Method for estimating the self-discharge of an electrochemical element of a battery, and associated methods and devices

Abstract

The present invention relates to a method for estimating the self-discharge of an electrochemical element to which a balancing current is applied, the method comprising the steps of: - obtaining: - the capacity of the electrochemical element; - first measurement values comprising measurements of the current and of the balancing current applied at first times; and - second measurement values comprising measurements of the state of charge at second times; - estimating the value of the self-discharge at a second estimation time by applying an estimation function to the obtained values, the estimation function calculating a first contribution corresponding to the variation in the state of charge and a second contribution corresponding to the charge buildup related to the first values.

Description

[0001]Method for estimating the self-discharge of an electrochemical element of a battery, associated methods and devices The present invention relates to a method for estimating the self-discharge of at least one electrochemical element of a battery. It also relates to methods implementing the aforementioned estimation method, namely a method for evaluating the dispersion of the state of charge and a method for controlling the current balancing of the electrochemical elements of a battery. The invention also relates to a computer, a management system and an associated battery. Typically, a battery comprises one or more current accumulators also called electrochemical generators,cells or elements. An accumulator is an electricity-producing device in which chemical energy is converted into electrical energy. The chemical energy comes from electrochemically active compounds deposited on at least one face of electrodes arranged in the accumulator. Electrical energy is produced by electrochemical reactions during a discharge of the accumulator. The electrodes, arranged in a container, are electrically connected to current output terminals which ensure electrical continuity between the electrodes and an electrical consumer with which the accumulator is associated. In order to increase the electrical power delivered, several sealed accumulators can be combined together to form a battery. Thus, a battery can be divided into modules, each module being composed of one or more accumulators connected together in series and / or in parallel. Thus,a battery may for example comprise one or more parallel branches of accumulators connected in series and / or one or more parallel branches of modules connected in series. A charging circuit is generally provided to which the battery can be connected to recharge the accumulators. In addition, an electronic management system comprising measurement sensors and an electronic control circuit, more or less advanced depending on the applications, may be associated with the battery. Such a system makes it possible in particular to organize and control the charging and discharging of the battery, to balance the charging and discharging of the different accumulators in the battery with respect to each other. To balance the charging and discharging of the different accumulators, it is necessary to be able to carefully obtain the current imbalance between these accumulators. For this,it is known to perform a complete discharge to determine the state of charge and then to compensate for the observed state of charge deviations by balancing the accumulators but this involves frequently stopping the complete system, which can be problematic for certain applications. There is therefore a need for a method for efficiently determining the quantities involved in the current balancing of electrochemical elements of a battery and for better controlling this balancing. In particular, a good knowledge of the self-discharge dispersions would allow more intelligent control of the balancing making it possible to compensate for these effects without having to perform a complete discharge. To this end, the description describes a method for estimating the self-discharge of at least one electrochemical element of a battery with respect to a reference value,a balancing current specific to the electrochemical element being applied to each electrochemical element, the method being implemented by a computer, the method comprising, for at least one electrochemical element, the steps of: - obtaining: - the capacity of the electrochemical element, - first measurement values, the first values ​​comprising measurements or estimations of the current of the electrochemical element and of the balancing current applied to the electrochemical element at first times, and - second measurement values, the second values ​​comprising measurements or estimations of the state of charge of the electrochemical element at second times, - estimation of the value of the self-discharge of the electrochemical element at a second time, called the second estimation time, by applying an estimation function on the capacity, of the first measurement values ​​and of the second measurement values,the estimation function calculating two contributions to the self-discharge, a first contribution corresponding to the variation in the state of charge of the electrochemical element between the second estimation time and a second earlier time and a second contribution corresponding to the accumulation of charge in the electrochemical element linked to the first measurement values ​​in the time interval between the second earlier time and the second estimation time. According to particular embodiments, the method for estimating the self-discharge has one or more of the following characteristics, taken in isolation or in all technically possible combinations: - the reference value is the self-discharge value of another electrochemical element, the other electrochemical element preferably being the electrochemical element for which the self-discharge value at the second earlier time is the highest,the estimation function also taking into account a third contribution corresponding to the variation in the state of charge of the other electrochemical element between the second estimation instant and the second previous instant and a fourth contribution corresponding to the accumulation of charge in the other electrochemical element linked to the first measurement values ​​in the time interval between the second previous instant and the second estimation instant. - the estimation function is a weighted sum of the contributions and the value of the self-discharge compared to the reference value at the second previous instant. - the contributions are weighted by the same gain coefficient, the gain coefficient depending on: - a first parameter, the first parameter being the product of the capacity of the electrochemical element with the time interval between the second previous instant and the second estimation instant, - a second parameter,the second parameter being an adjustable value, and possibly - a third parameter taking into account the uncertainty of the second measurements, the third parameter preferably depends on the ratio between the uncertainty of the second measurements and the value of the contributions, the gain coefficient preferably being a hyperbolic function which can be written in the form:^, ^3 ^^1^^12 + ^^22with P1 the first parameter, P2 the second parameter and P3 the third parameter. The description also relates to a method for evaluating the dispersion of the state of charge of a plurality of electrochemical elements of a battery, the plurality preferably comprising all of the electrochemical elements of the battery, the method for evaluating the dispersion of the state of charge being implemented by a computer and comprising the steps of: - implementing, for the plurality of electrochemical elements of the battery (10), the steps of a method for estimating the self-discharge of an electrochemical element of a battery with respect to a reference value, the method being as previously described, to obtain a self-discharge value with respect to the reference value for each electrochemical element of the plurality of electrochemical elements,and - determining the dispersion of the state of charge within the plurality at an evaluation time as the difference between the state of charge of the electrochemical element of the plurality which is the highest and the state of charge of the electrochemical element of the plurality which is the lowest, the determining step comprising the application of an evaluation function on values ​​used or obtained during the implementation step. According to particular embodiments, the method for evaluating the dispersion of the state of charge has one or more of the following characteristics, taken in isolation or in all technically possible combinations: - the battery is provided with a balancing circuit for applying a respective balancing current in each electrochemical element of the plurality of electrochemical elements, the balancing circuit having two states, an active state and an inactive state,the applied evaluation function being chosen from several sub-functions according to at least one predefined criterion, the at least one predefined criterion preferably being that the evaluation time corresponds to the second estimation time and the state of the balancing circuit at the evaluation time. - the sub-functions are chosen from: - a calculation sub-function calculating the maximum of the difference in the charge states at the evaluation time. - an estimation function applied to the values ​​obtained at the end of the implementation step, the estimation function calculating two contributions to the dispersion, a first contribution linked to the self-discharge specific to each electrochemical element and a second contribution linked to the maximum self-discharge of all the electrochemical elements of the plurality, and - a sub-function is an estimation function applied to the values ​​obtained at the end of the implementation step,the estimation function calculating three contributions to the dispersion, a first contribution linked to the self-discharge specific to each electrochemical element, a second contribution linked to the maximum self-discharge of all the electrochemical elements of the plurality and a third contribution linked to the balancing currents applied to the electrochemical elements of the plurality of electrochemical elements. - when a criterion according to which the evaluation instant corresponds to the second estimation instant is verified, the evaluation function chosen is the calculation sub-function, and when the balancing circuit is in the active state, the evaluation function chosen being the estimation sub-function calculating three contributions,the chosen evaluation function is the estimation sub-function calculating two contributions otherwise. The description also relates to a method for controlling the current balancing of the electrochemical elements of a battery, the battery being provided with a balancing circuit for applying a respective balancing current in each electrochemical element of the plurality of electrochemical elements, the balancing circuit having two states, an active state and an inactive state, the control method being implemented by a computer, the control method comprising the steps of: - implementing the steps of the method for evaluating the dispersion of the state of charge of a plurality of electrochemical elements of a battery, the evaluation method being as previously described, to obtain an evaluated dispersion, - comparing the evaluated dispersion with a dispersion threshold,and - controlling the state of the balancing circuit as a function of the result of the comparison by putting the balancing circuit in the active state if the evaluated dispersion is greater than or equal to the dispersion threshold possibly reduced by a hysteresis threshold and putting the balancing circuit in the inactive state otherwise. The description also relates to a method for controlling the current balancing of the electrochemical elements of a battery, the control method being implemented by a computer, the control method comprising the steps of: - implementing the steps of the method for evaluating the dispersion of the state of charge of a plurality of electrochemical elements of a battery, the evaluation method being as previously described, to obtain an evaluated dispersion, - comparing the evaluated dispersion with a dispersion threshold,and - control of the time interval between two second consecutive instants to be used for subsequent measurements depending on the result of the comparison. According to particular embodiments, the method for controlling the current balancing of the electrochemical elements has one or more of the following characteristics, taken in isolation or in all technically possible combinations: - the control step comprises an incrementation of the time interval by a first time increment when the evaluated dispersion is less than or equal to the threshold and a reduction of the time interval by a second time increment when the evaluated dispersion is less than or equal to the threshold, the time interval being set to a predefined value if the reduction of the second increment leads to a value less than the predefined value,the first time increment and the second time increment preferably being equal. - the method further comprises a step of testing the validity of the measurement, the test step comprising a test of the amplitude of variation of the dispersion with respect to a maximum possible variation value. The description also relates to a computer capable of implementing a method as previously described. The description also relates to a system for managing a plurality of electrochemical elements of a battery, the electrochemical elements having terminals, the management system comprising: - a balancing circuit capable of applying a respective balancing current to each of the electrochemical elements of the plurality of electrochemical elements, - for each electrochemical element of the plurality of electrochemical elements: - a sensor of the current delivered by the electrochemical element, - a sensor of the balancing current applied,and - a voltage sensor capable of measuring the voltage across the terminals of the electrochemical element, and - a computer as previously described. The description also relates to a battery comprising: - electrochemical elements, and - a management system as previously described. In the present description, the expression "suitable for" means indifferently "suitable for", "adapted to" or "configured for". Characteristics and advantages of the invention will appear on reading the description which follows, given solely by way of non-limiting example, and with reference to the appended drawings, in which: - Figure 1 is a schematic representation of an example of a battery comprising an electrochemical element, - Figure 2 is a graph illustrating an example of a state of charge - open circuit voltage characteristic of the electrochemical element of Figure 1,- Figure 3 is a block diagram representation of an exemplary implementation of a method for estimating the self-discharge of an electrochemical element, - Figure 4 is a flowchart of an exemplary implementation of a method for evaluating the dispersion of state of charge within a plurality of electrochemical elements, - Figure 5 is a flowchart of an exemplary implementation of a balancing control method, - Figure 6 is a flowchart of an exemplary implementation of another balancing control method, - Figure 7 is a flowchart of an exemplary implementation of yet another balancing control method, - Figure 8 is a block diagram representation of the arrangement of the methods of Figures 3 to 7, - Figures 9 to 12 presenting experimental results obtained by the Applicant by implementing the methods of Figures 3 to 7. A battery 10 is shown in the Figure 1. In a manner known per se,a battery is generally an arrangement of a plurality of electrochemical elements but for the sake of simplification of the subject, a case with a single electrochemical element is described in the following, knowing that the transposition to other arrangements is immediate. The battery 10 comprises an electrochemical element 12 and a management system 14 of the electrochemical element 12. As explained previously,an electrochemical element 12 is an electricity production device in which chemical energy is converted into electrical energy. The electrochemical element 12 therefore delivers a current and a voltage between two terminals. The electrochemical element 12 may have a state of charge SOC – open circuit voltage OCV characteristic as seen in FIG. 2. This characteristic is referred to as the SOC / OCV characteristic hereinafter. The state of charge is useful information for the management system 14 to optimize the use and lifespan of the battery 10. The state of charge is often designated by the abbreviation SOC which refers to the English term “State of Charge”. The open circuit voltage is often designated by the abbreviation OCV which refers to the English term “Open Circuit Voltage”. In FIG. 2, the state of charge SOC is expressed as a percentage of a maximum state of charge. The SOC / OCV characteristic may have,for example, four zones, a first zone Z1, a second zone Z2, a third zone Z3 and a fourth zone Z4. The first zone Z1 corresponds to the start of charging and the fourth zone Z4 to the end of charging. For the two intermediate zones, since the second zone Z2 and third zone Z3 correspond to a flat portion, the term flat portion (Z23) will be used in the following. The flat portion Z23 is a portion in which the open circuit voltage variation OCV is less than 30 mV for a variation of at least 10% of the state of charge SOC. Such a type of SOC / OCV characteristic is found in particular when the electrochemical element 12 is an electrochemical element comprising a cathodic active material chosen from the following groups or their mixtures: i) a compound of formula LixFe1-yMyPO4 where M is chosen from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo; and 0.8≤x≤1.2; 0≤y≤0.6,ii) a compound of formula LixMn1-y-zM'yM''zPO4, where M' and M'' are different from each other and are selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb and Mo, with 0.8≤x≤1.2; 0≤y≤0.6; 0.0≤z≤0.2, iii) a compound of formula LixMn2-y-zNiyMzO4-d-cFc where M represents one or more elements selected from the group consisting of B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Cu, Zn, Y, Zr, Nb, Ru, W and Mo; and 1≤x≤1.4; 0 <y≤0,6 ; 0≤z≤0,2 ; 0≤d≤1 ; 0≤c≤1, iv) un composé de formule LixMn2-y-zM'yM''zO4, où M' et M" sont choisis dans le groupe consistant en B, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb et Mo ;. M' et M" étant différents l’un de l’autre, et 1≤x≤1,4 ; 0≤y≤0,6 ; 0≤z≤0,2, et v) un composé de formule LiVPO4F. La matière active anodique n’est pas particulièrement limitée. Elle est un matériau capable d’insérer du lithium dans sa structure. Elle peut être choisie parmi des composés du lithium, des matériaux carbonés comme le graphite,coke, carbon black and glassy carbon. It can also be based on tin, silicon, carbon and silicon compounds, carbon and tin compounds or carbon, tin and silicon compounds. It can also be a lithiated titanium oxide such as Li4Ti5O, 12or a niobium titanium oxide such as TiNb2O7. Of course, these examples are non-limiting and the methods described later may be used for any type of electrochemical element 12. The management system 14 is a system suitable for managing the electrochemical element 12. The management system 14 comprises a balancing circuit 15 and, for each electrochemical element 12, a voltage sensor 16, a first current sensor 18 and a second current sensor 20. The balancing circuit 15 is suitable for applying a respective balancing current to each of the electrochemical elements 12 of the battery. The balancing circuit 15 has two states, namely an active state in which the balancing circuit 15 applies a balancing current and an inactive state in which the balancing circuit 15 does not apply the balancing current. The voltage sensor 16 is suitable for measuring the voltage across the terminals of the electrochemical element 12.The first current sensor 18 is suitable for measuring the current delivered by the electrochemical element 12. The second current sensor 20 is suitable for measuring the balancing current applied to the electrochemical element 12 by the balancing circuit 15. The computer 22 is suitable for implementing a plurality of methods which will be described later. The computer 22 is an electronic circuit designed to manipulate and / or transform data represented by electronic or physical quantities in registers of the computer and / or memories into other similar data corresponding to physical data in the memories of registers or other types of display devices, transmission devices or storage devices.As specific examples, the calculator 22 comprises a single-core or multi-core processor (such as a central processing unit (CPU), a graphics processing unit (GPU), a microcontroller and a digital signal processor (DSP)), a programmable logic circuit, such as an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), a programmable logic device (PLD) and programmable logic arrays (PLA), a state machine, a logic gate and discrete hardware components. An operation of the calculator 22 is now described with reference to FIG. 3 which is a flowchart illustrating an example implementation of a method for estimating the self-discharge of at least one electrochemical element 12 of the battery 10 relative to a reference value. According to the example described, the estimation method comprises an obtaining step and an estimation step.During an obtaining step, the calculator 22 obtains the capacity of the electrochemical element 12, first measurement values ​​and second measurement values. This obtaining step is shown diagrammatically in Figure 3 by the squares 30, 32 and 34 corresponding respectively to the capacity, the first values ​​and the second values. The capacity can be obtained by any means, in particular by implementing an estimation. For the following, the capacity is noted ^^. ^^ with i an index used to identify electrochemical elements 12. The capacity is thus the capacity of the ith electrochemical element 12 of the battery 10. Indeed, the electrochemical elements 12 are assumed here to be connected in series. The first measurement values ​​are values ​​of the current of the electrochemical element 12 and of the balancing current applied to the electrochemical element 12. In the following, ^^ ^ ^^ ^ ^^ ^^ and ^^ ^ ^^ ^^^ ^^respectively denote the current of the electrochemical element 12 of index i and the balancing current applied to the electrochemical element 12 of index i. Each of the first measurement values ​​are obtained at first times. According to the example described, the first measurement values ​​are obtained at regular intervals, this time interval being noted Δ ^^. According to the example described, the two current sensors 18 and 20 measure these current values. Alternatively, the first measurement values ​​are obtained by an estimation. The first measurement values ​​are therefore obtained either by measurements or by estimations or if this is relevant, for example because certain measurements are not feasible at certain first times, by measurements and estimations. The second measurement values ​​are values ​​of the state of charge of the electrochemical element. Each of the second measurement values ​​are obtained at second times.The second instants are denoted tn in the sequence, the index n being an integer. The second measurement values ​​can thus be denoted ^^ ^^ ^^. ^ ^^ ^, i designating the i-th electrochemical element 12 and n the second instant tn. As for the first measurement values, the second measurement values ​​are obtained either by measurements or estimations. For a measurement, it is appropriate to carry out a complete discharge or charge. Such an operation is often referred to as “taking a photo”. At the end of the obtaining step, the computer 22 thus has a plurality of values ​​for several physical quantities and will use these values ​​to estimate the self-discharge of the electrochemical element 12. During the estimation step, the computer 22 thus estimates the value of the self-discharge of the electrochemical element 12 at a second instant, called the second estimation instant, by applying an estimation function on the capacity, the first measurement values ​​and a second measurement value. This estimation step is symbolized by a rectangle 36.According to a first example of the obtaining step, the reference value is zero and the estimation function is an observer based on a formula which can be described as "coulometry". More precisely, the state of charge can be expressed by the following formula:. The estimation function therefore seeks to obtain the estimate of the self-discharge of the electrochemical element considered. Generally, the self-discharge current is expressed as a loss of capacity per month of the electrochemical element 12 considered. In other words, a loss of 3% of capacity per month of an electrochemical element of 180 Ah corresponds to a current of 24∗30= 7.5 ^^ ^^. It may also be specified here that the convention used for the current is the receiver convention, so that the discharge current is negative and thus the self-discharge currents to be estimated are negative currents. In this case, the estimation function calculates two contributions to the self-discharge, a first contribution and a second contribution. The first contribution corresponds to the variation in the state of charge of the electrochemical element 12 between the second estimation time and a second earlier time. The second contribution corresponds to the accumulation of charge in the electrochemical element 12 linked to the first measurement values ​​in the time interval between the second earlier time and the second estimation time. More precisely, the estimation function is a weighted sum of the contributions and the value of the self-discharge relative to the reference value at the second earlier time.The contributions are here weighted by the same gain coefficient. The estimation function is written mathematically:. Where: ^ ^ ^ corresponds to the value of ^^ ^^^^ ^^ ^^ + ^^ ^^ ^^ ^^ at times ^^ ^^ +^^Δ ^^, k being an integer varying between 0 and M-1, and^ is the gain coefficient. According to a first embodiment, the gain coefficient depends on two parameters, the first parameter being the product of the capacity of the electrochemical element with the time interval between the second previous instant and the second estimation instant and the second parameter being an adjustable value. In addition, the gain coefficient is a hyperbolic function that can be written in the form:^ ^1 ^^12 + ^^22 with P1 the first parameter and P2 the second parameter. As a particular example, the gain coefficient is given by: Where: ^ ^^ is a tuning parameter (second parameter P2) which allows to control the update speed of the observer. According to a second embodiment, the gain coefficient used also takes into account a third parameter, the third parameter taking into account the uncertainty of the second measurements. For example, the gain coefficient is a hyperbolic function that can be written in the form: ^^3 ^^1 ^^12 + ^^22 with P1 the first parameter, P2 the second parameter and P3 the third parameter. The gain coefficient then becomes:i i Ki = αn θ ΔtMn γ2 + (θiΔtM)2where ^^ ^ ^^ ^ denotes the third parameter and depends on the ratio between the uncertainty of the second measurements and the value of the contributions. According to a particular example, the third parameter ^^ ^ ^^^ verifies the following relation: Where: ^ is a tuning parameter, this parameter verifying 0 ≤ ^^ ^ ^^ ^ ≤ 1 ^ ^^1 and ^^2 represent, respectively, the uncertainty in the state of charge measurement at the second instants ^^ ^^+1 and ^^ ^^ , ^ ^^ ^^ ^^ ^^ ∑ ^ ^ ^ ^ =1^^ ^^ corresponds to the uncertainties in the measurements of the currents of the electrochemical elements 12 and self-discharge, ^ ^^ denotes a relatively small positive number, the role of which is to avoid division by 0 if the term is zero, and ^ denotes the error in the estimation of self-discharge, this value being given according to the following formula: The third parameter ^^ ^ ^^ ^ thus introduces a so-called dead zone mechanism into the estimation process. In other words, for there to be an update, the estimate ^^ ^ ^^ ^must be sufficiently large relative to the measurement uncertainties ^^1 + ^^ 2 .According to a second example, the estimation function is an observer based on a coulometry model and the reference value is the self-discharge value of a reference electrochemical element 12. According to a particular example, the reference electrochemical element 12 is the electrochemical element 12 for which the self-discharge value at the second prior instant is the highest. Such an estimation function thus allows the estimation of the self-discharge difference between the electrochemical element 12 considered and that of the reference electrochemical element 12. In addition to the first and second previous contributions, the estimation function according to the second example also takes into account two additional contributions. The third contribution corresponds to the variation in the state of charge of the reference electrochemical element between the second estimation instant and the second prior instant.The fourth contribution corresponds to the charge accumulation in the reference electrochemical element linked to the first measurement values ​​in the time interval between the second prior instant and the second estimation instant. Furthermore, as for the previous embodiment, the estimation function is a weighted sum of the contributions and the self-discharge value relative to the reference value at the second prior instant. The contributions are here weighted by the same gain coefficient. The estimation function is written mathematically:. Where: ^ Δ ^^ ^^ ^ ^ ^^̂ ^^ represents the estimate of the self-discharge deviation of the ith electrochemical element 12 relative to the self-discharge of the reference electrochemical element 12, ^ ^^ ^^ denotes the index of the reference electrochemical element, in the example described, due to the choice of the reference electrochemical element, the index ^^^^ checks the following relation:^^ ^^ = arg miin ^^ ^^ ^^ ^ ^^ ^ ^ Δ ^^ ^^ ^^ ^ ^^ ^ is the difference in state of charge between the ith electrochemical element and the reference electrochemical element, this value Δ ^^ ^^ ^^ ^ ^^ ^ thus verifying the following mathematical relationship: Δ^^ ^^ ^^ ^^ = ^^ ^^ ^^ ^^ − ^^ ^^ ^ ^^ ^^^^ ^^ ^ ^^As for the previous case, two examples can be considered in particular for the gain coefficient ^^ ^ ^ ^ ^ . According to the first example, the gain coefficient ^^ ^ ^^^ is always expressed as follows: In the case of the second example, the gain coefficient ^^ ^^^^ can be written: where ^^ ^ ^^ ^ = ^^ ^ ^^ ^ ^^ ^^ ^^ (1 − as before (the quantity ^^ still serving to avoid division by 0) but with a new formulation for the estimation ^^ ^ ^^ ^ Who devient : In each of the cases described above, a value for the self-discharge of an electrochemical element 12 is obtained at the end of the method. This is represented schematically in FIG. 3 by the circle 38. Such a method thus makes it possible to obtain a more reliable estimate of the self-discharge. The estimate of the self-discharge can be used for numerous applications. According to a first application, the estimate of the self-discharge is used to obtain the dispersion of the state of charge of the electrochemical elements. Thus, by way of example, the computer 22 can also implement a method for evaluating the dispersion of the state of charge of the electrochemical elements according to the flowchart illustrated in FIG. 4. The evaluation method comprises an implementation step and a determination step.During the implementation step, the calculator 22 implements the method for estimating the self-discharge for a plurality of electrochemical elements 12 of the battery 10. The plurality of electrochemical elements 12 preferably groups together all of the electrochemical elements 12 of the battery 10. The calculator 22 thus makes it possible to obtain a self-discharge value relative to the reference value for each electrochemical element 12 of the plurality. During the determination step, the calculator 22 determines the dispersion of the state of charge within the plurality at an evaluation time as the difference between the state of charge of the electrochemical element 12 of the plurality which is the highest and the state of charge of the electrochemical element 12 of the plurality which is the lowest.Alternatively, it would be possible to envisage a determination step during which the estimation of the self-discharge difference relative to the electrochemical element 12 having the greatest self-discharge value is used. According to the example of FIG. 4, the determination step comprises a test operation and an application operation. During the test operation, one or more predefined criteria are used. In the case described, two criteria are used. A first criterion consists of determining whether the evaluation time corresponds to a second estimation time. This test is represented in FIG. 4 by a diamond schematically representing a test of the value of the photo-taking variable PF. If the photo-taking variable PF has a first value, this indicates that the photo has been taken, whereas if the photo-taking variable PF has a second value, this indicates that the photo has not been taken.The photo-taking variable PF is thus, for example, a Boolean whose first value is TRUE (denoted O in Figure 4) and the second value is FALSE (denoted N in Figure 4). A second criterion used during the test operation is to test the state of the balancing circuit at the evaluation instant. This test is represented in Figure 4 by a diamond schematically representing a test of the value of the activation variable ABF. The activation variable ABF is a Boolean taking the value TRUE when the balancing circuit 15 is in the active state and FALSE otherwise. During the application operation, the computer 22 applies an evaluation function to values ​​used or obtained during the implementation step. The evaluation calculation function applied is chosen according to the result of the test operation.More precisely, the evaluation calculation function will be chosen from several sub-functions according to the predefined criterion(s) used during the test operation. In the illustrated case, the calculator 22 has three different sub-functions denoted respectively SFC, SFE1 and SFE2. When the first criterion is verified, the calculator 22 applies as calculation function an SFC calculation sub-function. The SFC calculation sub-function calculates the maximum of the difference in the states of charge of the electrochemical elements 12 of the plurality at the evaluation instant. Such an SFC calculation sub-function is written:. Where: o SS denotes the deviation in state of charge SS, i.e. the percentage difference between the most charged electrochemical element 12 and the least charged electrochemical element 12 within the plurality, and o MOWS corresponds to the maximum window for observing deviations in states of charge in the measurement zone, such a window can for example be set as the maximum state of charge level of the zone Z1 in FIG. 2. When the second criterion is satisfied but not the first criterion, the computer 22 applies as a calculation function a first estimation sub-function SFE1. The first estimation sub-function SFE1 is applied to the values ​​obtained at the end of the implementation step. Unlike the calculation sub-function SFC which uses measurements also used in the implementation step, the first estimation sub-function SFE1 uses the output data of the implementation step.The first estimation sub-function SFE1 estimates four contributions to the dispersion, namely: - a first contribution linked to the deviation in charge state within the plurality, - a second contribution linked to the self-discharge specific to each electrochemical element 12, - a third contribution linked to the maximum self-discharge of all the electrochemical elements 12 of the plurality, and - a fourth contribution linked to the balancing currents applied to the electrochemical elements 12 of the plurality of electrochemical elements 12. More precisely, in the example described, the first estimation sub-function SFE1 is written:. Where: ^ DT denotes the time interval between two iterations (in seconds), ^ SBWB denotes the amount of charge balanced by the balancing circuit 15 between the instants ^^ ^^ and ^^ ^^+1as a percentage of the state of charge of the electrochemical element 12 considered, this corresponding to the following formula:^^ ^^ ^^ ^^ = 100 × ^^ ^^ ^^ ^^, ^^ [%. ^^−13600 × ^^ ] ^^ where ^^ is the index of the electrochemical element 12 considered and is the balancing current applied to the electrochemical element 12 ^^ in amperes, ^ MSD%: maximum self-discharge estimated on all the electrochemical elements 12 constituting the plurality: MSD 100 ^^ ^^ ^^ ^^ℎ, ^^ % = 3600 mi∈aNx^^ [%. ^^−1] ^^ Where ^^ ^^ denotes the capacity of cell i in ^^ℎ and ^^ ^^ ^^ ^^ℎ, ^^ the self-discharge of the electrochemical element 12 estimated in ^^, and ^ CSD%,i: estimated self-discharge of the ith electrochemical element 12 considered: 100 × ^^ ^^ ^^ ^^ℎ, ^^ 00 × ^^ [% −1 36 . ^^ ] ^^When neither of the two criteria is verified, the computer 22 applies a second estimation sub-function SFE2. The computer 22 therefore applies this second estimation sub-function SFE2 when the balancing circuit 15 is in the active state and the evaluation time does not correspond to the second estimation time. As in the case of the first estimation sub-function SFE1, the second estimation sub-function SFE2 is an estimation function applied to the values ​​obtained at the end of the implementation step. The second estimation sub-function SFE2 estimates three contributions to the dispersion, namely: - a first contribution linked to the deviation in the state of charge within the plurality, - a second contribution linked to the self-discharge specific to each electrochemical element 12, and - a third contribution linked to the maximum self-discharge of all the electrochemical elements 12 of the plurality.More precisely, in the example described, the second estimation sub-function SFE2 is written: ^^ ^^ ^^2 = ^^ ^^( ^^) + ^^ ^^ ∗ ( ^^ ^^ ^^ − ^^ ^^ ^^) In each case, a measured or estimated value is thus obtained at the end for the deviation in state of charge SS by taking into account the best available values. The evaluation method makes it possible to obtain the most accurate value of the deviation in state of charge SS within the plurality of electrochemical elements 12. According to a second application, the estimation of the self-discharge is used to control the current balancing of the electrochemical elements. Two specific examples will now be described with reference to the flowcharts of FIGS. 5 and 6. According to a first example corresponding to FIG. 5, the computer 22 implements a method for controlling the current balancing of the electrochemical elements.The control method comprises an implementation step, a comparison step and a control step. During the implementation step, the computer 22 implements the previous evaluation method to obtain an evaluated dispersion. The upper part of the figure (before the brace) therefore corresponds to the resumption of the flowchart of figure 4. During the comparison step, the computer 22 compares the evaluated dispersion with an SST dispersion threshold. The SST dispersion threshold is a parameter fixing the difference in percentage of maximum charge state tolerated in the plurality at the time of taking photos. The SST dispersion threshold is chosen to guarantee that at the time of taking photos the elements are observable and prevents the available capacity from being too reduced. An SST dispersion threshold of between 1% and 5% usually makes it possible to meet these conditions.This operation is symbolized in Figure 5 by a diamond giving the result of the comparison (O for dispersion greater than the dispersion threshold SST and N for a dispersion less than the dispersion threshold SST). During the control step, the computer 22 controls the state of the balancing circuit 15 according to the result of the comparison. According to the example described, the computer puts the balancing circuit in the active state of the balancing circuit if the evaluated dispersion is greater than or equal to the dispersion threshold. This is symbolized in Figure 5 by a rectangle in which ABF = 1 is written. Otherwise, the computer 22 puts the balancing circuit 15 in the inactive state. This is illustrated schematically by the rectangle where ABF = 0 is written. According to a second example corresponding to Figure 6, an additional test is added.This test consists of verifying whether the evaluated dispersion is greater than or equal to the dispersion threshold reduced by a hysteresis threshold. If so, the computer 22 keeps the balancing circuit in the active state of the balancing circuit 15. Otherwise, the computer 22 puts the balancing circuit in the inactive state, as schematically illustrated by the rectangle where ABF = 0 is written. Such an additional test makes it possible to avoid blocking the balancing circuit 15 in active mode. The balancing control method described previously allows efficient management of the balancing circuit 15 so that it ensures good balancing of the state of charge of the electrochemical elements 12 within the plurality. According to a second example corresponding to FIG. 7, the computer 22 implements a balancing control method comprising an implementation step, a comparison step and a control step.The implementation and comparison steps are similar to the previous cases. The same remarks therefore apply here and are not repeated. During the control step, the computer 22 controls the time interval between two second consecutive instants to be used for subsequent measurements according to the result of the comparison, i.e. the time interval between two photos taken. Such a time interval will therefore be called in the following measurement interval to clarify the rest of the description. Ideally, this time interval must be as long as possible to interrupt the use of the battery 10 as little as possible. The control carried out by the computer 22 may consist of implementing one or more of the operations set out below. For example, according to a first operation, the measurement interval is incremented by a first time increment when the evaluated dispersion is less than or equal to a threshold.According to a second operation, the measurement interval is decreased by a second time increment when the evaluated dispersion is less than or equal to the threshold. According to a third operation, the measurement interval is set to a predefined value if the decrease of the second increment leads to a value less than the predefined value. In the three previous operations, the first time increment and the second time increment can advantageously be equal. According to more complex operations, the first time increment and the second time increment depend on other parameters, for example on the number of iterations already carried out in the implementation of the method or on the deviation of the evaluated dispersion from the threshold. The smaller the dispersion is compared to the threshold, the larger the first time increment can be. For the specific example of Figure 7, an example of sequences of actions is now described.During an initialization action, the measurement interval BP is initialized to an initial value T0. This action is represented in Figure 7 by a rectangle in which BP=T0 is indicated. As long as the time elapsed since the last photo was taken is less than the measurement interval BP, the computer 22 maintains the variable PR at a first value indicating that a discharge is not necessary. When the elapsed time becomes greater than or equal to, the value of the variable PR is modified to a second value. This second value corresponds to the fact that a discharge should be carried out. For example, the first value is 0 and the second value is 1, so that the first value corresponds to TRUE while the second value corresponds to FALSE. This action is represented schematically by a diamond in which Δ ^^ ≥ ^^ ^^ is indicated and rectangles indicating PR = 0 or PR = 1 depending on the case.According to a more elaborate embodiment, this action may include additional criteria, for example, if the dispersion of the state of charge exceeds a threshold, the change to 1 of the variable PR may also be imposed. This information on the dispersion of the state of charge comes here from the evaluation method described previously. The computer 22 then puts a test action to test whether the electrochemical element 12 is in the measurement zone, that is to say a zone adapted from the SOC / OCV characteristic of figure 2. Such a test action thus corresponds to a step of testing the validity of the measurement. This action is represented schematically by a diamond in which it is indicated "Z1 / Z4?". When the electrochemical element 12 is actually in the measurement zone, it is then tested whether the difference in state of charge with balancing is less than or equal to the MOWS value already explained previously.In Figure 7, this action is represented by a diamond with the indication SS ≤ ^^ ^^ ^^ ^^. If not, the measurement interval BP is reset to the initial value T0 as indicated in the corresponding rectangle in Figure 7. In the other case, the computer 22 implements a second test. This test determines whether the difference in the state of charge with balancing is less than or equal to the MUL value. The MUL value corresponds to the maximum tolerance allowed for the dispersion at the time of the measurement. The MUL value is less than or equal to the MOWS value. The MUL value can in particular be set to the value of the dispersion threshold SST. As previously, the action corresponding to the second test is represented by a diamond with the indication SS < ^^ ^^ ^^. If the condition is met, the measurement interval BP is incremented by a predefined time interval TS relative to the current value of the measurement interval denoted BP. nin the corresponding rectangle. The predefined time TS can be set to the desired value depending on the precision desired for obtaining the BP measurement interval. As a non-limiting example, a value equal to 1 day can be chosen. Otherwise, the current value of the BP measurement interval n is compared to the predefined time interval TS. If the difference result is positive (BP n > TS), the predefined time interval TS is removed the current value of the BP measurement interval n to get a new value for the BP measurement interval and otherwise the current value of the BP measurement interval nis maintained. Such a method is implemented iteratively until a value for the measurement interval BP is obtained that is satisfactory. Such a control method therefore makes it possible to dynamically adjust the measurement interval BP. This results in increased availability of the battery 10. With reference to FIG. 8 which is a block view of the methods just described, the method comprises four blocks: a first block B1 providing a self-discharge value for each electrochemical element 12, a second block B2 giving the value of the dispersion within the plurality of electrochemical elements 12, a third block B3 controlling the measurement interval and a fourth block B4 controlling the activation of the balancing circuit 15. These different blocks are independent in the sense that they only need the output of the previous block to be implemented. This means that any other way of providing the same value can be envisaged.Thus, by way of example, the actions of the third block B3 can be implemented with any method that makes it possible to obtain the value of the dispersion within the plurality of electrochemical elements 12, for example, a combination of blocks B1 and B2 corresponding to the method described, but also a combination of B2 with a block other than block B1 or a combination of completely different blocks. Similar remarks are valid for the second block B2 and the fourth block B4. In each case, blocks B1 to B4 are advantageously used to ensure better current balancing of the electrochemical elements 12. This has been shown in the context of experiments carried out by the Applicant and the results of which are visible in FIGS. 9 to 12. In these experiments, the battery comprises 5 electrochemical elements 12 in series. The final time considered is 180 days.The capacities and self-discharge of each electrochemical element 12 are given in the following table: [Table 1] Element E1 E2 E3 E4 E5 Capacity (. A.h) 199.64 199.5 199.09 194.95 196.33 Self-discharge ( mA)2.5 1 2 6.5 5.5 The corresponding self-discharge curves for each of the electrochemical elements 12 are shown in Figure 9, which shows the evolution of the current over time. This information makes it possible to obtain the self-discharge dispersion by implementing the evaluation method. The tolerance for the maximum state-of-charge dispersion within the battery 10 is set to 1%. With the implementation of the method, the results of Figures 10 and 11 are obtained, to be compared with the result of Figure 12, corresponding to current balancing without implementing the method. Figures 10 and 12 represent the evolution of the state-of-charge dispersion over time, and in dotted lines the times at which a photo is taken. Figure 11 shows how the BP measurement interval increases over time.This comparison shows that the method allows a reduction of a factor of 4 in the dispersion of the state of charge as well as a gain of a factor of 3 on the BP measurement interval. The methods described therefore allow better balancing to be obtained with spaced photos.

Claims

CLAIMS 1. Method for estimating the self-discharge of at least one electrochemical element (12) of a battery (10) relative to a reference value, a balancing current specific to the electrochemical element (12) being applied to each electrochemical element (12), the method being implemented by a computer (22), the method comprising, for at least one electrochemical element (12), the steps of: - obtaining: - the capacity of the electrochemical element (12), - first measurement values, the first values comprising measurements or estimations of the current of the electrochemical element (12) and of the balancing current applied to the electrochemical element (12) at first times, and - second measurement values, the second values comprising measurements or estimations of the state of charge of the electrochemical element (12) at second times,- estimation of the value of the self-discharge of the electrochemical element (12) at a second instant, called the second estimation instant, by applying an estimation function to the capacity, the first measurement values and the second measurement values, the estimation function calculating two contributions to the self-discharge, a first contribution corresponding to the variation in the state of charge of the electrochemical element (12) between the second estimation instant and a second earlier instant and a second contribution corresponding to the accumulation of charge in the electrochemical element (12) linked to the first measurement values in the time interval between the second earlier instant and the second estimation instant.

2. Method for estimating the self-discharge according to claim 1, in which the reference value is the self-discharge value of another electrochemical element (12), the other electrochemical element (12) preferably beingthe electrochemical element (12) for which the self-discharge value at the second prior instant is the highest, the estimation function also taking into account a third contribution corresponding to the variation in the state of charge of the other electrochemical element (12) between the second estimation instant and the second prior instant and a fourth contribution corresponding to the accumulation of charge in the other, electrochemical element (12) linked to the first measurement values in the time interval between the second prior time and the second estimation time.

3. Method for estimating self-discharge according to claim 1 or 2, wherein the estimation function is a weighted sum of the contributions and the value of the self-discharge relative to the reference value at the second prior time. 4.Method for estimating self-discharge according to claim 3, in which the contributions are weighted by the same gain coefficient, the gain coefficient depending on: - a first parameter, the first parameter being the product of the capacity of the electrochemical element (12) with the time interval between the second previous instant and the second estimation instant, - a second parameter, the second parameter being an adjustable value, and possibly - a third parameter taking into account the uncertainty of the second measurements, the third parameter preferably depends on the ratio between the uncertainty of the second measurements and the value of the contributions, the gain coefficient preferably being a hyperbolic function which can be written in the form: ^. ^3 ^^1^^12 + ^^22with P1 the first parameter, P2 the second parameter and P3 the third parameter.

5. Method for evaluating the dispersion of the state of charge of a plurality of electrochemical elements (12) of a battery (10), the plurality preferably comprising all of the electrochemical elements (12) of the battery (10), the method for evaluating the dispersion of the state of charge being implemented by a computer (22) and comprising the steps of: - implementing, for the plurality of electrochemical elements (12) of the battery (10), the steps of a method for estimating the self-discharge of an electrochemical element (12) of a battery (10) with respect to a reference value according to any one of claims 1 to 4, to obtain a self-discharge value with respect to the reference value for each electrochemical element of the plurality of electrochemical elements (12), and - determining the dispersion of the state of charge within the plurality at an evaluation time as the difference between the state of charge of the electrochemical element (12) of the plurality which is the highest and the state of charge of the electrochemical element (12) of the plurality which is the lowest, the determining step comprising the application of an evaluation function on values used or obtained during the implementation step. 6.A method for evaluating the dispersion of the state of charge according to claim 5, wherein: the battery (10) is provided with a balancing circuit (15) for applying a respective balancing current in each electrochemical element (12) of the plurality of electrochemical elements (12), the balancing circuit (15) having two states, an active state and an inactive state, the applied evaluation function being chosen from several sub-functions according to at least one predefined criterion, the at least one predefined criterion preferably being that the evaluation time corresponds to the second estimation time and the state of the balancing circuit (15) at the evaluation time.

7. A method for evaluating the dispersion of the state of charge according to claim 6, wherein the sub-functions are chosen from: - a calculation sub-function calculating the maximum of the difference in the states of charge at the evaluation time.- an estimation function applied to the values obtained at the end of the implementation step, the estimation function calculating two contributions to the dispersion, a first contribution linked to the self-discharge specific to each electrochemical element (12) and a second contribution linked to the maximum self-discharge of all the electrochemical elements (12) of the plurality, and - a sub-function is an estimation function applied to the values obtained at the end of the implementation step, the estimation function calculating three contributions to the dispersion, a first contribution linked to the self-discharge specific to each electrochemical element (12), a second contribution linked to the maximum self-discharge of all the electrochemical elements (12) of the plurality and a third contribution linked to the balancing currents applied to the electrochemical elements (12) of the plurality of electrochemical elements.A method for assessing state of charge dispersion according to any one of claims 6 and 7, wherein:. - when a criterion according to which the evaluation instant corresponds to the second estimation instant is verified, the evaluation function chosen is the calculation sub-function, and - when the balancing circuit (15) is in the active state, the evaluation function chosen being the estimation sub-function calculating three contributions, the evaluation function chosen is the estimation sub-function calculating two contributions otherwise.

9. Method for controlling the current balancing of the electrochemical elements (12) of a battery (10), the battery (10) being provided with a balancing circuit (15) making it possible to apply a respective balancing current in each electrochemical element (12) of the plurality of electrochemical elements (12), the balancing circuit (15) having two states, an active state and an inactive state, the control method being implemented by a computer (22),the control method comprising the steps of: - implementing the steps of the method for evaluating the dispersion of the state of charge of a plurality of electrochemical elements (12) of a battery (10) according to any one of claims 5 to 8, to obtain an evaluated dispersion, - comparing the evaluated dispersion with a dispersion threshold, and - controlling the state of the balancing circuit (15) as a function of the result of the comparison by putting the balancing circuit (15) in the active state if the evaluated dispersion is greater than or equal to the dispersion threshold possibly reduced by a hysteresis threshold and putting the balancing circuit (15) in the inactive state otherwise.

10. Method for controlling the current balancing of the electrochemical elements (12) of a battery (10), the control method being implemented by a computer (22),the control method comprising the steps of: - implementing the steps of the method for evaluating the dispersion of the state of charge of a plurality of electrochemical elements (12) of a battery (10) according to any one of claims 5 to 8, to obtain an evaluated dispersion, - comparing the evaluated dispersion with a dispersion threshold, and - controlling the time interval between two second consecutive instants to be used for subsequent measurements according to the result of the comparison.

11. The balancing control method according to claim 10, wherein the control step comprises an incrementation of the time interval by a first time increment when the evaluated dispersion is less than or equal to the threshold and a reduction of the time interval by a second time increment when the evaluated dispersion is less than or equal to the threshold, the time interval being set to a value, predefined if the decrease of the second increment leads to a value lower than the predefined value, the first time increment and the second time increment preferably being equal.

12. Method for controlling the balance according to claim 10 or 11, in which the method further comprises a step of testing the validity of the measurement, the testing step comprising a test of the amplitude of variation of the dispersion with respect to a maximum possible variation value.

13. Computer (22) capable of implementing a method according to any one of claims 1 to 12. 14.Management system (14) for a plurality of electrochemical elements (12) of a battery (10), the electrochemical elements (12) having terminals, the management system (14) comprising: - a balancing circuit (15) capable of applying a respective balancing current to each of the electrochemical elements (12) of the plurality of electrochemical elements (12), - for each electrochemical element (12) of the plurality of electrochemical elements (12): - a sensor of the current (18) delivered by the electrochemical element (12), - a sensor of the balancing current (20) applied, and - a voltage sensor (16) capable of measuring the voltage at the terminals of the electrochemical element (12), and - a calculator (22) according to claim 13.

15. Battery (10) comprising: - electrochemical elements (12), and - a management system (14) according to claim 14.

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