Method for detecting a risk of failure through imbalance of a device for storing energy comprising a set of stages of electrochemical cells

Abstract

Method for detecting a risk of failure through imbalance of a device (1) for storing energy comprising a set of stages (2) that are electrically connected to one another in series and that consist of electrochemical cells that are electrically connected to one another in parallel, characterized in that it comprises a step (E2) of determining a first function (f1) characterizing correct operation of at least one stage, the first function defining a relationship between, on the one hand, a quantity relating to an amount of charge flowing through the at least one stage, and on the other hand, a voltage across the terminals of the at least one stage, the first function being defined in a given voltage range (P).

Description

Description Title of the invention: Method for detecting a risk of failure due to imbalance in an energy storage device comprising a set of electrochemical cell stages Technical field of the invention

[0001] The invention relates to the field of monitoring energy storage devices comprising a series of electrochemical cell stages, each stage comprising one or more electrochemical cells in parallel, particularly lithium-ion cells. More specifically, the invention relates to a method for detecting a risk of failure due to imbalance between the series-connected stages of such an energy storage device. The invention also relates to monitoring equipment configured to implement such a detection method. Prior art

[0002] Some energy storage devices comprise a series of electrochemical cell stages, often lithium-ion, connected in series to achieve a desired target voltage. Each stage includes one or more electrochemical cells connected in parallel to achieve a desired target capacity. For various reasons, a state-of-charge imbalance (SOC) can occur between the series-connected stages. This imbalance is commonly referred to as "cell imbalance."Possible reasons include variations in charge state during cell assembly, variations in self-discharge, capacitance, or resistance between cells, which can themselves result from manufacturing variations, or variations in operating conditions leading to different aging kinetics. Once observed, these imbalances are usually corrected by an electronic balancing system. However, sometimes the imbalance is so severe that it cannot be compensated.

[0003] This imbalance can then lead to one stage prematurely reaching its maximum charging capacity, or its maximum discharge capacity, before the other stages in series. If charging of the stage continues after it has reached its maximum charging capacity, or its maximum discharge capacity, it can result in overcharging or under-discharging. These conditions can induce undesirable heating of the affected stage in series, cause thermal runaway, or even lead to a fire in the entire energy storage system.

[0004] To detect a risk of imbalance in an electrochemical stage of an energy storage device, the most common method is based on observing the voltage across each stage in series during a complete charge or discharge of the electrochemical storage system. The stage of electrochemical cells exhibiting a risk of imbalance, or already unbalanced, generally has a voltage across its terminals that is significantly different from the voltage across the other stages and can thus be identified.

[0005] However, this method has drawbacks. The observed voltage variations are themselves dependent on operating conditions, namely temperature and charge / discharge current. These observed voltage variations also depend on the state of charge and state of health (SOH) at the time of observation. Finally, variations in state of charge do not necessarily translate into voltage variations, particularly in the case of Lithium Iron Phosphate (LFP) batteries, which have iron phosphate on the positive electrode and exhibit a very stable voltage over a wide operating range, or in other words, a wide range of state of charge. Therefore, in practice, this method appears difficult to calibrate to avoid false alarms, and may even be insufficient in certain configurations.

[0006] On the other hand, voltage differences between stages are sometimes too small or only become significant very late in operation, potentially leading to a safety issue once detected. Thus, when a risk of imbalance is detected in this way, it is generally necessary to urgently shut down the energy storage system, which severely disrupts the various connected equipment. Therefore, current state-of-the-art detection methods do not allow for simple and reliable maintenance of energy storage systems.

[0007] Health status indicators are also known, providing an indication of the aging status of an energy storage device or a stage within that device. Such indicators are complex to calculate and cannot detect a risk of failure due to imbalance in at least one stage of electrochemical cells within the energy storage device.

[0008] In parallel, with the increasing prevalence of equipment incorporating energy storage units, particularly automobiles equipped with lithium-ion batteries, there is a growing number of second-life energy storage units available for stationary energy storage. These units can be used to store electrical energy produced by intermittent energy sources (such as solar or wind power) for gradual release. These individual energy storage units are combined and electrically connected to form a larger-capacity energy storage system.Because the energy storage units that make up such energy storage devices may have different levels of wear or age, the risk of observing an imbalance between the energy storage units is particularly important. Presentation of the invention

[0009] The object of the invention is to provide a method for detecting a risk of failure by imbalance of an energy storage device comprising a set of stages of electrochemical cells, the stages being electrically connected in series, the detection method remedying the above disadvantages and improving the detection methods known in the prior art.

[0010] More specifically, an object of the invention is a method for detecting a risk of failure due to imbalance which can be implemented during partial charges and / or discharges of the energy storage device and which allows such a risk to be detected early. Summary of the invention

[0011] The invention relates to a method for detecting a risk of failure due to imbalance in an energy storage device comprising a set of stages electrically connected in series and made up of electrochemical cells electrically connected in parallel, the detection method comprising: - a step of determining a first function characterizing the correct operation of at least one stage, the first function defining a relationship between, on the one hand, a quantity relating to a quantity of charges circulating in the at least one stage and, on the other hand, a voltage across the terminals of the at least one stage, the first function being defined over a given voltage range, - a step of determining a second function characterizing the operation of a stage with the lowest voltage across its terminals among all the stages of the energy storage device, the second function defining a relationship between, on the one hand, said quantity relating to a quantity of charge flowing in the stage with the lowest voltage across its terminals among all the stages of the energy storage device and, on the other hand, a voltage across the terminals of this stage, the second function being defined over said voltage range, then - a step of calculating a difference in amplitude value or a difference in integral value between said first function and said second function, then - a step of comparing said difference to a threshold.

[0012] The first function can define a relationship between, on the one hand, an average over all stages of the energy storage device of said quantity relating to a quantity of charges circulating in each stage and, on the other hand, an average voltage across the terminals of each stage of the energy storage device.

[0013] This quantity, relative to a quantity of charges circulating in a stage, can be an incremental capacity of that stage.

[0014] Said voltage range may include a lower limit and an upper limit, the lower limit corresponding to a first inflection point of the first function and / or the upper limit corresponding to a second inflection point of the first function, the first inflection point and the second inflection point being positioned on either side of a maximum value reached by the first function.

[0015] The first function can reach a maximum value for a given voltage value, and said voltage range can include a lower limit and an upper limit, the lower limit being strictly greater than the voltage value for which the first function reaches the maximum value or the upper limit being strictly less than the voltage value for which the first function reaches the maximum value.

[0016] The detection process may include a step of defining said voltage range comprising: - a sub-step of calculating an offset of the first function compared to a previous iteration of the detection process, then, - a sub-step of calculating a lower limit and an upper limit of the voltage range as a function of the previously calculated offset.

[0017] The said first function and / or the said second function can be determined: - either during a charging or discharging phase of the energy storage device under a slow regime, in particular a regime less than or equal to C / 5, - either during a charging or discharging phase of the energy storage device under a rapid regime, in particular a regime strictly greater than C / 5, the step of determining the first function and / or the second function then including a sub-step of filtering the quantity relating to a quantity of charges circulating in a stage.

[0018] This difference can be equal to: - the difference between an integral of the first function over said voltage range, and an integral of the second function over said voltage range, or - the difference between a maximum value of the first function over said voltage range and a maximum value of the second function over said voltage range.

[0019] Said first function and / or said second function can be determined during a partial charge or discharge of the energy storage device, said voltage range including a lower limit corresponding to a state of charge of the energy storage device greater than or equal to 25% and / or said voltage range including an upper limit corresponding to a state of charge of the energy storage device less than or equal to 75%.

[0020] The step of comparing said difference to a threshold may include: - a sub-step comparing said difference to a first threshold and a second threshold, the second threshold being strictly greater than the first threshold, then - a sub-step of memorizing a first warning indicator showing a moderate risk, if said difference is greater than or equal to the first threshold and strictly less than the second threshold, and - a sub-step of memorizing a second warning indicator indicating a high risk, if said difference is greater than or equal to the second threshold.

[0021] The first threshold can be determined based on an observed dispersion of said quantity relative to a quantity of charges circulating in a floor, and the second threshold can be determined based on an admissible overload by the floor before thermal runaway.

[0022] The invention also relates to monitoring equipment for an energy storage device comprising a set of electrochemical stages electrically connected in series, the monitoring equipment comprising hardware and software means configured to implement the method of detecting a risk of failure by imbalance of the energy storage device as defined above.

[0023] The invention also relates to a computer program product comprising program code instructions recorded on a computer-readable medium to implement the steps of the detection process as defined above when said program is running on a computer.

[0024] The invention also relates to a computer-readable data recording medium on which is recorded a computer program comprising program code instructions for implementing the detection method as defined above. Presentation of the figures

[0025] These objects, features and advantages of the present invention will be described in detail in the following description of a particular embodiment, given by way of non-limiting example, with reference to the accompanying figures, among which: Figure 1 is a schematic view of an energy storage device to which monitoring equipment is connected according to an embodiment of the invention. Figure 2 is a synoptic diagram of a method for detecting a risk of failure due to imbalance of an energy storage device according to an embodiment of the invention. Figure 3 is a graph representing the incremental capacity of series stages of the energy storage device as a function of a voltage across each stage (or group of stages). Figure 4 is a graph representing the voltage across the series stages (or group of stages) of the energy storage device as a function of the amount of electrical charge accumulated by each stage (or group of stages). Figure 5 is a graph illustrating the time evolution of a maximum incremental capacitance value of the stage (or group of stages) with the highest voltage across its terminals and of the stage (or group of stages) with the lowest voltage across its terminals. Figure 6 is a graph illustrating the time evolution of an incremental integral capacitance value of the stage (or group of stages) with the highest voltage across its terminals and of the stage (or group of stages) with the lowest voltage across its terminals. Figure 7 is a graph illustrating the time evolution of a difference from an average of the amplitude of a function characterizing the stage with the lowest voltage across its terminals and of a function characterizing the stage with the highest voltage across its terminals. Detailed description

[0026] Figure 1 schematically illustrates an energy storage device 1 comprising a set of stages 2 of electrically connected electrochemical cells. The stages 2 are electrically connected in series. Each stage 2 may include one or more electrochemical cells 3, also called "accumulators" or "rechargeable batteries," electrically connected in series and / or parallel. Each cell 3 comprises a positive electrode, or cathode, and a negative electrode, or anode. The cathodes of the individual cells 3 are connected directly or indirectly to a positive terminal of a stage 2. Similarly, the anodes of the individual cells 3 are connected directly or indirectly to a negative terminal of a stage 2. The positive and negative terminals of each stage 2 are connected, respectively, directly or indirectly, to a positive and negative terminal of the energy storage device 1.The different stages can be assembled in a removable manner within the energy storage device, so that they can be removed and / or replaced.

[0027] According to the embodiment illustrated in Figure 1, the energy storage device 1 comprises four stages 2 electrically connected in series. Each stage 2 comprises six cells 3 electrically connected in parallel. Alternatively, the number of stages 2 and / or cells 3 could be different. Advantageously, all stages 2 comprise the same number and arrangement of cells 3. Thus, they can have substantially identical theoretical operating modes, including comparable voltages across their terminals and capacitance.

[0028] The stages 2 and / or cells 3 comprising the energy storage device 1 may, respectively, be second-life stages and / or cells, meaning stages and / or cells resulting from a remanufacturing process after having been integrated into a primary system. For example, the energy storage device 1 may consist of a set of batteries from electric or hybrid vehicles. These batteries may have been used to store energy for vehicle propulsion during their first life and then removed for reuse once the vehicle was no longer operational. The energy storage device 1 may be designed to store electrical energy produced by an intermittent energy source (e.g., solar or wind power).

[0029] The cells 3 composing the energy storage device 1 are preferably lithium-ion cells. In such cells, lithium ions can be reversibly exchanged between the positive and negative electrodes. All the cells 3 of the same energy storage device 1 preferably have the same chemical composition. The negative electrode may comprise a graphite-based material (LixC6) or a lithium titanate (LTO)-based material. The positive electrode may be based on one of the following materials: - Lithium Iron Phosphate (LFP), - Lithium Nickel Manganese Cobalt Oxide (NMC), - Lithium Cobalt Oxide (LCO), - Lithium Nickel Cobalt Aluminum Oxide (NCA), - a mixture of Lithium Cobalt Oxide and Lithium Nickel Cobalt Aluminum Oxide (Blend LCO-NCA). Alternatively, the cells 3 comprising the energy storage device 1 could be of the sodium-ion type. In any case, the individual cells 3 and the stages 2 that comprise the cells 3 are designed to operate in a balanced manner. An imbalance in a stage 2 can lead to performance losses, or even thermal runaway in that stage, and thus to a failure of the energy storage device 1.

[0030] The energy storage device 1 also includes an electronic control system 4, commonly referred to as a BMS (Battery Management System), which is configured to control the state and / or operation of the energy storage device 1. The electronic control system 4 can be configured to control each cell 3 individually or a group of cells 3 connected together as a stage 2. In particular, according to the embodiment presented, the electronic control system 4 is configured to determine and / or measure in real time (i.e., instantaneously or near-instantaneously) the following data: - an average voltage U_moy, equal to the average of the voltages across the different stages 2; - a minimum voltage U_min, equal to the voltage across the terminals of stage 2 with the lowest voltage among all the stages 2; - a maximum voltage U_max, equal to the voltage across the terminals of stage 2 with the highest voltage among all the stages 2; - an electric current I of charge or discharge passing through the energy storage device 1. Advantageously, a large majority of batteries or energy storage units produced or in use worldwide include an electronic control system 4 that is already configured to provide this data. Therefore, it is not necessary to modify existing electronic control systems 4 to implement the invention.

[0031] Note that, since the different stages 2 are connected in series, the electric current flowing through the energy storage device 1 is equal to the electric current flowing through each of the stages 2. Furthermore, the electronic control system 4 can also be configured to provide other data, including the voltage across each stage of the energy storage device 1, the state of charge of the energy storage device 1 (commonly referred to as SOC), the state of health of the energy storage device 1 (commonly referred to as SOH), etc.

[0032] The electronic control system 4 is connected via a data exchange network to a monitoring device 5 according to one embodiment of the invention. The monitoring device 5 includes, in particular, a memory 6, a microprocessor 7, and an input / output interface 8 configured to receive data from the electronic control system 4 and configured to communicate with a human-machine interface 9, for example, a computer equipped with a screen. The memory 6 is a data storage medium comprising instruction codes which, when executed by the microprocessor 7, cause the microprocessor to implement a method for detecting a risk of failure due to imbalance in the energy storage device 1, according to one embodiment of the invention.

[0033] The monitoring equipment 5 can be connected to the electronic control system 4 via a data exchange network such as the internet. Alternatively, the monitoring equipment 5 can be integrated into a housing connected to the electronic control system 4 by a direct wired connection, or even integrated directly into the electronic control system 4.

[0034] A first embodiment of a method for detecting a risk of failure due to imbalance in the energy storage device 1 according to the invention is now described. The method relies on data calculated or measured by the electronic control system 4 during a charging or discharging phase of the energy storage unit 1. Advantageously, the method does not require a complete charge or discharge of the energy storage device 1. On the contrary, only a partial charge or discharge is sufficient for the implementation of the method. For example, the method can be implemented during a charge or discharge in which the state of charge of the energy storage device 1 varies between 25% and 75% of its total charging capacity. The determination method can be broken down into five steps E1, E2, E3, E4, E5, schematically represented in Figure 2.

[0035] In a first step El, the electronic control system 4 transmits the values ​​of the following quantities to the monitoring equipment 5: - the U_min voltage of stage 2 exhibiting the lowest voltage, - the U_max voltage of stage 2 exhibiting the lowest voltage, - the average voltage U_moy across the terminals of the different stages, - the electric current I flowing in the energy storage device. These values ​​can, for example, be transmitted in the form of time series, periodically and / or at the end of each charging or discharging phase of the energy storage device 1.

[0036] In a second step E2, a first function fl, called the reference function, is determined, characterizing the correct operation of at least one stage 2. "Correct operation" is understood to mean the normal or nominal operation of at least one stage 2, that is, the operation of a non-faulty stage. According to the first embodiment, the first function fl is equal to an average function fl_moy calculated on the basis of all the stages of the energy storage device 1. This first embodiment therefore relies on the assumption that the average of all the stages is representative of correct operation. It may be agreed that this embodiment can only be implemented for an energy storage device comprising a sufficient number of stages so that the average calculated over all the stages accurately reflects, according to statistical laws, correct operation.Alternatively, and as we will see later, other methods for determining a reference function can be proposed.

[0037] In general, the first function fl is a mathematical function, representable on a graph such as the one in Figure 3, and which can be defined by a set of points. In particular, the first function defines a relationship between, on the one hand, an incremental capacitance of at least one stage and, on the other hand, a voltage across that at least one stage. The incremental capacitance of a stage is defined by the ratio of the differential charge quantity of that stage to the differential voltage across that stage.

[0038] The average function fl_moy can be determined as follows: first, the quantity of charge Q flowing through each stage is determined by integrating the value of the electric current I over a charging or discharging period. This yields a function establishing a relationship between the quantity of charge Q and an elapsed time. This function is combined with a function establishing a relationship between the average voltage U_moy and the elapsed time. A primitive function Fl (shown in Figure 4) can thus be constructed, defining a relationship between the average quantity of charge flowing through each stage and the average voltage U_moy. This primitive function can then be differentiated with respect to the average voltage U_moy to obtain the average function fl_moy. This provides an average incremental capacity of the energy storage device.The average incremental capacity is therefore a quantity relative to the amount of charge flowing through each stage. Defining the average function fl_moy based on an average of all the stages of the storage device makes the detection process more robust, and in particular maintains effective detection even when one of the stages has an abnormally high voltage across its terminals.

[0039] In Figure 3, the incremental capacitance is represented on the vertical axis and expressed in Ampere-hours per volt. The voltage is represented on the horizontal axis and expressed in volts. In Figure 4, the amount of charge flowing through a stage is represented on the x-axis and expressed in Ampere-hours. The voltage across a stage is represented on the y-axis and expressed in Volts. In the example shown, the voltage across each stage varies generally between 3.5V and 3.9V during a partial charge of that stage. The amount of charge accumulated by this stage during this charge is approximately 30Ah.

[0040] Alternatively, to determine the average function fl_moy, one can determine, for each stage, the function defining the relationship between the incremental capacity of that stage and the voltage across that stage. Then, one can perform an arithmetic mean of the functions determined for each stage. This method allows for more precise detection but requires more computational resources. Furthermore, this method requires that the electronic control system 4 provide the voltage across each stage of the energy storage device.

[0041] According to an alternative embodiment of the second stage E2, the at least one stage whose operation is correct could be defined as the stage 2 whose voltage across its terminals is closest to the average voltage of all the stages 2 of the energy storage device 1.

[0042] According to other embodiments of the second step E2, the first function could be defined differently, for example by means of a theoretical function or by identifying by any means one or more stages of the energy storage device 1 which is functioning correctly and by determining the relationship between the incremental capacitance flowing in this or these stages and the voltage across this or these stages.

[0043] According to another embodiment of the second step E2, the first function could take a different form than a function defining a relationship between the incremental capacitance and a voltage across at least one stage. Referring to Figure 4, the first function could express a voltage across at least one stage as a function of the amount of charge flowing through that stage. Thus, the first function could correspond to the primitive function Fl defined above.

[0044] Finally, at the end of the second step E2, we obtain a first function, representative of the normal operation of one or more stages. This first function can be determined using several different methods, but they all share the common feature of defining a relationship between, on the one hand, a quantity relating to the amount of charge flowing through a stage and, on the other hand, a voltage across that stage. This first function is therefore a reference function and serves as a basis for comparison to determine whether a particular stage presents a risk of failure due to imbalance.

[0045] In a third step E3, a second function f2 is determined to be compared with the first function defined previously. The third step can be executed before or after the second step E2, or even in parallel with the second step E2. According to the first embodiment, the second function f2 establishes a relationship between On the one hand, the incremental capacity of the stage with the lowest voltage across its terminals among all the stages of the energy storage device, and on the other hand, the voltage U_min across this stage. The method for determining the second function f2 can be analogous to the method for determining the first function. In particular, the second function f2 can be obtained by determining the amount of charge flowing through the stage with the lowest voltage across its terminals. We can thus construct the primitive function F2, which defines a relationship between this amount of charge and the voltage U_min. This primitive function F2 can then be differentiated to obtain the function f2. As illustrated in Figure 3, the second function f2 is intended to be compared with the average function fl_moy.

[0046] According to one embodiment, the detection method could also be implemented by comparing the primitive functions Fl and F2 defined previously. In this scenario, the first and second functions would be equal to the primitive function Fl and the primitive function F2, respectively. Generally, the graph on which the second function can be represented is identical to the graph on which the first function can be represented. In other words, the shape of the first function is the same as the shape of the second function, thus allowing for a direct comparison between the two.

[0047] Note that the amount of charge circulating in each stage is calculated more accurately during a charging or discharging phase of the energy storage device at a slow rate, specifically a rate less than or equal to C / 5, i.e., with a charging current sufficient to fully recharge the energy storage device in at least five hours. Alternatively, the amount of charge circulating in each stage can also be calculated during a charging or discharging phase of the energy storage device at a faster rate, specifically a rate strictly greater than C / 5. In this case, steps E2 and E3 for determining the first function and / or the second function advantageously include a substep for filtering the quantity of charge circulating in a stage.

[0048] The first and second functions are defined over a given voltage range P. Since the first function is determined during a partial charge or discharge, this voltage range can be limited compared to the voltage amplitude across a stage between a partially charged and a partially discharged state. For example, with reference to Figure 4, the voltage range P can be defined approximately between 3.55V and 3.75V, while the voltage across a stage is likely to vary between 2.70V in the fully discharged state and 4.2V in the fully charged state. The voltage amplitude of this voltage range can be, for example, less than or equal to 50% of the voltage amplitude between the minimum and maximum voltages across a stage.

[0049] When the first function expresses an incremental capacitance as a function of a voltage, it reaches a maximum value VMl, or in other words, a peak amplitude, for a given voltage. Advantageously, the voltage range P is defined to include the voltage for which the first function reaches the maximum value VMl. Furthermore, the first function may also include a first inflection point 11 and a second inflection point 12 on either side of the maximum value VMl. The voltage range P can be defined to include the voltages for which the first function reaches the first inflection point 11 and the second inflection point 12.In particular, the voltage range P can be defined such that its lower limit U_inf corresponds to the voltage value at which the first inflection point 11 is reached, and / or such that its upper limit U_sup corresponds to the voltage value at which the second inflection point 12 is reached. As we will see later, such a definition provides a good compromise between, on the one hand, a relatively narrow voltage range, which allows the detection method to be implemented during partial charging or discharging phases. As a side note, to identify the first and second inflection points, the first function can optionally be filtered, in particular smoothed, to eliminate any unwanted variations.

[0050] Advantageously, the voltage range P can be defined dynamically. Indeed, it is observed that the voltage at which the maximum value VMl is reached can vary depending on several parameters, notably the overall age of the energy storage device 1 and / or its temperature. Thus, a gradual shift, or drift, in the voltage at which the maximum value VMl is reached can occur. The detection method can therefore optionally include a step E6 for defining the voltage range P. This step may include a first substep E61 for calculating a shift in the first function compared to a previous iteration of the detection method. This shift can, for example, be calculated by observing the shift in the voltage at which the maximum value VMl of the first function is reached.Alternatively or complementarily, this offset can be calculated based on an estimate of the state of health (SOH) of the energy storage device 1 and / or based on its temperature. Then, in a second substep E62, a new lower and upper limit of the voltage range P can be calculated based on the previously calculated offset. Specifically, each new limit can be calculated by applying to the existing limit an offset corresponding to the voltage offset at which the maximum value VMl is reached. The dynamic definition of the voltage range P ensures that the detection method remains effective over time and under a wide variety of operating conditions.

[0051] Next, in a fourth step E4, we calculate a difference between the first function fl and the second function f2. There are several ways to quantify a such a difference. According to a first embodiment, the difference between the first function and the second function can be equal to the difference between an integral of the first function over said voltage range P and an integral of the second function over said voltage range P. In other words, the difference is then equal to the area A defined between the first function f1 and the second function f2 over the voltage range P. The integral of an incremental capacitance function of a stage over the voltage range P can represent a regional capacitance of that stage. Advantageously, when the function defines a relationship between the incremental capacitance of a stage and the voltage across that stage, and when the voltage range P is defined such that its lower and upper limits correspond to the two inflection points 11 and 12 as described previously, the voltage range thus defined allows for improved sensitivity of the detection.Indeed, we observe that it is essentially between the two inflection points 11 and 12 that the differences between the first function and the second function are the most important.

[0052] Another advantage of determining this difference based on integral calculus is that this method can be implemented over any voltage range, including a range where the incremental capacitance curve does not reach its maximum value. Indeed, observing the first function fl around its maximum value VMl is not essential for implementing the detection method. Thus, the detection method makes it possible to detect an imbalance even when the energy storage device undergoes incomplete charge and discharge cycles. Alternatively, the lower limit U_inf and the upper limit U_sup can also be determined in such a way as to exclude the voltage at which the first function reaches the maximum value VMl.In other words, the lower limit U_inf can be strictly greater than the voltage value for which the first function reaches the maximum value, or the upper limit U_sup can be strictly less than the voltage value for which the first function reaches the maximum value.

[0053] As a point of note, the detection method advantageously allows the detection of an imbalance that does not cause a shift in the voltage for which the maximum value of the incremental capacitance function is reached.

[0054] In another embodiment, the difference between the first and second functions can be calculated by finding the difference in amplitude between these two functions. This difference in amplitude can be equal to the largest amplitude difference observed for a given voltage within the voltage range P. Alternatively, the difference between the maximum value VM1 of the first function over said voltage range and a maximum value VM2 of the second function over said voltage range can be calculated. In this embodiment, it is sufficient to simply identify the maximum values ​​of the first and second functions within the voltage range P.

[0055] Alternatively, one can also calculate the difference between the minimum value of the first function over the given voltage range and the minimum value of the second function over the same voltage range. Calculating the difference in amplitude between the first function fl and the second function f2 is particularly simple and computationally efficient.

[0056] When the first function expresses a voltage across at least one stage as a function of the amount of charge flowing through that stage, as illustrated in Figure 4, this difference can be equal to the maximum amplitude difference between the first function Fl and the second function F2 over the considered voltage range P. In this case, this amplitude difference is a voltage difference. Figure 4 shows that this difference (identified by dU) is particularly significant at low charge levels. This method therefore makes it possible to detect an imbalance during the initial charging phases of the energy storage device, and especially well before the incremental capacitance curves reach their maximum value.

[0057] In a fifth step E5, the difference calculated in the fourth step is compared with a threshold. Then, if the difference is strictly greater than this threshold, a warning can be stored in the memory 6 of the monitoring equipment 5. This warning can be read by the human-machine interface 9. Then, the human-machine interface 9 can generate an alert message indicating that a stage of the energy storage device is at risk of failure due to imbalance.

[0058] Advantageously, comparing functions that define a relationship between a quantity of charge circulating in the stage and a voltage across the stage allows for the very early detection of a drift that foreshadows a risk of thermal runaway. For example, energy storage devices have been observed where simply monitoring the voltage across the different stages did not allow for the identification of any anomaly several months before a failure occurred. In contrast, implementing the method according to the invention on this energy storage device makes it possible to identify a risk of failure due to imbalance several months before it occurs. Furthermore, the detection method generally makes it possible to identify the stage of the energy storage device responsible for this anomaly. The stage in question can then be easily removed or replaced during maintenance.

[0059] Figure 5 is a graph comprising a first curve Cl illustrating the temporal evolution of the difference between the maximum value of the average function fl_moy and the maximum value of the second function f2 over a period of two years with an energy storage device 1 comprising a stage exhibiting an imbalance failure (the T0 mark indicates the beginning of the two-year period, the T1 mark indicates a duration of one year from T0, and the T2 mark indicates a duration of two years from T0). During slightly less than one year (i.e., over the first six points of Cl), The Cl curve reaches very high values, indicating a significant difference between the average function fl_moy and the function f2. Then, during an intervention shortly before the one-year mark, represented by a dashed line R, the faulty stage was replaced. At that point, the Cl curve oscillates around a value close to zero, meaning that the stage with the lowest voltage across its terminals exhibits operation close to the average operation of all stages. The graph in Figure 5 includes a second curve C2 illustrating the temporal evolution of the difference between the maximum value of the average function fl_moy and the maximum value of a function f_max over the same two-year period with the same energy storage device 1.The function f_max is defined as the function defining the relationship between, on the one hand, the incremental capacity of the module with the highest voltage across its terminals among all the modules 2 of the energy storage device 1, and on the other hand, the voltage U_max. The curve C2 shows a significantly smaller amplitude than the curve Cl up to the date of intervention.

[0060] Similarly, Figure 6 is a graph including a third curve, C3, illustrating the time evolution of the difference between the integral of the mean function fl_moy over the range P and the integral of the function f2 over the range P, over a two-year period, with an energy storage device 1 comprising a stage exhibiting an imbalance failure. As before, the curve C3 reaches very high values ​​until the intervention date, which occurred shortly before one year prior, when the failed stage was replaced. Following this intervention, the curve C3 oscillates around a value close to zero. The graph in Figure 5 includes a fourth curve, C4, illustrating the time evolution of the difference between the integral of the mean function fl_moy over the range P and the integral of the function f_max over the range P, over a two-year period, with the same energy storage device 1.Curve C4 shows a significantly smaller amplitude than curve C3 up to the date of intervention.

[0061] Figure 7 is a graph including a fifth curve, C5, illustrating the time evolution of the difference between the minimum value of function Fl and the minimum value of function F2 over a two-year period with an energy storage device 1 comprising a stage exhibiting an imbalance failure. In other words, the fifth curve, C5, expresses the time evolution of the voltage difference dU shown in Figure 4. Until the intervention date (represented by line R), curve C5 reaches significant values, notably between 100 mV and 300 mV. After the intervention, curve C5 oscillates around a value close to zero. The graph in Figure 7 also includes a sixth curve, C6, illustrating the time evolution of the difference between the minimum value of function Fl and the minimum value of a function F_max over a two-year period with the same storage device. Energy 1. The function F_max is defined as the function defining a relationship between the quantity of charge flowing in the module with the highest voltage across its terminals and the voltage U_max. Curve C9 shows a significantly smaller amplitude than curve C5 up to the date of intervention.

[0062] Advantageously, the threshold against which the difference is compared is non-zero. Indeed, due to various factors generating some dispersion in the operation of the stages, the difference calculated during the fourth step E4 can be non-zero even if no stage is faulty. This difference is notably observed on curves C2 and C4 presented above.

[0063] According to an improvement of the invention, the fifth step E5 may include - a step E51 comparing said difference to a first threshold and a second threshold, the second threshold being strictly greater than the first threshold, then - a step E52 for memorizing a first warning indicator showing a moderate risk, if said difference is greater than or equal to the first threshold and strictly less than the second threshold, and - a step E53 of memorizing a second warning indicator indicating a high risk, if said difference is greater than or equal to the second threshold. The first witness and the second witness are intended to be recorded in the memory 6 of the monitoring equipment 5. These witnesses can then be consulted by the human-machine interface 9 in order to produce an alert message adapted to the situation.

[0064] Advantageously, the first threshold is determined based on a normal dispersion of the quantity in question relative to a quantity of charge circulating in a stage. This first threshold can, for example, be determined experimentally by observing correctly functioning stages in an energy storage device. The first threshold can thus be defined as equal to, or slightly greater than, the largest difference (as calculated in step E4) observed over a sufficiently long period in an energy storage device where all stages are functioning correctly.

[0065] The second threshold can be determined based on the permissible overload of the electrochemical cells used. The permissible overload refers to the percentage of charge a cell can withstand before irreversible degradation occurs, a value then converted, if necessary, to the permissible charge for a stage. Indeed, it has been observed that the difference between the integral of the average function (fl_moy or f_max) and the integral of the second function corresponds approximately to an excessive charge for the stage. In other words, if this difference exceeds the permissible overload, then thermal runaway will certainly occur. Therefore, the second threshold can be advantageously defined as a fraction of the permissible overload, for example, 50% of the permissible overload.

[0066] Finally, thanks to the invention, we have a method for detecting the risk of failure due to imbalance in a stage of an energy storage device which can be implemented during partial charging and / or discharging. Compared to known methods, this process allows for the early detection of imbalances, leading to improved maintenance of the energy storage device. In particular, the process enables the detection of imbalances that were previously difficult to detect, notably imbalances caused by a state of charge (SOC) deviation in chemistries that do not exhibit a significant relationship between voltage and state of charge (SOC), such as Li-ion LFP chemistry, or those caused by a state of health (SOH) deviation in the different stages of the energy storage device.

[0067] The invention has the advantage of not requiring any prior characterization of the energy storage device or a similar energy storage device. Indeed, according to the invention, the function characterizing the correct operation of at least one stage is established directly with the energy storage device for which a risk of failure due to imbalance is to be detected. According to the invention, the operation of the stage exhibiting the lowest voltage across its terminals is compared with the operation of other stages of the same energy storage device. The method according to the invention is therefore much simpler to implement than previously known detection methods. The method can be implemented on any energy storage device comprising a set of stages electrically connected in series, without characterization or calculation of the theoretical correct operation of this energy storage device.The monitoring equipment implementing the detection method according to the invention is thus "plug-and-play", that is to say, it is functional as soon as it is connected to the electronic control system of an energy storage device.

Claims

Demands

1. A method for detecting a risk of failure due to imbalance in an energy storage device (1) comprising a set of stages (2) electrically connected in series and made up of electrochemical cells electrically connected in parallel, characterized in that it comprises: - a step (E2) of determining a first function (fl) characterizing a correct operation of at least one stage, the first function defining a relationship between on the one hand a quantity relating to a quantity of charges circulating in the at least one stage and on the other hand a voltage across the terminals of the at least one stage, the first function being defined over a given voltage range (P), - a step (E3) of determining a second function (f2) characterizing the operation of a stage with the lowest voltage across its terminals among all the stages of the energy storage device (1), the second function defining a relationship between, on the one hand, said quantity relating to a quantity of charge flowing in the stage with the lowest voltage across its terminals among all the stages of the energy storage device and, on the other hand, a voltage across the terminals of this stage, the second function being defined over said voltage range, then - a step (E4) of calculating a difference in amplitude value or a difference in integral value between said first function and said second function, then - a step (E5) of comparing said difference to a threshold.

2. A detection method according to the preceding claim, characterized in that the first function (fl) defines a relationship between, on the one hand, an average over all the stages of the energy storage device of said quantity relating to a quantity of charges circulating in each stage and, on the other hand, an average voltage across the terminals of each stage of the energy storage device.

3. A detection method according to any one of the preceding claims, characterized in that said quantity relating to a quantity of charges circulating in a stage is an incremental capacity of that stage.

4. A detection method according to the preceding claim, characterized in that said voltage range (P) comprises a lower limit (U_inf) and an upper limit (U_sup), the lower limit corresponding to a first inflection point (11) of the first function and / or the upper limit corresponding to a second inflection point (12) of the first function, the first inflection point and the second inflection point being positioned on either side of a maximum value (VMl) reached by the first function.

5. A detection method according to claim 3, characterized in that the first function reaches a maximum value (VM1) for a given voltage value (U_VM1), and in that said voltage range (P) comprises a lower limit (U_inf) and an upper limit (U_sup), the lower limit (U_inf) being strictly greater than the voltage value for which the first function reaches the maximum value or the upper limit (U_sup) being strictly less than the voltage value for which the first function reaches the maximum value.

6. A detection method according to any one of the preceding claims, characterized in that it comprises a step (E6) of defining said voltage range (P) comprising: - a substep (E61) of calculating an offset of the first function (fl) compared to a previous iteration of the detection process, then, - a sub-step (E62) of calculating a lower limit (U_inf) and an upper limit (U_sup) of the voltage range as a function of the previously calculated offset.

7. A detection method according to any one of the preceding claims, characterized in that said first function (fl) and / or said second function (f2) are determined: - either during a charging or discharging phase of the energy storage device under a slow regime, in particular a regime less than or equal to C / 5, - either during a charging or discharging phase of the energy storage device under a rapid regime, in particular a regime strictly greater than C / 5, the step (E2, E3) of determining the first function and / or the second function then including a sub-step of filtering the quantity relating to a quantity of charges circulating in a stage.

8. A detection method according to any one of the preceding claims, characterized in that said difference is equal to: - the difference (A) between an integral of the first function (fl) over said voltage range (P), and an integral of the second function (f2) over said voltage range, or - the difference between a maximum value (VM1) of the first function (fl) on said voltage range and a maximum value of the second function (f2) on said voltage range.

9. A detection method according to any one of the preceding claims, characterized in that said first function (fl) and / or said second function (f2) are determined during a partial charge or discharge of the energy storage device (1), said voltage range (P) comprising a lower limit (U_inf) corresponding to a state of charge of the energy storage device greater than or equal to 25% and / or in that said voltage range comprising an upper limit (U_sup) corresponding to a state of charge of the energy storage device less than or equal to 75%.

10. A detection method according to any one of the preceding claims, characterized in that the step (E5) of comparing said difference to a threshold comprises: - a substep (E51) comparing said difference to a first threshold and a second threshold, the second threshold being strictly greater than the first threshold, then - a sub-step (E52) for memorizing a first warning indicator showing a moderate risk, if said difference is greater than or equal to the first threshold and strictly less than the second threshold, and - a sub-step (E53) of memorizing a second warning indicator showing a high risk, if said difference is greater than or equal to the second threshold.

11. A detection method according to the preceding claim, characterized in that the first threshold is determined as a function of an observed dispersion of said quantity relative to a quantity of charges circulating in a stage, and in that the second threshold is determined as a function of an admissible overload by the stage before thermal runaway.

12. Monitoring equipment (5) for an energy storage device (1) comprising a set of electrochemical stages (2) electrically connected in series, characterized in that it comprises hardware (6, 7, 8) and software means configured to implement the method for detecting a risk of failure by imbalance of the energy storage device according to one of the preceding claims.

Images