Method for detecting a risk of malfunction through imbalance of a device for storing energy comprising a set of levels of electrochemical cells

Abstract

The invention relates to a method for detecting a risk of malfunction through imbalance of a device (1) for storing energy comprising a set of levels (2) electrically connected to one another in series and consisting of electrochemical cells (3) electrically connected to one another in parallel, characterised in that it comprises: - a step (E2) of determining a first function (f1) characterising a correct operation of at least one level, - a step (E3) of determining a second function (f2) characterising the operation of a level having the lowest voltage at its terminals out of the set of levels of the device (1) for storing energy, - a step (E4) of calculating a difference between the first function (f1) and the second function (f2), and then - a step (E5) of comparing the difference with a threshold, or several thresholds.

Description

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

[0001] The invention relates to the field of monitoring energy storage devices comprising a set of stages of electrochemical cells, the stages being electrically connected in series, each stage comprising one to several electrochemical cells in parallel, in particular lithium-ion type cells. More specifically, the invention relates to a method for detecting a risk of failure due to imbalance between the stages in series of such an energy storage device. The invention also relates to monitoring equipment configured to implement such a detection method. State of the prior art

[0002] Some energy storage devices comprise a set of stages of electrochemical cells, in particular lithium-ion cells, electrically connected in series to obtain a desired target voltage, each stage comprising one or more electrochemical cells electrically connected in parallel to obtain a desired target capacity. For various reasons, a state-of-charge imbalance, commonly referred to as SOC (from the English "State-of-Charge"), may appear between the stages in series. This imbalance is commonly referred to as "cell imbalance".Possible reasons include state-of-charge dispersion problems during cell assembly, self-discharge, capacity, or resistance dispersion problems between cells, which may themselves be the result of cell manufacturing dispersion problems, or problems with dispersion of operating conditions leading to different aging kinetics. Once observed, these imbalances are most often corrected by an electronic balancing system. However, sometimes the imbalance is such that it cannot be compensated.

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

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

[0005] However, this method has drawbacks. The observed voltage differences are themselves a function of the usage conditions, namely the temperature and the charge and discharge current. These observed voltage differences are also a function of the states of charge and the states of health (commonly referred to as SOH, from the Anglicism "State-of-Health") considered at the time of their observation. Finally, state of charge differences do not necessarily translate into voltage differences, particularly in the case of Lithium Iron Phosphate (LFP) batteries, i.e. based on iron phosphate at the positive electrode, which have a very stable voltage value over a wide operating range, in other words over a wide state of charge range. This method therefore appears in practice difficult to calibrate to avoid false alarms, and therefore insufficient in certain configurations.

[0006] On the other hand, voltage differences between stages are sometimes too small or only become sufficiently large very late in the usage conditions, likely to cause a safety problem once detected. Thus, when a risk of imbalance is detected in this way, it is generally necessary to urgently interrupt the use of the energy storage device, which seriously disrupts the various equipment connected to it. The detection methods known from the state of the art therefore do not allow simple and serene management of the maintenance of energy storage devices.

[0007] Health status indicators are also known which provide an indicator of the aging status of an energy storage device or of a stage comprising the energy storage device. Such indicators are complex to calculate and do not make it possible to detect a risk of failure due to imbalance of at least one stage of electrochemical cells comprising the energy storage device.

[0008] At the same time, with the widespread use of equipment incorporating an energy storage unit, particularly motor vehicles incorporating a lithium-ion battery, there is an increasingly large quantity of so-called second-life energy storage units, which can be used for stationary energy storage, particularly for storing electrical energy produced by an intermittent energy production source (for example, solar or wind energy) with a view to releasing this energy gradually. These different energy storage units are grouped together and electrically connected together to form a larger-capacity energy storage device.As 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 significant. Presentation of the invention

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

[0010] More specifically, a first object of the invention is a method for early detection of a risk of failure due to imbalance of an energy storage device.

[0011] A second 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. Summary of the invention

[0012] The invention relates to a method for detecting a risk of failure due to imbalance of an energy storage device comprising a set of stages electrically connected to each other in series and consisting of electrochemical cells electrically connected to each other in parallel, the detection method comprising: - a step of determining a first function characterizing 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 time elapsed during a charge or a discharge of the at least one stage, - a step of determining a second function characterizing the operation of a stage having 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 charges circulating in the stage having the lowest voltage across its terminals among all the stages of the energy storage device and, on the other hand, a time elapsed during a charge or a discharge of the stage having the lowest voltage across its terminals, then - a step of calculating a difference between said first function and said second function, then - a step of comparing said difference to a threshold.

[0013] The first function can define a relationship between, on the one hand, an average over all the stages of the energy storage device of a quantity relating to a quantity of charges circulating in each stage and, on the other hand, a time elapsed during a charge or a discharge of all the stages of the energy storage device.

[0014] Said quantity relative to a quantity of charges circulating in a stage can be an incremental capacity of this stage.

[0015] The step of calculating a difference between said first function and said second function may comprise a sub-step of calculating an integral value of a difference between the first function and the second function.

[0016] The step of calculating a difference between said first function and said second function may comprise: - a sub-step of estimating an extremum reached by the first function, - a sub-step of estimating an extremum reached by the second function, then - a sub-step of calculating a difference between the extremum reached by the first function and the extremum reached by the second function.

[0017] The step of calculating a difference between said first function and said second function may comprise: - a sub-step of detecting an extremum reached by the first function, - a sub-step of estimating a charge or discharge duration at the end of which the first function reaches its extremum, - a sub-step of detecting an extremum reached by the second function, - a sub-step of estimating a charge or discharge duration at the end of which the second function reaches its extremum, - a sub-step of calculating a difference between the charge or discharge duration at the end of which the first function reaches its extremum and the charge or discharge duration at the end of which the second function reaches its extremum.

[0018] The step of calculating a difference between said first function and said second function may comprise: - a sub-step of calculating an average charging current of the energy storage device between a time when the first function reaches its extremum and a time when the second function reaches its extremum, then - a sub-step of calculating a state of charge imbalance by the formula: D_SOC = D_T x l_moy / Q, where: D_SOC denotes state of charge imbalance, D_T denotes the difference between the charge or discharge time at the end of which the first function reaches an extremum and the charge or discharge time at the end of which the second function reaches a corresponding extremum, l_moy denotes the average charge current, and Q denotes the total remaining capacity of the cell stage considered, then - a step of comparing said state of charge imbalance to a threshold.

[0019] The step of calculating a difference between said first function and said second function may comprise: - a sub-step of estimating a first extremum reached by the first function, - a sub-step of estimating at least a second extremum reached by the first function, - a sub-step of estimating a charge or discharge duration at the end of which the first function reaches its first extremum, - a sub-step of estimating a charge or discharge duration at the end of which the first function reaches its second extremum, - a sub-step of estimating a first extremum reached by the second function, - a sub-step of estimating at least a second extremum reached by the second function, - a sub-step of estimating a charge or discharge duration at the end of which the second function reaches its first extremum, - a sub-step of estimating a charge or discharge duration at the end of which the second function reaches its second extremum, then: - a sub-step of calculating a difference between the first extremum of the first function and the first extremum of the second function, and / or - a sub-step of calculating a difference between the second extremum of the first function and the second extremum of the second function, and / or - a sub-step of calculating a difference between the charge or discharge duration at the end of which the first function reaches its first extremum and the charge or discharge duration at the end of which the second function reaches its first extremum, and / or - a sub-step of calculating a difference between the charge or discharge duration at the end of which the first function reaches its second extremum and the charge or discharge duration at the end of which the second function reaches its second extremum.

[0020] Said first function and / or said second function can be determined: - either during a charging or discharging phase of the energy storage device at a slow rate, in particular a rate less than or equal to C / 5, - either during a charging or discharging phase of the energy storage device according to 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 comprising a sub-step of filtering the quantity relating to a quantity of charges circulating in a stage.

[0021] The step of comparing said difference to a threshold may comprise: - a sub-step of comparing said difference to a first threshold and to a second threshold, the second threshold being strictly greater than the first threshold, then - a sub-step of memorizing a first warning indicator indicating a moderate risk, if said difference is greater than or equal to the first threshold and strictly less than at the second threshold, and - a sub-step of storing a second warning indicator indicating a high risk, if said difference is greater than or equal to the second threshold.

[0022] The first threshold may be determined based on an observed dispersion of said difference, and the second threshold may be determined based on an admissible overload by at least one stage of electrochemical cells of the energy storage device.

[0023] The invention also relates to equipment for monitoring 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 for detecting a risk of failure due to imbalance of the energy storage device as defined previously. Presentation of figures

[0024] These objects, characteristics and advantages of the present invention will be explained in detail in the following description of a particular embodiment made without limitation in relation to the attached figures among which: Figure 1 is a schematic view of an energy storage device to which monitoring equipment is connected according to one embodiment of the invention. Figure 2 is a block diagram of a method for detecting a risk of failure due to imbalance of an energy storage device according to one embodiment of the invention. Figure 3 is a graph showing the incremental capacity of different stages of the energy storage device as a function of time elapsed during a charge of the energy storage device. Figure 4 is a graph representing the incremental capacity of the different stages of the energy storage device as a function of the voltage across these stages during a charge of the energy storage device. Detailed description

[0025] Figure 1 schematically illustrates an energy storage device 1 comprising a set of stages 2 of electrochemical cells electrically connected to each other. The stages 2 are electrically connected in series. Each stage 2 may comprise one or more electrochemical cells 3, also called "accumulators" or "rechargeable batteries", electrically connected to each other in series and / or in parallel. Each cell 3 comprises a positive electrode, or cathode, and a negative electrode, or anode. The cathodes of the different cells 3 are connected directly or indirectly to a positive terminal of a stage 2. Similarly, the anodes of the different 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 removably assembled in the energy storage device, so that they can be removed and / or replaced.

[0026] 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 an identical number and arrangement of cells 3. Thus, they can comprise substantially identical theoretical operating modes, in particular voltages at their terminals and a capacity which are comparable.

[0027] The stages 2 and / or the cells 3 making up the energy storage device 1 may possibly be respectively so-called second-life stages and / or cells, that is to say stages and / or cells resulting from a re-manufacturing process after having been integrated within a first system. For example, the energy storage device 1 may be composed of a set of batteries from electric or hybrid motor vehicles. These batteries may have been used to store energy for the propulsion of the vehicle during a first life, then have been dismantled for a second life when the vehicle was used. The energy storage device 1 may be intended to store electrical energy produced by an intermittent energy production source (for example solar or wind energy).

[0028] The cells 3 making up the energy storage device 1 are preferably lithium-ion type cells. In such cells, lithium ions can be reversibly exchanged between the positive electrode and the negative electrode. 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-based material (LTO). 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 making up the energy storage device 1 could be of the sodium-ion type. In any event, the different cells 3 and the stages 2 which comprise the cells 3 are intended to operate in a balanced manner. The imbalance of a stage 2 can lead to losses of performance, or even thermal runaway of this stage and therefore to a failure of the energy storage device 1.

[0029] The energy storage device 1 also includes an electronic control system 4, commonly referred to as BMS (acronym for "Battery Management System"). 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 set of cells 3 connected together in the form of a stage 2. In particular, according to the embodiment presented, the electronic control system 4 is configured to determine and / or measure the following data: - an average voltage U_moy, equal to the average of the voltages at the terminals of the different stages 2; - a minimum voltage U_min, equal to the voltage across stage 2 with the lowest voltage among all stages 2; - a maximum voltage U_max, equal to the voltage across stage 2 with the highest voltage among all 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 service throughout the world comprise an electronic control system 4 which is already configured to provide this data. It is therefore not necessary to modify the existing electronic control systems 4 to implement the invention.

[0030] As a note, since the different stages 2 are assembled in series, the electric current passing through the energy storage device 1 is equal to the electric current passing through each of the stages 2. In addition, 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.

[0031] The electronic control system 4 is connected via a data exchange network to monitoring equipment 5 according to one embodiment of the invention. The monitoring equipment 5 comprises in particular a memory 6, a microprocessor 7, 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 recording medium comprising instruction codes which, when executed by the microprocessor 7, cause the latter to implement a method for detecting a risk of failure due to imbalance of the energy storage device 1, according to one embodiment of the invention.

[0032] The monitoring equipment 5 may 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 box connected to the electronic control system 4 by a direct wired connection, or even be integrated into the electronic control system 4.

[0033] A first embodiment of a method for detecting a risk of failure due to imbalance of the energy storage device 1 according to the invention is now described. The method is based 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 charging or discharging of the energy storage device 1. On the contrary, only a partial charging or discharging is sufficient for implementing the method. For example, the method can be implemented during a charging or discharging 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 shown schematically in FIG. 2.

[0034] In a first step El, the electronic control system 4 transmits to the monitoring equipment 5 the values ​​of the following quantities: - the voltage U_min of stage 2 with the lowest voltage, - the voltage U_max of stage 2 with the highest voltage, - the average voltage U_moy at the terminals of the different stages, - the electric current I flowing in the energy storage device. These values ​​can be transmitted, for example, in the form of time series, periodically and / or at the end of each charging or discharging phase of the energy storage device 1.

[0035] In a second step E2, a first function fl, called the reference function, is determined, characterizing correct operation of at least one stage 2. By "correct operation", we mean normal or nominal operation of at least one stage 2, that is to say the operation of a non-failing stage. According to the first embodiment, the first function fl is equal to an average function calculated on the basis of all the stages of the energy storage device 1. This first embodiment is therefore based on the assumption that the average of all the stages is representative of correct operation. Defining the first function on the basis of an average of all the stages of the storage device makes it possible to make the detection method more robust, and in particular to maintain effective detection even when one of the stages has an abnormally high voltage at its terminals.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 correctly reflects, according to the laws of statistics, correct operation. Alternatively, and as we will see later, other methods for determining the reference function may be proposed.

[0036] Generally speaking, the first function fl is a mathematical function, representable on a graph such as the graph in Figure 3, and which can be defined by a set of points. The first function defines a relationship between, on the one hand, a quantity relating to a quantity of charges circulating in at least one stage (represented on the ordinate in Figure 3), and on the other hand, a time elapsed during a charge or discharge of the at least one stage (represented on the abscissa in Figure 3, and expressed for example in hours). In this case, the functions represented in Figure 3 are representative of a charge of the energy storage device: the state of charge increases as one progresses along the abscissa axis. These functions can therefore be determined during a charging phase of the energy storage device.Alternatively, these functions can also be calculated during discharges of the energy storage device by reversing the orientation of the abscissa axis. In the event that said quantity relating to a quantity of charges circulating in at least one stage changes sign during the discharge phases compared to the charge phases, an absolute value of this quantity is advantageously used. In the event that the first function fl is equal to an average function calculated on the basis of all the stages, said at least one stage corresponds to all the stages of the energy storage device 1.

[0037] According to a preferred embodiment, the quantity relating to a quantity of charges circulating in at least one stage is equal to an incremental capacity (dQ / dU, expressed for example in Ampere-hours per volt) of the at least one stage. The incremental capacity of a stage is defined by a ratio of a differential in the quantity of charges dQ of this stage to a voltage differential dU across this stage. Alternatively, the quantity relating to a quantity of charge circulating in at least one stage could be defined differently. It could for example be equal to dU / dQ, or to a function derived from dQ / dU or dU / dQ. This function could even be defined so as to be independent of the voltage differential dU across this stage.

[0038] The first function fl can be determined as follows: first, a first intermediate function is calculated defining a relationship between the electric current I flowing in the energy storage device and the time elapsed during a charging or discharging period of the energy storage device. Then, a second intermediate function is calculated defining a relationship between a quantity of charges Q flowing in each stage and the time elapsed by integrating the first intermediate function over the charging or discharging period considered. This second intermediate function is combined with a third intermediate function establishing a relationship between the average voltage U_moy and the time elapsed. A fourth intermediate function can thus be calculated defining a relationship between the quantity of charges Q flowing in each stage and the average voltage U_moy.Then we calculate a fifth intermediate function by differentiating the fourth intermediate function. relative to the average voltage U_moy. The fifth intermediate function is therefore a function of the type dQ / dU_moy = f(U_moy). Finally, this fifth intermediate function is combined with the third intermediate function establishing a relationship between the average voltage U_moy and the elapsed time t so as to obtain the first function fl. The first function is therefore a function of the type dQ / dU_moy = f(t).

[0039] Alternatively, to determine the first function fl, one can determine for each stage the function defining the relationship between the incremental capacity of this stage and the elapsed time. Then, one can perform an arithmetic average of the functions determined for each stage. This method allows for more accurate detection but requires more computing resources because the calculations are repeated for each stage of the energy storage device. In addition, this method requires that the electronic control system 4 provides the voltage across each stage of the energy storage device.

[0040] According to an alternative embodiment of the second step E2, the at least one stage whose operation is correct could be defined as stage 2 whose voltage at its terminals is closest to the average voltage of all the stages 2 of the energy storage device 1.

[0041] According to other variant 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 are operating correctly and by determining the relationship between the incremental capacity circulating in this or these stages and the time elapsed during a charge or a discharge.

[0042] Finally, at the end of the second step E2, a first function fl is obtained, representative of normal operation of one or more stages. This first function can be determined using several different methods but which have the common point of defining a relationship between, on the one hand, a quantity relating to a quantity of charges circulating in a stage and, on the other hand, a time elapsed during a charge or discharge of the energy storage device. 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.

[0043] In a third step E3, a second function f2 is determined to be compared with the first function previously defined. As with the first function fl, the second function f2 is a mathematical function, representable on a graph such as the graph in Figure 3, and which can be defined by a set of points. The third step E3 can be executed before or after the second step E2 or in parallel with the second step E2. The second function f2 defines a relationship between, on the one hand, the magnitude relative to a quantity of charges circulating in the stage having the lowest voltage across its terminals among all the stages of the energy storage device and on the other hand a time elapsed during a charge or a discharge of this stage. In particular, according to the preferred embodiment, the quantity relating to the quantity of charges circulating in the stage having the lowest voltage across its terminals is equal to the incremental capacity of this stage.

[0044] The method for determining the second function f2 may be analogous to the method for determining the first function fl. The second function f2 may be determined as follows: first, a first intermediate function defining a relationship between the electric current I flowing in the energy storage device and the time elapsed during a charging or discharging period of the energy storage device is calculated. Then, a second intermediate function defining a relationship between a quantity of charges Q flowing in the stage having the lowest voltage across its terminals and the time elapsed is calculated by integrating the first intermediate function over the charging or discharging period. This second intermediate function is combined with a third intermediate function establishing a relationship between the minimum voltage U_min and the time elapsed.We can thus calculate a fourth intermediate function defining a relationship between the quantity of charges Q circulating in the stage with the lowest voltage at its terminals and the minimum voltage U_min. Then we calculate a fifth intermediate function by deriving the fourth intermediate function relative to the minimum voltage U_min. The fifth intermediate function is therefore a function of the type dQ / dU_min = f(U_min). Finally, this fifth intermediate function is combined with the third intermediate function establishing a relationship between the average voltage U_min and the elapsed time so as to obtain the second function f2. The second function is therefore a function of the type dQ / dU_min = f(t).

[0045] Generally speaking, the graph on which the second function is representable is identical to the graph on which the first function is representable. In other words, the shape of the first function is the same as the shape of the second function so as to allow a comparison of these two functions.

[0046] As a note, the quantity of charges circulating in each stage is calculated more precisely during a charging or discharging phase of the energy storage device according to a slow regime, in particular a regime less than or equal to C / 5, that is to say with a charging current allowing the energy storage device to be completely recharged in at least five hours. Alternatively, the quantity of charges circulating in each stage can also be calculated during a charging or discharging phase of the energy storage device according to a faster regime, in particular a regime strictly greater than C / 5. In this case, the steps E2 and E3 of determining the first function and / or the second function advantageously comprising a sub-step of filtering the quantity relating to a quantity of charges circulating in a stage.

[0047] The first function fl and the second function f2 are defined over a given charging or discharging period which may correspond to a partial charging or discharging of the energy storage device. Thus, the period over which the first function fl and the second function f2 are defined may be restricted compared to the total time required to fully charge or discharge the energy storage device, with the same charging regime, from a state of charge of 0% in the case of charging or respectively from a state of charge of 100% in the case of discharging. For example, the period over which the first function fl and the second function f2 are defined may be less than or equal to 75%, or even less than or equal to 50%, or even less than or equal to 25% of the total duration.Preferably, the period over which the first function fl and the second function f2 are defined is sufficient to identify at least one extremum of the first function and at least one extremum of the second function, or even at least two extremums of the first function and at least two extremums of the second function, or even three extremums of the first function and three extremums of the second function.

[0048] As can be clearly seen in Figure 3, when the period over which the first function fl and the second function f2 are defined is sufficiently large, the first function fl and the second function f2 have a specific appearance: each of these two functions successively reaches two maximum values, referenced VM11 and VM12 for the first function fl and VM21 and VM22 for the second function f2. The maximum values ​​VM11, VM12, VM21 and VM22 are reached respectively at the end of a duration T11, T12, T21 and T22. When the first function fl and the second function f2 are established during a charge of the energy storage device, the first maximum value VM11 reached by the first function is generally less than or equal to the second maximum value VM12 reached by the first function.Similarly, the first maximum value VM21 of the second function is generally less than or equal to the second maximum value VM22 of the second function. Furthermore, a gap is observed between the first function fl and the second function f2. In particular, the maximum values ​​VM11 and VM12 of the first function are strictly greater than the maximum values ​​VM21 and VM22 of the second function, respectively. In addition, the durations T11 and T12 are strictly less than the durations T21 and T22, respectively. Between its two maximum values ​​VM11 and VM12, the first function fl reaches a minimum value VM13 at the end of a duration T13. Similarly, between its two maximum values ​​VM21 and VM22, the second function f2 reaches a minimum value VM23 at the end of a duration T23. Note that the minimum value VM13 is strictly less than the maximum value VM23 and that the duration T13 is strictly less than the duration T23.The two maximum values ​​VM11 and VM12 and the minimum value VM13 constitute three extrema of the first function. Similarly, the two maximum values ​​VM21 and VM22 and the minimum value VM23 constitute three extrema of the second function. Furthermore, we observe. also that there are times when the first function is strictly greater than the second function and other times when the first function is strictly less than the second function.

[0049] In a fourth step E4, a difference is calculated between said first function fl and said second function f2. There are several ways of quantifying such a difference. According to a first embodiment, the fourth step E4 comprises a sub-step of calculating an integral value of a difference between the first function fl and the second function f2. This integral calculation can in particular be carried out over the entire period over which the first function and the second function are defined. This calculation therefore amounts substantially to calculating the area defined between the first function and the second function. Advantageously, the integral calculation can be based on an absolute value of the difference between the first function fl and the second function f2.Thus, the integral calculation of the periods where the first function is strictly greater than the second function is added to the integral calculation of the periods where the first function is strictly less than the second function. This allows the differences between the first function and the second function to be clearly highlighted, which improves the sensitivity of the detection. An advantage of determining said difference on the basis of an integral calculation is that this method can be implemented over any charging or discharging period, including a partial charging or discharging period in which the functions fl and / or f2 do not reach all their extrema. Thus, the detection method makes it possible to detect an imbalance even when the energy storage device undergoes incomplete charging and discharging cycles.Another advantage of determining the said difference on the basis of an integral calculus is that this method makes it possible to detect a difference even when the maximum values ​​VM11 and VM12 of the first function are substantially equal to the maximum values ​​VM21 and VM22 of the second function (we then have only a time shift between the two functions fl and f2). Similarly, this method makes it possible to detect a difference even when the durations T11 and T12 are substantially equal to the durations T21 and T22 of the second function (we then have only an amplitude shift between the two functions fl and f2).

[0050] According to another embodiment of the fourth step E4, the difference between the first function f1 and the second function f2 can be calculated by a difference in amplitude between these two functions. In this case, the fourth step comprises a sub-step of estimating at least one extremum (in particular the values ​​VM11, VM12 or VM13) reached by the first function, and a sub-step of estimating at least one extremum reached by the second function (respectively the values ​​VM21, VM22 or VM23). Then, the fourth step E4 comprises a sub-step of calculating a difference between the extremum reached by the first function and the extremum reached by the second function. This difference is therefore equal to VM11 - VM21, or to VM12 - VM22, or even to VM13 - VM23. The difference can also be equal to a result calculated as a function of the three differences at VMll - VM21, VM12 - VM22, and VM13 - VM23, or even equals a result calculated based on two differences among these three differences.

[0051] Due to the similarity in appearance of the first function and the second function, it is appropriate to compare the extrema VM11, VM12 and VM13 respectively with the extrema VM21, VM22 and VM23. Calculating a difference between non-corresponding extrema would lead to an aberrant result which can be filtered so as not to rely on such a comparison for the method. Implementing the fourth step E4 by comparing the maximum or minimum values ​​reached by the first function and by the second function has the advantage of being computationally efficient and simple to implement.

[0052] According to another embodiment of the fourth step E4, the difference between the first function fl and the second function f2 can be calculated by a time difference between the two functions fl and f2. In this case, the fourth step comprises a sub-step of detecting an extremum VM11, VM12, VM13 reached by the first function fl followed by a sub-step of estimating the duration T11, T12, T13 of charge or discharge at the end of which the first function reaches its extremum. Similarly for the second function: the fourth step comprises a sub-step of detecting an extremum VM21, VM22, VM23 reached by the second function, then a sub-step of estimating the duration T21 T22, T23 of charge or discharge at the end of which the second function reaches its extremum. Then, at least one of the differences T11 -T21, and / or T12 -T22, and / or T13 - T23 is calculated. The difference can thus be equal to Til - T21, or to T12 - T22, or even to T13 - T23.The difference can also be equal to a result calculated based on the three differences at T11 - T21, T12 - T22, and T13 - T23, or equal to a result calculated based on two of these three differences. This variant has the advantage of detecting an imbalance between the stages which mainly results in a time lag between the two functions. This allows us to detect an imbalance in the charging and discharging inertia of the different stages.

[0053] According to yet another variant embodiment of the fourth step, said difference between the first function and the second function may be equal to any result calculated as a function of all or part of the six differences VMll - VM21, VM12 - VM22, VM13 - VM23, TU - T21, T12 - T22, and T13 - T23.

[0054] In a fifth step E5, the difference calculated during the fourth step is compared with a threshold. Then, if the difference is strictly greater than said 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 presents a risk of failure due to imbalance.

[0055] Advantageously, the comparison of functions defining a relationship between a quantity relative to a quantity of charges circulating in a stage and a time elapsed at the during a charge or discharge of the energy storage device makes it possible to detect very early on a drift announcing a risk of thermal runaway. Energy storage devices 1 have thus been observed in which the simple observation of the voltage at the terminals of the different stages did not make it possible to identify any anomaly several months before a failure occurs. On the other hand, the implementation of the method according to the invention on this energy storage device makes it possible to identify a risk of failure by imbalance several months before it occurs. In addition, 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 easily be removed or replaced during a maintenance operation.

[0056] Figure 4 illustrates comparatively two functions fl' and f2' characterizing respectively a correct operation of at least one stage and the operation of a stage having the lowest voltage at its terminals among all the stages of the energy storage device. The two functions fl' and f2' establish a relationship between an incremental capacity dQ / dU (on the ordinate) and a voltage U at the terminals of the stage concerned. The functions fl' and f2' are constructed on the basis of the same energy storage device as previously described and during the same charging phase as that used to calculate the functions fl and f2 represented in Figure 3. It can be seen that the difference between the first function fl' and f2' in Figure 4 is significantly less noticeable than the difference between the functions fl and f2 in Figure 3.In particular, the functions fl and f2 are shifted from each other both in the abscissa and in the ordinate, whereas the functions fl' and f2' are shifted only in the ordinate. Compared to a method based on functions establishing a relationship between the incremental capacity and a voltage, the method just described, based on functions establishing a relationship between the incremental capacity and an elapsed charge or discharge time, makes it possible to better highlight a difference in behavior between the different stages and therefore to detect a risk of failure earlier.

[0057] According to an improvement of the invention, the fifth step E5 may comprise: - a step E51 of comparing said difference with a first threshold and a second threshold, the second threshold being strictly greater than the first threshold, then - a step E52 of storing a first warning indicator indicating 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 storing a second warning indicator indicating a high risk, if said difference is greater than or equal to the second threshold. The first indicator and the second indicator are intended to be recorded in the memory 6 of the monitoring equipment 5. These indicators can then be consulted by the human-machine interface 9 in order to produce an alert message adapted to the situation.

[0058] Advantageously, the first threshold is determined as a function of a normal dispersion of said quantity relative to a quantity of charges circulating in a stage. This first threshold can for example be determined experimentally by observing stages operating correctly in an energy storage device. The first threshold can thus be defined as equal to or slightly greater than the largest difference (as calculated during step E4) observed over a sufficiently long period, with an energy storage device in which all the stages operate correctly. The first threshold can also correspond to a threshold beyond which the electronic control system 4 can no longer compensate for imbalances between the different stages of the energy storage device.

[0059] The second threshold can be determined based on an allowable overload by at least one stage of electrochemical cells in the energy storage device. The allowable overload refers to the percentage of charges that a stage is capable of supporting before irreversible degradation. In other words, if this difference exceeds the allowable overload, then thermal runaway will definitely occur. The second threshold can therefore advantageously be defined as a fraction of the allowable overload.

[0060] According to another improvement of the invention, the fourth step E4 may comprise a sub-step of calculating an average charging current l_moy of the energy storage device between the instant when the first function f1 reaches one of the extrema VM11, VM12, VM13 and the instant when the second function f2 reaches one of the corresponding extrema VM21, VM22, VM23. Preferably, the charging current I (the value of which is provided by the electronic control system 4) is substantially constant between these two instants and the average charging current l_moy is equal to this value. Alternatively, the charging current I may undergo certain variations between these two instants and in this case a time average may be calculated.Then, the fourth step E4 may comprise a sub-step of calculating a state of charge variation D_SOC by means of a multiplication of the difference between the duration Til, T12, T13 of charge or discharge at the end of which the first function reaches one of its extremes and the duration T21 T22, T23 of charge or discharge at the end of which the second function reaches the corresponding extreme VM21, VM22, VM23 with the average charge current l_moy. In other words, the state of charge imbalance D_SOC may be calculated by the following formula: D SOC = D_T x l_moy / Q where Q denotes the total remaining capacity of the cell stage concerned, i.e. the total capacity of the stage considered during the implementation of the method, and where D_T is equal to the result of the subtraction T11-T21 or T12-T22 or T13-T23. Thus, the state of charge imbalance D_SOC is equal to a ratio between two quantities of charges and can be expressed as a percentage.Then, during a fifth step E5, we can compare the state of charge imbalance D SOC to a threshold, or to several thresholds to quantify the level of criticality. of the alert. In particular, the state of charge imbalance D_SOC can be compared to a first threshold and a second threshold as explained previously. The first threshold can be, for example, between 5% and 20%. The second alert threshold, strictly higher than the first alert threshold, can be, for example, between 20% and 40%.

[0061] Finally, thanks to the invention, there is provided a method for early detection of a risk of failure due to imbalance of a stage of an energy storage device which can be implemented during partial charges and / or discharges. Compared to known methods, this method makes it possible to detect an imbalance early, which allows for better maintenance of the energy storage device. In particular, the method makes it possible to detect imbalances that were previously difficult to detect, in particular imbalances caused by a difference in state of charge (SOC) on chemistries that do not have a notable relationship between voltage and state of charge (SOC) such as a Li-ion LFP chemistry, or caused by a difference in state of health (SOH) of the different stages of the energy storage device.

[0062] 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, said function characterizing correct operation of at least one stage is established directly with the energy storage device for which it is sought to detect a risk of failure due to imbalance. According to the invention, the operation of the stage having the lowest voltage at 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 together in series, without characterization or calculation of a theoretically 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 that it is functional as soon as it is connected to the electronic control system of an energy storage device.

Claims

Claims ) Method for detecting a risk of failure due to imbalance of an energy storage device (1) comprising a set of stages (2) electrically connected to each other in series and consisting of electrochemical cells (3) electrically connected to each other in parallel, characterized in that it comprises: - a step (E2) of determining a first function (fl) characterizing 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 time elapsed during a charge or a discharge of the at least one stage, - a step (E3) of determining a second function (f2) characterizing the operation of a stage having 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 charges circulating in the stage having the lowest voltage across its terminals among all the stages of the energy storage device and, on the other hand, a time elapsed during a charge or a discharge of the stage having the lowest voltage across its terminals, then - a step (E4) of calculating a difference between said first function (fl) and said second function (f2), then - a step (E5) of comparing said difference with a threshold. ) 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 a quantity relating to a quantity of charges circulating in each stage and, on the other hand, a time elapsed during a charge or a discharge of all the stages of the energy storage device. ) Detection method according to 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 this stage.) Detection method according to one of the preceding claims, characterized in that the step (E4) of calculating a difference between said first function and said second function comprises a sub-step of calculating an integral value of a difference between the first function (fl) and the second function (f2). ) Detection method according to one of the preceding claims, characterized in that the step (E4) of calculating a difference between said first function (fl) and said second function (f2) comprises:. - a sub-step of estimating an extremum (VM11, VM12, VM13) reached by the first function, - a sub-step of estimating an extremum (VM21, VM22, VM23) reached by the second function, then - a sub-step of calculating a difference between the extremum (VMll, VM12, VM13) reached by the first function and the extremum (VM21, VM22, VM23) reached by the second function. ) Detection method according to one of the preceding claims, characterized in that the step (E4) of calculating a difference between said first function (fl) and said second function (f2) includes: - a sub-step of detecting an extremum (VMll, VM12, VM13) reached by the first function (fl), - a sub-step of estimating a duration (Til, T12, T13) of charge or discharge at the end of which the first function reaches its extremum (VMll, VM12, VM13), - a sub-step of detecting an extremum (VM21, VM22, VM23) reached by the second function (f2), - a sub-step of estimating a duration (T21 T22, T23) of charge or discharge at the end of which the second function reaches its extremum (VM21, VM22, VM23), - a sub-step of calculating a difference between the duration (Til, T12, T13) of charge or discharge at the end of which the first function reaches its extremum and the duration (T21 T22, T23) of charge or discharge at the end of which the second function reaches its extremum (VM21, VM22, VM23). ) Detection method according to the preceding claim, characterized in that the step (E4) of calculating a difference between said first function (fl) and said second function (f2) comprises: - a sub-step of calculating an average charging current of the energy storage device between a time when the first function reaches its extremum (VMll, VM12, VM13) and a time when the second function reaches its extremum (VM21, VM22, VM23), then - a sub-step of calculating a state of charge imbalance by the formula: D_SOC = D_T x l_moy / Q, where: D_SOC denotes state of charge imbalance, D_T denotes the difference between the duration (Til, T12, T13) of charge or discharge at the end of which the first function reaches an extremum and the duration (T21 T22, T23) of charge or discharge at the end of which the second function reaches a corresponding extremum (VM21, VM22, VM23), l_moy denotes the average charge current, and Q denotes the total remaining capacity of the cell stage considered, then - a step (E5) of comparing said state of charge imbalance with a threshold. ) Detection method according to one of the preceding claims, characterized in that the step (E4) of calculating a difference between said first function (fl) and said second function (f2) comprises: - a sub-step of estimating a first extremum (VMll) reached by the first function, - a sub-step of estimating at least one second extremum (VM12, VM13) reached by the first function, - a sub-step of estimating a duration (Tll) of charge or discharge at the end of which the first function reaches its first extremum (VMll), - a sub-step of estimating a duration (T12, T13) of charge or discharge at the end of which the first function reaches its second extremum (VM12, VM13), - a sub-step of estimating a first extremum (VM21) reached by the second function, - a sub-step of estimating at least one second extremum (VM22, VM23) reached by the second function, - a sub-step of estimating a duration (T21) of charge or discharge at the end of which the second function reaches its first extremum (VM21), - a sub-step of estimating a duration (T22, T23) of charge or discharge at the end of which the second function reaches its second extremum (VM22, VM23), then: - a sub-step of calculating a difference between the first extremum (VM11) of the first function and the first extremum (VM21) of the second function, and / or - a sub-step of calculating a difference between the second extremum (VM12, VM13) of the first function and the second extremum (VM22, VM23) of the second function, and / or - a sub-step of calculating a difference between the duration (Tll) of charge or discharge at the end of which the first function reaches its first extremum (VM11) and the duration (T21) of charge or discharge at the end of which the second function reaches its first extremum (VM21), and / or - a sub-step of calculating a difference between the duration (T12, T13) of charge or discharge at the end of which the first function reaches its second extremum (VM12, VM13) and the duration (T22, T23) of charge or discharge at the end of which the second function reaches its second extremum (VM22, VM23). ) Detection method according to 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 at a slow rate, in particular a rate less than or equal to C / 5, - either during a charging or discharging phase of the energy storage device according to 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 comprising a sub-step of filtering the quantity relating to a quantity of charges circulating in a stage. 0) Detection method according to one of the preceding claims, characterized in that the step (E5) of comparing said difference with a threshold comprises: - a sub-step (E51) of comparing said difference with a first threshold and a second threshold, the second threshold being strictly greater than the first threshold, then - a sub-step (E52) of storing a first warning indicator indicating 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 storing a second warning indicator indicating a high risk, if said difference is greater than or equal to the second threshold. 1) Detection method according to the preceding claim, characterized in that the first threshold is determined as a function of an observed dispersion of said difference, and in that the second threshold is determined as a function of an overload admissible by at least one stage of electrochemical cells of the energy storage device. 2) Monitoring equipment (5) of an energy storage device (1) comprising a set of electrochemical stages (2) electrically connected in series, characterized in that it comprises hardware means (6, 7, 8) and software 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.

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