostic detection of a defective storage element.

[0019] An electrified vehicle comprising such a battery system is further provided.

[0020] There is further provided a control unit comprising means specifically configured to implement the limitation method according to the invention.

[0021] Further provided is 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 limitation method according to the invention.

[0022] 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 limitation method according to the invention.

[0023] The limitation method has the advantage of being a fully software-based solution and not requiring the implementation of a specific sensor. The invention makes it possible to temporarily adapt the parameters of a balancing function of the energy storage elements of a battery system in order to promote the detection of a defective storage element. The invention improves the efficiency of diagnostics. The safety and protection of battery systems is thus improved.

[0024] 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:

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

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

[0027] [Fig.3] schematically represents by a functional algorithm sequence an embodiment of the method for limiting a balancing according to the invention.

[0028] 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 trucks, 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.

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

[0030] In [Fig.l], a battery system 1 configured to implement the balancing limitation method is shown schematically. The system comprises a set of energy storage elements C1, C2 to Cn electrically arranged in series so as to deliver an electrical voltage to connection terminals. Each storage element C1 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 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.

[0032] Furthermore, the battery system comprises a control unit 2 (designated by the acronym BMS for “Battery Management System” or TBCU for “Traction Battery Control Unit”) adapted to supervise the electrical parameters 9 specific to the battery and to 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 state of charge of the battery expressed by a ratio between the quantity of energy stored at a given instant and the maximum quantity of energy 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 Health”), which designates the battery aging level parameter expressing a ratio between the maximum quantity of electricity storable at a given time and the maximum quantity of electricity storable in the new state of the battery.

[0033] Furthermore, the battery system 1 comprises 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 C1 to Cn of the assembly. 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 circuits of switches and resistors that can be controlled so as to discharge the most charged cells into a resistor to homogenize the charge of the cells. Alternatively, the balancing device may be of the active type and comprises circuits of switches and capacitors arranged so as to control energy transfers between adjacent cells, or groups of cells.

[0034] Preferably, the balancing device is activated when the battery system is in the started state and when the computer of the control unit of the battery system is started. The balancing is operable in the charging, discharging, relaxation and resting phases of the cells. The balancing can be activated even when the battery system 1 is stopped. For this purpose, each storage element comprises an individual self-powered control circuit allowing a discharge of the electrochemical cell even when the computer of the control unit of the battery system is switched off.

[0035] More specifically, the balancing of a storage element takes place in the following manner. The balancing device is capable of measuring the voltage of each storage element C1 to Cn and of determining the voltage difference between each storage element and the most charged storage element. For each element for which the difference is greater than a balancing threshold, for example 0.2 volts, the balancing device identifies a cell to be discharged and determines a necessary balancing duration. This duration can be determined as a function of the voltage difference and the value of the discharge resistance. Then, the balancing device commands an individual discharge instruction where the instruction is activated during the balancing duration, for example a switch closing instruction to discharge energy into a resistor.

[0036] Furthermore, the balancing device is configured to monitor and store in a non-volatile memory the total balancing duration that has been commanded for each cell over a determined monitoring period.

[0037] Among the electrical protection and safety functions, the battery system further comprises a device 4 for detecting a defective storage element among the set C1 to Cn. The detection device 4 is capable of identifying a storage element among the set comprising a defective cell from the measurement in real time of the electrical parameters 9. More precisely the detection device 4 is configured to implement at least one diagnostic mode.

[0038] A diagnostic mode 41 consists of measuring a parameter PI representative of the difference in state of charge between the most charged storage element and the least charged one of the set of storage elements C1 to Cn. If this difference is greater than a first threshold S1, then at least one storage element is reported as being defective, the least charged one. The fault is reported to the control unit of the battery system and the vehicle supervisor to signal it via a warning light and / or to prohibit starting of the vehicle.

[0039] In [Fig.2], a graph illustrates the detection strategy of the diagnostic mode, in which the 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 abscissa axis. The voltages are for example the no-load voltages at the start of the battery system. The set comprises 100 storage elements in this non-limiting example. On the ordinate, the individual voltage values ​​of each storage element are represented by points. In this example, the storage element Ck is defective because the voltage deviates significantly from the other cells. This is the sign of a self-discharge greater than the other cells. The first threshold SI, represented by a double arrow, is a value in voltage difference for example between 0.2 volts and 1 volt.The first threshold S1 is determined experimentally and is recorded in the memory of the control unit 2. SI is calibrated according to the storage element balancing strategy implemented by the control unit 2. In one variant, the parameter PI representative of the state of charge deviation can be estimated from the deviation between the average value of the voltages and the minimum voltage measured, said voltages being included among the measured voltages of all the storage elements Cl to Cn. In another variant, the diagnostic mode can consist of comparing a maximum state of charge deviation with respect to a first threshold SL. The detection strategy is then based on state of charge estimation models based on measurements of the exchanged current.

[0040] Furthermore, it is provided that the battery system 1 is intended to communicate with a communication bus 7, for example of the CAN (“Control Area Network”) type in order to exchange data with external systems, for example a supervisor 5. Furthermore, the battery system 1 is capable of signaling the fault state of a storage element, or an alert and diagnostic message to the supervisor 5, or even ordering the temporary shutdown of a system powered by the battery system 1.

[0041] Some or all of the functions of the battery system 1 are controlled by the control unit 2. The control unit 2 may comprise at least one processor which executes instructions stored in a computer-readable medium such as non-volatile memory. The control unit 2 may also be 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 other hardware processor.

[0042] In [Fig. 3], an embodiment of the limitation method is schematically represented by a sequence of a functional algorithm. In a first step E1, the battery system is in operation and the balancing device is activated permanently, i.e. when the battery system is started, and also preferably, when it is stopped.

[0043] In a second step E2, the control unit detects the activation of a limitation of the balancing function. The limitation is activated in a programmed manner on a scheduled date, preferably once or twice a year. The limitation may be conditioned on a balancing state of the storage elements, preferably when the balancing state of the storage elements of the set is reached, that is to say that the voltage difference between the most charged and least charged storage element is less than a balancing threshold, expressed in voltage. The limitation may be activated at the request of the control unit.

[0044] Then in a third step E3, the method comprises activating a limitation of a balancing duration of at least one storage element among the set during a limitation period. The balancing duration of only part or all of the storage elements of the set can be controlled. The limitation period has a duration of between three weeks and six weeks for example. The limitation period aims to reduce the balancing effect to promote the action of the function of detecting a defective storage element. Preferably, the limitation period is configured to a duration of four weeks. It has been observed that this duration makes it possible to observe and discriminate a defective element from the others.The duration of the limitation period and the maximum permitted duration of the balancing action during this period are jointly configured so as to favor the action of the diagnostic detection of a defective element following this period.

[0045] The limitation of the balancing duration can be carried out according to two embodiments where the duration of the balancing action is controlled by an opening and closing instruction of a switch of a balancing circuit according to the necessary balancing time and the maximum authorized balancing duration.

[0046] In a first embodiment, the limitation E3 consists of deactivating the balancing during the limitation period. According to this first embodiment, the Deactivation is only permitted when the difference between the maximum and minimum voltages of the storage elements in the set is below the balancing threshold. In this way, any existing imbalance is avoided if necessary.

[0047] In a second embodiment, the limitation E3 consists of limiting the balancing action to a maximum duration during the limitation period. The maximum duration is configured according to an acceptable unbalancing level and the diagnostic mode implemented at the end of the limitation period. For example, the maximum balancing duration is between ten hours and thirty hours during a limitation period of between three weeks and six weeks. Several combinations of maximum durations and duration of the limitation period are configurable in these ranges of values ​​to favor the diagnostic action. This embodiment makes it possible to calibrate the balancing action according to the diagnostic mode implemented. The balancing duration is monitored by a counter specific to each storage element. Once the maximum duration is reached for a given period, balancing is deactivated.

[0048] Whether for the first limitation mode or the second limitation mode, the limitation E3 of the balancing duration is programmed so as to be activated only once annually or only twice annually. The limitation can be programmed automatically or manually by the user of the vehicle, in anticipation of a prolonged period of shutdown, for example during holidays.

[0049] Then, in step E4, when the control unit detects that the limitation period is finalized, the method comprises the control E5 of the diagnostic mode at the end of the limitation period. The diagnostic mode is triggered immediately following the end time of the limitation period. This sequence of the method makes it easier to detect a defective storage element.

[0050] In a non-limiting embodiment of the diagnostic mode, the detection device measures the charge (or voltage) difference between the most charged storage element and the least charged one of all the storage elements of the battery system, and compares the difference with the first threshold SL. 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 elements. The threshold SI is for example between 0.2 volts and 1 volt. When the difference is greater than SI, an alert is signaled. Alternatively, the charge difference can be estimated from models and estimators of the battery system.

[0051] Once the diagnosis is performed, the battery system returns to step E1 in a default configuration of the balancing function.

[0052] The detection method applies to battery systems comprising a device for balancing the state of charge of the storage elements, and is preferably implemented for an electrified vehicle battery system, in particular a motor vehicle. The battery system is the traction battery system supplying at least one electric motor machine.

[0053] 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 limiting a balancing of an energy storage element among a set of storage elements (Cl-Cn) of a battery system (1) for carrying out a diagnosis for detecting a defective storage element, said battery system (1) comprising a balancing device (3, 8) and a detection device (4) capable of controlling a diagnostic mode (41) of the storage elements, the method being implemented by a control unit (2) of the battery system (1) and being characterized in that it comprises the following steps: - activating a limitation (E3) of a balancing duration of at least one storage element among the set (Cl-Cn) during a limitation period, - controlling (E5) the diagnostic mode at the end of the limitation period.

2. Method according to claim 1 in which the activation of the limitation (E3) of the balancing duration is programmed so as to be activated only once annually or only twice annually.

3. A method according to claim 1 or 2, wherein the limitation period has a duration of between three weeks and six weeks.

4. A method according to any one of claims 1 to 3, wherein the limitation (E3) of the balancing duration consists of deactivating the balancing during the limitation period.

5. Method according to claim 4 in which the deactivation of the balancing is authorized only when the difference between the maximum voltage and the minimum voltage of the storage elements of the set is less than a balancing threshold.

6. A method according to any one of claims 1 to 3, wherein the limitation (E3) of the balancing duration consists of limiting the balancing action by a maximum duration during the limitation period.

7. The method of claim 6, wherein the maximum balancing time is between ten hours and thirty hours during the limitation period.

8. 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, a detection device (4) capable of controlling a diagnostic mode of the storage elements and a control unit (2), characterized in that the control unit (2) is configured to implement the balancing limitation method according to any one of claims 1 to 7 for carrying out a diagnostic detection of a defective storage element.

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

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