Method for determining the state of health (SOH) of a Li-ion battery cell.

Abstract

A method for determining the state of health (SOH) of a Li-ion battery cell, intended for operation within a specific voltage range for a given application, comprising: - defining a measurement voltage range and - when the cell voltage is within this range: - measuring the current across the cell, - measuring the cell temperature, - determining, within the measurement voltage range, one of the following values: o the total discharge charge acceptance (Qd-T) and o the total charge acceptance (Qc-T); and - determining the cell's state of health (SOH) based on a predetermined relationship, within the measurement voltage range, between the total discharge charge acceptance (Qd-T), respectively the total charge acceptance (Qc-T), and the cell's state of health (SOH). Figure for the abstract: Fig. 6

Description

Description Title of the invention: Method for determining the state of health (SOH) of a Li-ion battery cell.

[0001] — The present invention relates to the field of Li-ion batteries. For the proper func- operation of the applications for which they are used, it is essential to know the storage capacity of a battery.

[0002] — The electrical storage capacity of a battery is measured in ampere-hours (Ah), or in energy, that is to say in Watt-hours (Wh). It allows the autonomy to be determined of the battery.

[0003] — The autonomy of a battery depends on its initial storage capacity, but also of his state of health and his state of charge.

[0004] — The state of health of a battery, or SOH for State Of Health by anglicism, which related to the usable energy output from the battery, is a percentage which corresponds to the total amount of energy dischargeable into the battery at a given time, relative to the total amount of energy that can be discharged into the battery during its manufacture. This allows to take into account the impact of battery aging on its capacity storage.

[0005] = The state of charge of a battery, or SOC for State Of Charge by anglicism, is a percentage which corresponds to the quantity of energy which can be discharged into the battery at a given time, relative to the maximum amount of energy that can be discharged in the battery at said time t. For a given energy stored in the battery and can be restored, the state of charge is therefore linked to the state of health of the battery.

[0006] However, according to the present invention, the calculation of the SOC is not obligatory for determine the SOH of a battery, which is particularly advantageous and described later- laughingly.

[0007] — Charge acceptance is the amount of electricity in ampere-hours that a battery with a specific state of charge can absorb, at a temperature and current of determined load, and for a given duration.

[0008] — The initial storage capacity in returnable energy of a battery is known and typically corresponds to a health status of 100%. To calculate your health status at at a later time, there are many known techniques.

[0009] = The simplest is to fully charge the battery at time t, then perform a complete discharge. The measurement of the energy released during this full discharge brought back to the initial storage capacity of the battery gives a reliable health status. However, besides the energy loss that this technique involves, the time needed to implement it is often too long. Other methods have been developed, for example the so-called ICA method based on a calculation of the derivative of the charge on the voltage, described in particular in the applicant's patent EP3080625B1 and which works very well on batteries which have notable voltage variations, typically when fully charged or fully discharged. There is also a method of no-load voltage with several states of charge and mathematical calculation of the no-load voltages of each electrode, for example as described in the applicant's patent EP3080624B 1, but which requires a waiting time in which the battery is left at rest, which is relatively long for measuring the no-load voltages. It is common for the variation of a cell's voltage as a function of its SOC, whether in charge or discharge, to have 3 ranges: a steep slope range in the ranges of values ​​close to full charge, a steep slope range in the ranges of values ​​close to full discharge, and a plateau range between these two extreme ranges, as illustrated in [Fig.1]. On the plateau, the voltage is "relatively constant", that is to say that the voltage variation at the terminals of the cell, between two successive measurements, is less than a predetermined threshold value. This amounts to considering that the slope of the voltage variation is low, that is to say less than a predetermined threshold value. The present invention makes it possible to determine the SOH of a cell, whatever the voltage, but it is advantageously implemented for voltages corresponding to the voltage plateau. It also allows implementation on a reduced portion of the voltage plateau, corresponding to a reduced variation range of the SOC, and is therefore particularly applicable in the context of battery applications not performing complete charge and discharge cycles. It is advantageously implemented: 1. for batteries connected to the electricity network, in particular batteries implemented in the field of renewable energies (solar, wind, hydraulic, geothermal, biomass, etc.), and for example batteries of inverters; 2. for batteries of autonomous systems (individual houses / buildings autonomous, satellites, portable electric devices, etc.). The present invention can be implemented in charging or discharging the battery. Preferably, the charge is controlled, carried out at constant current and constant voltage (CC-CV). In this context, the present invention relates, according to a first of its objects, to a method for determining the state of health (SOH) of a Li-ion battery cell, the cell being intended to be operated in a usage voltage range for a usage application, the method comprising steps consisting of: - define a measurement voltage range included in the usage voltage range, and delimited by a first predetermined measurement voltage value and a second predetermined measurement voltage value, and - when the voltage across the cell terminals is between the first predetermined measurement voltage value and the second predetermined measurement voltage value: - measure the intensity value at the terminals of the cell, - measure the cell temperature value, - determine one of the values ​​among: o the total charge-discharge acceptance (Qd-T) over the measurement voltage range 0 total charge acceptance under load (Qc-T) over the measurement voltage range; the total charge acceptance in discharge (Qd-T), respectively in charge (Qc-T), being determined by the sum of the values ​​Qd (tk), respectively Qc (tk), between the first predetermined measurement voltage value and the second predetermined measurement voltage value, the value Qd (tk), respectively Qc (tk), being the value of the charge acceptance in discharge, respectively in charge, corresponding to the quantity of Ampere-hour discharged, respectively charged, of the cell at time k (tk) or during a time step equal to tk; and - determine the state of health (SOH) of the cell on the basis of a predetermined relationship, over the measurement voltage range, between the total charge acceptance in discharge (Qd-T), respectively in charge (Qc-T), and the state of health (SOH) of the cell. We can predict that: - If the cell is discharging, then the value Qd (tk) is determined according to the formula Qd (tk) = a(T,C) * I(tk); - If the cell is charging, then the value Qc (tk) is determined according to the formula Qc (tk) = B(T,C)* I(tk); With a(T,C) and B(T,C) coefficients whose values ​​depend at least on the temperature (T) and possibly also on the intensity (C), and which are pre-recorded in a memory, I(tk) the value of the current at time k (tk) or during a time step equal to tk. It can be expected that the variation of the voltage as a function of the state of charge (SOC) of the cell is known, and in which the measurement voltage range is defined, in furthermore, so as to meet at least one of the following criteria: - the measuring voltage range is chosen so that the variation in voltage as a function of the cell's state of charge (SOC) is less than a predefined threshold value, - the measurement voltage range is within a range representative of a state of charge (SOC) of the cell between 10% and 90%, and preferably between 45% and 55%, and - the measurement voltage range is chosen from a voltage range crossed by the cell in the context of its usage application at a frequency greater than or equal to a predetermined threshold value. It can be provided that in the measurement voltage range, the difference between the first predetermined measurement voltage value and the second predetermined measurement voltage value is between 0.05V and 0.20V, and preferably between 0.05V and 0.10V. It is also possible to provide a test step, prior to the step of determining the state of health (SOH) of the cell on the basis of a predetermined relationship, over the measurement voltage range, between the total charge acceptance in discharge (Qd-T), respectively in charge (Qc-T), and the state of health (SOH) of the cell, said tests being implemented on a representative cell to determine said relationship between the state of health (SOH) of the cell and the total charge acceptance in discharge (Qd-T), respectively in charge (Qc-T). It is also possible to provide a step of calibrating the total capacity of the Li-ion battery cell, said cell being characterized by an initial capacity Ctot_ref (t0), the calibration step comprising steps consisting of: - detect an end of charge phase, respectively of discharge, and when detecting an end of charge phase, respectively of discharge: - recalibrate the total cell capacity as Ctot_ref (tk)=SOH* Ctot_ref (tO); and define Cr (tk)= Ctot_ref (tk), respectively Cr (tk)= 0; and - Define the values ​​Qd (tk) = Qc (tk) = 0, with Ctot_ref (t0): the value of the total storage capacity of the cell considering a complete discharge under the reference conditions, at the initial time (t0), Ctot_ref (tk): the value of the total storage capacity of the cell considering a complete discharge under reference conditions, at time k (tk) or during a time step equal to tk, Cr (tk): the remaining stored capacity considering a complete discharge under reference conditions, at time k (tk) or during a time step equal to tk, and SOH: The determined state of health (SOH) value of the cell. It is also possible to provide a step of determining the state of charge (SOC) of the cell, by steps consisting of determining one of the values ​​among: 0 the value Qd (tk), the value of the charge acceptance corresponding to the quantity of Ampere-hours discharged from the cell at time k (tk), or during a time step equal to tk; and calculate the value Cr (tk), the remaining stored capacity considering a complete discharge under the reference conditions, at time k (tk) such that Cr (tk) = Cr (tk-1) - Qd (tk); 0 the value Qc (tk), the value of the charge acceptance corresponding to the quantity of Ampere-hours charged at time k (tk), or during a time step equal to tk; and calculate the value Cr (tk) at time k (tk) such that Cr (tk) = Cr (tk-1) + Qe (tk). An initialization step may further be provided, comprising steps consisting of: - determine the value of a constant Ctot_ref (t0), corresponding to the total storage capacity of the cell at an initial time (t0), considering a complete discharge under the reference conditions; - initialize the value of a variable Ctot_ref (tk), corresponding to the total storage capacity of the cell at a later time k (tk), considering a complete discharge under the reference conditions, such that Ctot_ref (tk) = Ctot_ref (t0); - determine the cell's state of charge (SOC) value; - initialize the value of a variable Cr (tk), corresponding to the remaining stored capacity at time k (tk), as a function of the value of the state of charge (SOC), such that: Cr (tk) = 0, if the determined state of charge (SOC) is equal to 0%; Cr(tk) = Ctot_ref(tk), if the determined state of charge (SOC) is equal to 100%; and Cr (tk) = X, if the determined state of charge (SOC) is equal to X / Ctot_ref (tk). - Initialize to 0 the value of a variable Qd (tk), corresponding to the charge acceptance in discharge of the cell, and - Initialize to 0 the value of a variable Qc (tk), corresponding to the charge acceptance of the cell. In addition, steps can be planned consisting of: - measure the current value (T) at the terminals of the cell; - compare the measured current value (I) to a reference value (Iref) which may not be zero; - determine the cell charging mode, such as: o SiI <Iref, alors la cellule est en mode de décharge ; o If I > Iref, then the cell is in charging mode; o If I = Iref for a predetermined duration, then the cell is in neutral mode. In addition, steps can be planned consisting of: Oo If the cell is in discharge mode, then determine the Qd (tk) value, and calculate the Cr value (tk); o If the cell is in charge mode, then determine the Qc (tk) value, and calculate the Cr (tk) value. We can predict that: - If the cell is in discharge mode, then the value Qd (tk) is determined according to the formula Qd (tk) = a(T, C) * I(tk); and - If the cell is in charging mode, then the value Qc (tk) is determined according to the formula Qc (tk) = B(T, C)* I(tk); a(T, C) and B(T, C) being coefficients whose values ​​recorded in a memory, depend at least on the temperature (T) and possibly on the intensity (C). It can be provided that the method according to the invention is implemented in the usage voltage range in which the cell is intended to be operated, and which is between a first predetermined value corresponding to a state of charge (SOC) greater than or equal to 10%; and a second predetermined value corresponding to a state of charge (SOC) less than or equal to 90%. According to another of its objects, the invention also relates to a computer program, comprising program code instructions for implementing the method according to the invention when said program is executed on a computer. The invention also relates to a battery management system, comprising a processor configured to be able to execute the computer program according to the invention. Other characteristics and advantages of the present invention will appear more clearly on reading the following description given by way of illustrative and non-limiting example and made with reference to the appended figures. Figures are not necessarily to scale. DESCRIPTION OF THE DRAWINGS [Fig.1] illustrates the voltage variation as a function of the state of charge for a Li-ion battery, [Fig.2] illustrates an embodiment of a model of coefficient values ​​œ according to the invention, as a function of the intensity (regime) and the temperature, [Fig.3] illustrates an embodiment of a model of values ​​of the discharged capacity Cr according to the invention, as a function of the intensity (regime) and the temperature, [Fig.4] illustrates an embodiment of a model of coefficient values ​​(B according to the invention, as a function of the intensity (regime) and the temperature, [Fig.5] illustrates an embodiment of a model of values ​​of the charged capacity Cr according to the invention, as a function of the intensity (regime) and the temperature, [Fig.6] illustrates a model of variation of the SOH as a function of the charge acceptance Q according to the invention. Detailed description The present invention is intended in particular to be implemented in a battery management system, known by the acronym BMS for Battery Management System. It comprises an algorithm implemented in the form of a computer program. For the sake of simplicity, the terms "battery" and "cell" are used here interchangeably. The invention aims to determine the state of health (SOH) of a battery, through the measurement of its total charge or discharge acceptance between two predetermined voltage values. This measurement of total charge or discharge acceptance can be carried out outside of any context of calculating the SOC of the battery, the determination of the state of health (SOH) of a battery according to the invention not being influenced by the SOC. Although the determination of the state of health (SOH) can be integrated within the BMS, it can however be carried out in parallel and in addition to the SOC calculation functions implemented in the BMS. As such, the embodiment described below will also include the description of the SOC calculations. Conventionally, the calculation of the SOC of a battery requires a first initialization at an initial time t0 allowing certain permanent, non-modifiable values ​​to be defined, the initialization then allowing a SOC calculation loop and possible calibration to be supplied. The SOC of a battery indicates its state of charge, that is to say the percentage that the remaining storage capacity of the battery represents in relation to the total storage capacity that can be returned at time t. A battery's SOH indicates its state of health, i.e., the level of aging of the battery, usually expressed as a percentage of the total initial storage capacity. The storage capacity is expressed in Ah and similarly the storage energy is expressed in Wh. First initialization for SOC calculation We consider t0 as the instant of first initialization of the algorithm according to the invention. First, we determine the Crr_rr (ta) value, which is the initial storage capacity of the cell. This value is known or measured, for example, in the factory. For example, it is equal to 2.5 Ah. We then determine at time t, the value Cu ret (t:), which is the storage capacity total at time t, of said cell, such that Craret (tr) = Crotret (to), assuming a start with an SOH = 100% at the first start of the system in its destination application. By "start", we mean the moment when the BMS is initialized and therefore the moment of first use of the battery with this BMS. Typically, this is the first use of the battery after marketing. In the case of a second-life battery, this may be different. We can then initialize the value of C, (t,), the remaining stored capacity at time ty. according to one of the following 3 cases: C, (t;) = 0, with a start with a SOC = 0%, for example when we know the battery is completely discharged; C (ti) = Crorret (t,), with a start with a SOC = 100%, for example when we know the battery is fully charged; and C, (t;) = X, with a start with a SOC = X / Crorret (tr). With in this case t, = tosi SOH = 100%. The value Ç, (t,) can be initialized in these 3 different ways, depending on the SOC value. There is a good chance that the battery is fully charged before installation in its destination application. In this case, the value of C, (tr ) is initialized to C,orer (tu), corresponding to a SOC of 100%. But if the SOC value is unknown, it can be initialized to a predetermined value, for example 75%, and wait until a full charge is obtained before recalibrating it to 100%. Finally, we determine the value Q; (ty), the quantity of Ah discharged at time t,, which in this case is equal to Q, (t,), the quantity of Ah charged at time t,, initialized to 0 since the battery is at this stage ty neither charging nor discharging. At least the initial value Correr(to) is stored in a memory. We can also store the value C,(t,). The value Core<(to) is essential since it has a direct impact on the value of the SOC, therefore a direct impact on energy management, essential to avoid excessively accelerated aging of the battery. The values ​​of Crorret (to), Crorret (fa), Er (ta), Qa (1), Q: (tx), can also be stored in a memory. Calibration of the SOC calculation A calibration step can be planned. The calibration step can be implemented at any time. It is preferably based on a prior determination of the SOH according to the invention, described later. Once the determination of the SOH (t,) at time t has been carried out according to the invention, it is possible to recalibrate the calculation of the SOC in the following way: We calculate Crotret (t:) = SOH(t,) % Crotret (to). If the BMS detects an end of discharge, then we consider that C, (t,) = 0. If the BMS detects an end of charge, then we consider that C, (t,) = Crorret (tr » We can then recalculate the SOC (t,) such that SOC (t4) = Cr(t,)7 Crorret (tr) We can then calibrate the Q value; (t4) = Q. (t,) = 0. The quantity of Ah discharged at time t, is equal to the quantity of Ah charged at time t,, Calibrated to O, which completes the calibration step. In this calibration phase, the SOC is recalibrated to 100%, respectively 0%, each time the end of charge criterion, respectively end of discharge, is reached by the BMS, which allows whenever possible to correct any drifts of an Ah-metric algorithm. Battery charging mode It is planned to determine the battery charging mode, i.e. whether it is charging, discharging, or neither, i.e. in neutral mode. To do this, we plan to measure the intensity I at the battery terminals. If < Les then the battery is considered to be in discharge mode; If > Les then the battery is considered to be in charge mode; Silly = lys for a predetermined duration, then the battery is considered to be in neutral mode. Lerest is a reference intensity value. For example Ie: = 0. However, it may be desirable to define a non-zero value for Lx, in order to take into account in particular the current linked to the self-consumption of the system integrating the battery, which is generally on standby during the implementation of the invention. Calculation of SOC Once the battery charging mode is determined, the SOC can be calculated as follows. For discharge mode: First, we determine the value Q; (t,), the quantity of Ampere-hours discharged at time t,, or during a time step equal to t,. The quantity of Ah can be measured or calculated by integrating the current as a function of time. In particular, the value Q: (t,) is calculated as a function of temperature, and discharge intensity, as described later. We can then calculate the value C, (t,), which is the remaining stored capacity, considering a complete discharge under the reference conditions. The value C, (t,) at time t, depends on: of the value C, (t,-1) at the previous time step t,,, and of the Q value; (tr). In this case, we plan to calculate the value C, (t,) according to the following equation: C:(t) = C (tir) - Qu (ti). The value Q; (t,) is subtracted from the value C, (t,.1) since the battery is in discharge mode. Depending on the needs, at least one of the Q; (t,) values, or even all of the Q; (t,) values, can be stored in a memory at each ty. We can also overwrite the value Q; (t,.1) with that of Q; (t,). We can finally reset the value Q; (t,). In some cases, it may happen that Ç, (t,) = Ô before the BMS end-of-discharge criterion is reached. In this case, it is possible: either indicate a stabilized SOC at 0% while continuing to integrate the Ah until the end of discharge criterion is reached; or to indicate a SOC < 0%. Advantageously, the end of discharge criterion is a privileged moment to trigger a possible calibration of the SOC. In this case, the calibration is preferably implemented at SOC=0%. For charging mode: We first calculate the value Q; (t«), the quantity of Ampere-hours charged at time t,, or during a time step equal to t,. In particular, the value Q.(t,) is calculated as a function of temperature, and possibly intensity, as described later. We can then calculate the value C, (t,), which is the remaining stored capacity, considering a complete discharge under the reference conditions. The value C, (t,) at time t, depends on: of the value C, (t,-1) at the previous time step t,,, and of the value Q. (t;). In this case, we plan to calculate the value C, (t,) according to the following equation: C (ti) = C, (tr) + Q. (ti). The value Q, (t;) is added to the value C, (t,,) since the battery is in charging mode. Depending on the needs, at least one of the Q.(t,) values, or even all of the Q,(t,) values, can be stored in a memory at each time. We can also overwrite the value Q, (t,1) with that of Q. (t,). We can finally reset the Q value to 0; (1). In some cases, it may happen that C, (t4) > Crorrer (tx), and this, before the end of charge criterion of the BMS is reached. In this case, it is possible: either indicate a stabilized SOC at 100% while continuing to integrate the Ah until the end of charge criterion is reached; or to indicate a SOC greater than 100%. Advantageously, the end of charge criterion is a privileged moment to trigger a possible calibration of the SOC. In this case, the calibration is preferably implemented at SOC=100%. At any time t, during the operation of a battery, its state of charge SOC can be calculated according to the following equation: SOC (t,) = 100 # C, (t,) / Crorret (tk) Load acceptance At each measurement step t,, corresponding to the time step considered, we calculate the value of the charge acceptance, Q; in discharge or Q, in charge, in Ah. The calculation of charge acceptance can be implemented by integration over time of the measurement of the current, of the intensity, between two predefined voltages, which typically require several time steps t,. In fact, for a charge (or a discharge), the value of the charge acceptance, also called charge (or discharge) capacity, depends in particular on the temperature, but also on the charged (or discharged) intensity. It therefore seems sensible to take into account at least the influence of temperature, and possibly of intensity, in the calculation of charge acceptance. For this purpose, it is planned to assign a coefficient to the current measurement. This coefficient, called “[3” in charge and “à” in discharge, is in this case non-linear, as described below. Impact of temperature In practice, the current intensity and the temperature change and both have an impact on the measurement of Ah charged or discharged. It is therefore necessary to establish a reference current and temperature. The current intensity is measured at the battery terminals. The temperature is that of the cell core, which is approximated by the skin temperature, for example obtained using a thermocouple attached to the wall of the battery cell. At each time step, and in this case in real time, we measure the current intensity, the voltage at the battery terminals, and the skin temperature. The temperature of a cell has an impact on its storage capacity. For example, for a discharge at 25°C, a capacity of 10 Ah is measured. But if the same cell is discharged at -20°C, then the capacity measurement is different, in this case lower, for example, 1 Ah. It is therefore appropriate to establish a reference temperature value. In practice, since the temperature value varies during current measurements, it is appropriate to bring the measured temperature value (variable) back to the reference temperature value (constant), in this case using a matrix of reference which allows a multiplicative factor to be applied to each measurement, hereinafter the coefficient à or fB, the value of which depends on the temperature. As is well known, the Ç regime, or C-rate by anglicism, is an intensity which is the current draw made on the battery. 1C corresponds to a discharge of the battery in lh. With a 10Ah battery, a 1C discharge means the battery can be discharged at 10A; a 10C discharge means the battery can be discharged at 1A; a 10C discharge means the battery can be discharged at 100A; etc. The regime is therefore a way of noting an intensity or electric current. For simplification, the notions of regime, intensity and current are therefore used here interchangeably. In this case, the current regime values ​​are between C / 20 and 3C, which correspond to the values ​​between which a Li-ion battery can cycle. In this case, the value of the measured intensity, or of the measured ampere-hours, is multiplied by a coefficient a or B, the value of which depends on the value of the measured temperature and which may also depend on the value of the measured current. The coefficient a or B makes it possible to establish an equivalence with the values ​​of the reference temperature and current. In this case, we have a reference table for charging (coefficient [B) and a reference table for discharging (coefficient a), preferably between a temperature above -20°C and below +70°C, which are the usage values ​​for the temperature of a Li-ion battery. In the case of battery discharge, linked to use, the discharged current is generally of a variable nature. For this reason, a coefficient will be used for the discharge "whose value depends on the value of the measured temperature and the value of the measured current. Since charging a Li-ion battery is generally carried out at a constant and pre-determined current over a large part of its SOC range (excluding the end of charge zone, typically SOC>90%), the calculation of charge acceptance over this range can be simplified by applying a coefficient 8 whose value depends only on the temperature (the current value being known and fixed). In a thermal environment, temperature control is not mandatory. The same applies if the charge or discharge current is imposed. In this case, a single correction factor is used, i.e., a single coefficient value. In the case of the SOC calculation, we calculate at each time step t, the quantity of Ah which has passed through, which allows the SOC to be updated at each time step. Alternatively, for the calculation of the SOC, it is possible to sum over several time steps t, the charge acceptance, in charge or in discharge, and to calculate the SOC as described previously that every N time steps, for N natural number >1, or when necessary for the system. The previous calculations thus allow the BMS system to calculate the SOC of the battery at any time. Usage voltage range and measurement voltage range for SOH calculation It is interesting to be able to correct the SOC measurement by integrating a determination of the state of health (SOH) of the battery during the calibration step described previously. This determination of the SOH allows the taking into account of battery aging in the estimation at any time of the SOC(t,) according to the previous calculation, as well as the remaining capacity of the battery Cho.rer (tr). Taking battery aging into account in the Croiret (tx) also makes it possible to correct any SOC measurement made by other estimation modes that do not take into account the gradual decline over time in the overall capacity of the battery. This may be an SOC estimate provided by a BMS that does not implement the calculation mode described above, in particular the calibration step described above. Since knowledge of the Cootret (t,) provides important information to the user on the maximum capacity of the battery to restore energy, and therefore to provide a service, the determination of the C,oret (t:) is also of interest to the user outside of any context of calculating the state of charge (SOC) described previously. Finally, the state of health (SOH) of the battery can be an important criterion for the user to determine when excessive aging of the battery will require its replacement. For example, a battery with SOH <80% can be considered as requiring replacement. This does not require any calculation of C vor ref (t,) or SOC(tk). A method for determining the SOH of a battery is therefore proposed here. Advantageously, the calculations of the Core: (ty), and and the SOC described previously can also be implemented. sSoH The calculation of the SOH according to the invention is carried out between two predetermined voltage values. To do this, the usage voltage range is defined as the voltage range used by the battery in its application. Within this usage voltage range, a measurement voltage range is defined extending between a first predetermined measurement voltage value and a second predetermined measurement voltage value. The measuring voltage range is chosen as being regularly traversed inte- generally by the battery, during a charging and / or discharging phase. "Regularly" means that it is fully covered at a frequency equal to and preferably higher than the desired frequency for evaluating the SOH. Preferably, the two predetermined measurement voltage values ​​are within the battery voltage plateau, and typically within an equivalent SOC range of between 10% and 90%. To ensure regular measurement, the two predetermined measurement voltage values ​​are chosen here as being close to an equivalent of 50% of the battery SOC, for example between 40% and 60% of the SOC. Preferably, the voltage interval between the two predetermined measurement voltage values ​​is of limited amplitude, in order to ensure a complete and rapid path of the latter during charging or discharging and therefore a rapid evaluation of the SoH. If the measuring voltage range is too small, for example less than 0.05V, there is a risk of generating too many errors in the SOH evaluation. If the measuring voltage range is too large, for example greater than 0.20V, there is a risk that the SOH determination time will be too long. It is thus chosen between 0.05V and 0.20V, and preferably between 0.5V and 0.10V. Tests were carried out by the applicant with a measurement step of 0.05V, resulting in measurement results in around twenty seconds. In this case, the two predetermined measurement voltage values ​​are 3.2V and 3.25V, i.e. a measurement voltage range of 0.05V close to an equivalent of 50% of the SOC of an LFP battery. The SOH determination can advantageously take place during normal use of the battery, without disturbing said use, when the battery voltage is charging and / or discharging through the measurement voltage range. In the case of a system also performing a SOC calculation, the SOH determination can be done in parallel with the SOC determination. The determination of the SOH is carried out during, and at the end of, the charging or discharging journey between the first predetermined measurement voltage value and the second predetermined measurement voltage value. During the measurement voltage range, the charge acceptance, in charge or discharge, is calculated between these two predetermined voltages. To do this, the quantity of Ah that has passed through is calculated at each time step t, and this quantity is summed over all the time steps t, between the two predetermined voltages, i.e. in this case for 0.05V, between 3.2V and 3.25V. Total charge acceptance The determination of the charge acceptance between the two measuring voltages is carried out using the same approach as that presented previously for the calculation of the SOC, i.e.: by measuring at each time step t, the intensity of the charged current or discharged, as well as the battery temperature, by calculating the quantity of Ah which has passed over the time step ty The quantities of Ah which have passed through are summed over the entire measurement voltage range in order to obtain the total charge acceptance, whether charging or discharging, over it. Here we will note Qu-r and Q.-r the total charge acceptances, in discharge or in charge respectively, resulting from the sums of the respective Q; (t,) and Q. (t,), resulting from the different time steps t, necessary to cover the measurement voltage range. When calculating the quantity of Ah that has passed through, the measured current is corrected for current and temperature as in the SOC calculation described previously. Knowing the regime, by measuring the current that passes through, and the temperature, thanks to the reference table corresponding to the charging or discharging mode, it is possible to compensate for temperature and intensity, that is to say to apply to the measurement of the intensity I, the appropriate value of the coefficient à or PB. For example, [Fig.2] or [Fig.4] can also be presented in tabular form. For example, the reference temperature value is 25 °C at a C / 2 regime. Under these conditions, œ = 1. For any other condition, the value of the coefficient œ is given by the temperature coupled with the regime. The same applies to the coefficient B. For example, if the temperature is 10°C and at the same regime C / 2 then œ = 1.051. If the temperature is 30°C and the regime equal to 2C, then œ = 1.012; etc. The use of correction coefficients "and" allows the influence of the current regime and the effects of temperature to be taken into account, both in discharge (a) and in charge (B). Thanks to these coefficients, each Ah charged and discharged is reported to a fixed reference capacity. Determination of SOH The previous step of traversing the measurement voltage range in charge or discharge made it possible to determine the total charge acceptance, in charge or discharge, over the measurement voltage range: Q4r where Q..r It was noted by the applicant that the acceptance of charge or discharge over a predetermined voltage range of a battery was representative of the state of health (SOH) of the latter. Through a campaign of preliminary measurements on batteries representative of those used in the application, it is therefore possible to determine a correspondence relationship between the charge acceptance over the measurement voltage range, in charging or discharging, and the state of health (SOH) of the battery. Following such a test campaign, it is possible, for example, to plan to record in a memory a predefined chart between the SOH value and the load acceptance, in this case in the form of a straight line, as illustrated in [Fig.6]. This relationship can also be analytical in nature, i.e. a calculation formula, and implemented in a BMS memory. By reporting the value of the total charge acceptance Q47 or Qr calculated previously, we can thus determine the value of the SOH. The invention is not limited to the previously described embodiments. The sampling of the measurements, i.e. the value of the time step t,, can be re-evaluated depending on the expected precision. The maps at and [3 can be adjusted according to the end of charge and end of discharge criteria defined by the BMS. In the case of a battery module comprising several elements in series and several thermocouples, it is preferable to select the lowest temperature, in order to establish a diagnosis based on the most limiting conditions. Nomenclature Input data Uiowena: End of discharge voltage ": Coefficient allowing to take into account the impact at least of the temperature on the discharge fB: Coefficient allowing to take into account the impact of at least the temperature on the load Coirer (to): Initial storage capacity, fixed in the initialization phase and modified once by the value calculated during the first successful check, the check being a first full charge or full discharge allowing validation by measurement that the value Cor ret (ty) is indeed equal at start-up to the value C,uret (to). Measured values I: Current at the terminals of the cell T: Cell skin temperature U: Voltage across the cell terminals Calculated values C, (t): Remaining stored capacity, considering a complete discharge under reference conditions Coiret (tx): Total storage capacity considering a complete discharge under reference conditions Qa (t,): acceptance of charge in discharge, quantity of Ah discharged at time ty, during a time step equal to t, for the SOC function Qe (ty): charge acceptance in charge, quantity of Ah charged at time t, during a time step equal to t, for the SOC function Qar: total charge acceptance over the measurement voltage range, in discharge Qa-r: total charge acceptance over the measuring voltage range, under load SOC (t,): State of charge of the cell at time t, SOH (t;): State of health of the cell at time t,

Claims

Claims

1. Method for determining the state of health (SOH) of a cell Li-ion battery, the cell being intended to be operated in a range of usage voltage for a usage application, characterized in that the method comprises steps consisting of: - define a measurement voltage range within the range of usage voltage, and delimited by a first predetermined value of measuring voltage and a second predetermined voltage value of measure, and - when the voltage across the cell terminals is between first predetermined measurement voltage value and the second predetermined measurement voltage value: - measure the intensity value at the terminals of the cell, - measure the cell temperature value, - determine one of the values ​​among: the total charge acceptance in discharge (Q;-r) over the range of measuring voltage; the total charge acceptance under load (Qr) over the range of measuring voltage; the total charge acceptance in discharge (Q4.r), respectively in charge (Qr), being determined by the sum of the values ​​Q; (t,), respec- tively Q, {t,), between the first predetermined voltage value of measurement and the second predetermined measurement voltage value, the value Q; {t,), respectively Q. (t,), being the value of the acceptance from charge to discharge, respectively in charge, corresponding to the quantity of ampere-hours discharged, respectively charged, of the cell at time k (t,) or during a time step equal to t,; and determine the state of health (SOH) of the cell based on a predetermined relationship, over the measuring voltage range, between the total charge acceptance in discharge (Qa-r), respec- tively in charge {(Q.- ), and the state of health (SOH) of the cell.

2. A method according to claim |, wherein: If the cell is discharging, then the value Q; (t,) is de- terminated according to the formula Q; (t,) = a(T,C) # I(t,) ; If the cell is charging, then the value Q,(t,) is determined according to the formula Q. (t,) = B(T,C)* I(t4); With a(T,C) and B(T,C) coefficients whose values ​​depend on the less than the temperature (T) and possibly also the intensity (C), and which are pre-recorded in a memory, I(t,) the value of the current at time k(t,) or during a time step equal to ty.

3. Method according to any one of the preceding claims, in which the variation of the voltage according to the state of charge (SOC) of the cell is known, and in which the measurement voltage range is further defined so as to meet at least one of the criteria following: - the measuring voltage range is chosen so that the variation of the voltage depending on the cell's state of charge (SOC) is lower to a predefined threshold value, - the measuring voltage range is within a representative range indicating a cell state of charge (SOC) between 10% and 90%, and preferably between 45% and 55%, and - the measuring voltage range is chosen within a voltage range crossed by the cell as part of its application of use to a frequency greater than or equal to a predetermined threshold value.

4. A method according to any preceding claim, in which in the measuring voltage range, the gap between the first predetermined measurement voltage value and the second predetermined value completed measurement voltage is between 0.05V and 0.20V, and pre- preferably between 0.05V and 0.10V.

5. A method according to any preceding claim, further comprising a testing step, prior to the de- termination of the cell's state of health (SOH) based on a predetermined relationship, over the measuring voltage range, between the total charge acceptance in discharge (Q;r), respectively in charge (Qr), and the state of health (SOH) of the cell, said tests being implemented on a representative cell to determine said relationship between cell state of health (SOH) and cell acceptance total charge in discharge (Q; 1), respectively in charge (Q..1).

6. Method according to any one of the preceding claims, further comprising a step of calibrating the total capacity of the Li-ion battery cell, said cell being characterized by a initial capacity Crorrer (to), the calibration step comprising steps consisting of: - detect an end of charge or discharge phase, respectively, and when detecting an end of charge phase, respectively dump : - recalibrate the total cell capacity as Ceres(t)=SOH*# Crorres (to) ; and define C, (t1) = Cror rer (t4), respectively C, (t,) = O ; and - Define the Q values; (t,) = Q. (t,) = 0, with Coot re: (to): the value of the total storage capacity of the cell considering a complete discharge under reference conditions, at initial time (to), Crotre: (t4): the value of the total storage capacity of the cell considering a complete discharge under reference conditions, at time k (t,) or during a time step equal to t,, C, (t,): the remaining stored capacity considering a complete discharge under reference conditions, at time k {t,) or during a step of time equal to t, and SOH: the value of the determined state of health (SOH) of the cell.

7. The method of claim 6, further comprising a step of de- termination of the cell's state of charge (SOC), by steps consisting of determining one of the values ​​among: the value Q; (t;), the value of the charge acceptance cor- corresponding to the quantity of ampere-hours discharged from the cell at time k (t,), or during a time step equal to t,; and calculate the value C, (t;), the remaining stored capacity considering a complete discharge under the conditions of reference, at time k (t,) such that C, (t,) = C, (te-1) - Qa (ta) 3 the value Q. (t,), the value of the charge acceptance cor- corresponding to the quantity of Ampere-hours charged at time k (t x), Or during a time step equal to t, ; and calculate the value C, (t,) at time k (t,) such that C, (t:) = C, (ter) + Q. (ti).

8. A method according to any preceding claim, further comprising an initialization step, comprising steps consisting of: determine the value of a constant Crorret (to), Corresponding to the total storage capacity of the cell at an initial time (to), considering a complete discharge under the conditions of reference ; initialize the value of a variable Crorrr(t,), corresponding to the total storage capacity of the cell at a later time k (t;), considering a complete discharge under the conditions of reference, such as Crorer (te) = Chotret (to); determine the cell's state of charge (SOC) value; initialize the value of a variable C, (t,), corresponding to the remaining stored capacity at time k(t,), as a function of the state of charge (SOC) value, such as: C, (t,) = 0, if the determined state of charge (SOC) is equal to 0%; C (te) = Crorrer (tr), if the determined state of charge (SOC) is equal to 100%; ct C, (t,) = X, if the determined state of charge (SOC) is equal to X / Crorret (tk). Initialize to O the value of a variable Q; (t;), corresponding to the cell's charge-discharge acceptance, and Initialize to O the value of a variable Q. (t,), corresponding to the cell's charge acceptance.

9. A method according to any preceding claim, further comprising steps of: measure the current value (I) at the terminals of the cell; compare the measured current value (I) to a value of reference (Iref) possibly non-zero; determine the cell charging mode, such as: Yes <Iref, alors la cellule est en mode de décharge ; If I > Iref, then the cell is in charging mode; If T = Tref for a predetermined duration, then the cell is in neutral mode.

10. The method of claim 9, further comprising steps consisting of: If the cell is in discharge mode, then determine the value Q; (t,), and calculate the value C, (ty): If the cell is in charge mode, then determine the value OQ-(t,), and calculate the value C,(t,).

11. The method of claim 10, wherein: If the cell is in discharge mode, then the value Q; (t,) is determined according to the formula Q; (t;) = a(T, C) *# I(t,) ; and If the cell is in charging mode, then the value Q,(t,) is determined according to the formula Q, (t,) = P(T, C)* I(t,) ; a(T, C) and (B(T, C) being coefficients whose recorded values in a memory, depending at least on the temperature (T) and possibly actually of the intensity (C).

12. A method according to any preceding claim, ca- characterized in that it is implemented in the operating voltage range in which the cell is intended to be operated, and which is included between a first predetermined value corresponding to a state of load (SOC) greater than or equal to 10%; and a second predetermined value completed corresponding to a state of charge (SOC) less than or equal to 90%.

13. Computer program, comprising code instructions of program for implementing the method according to any of the preceding claims when said program is executed on a computer.

14. | Battery management system, comprising a processor configured to be able to execute the computer program according to the claim 13.