Method for monitoring the health status of a battery

Abstract

Method (100) for monitoring the SOH of a battery, comprising the steps of: Initial charging (110) of the battery until a first predetermined stop voltage U is reached, the initial charging of the battery being carried out by applying a constant current, Resting (120) of the battery for a suitable time so that at the end of the resting period: a battery voltage is equal to, or sufficiently close to, an open circuit voltage OCV of the battery, and a stable battery temperature, obtaining (130) a battery indicator from among: the open circuit voltage OCV of the battery, a DC resistance DCR of the battery, obtaining (140) a battery model taking as input the battery indicator and providing as output the SOH of the battery, and determining (150) the SOH of the battery by providing the obtained indicator (130) to the obtained model (140).Figure to be published with the abbreviation: Figure 1.

Description

Title of the invention: Method for monitoring the health status of a battery TECHNICAL FIELD OF THE INVENTION

[0001] The technical field of the invention is that of the monitoring and management of electric batteries, in particular batteries on board a vehicle, especially in an aircraft.

[0002] The present invention relates in particular to a method for monitoring the health status of a battery. TECHNOLOGICAL BACKGROUND OF THE INVENTION

[0003] Climate change is a major concern for many legislative and regulatory bodies worldwide. Indeed, various restrictions on carbon emissions have been, are being, or will be adopted by various states. In particular, an ambitious standard applies to both new types of aircraft and those currently in operation, requiring the implementation of technological solutions to bring them into compliance with current regulations. Civil aviation has been actively working for several years now to contribute to the fight against climate change.

[0004] Technological research efforts have already led to very significant improvements in the environmental performance of aircraft. The Applicant takes into account the factors impacting all phases of design and development in order to obtain aeronautical components and products that are less energy-intensive, more environmentally friendly, and whose integration and use in civil aviation have moderate environmental impacts, with the aim of improving the energy efficiency of aircraft. The Applicant is constantly working to reduce its climate impact by employing methods and operating virtuous development and manufacturing processes that minimize greenhouse gas emissions to the minimum possible in order to reduce the environmental footprint of its activity.

[0005] This sustained research and development work focuses on new generations of aircraft engines, the weight reduction of aircraft, particularly through the materials used and lighter on-board equipment, the development of the use of electrical technologies to provide propulsion, and finally aviation biofuels.

[0006] In this context, the use and management of electric batteries in aircraft must be optimized. The state of health of a battery, also commonly called SOH for "State of Health" (SOH) can be defined based on the loss of capacity or gain in resistance of a battery throughout its lifespan. In this application, SOH is defined as: [00 ° 7] SQH = x 1 go % ■initiated

[0008] With: * current' 'a Current maximum battery capacity * (-'initial' 'a Initial maximum battery capacity

[0009] Depending on the health status of a battery, its use can be modified. For example, in the transportation sector, batteries are usually used up to a threshold health status of between 70% and 80%. This initial use corresponds to what is called the "first life" of a battery. Below this threshold health status, the loss of capacity tends to accelerate, but the battery can continue to be used in so-called "second life" applications, such as "smart grid" type energy networks.

[0010] For embedded applications, particularly in aircraft, it is therefore essential to measure or estimate the health status of a battery. Known methods exist for monitoring the battery's health status without requiring the battery to be removed from the device in which it is installed.

[0011] A first known method, disclosed in Shen, P., Ouyang, M., Lu, L., Li, J., & Feng, X. (2018). The co-estimation of State of Charge, State of Health, and State of Function for lithium-ion batteries in electric vehicles. IEEE Transactions on Vehicular Technology, 67(1), 92-103, proposes estimating the state of health of a battery based on a recursive least squares method with a forgetting factor. This method is very comprehensive because it also allows for the estimation of the state of charge and the state of operation of the battery. However, this method is complex to implement.

[0012] A second known method, described in Zhou, Y., Huang, M., & Pecht, M. (2018, August 27). An Online State of Health Estimation Method for Lithium-ion Batteries Based on Integrated Voltage. 2018 IEEE International Conference on Prognostics and Health Management, ICPHM 2018, reveals a linear correlation between the state of health of a battery and the voltage integral during a portion of the battery's charge. However, this method requires considering a very wide range of battery state-of-charge levels (e.g., between 10% and 90% charge), which is not always done in every application. Furthermore, the range of battery state-of-charge levels required to implement this method is not compatible with certain uses.

[0013] There is therefore a need to provide a method for determining the health status of a battery that at least partially resolves the drawbacks of prior art methods. Summary of the invention

[0014] The invention offers a solution to the problems mentioned above, by allowing the determination of a SOH state of health of a battery with a method that is simple to implement and compatible with a common use of the battery, for example on board an aircraft.

[0015] One aspect of the invention relates to a method for monitoring the state of health (SOH) of a battery, comprising the steps of: • Initial battery charging until a predetermined first stop voltage (U) is reached, with the initial battery charging being carried out by applying a constant current, • The battery is put into a resting state for a suitable duration so that at the end of the resting state: • a battery voltage that is equal to, or sufficiently close to, the battery's open-circuit voltage (OCV), and • a stable battery temperature, • obtaining a battery indicator from among: • the open-circuit voltage (OCV) of the battery, • a DC resistance (DCR) of the battery, • obtaining a battery model that takes the battery indicator as input and provides the battery's state of health (SOH) as output, and • Determination of the SOH health status of the battery by providing the obtained model with the indicator obtained.

[0016] Thanks to the invention, battery health monitoring can be performed easily and directly in an embedded system. Indeed, the implementation of the method according to the invention is economical in terms of computing resources and does not require a complete discharge and / or charge of the battery, as this is not operationally feasible. The method of the invention makes it possible to obtain an estimate of the health of the electrical cell by scanning a relatively narrow range of cell charge states.

[0017] The method according to the invention is based, in a first implementation, on a clear relationship between the battery's state of health and the voltage measured between two steps of a specific charging protocol. The battery's state of health also has a clear relationship, used in a second implementation, with the resistance internal battery calculated at the beginning of the second stage of the specific protocol's charging process.

[0018] In addition to the characteristics mentioned in the preceding paragraph, the monitoring method according to one aspect of the invention may have one or more additional characteristics from among the following, considered individually or in all technically possible combinations: • The process also includes an additional battery charge: • for a predetermined period, and / or • until a second predetermined maximum voltage U is reached, and / or • a predetermined state of charge, • The process further includes modifying the battery's operating conditions based on the determined SOH (State of Health), the modification of the battery's operating conditions including: • when the determined SOH (State of Health) is below a predetermined health threshold value, replace the battery with a new one, and / or • Charge the battery, during supplemental charging, at a current intensity less than or equal to a maximum current intensity, the maximum current intensity being determined from the determined SOH (State of Health) rating. • The battery rest period is equal to or greater than one hour, • The battery indicator is the open-circuit voltage (OCV) of the battery, • The battery indicator is the DC resistance DCR of the battery, and obtaining the DC resistance DCR of the battery is carried out at the beginning of the additional battery charging.

[0019] A second aspect of the invention relates to an electric battery management system comprising means for implementing the method according to the invention.

[0020] A third aspect of the invention relates to an aircraft comprising an electric battery and a battery management system according to the invention.

[0021] A fourth aspect of the invention relates to a computer program comprising instructions which, when the program is executed by a computer, lead the computer to implement a process according to the invention.

[0022] A fifth aspect of the invention relates to a computer-readable data carrier on which the computer program according to the invention is recorded.

[0023] The invention and its various applications will be better understood by reading the following description and examining the accompanying figures. BRIEF DESCRIPTION OF THE FIGURES

[0024] The figures are presented for illustrative purposes only and are in no way limiting of the invention. • Fig. 1 shows a block diagram of an example of a method for monitoring the SOH health status of a battery according to the invention. • Fig. 2 is a graph illustrating an example of the evolution of a battery's SOH health status as a function of the battery's OCV open circuit voltage. • Fig. 3 is a graph showing an example of the relationship between a battery SOH health state and a DC resistance DCR. • Fig. 4 is a graph illustrating an example of current profile and battery voltage profile during a charging protocol incorporating an example of a method according to the invention. DETAILED DESCRIPTION

[0025] Figure 1 is a block diagram illustrating the steps of an example of method 100 for monitoring the SOH (State of Health) of an electric battery. The optional steps of the example of method 100 are indicated by a dashed rectangle.

[0026] The method 100 can be implemented by a computer, a processor, or a microprocessor. For example, the method 100 can be implemented by a microprocessor included in a management system for one or more electric batteries of an embedded system such as an aircraft. The management system preferably has the structure of a computer (in this case, an onboard computer) and / or a computer. It comprises an electronic circuit (in one or more parts) equipped with at least one non-volatile memory and a processor for executing logical operations. It may also include one or more other memories, of the random access memory (RAM) type or another type, and one or more other processors. The management system may also include measurement means: • the current applied to the electric battery or batteries, • the temperature of the electric battery(ies), and • the voltage of the electric battery or batteries.

[0027] By "computer-implemented," it is meant that the steps, or virtually all of the steps, are executed by at least one computer or processor or other similar system. Thus, steps are carried out by the computer, possibly in a fully automatic or semi-automatic manner. In some examples, the triggering of at least some of the steps of the process can be achieved through user-computer interaction. The level of user-computer interaction required may depend on the level of automation planned and weighed against the need for implement the user's wishes. In examples, this level can be defined by the user and / or predefined.

[0028] A typical example of computer implementation of a process consists of executing the process with a system adapted for this purpose. The system may include a processor coupled with memory and a graphical user interface (GUI), with a computer program comprising instructions for implementing the process being stored in memory. The memory may also store a database. Memory is any hardware adapted for such storage, possibly comprising several distinct physical parts.

[0029] A first step 110 of the process 100 comprises the initial charging of the battery. The term "initial" simply means, in this application, that this charge is the first charge of the process 100. The process 100 may therefore include at least one other charge, called a supplementary charge or second charge. This initial charge 110 is carried out by applying a constant current. The initial charge stops when the battery voltage is equal to, or greater than, a predetermined first stop voltage U.

[0030] In one example, the first predetermined stop voltage U is between 3.5 and 4.1 volts, and preferably between 3.75 and 3.85 volts.

[0031] A second step 120 of the process 100 includes putting the battery to rest. This rest period must be long enough so that two conditions are met during or at the end of this rest period. The first condition is that the battery voltage is equal to, or sufficiently close to, the open-circuit voltage (OCV) of the battery. A battery voltage sufficiently close to the OCV of the battery is a voltage equal to the OCV of the battery with a difference of less than XX volts. The second condition is that the battery temperature is stable, i.e., that it no longer varies over time or at least that the temperature variation is less than 0.5°C per hour.

[0032] In an example, compatible with the previous example, the battery standby time 120 is equal to or greater than one hour.

[0033] A third step 130 of the process 100 comprises obtaining a battery indicator. The battery indicator may be: • the open circuit voltage (OCV) of the battery, expressed in volts, or • the DC resistance DCR, for “Direct Current Resistance” in English, of the battery, expressed in milliOhms (mQ).

[0034] The term “obtaining” means, in this application, receiving and / or measuring and / or calculating.

[0035] As a reminder, DC resistance DCR is the inherent static resistance of the conductor, i.e., the resistance of the battery when a certain amount of direct current is applied to the battery. DC resistance DCR is calculated according to Ohm's law. DC resistance DCR is the resistance calculated after a predefined interval At. For example, we can speak of a DCR of 10s for a DC resistance DCR calculated 10 seconds after the current has been applied.

[0036] A fourth step 140 of the process 100 comprises obtaining a battery model that takes the battery indicator as input and provides the battery's state of health (SOH) as output. In an example consistent with the preceding examples, the model may include a function that defines the battery's SOH as a function of the battery indicator. For example, a function f that yields an equation of the following form:

[0037] SOH = ^indicator, T, I)

[0038] With: • indicator, the battery indicator • T, the battery temperature, in °C, and • I, the intensity of the electric current applied to the battery, in amperes.

[0039] In this example, the equation SOH = f(indicator, T, l) can therefore be fitted to experimental data, i.e., to the battery temperature T and the current I applied to the battery. For example, it is possible to perform additional measurements of the battery temperature and the current applied to the battery during step 130. With these additional measurements, it is possible, for example, to recalibrate the model according to the temperature at which charging is carried out and according to the current applied to the battery during charging.

[0040] In one example, to obtain the battery model taking the battery indicator as input and providing the battery's state of health (SOH) as output, it is possible to age a set of calibration batteries and perform a set of measurements during the aging of this set of calibration batteries in order to establish equations SOH = f(OCV)^^ SOH = f(DCR). The calibration batteries are similar to the battery for which method 100 is implemented. The term "similar" means that the calibration batteries have open-circuit voltage, internal impedance, capacity, and chemical characteristics identical to the battery whose state of health is monitored with method 100. The set of calibration batteries can include between 10 and 100 calibration batteries. The aging of each calibration battery is carried out under different aging conditions. For example, the batteries Calibration batteries are aged by being subjected to electrical stress. This aging process is called cycling aging. For this type of aging, the aging conditions can vary depending on the ambient temperature, the charge and / or discharge current, the average state of charge during the cycles, and the depth of discharge during each cycle. During the aging of the calibration batteries, intermediate tests may be performed. These intermediate tests may include: • Measurement of capacity during continuous discharge • the application of a two-stage charging protocol comprising by for example steps 110, 120, 130 and 160.

[0041] These tests therefore make it possible to generate the battery model, taking the battery indicator as input and providing the battery's state of health (SOH) as output by establishing the equations SOH = f(OC V) and / or SOH = f(DCR). Once the battery model has been obtained through the tests, it can obviously be stored and reused many times to monitor the SOH of a battery similar to the calibration batteries. In other words, step 140 can consist of generating the battery model, for example, when no battery model has been previously generated, or simply of receiving this battery model when it has been previously generated.

[0042] In a first implementation mode, the indicator used is the open-circuit voltage OCV. In this first implementation mode, the model takes as input the open-circuit voltage OCV obtained in step 130 and provides as output the state of health (SOH) of the battery.

[0043] In an example of this first implementation mode, the model may include the following linear equation:

[0044] SOH(OCV) = 222^ OCV+m2

[0045] With: • OCV, the open-circuit voltage OCV obtained in step 130 . 222 j 2222, the coefficients to be determined.

[0046] These coefficients ffli can be determined when the battery model is generated, for example using the example described above, using an optimization algorithm such as the least squares method in order to fit the equation to the experimental data obtained.

[0047] Figure 2 is a graph illustrating an example of the evolution of the state of health (SOH) of a battery, represented on the vertical axis and expressed as a percentage of the initial maximum capacity (Cinitiale) of the battery, as a function of the circuit voltage. open OCV obtained in step 130. Thus it is possible to observe on [Fig.2] that the relationship between these two variables is quasi-linear.

[0048] In a second implementation, the indicator used is the DC resistance DCR. In this second implementation, the model takes as input the DC resistance DCR calculated in step 130 and outputs the battery's state of health (SOH). In this second implementation, obtaining the DC resistance DCR is preferably performed at the beginning of a supplementary battery charge, carried out, for example, in step 160 of process 100.

[0049] In an example of this second implementation mode, the model may include the following second-order polynomial equation:

[0050] SOH(DCR) = DCR 2 +n2*DCR + n3

[0051] With: • DCR, the DC resistance DCR obtained in step 130

[0052] nl, -¾ n3 ies coefficients to be determined.

[0053] These coefficients nl, ^3 can be determined when the battery model is generated, for example using the example described above, using an optimization algorithm such as the least squares method in order to fit the equation to the experimental data obtained.

[0054] In this second implementation mode, the DC resistance DCR can be calculated as follows:

[0055] ^^Rm- Kt)

[0056] With: • t, the present moment, * 'c constant current applied to the battery • At t, the time interval between the present instant t and the start of the application of the current / (f).

[0057] It is worth noting that, generally, the DC resistance DCR is calculated for short time periods, ranging from 1 to 60 seconds. In the equation presented above, the variation of the open-circuit voltage OCV is neglected, which is reasonable for small variations in the state of charge SOC of the battery, and therefore for a short time interval A t, i.e., less than or equal to 60 seconds, for example.

[0058] When the state of charge (SOC) of the battery varies significantly, the following equation is preferable:

[0059] _ I

[0060] Thus, when the battery's state of charge (SOC) varies significantly, the battery's SOC can be obtained beforehand, for example, by using a battery SOC estimator. The battery's SOC can then be used to determine the open-circuit voltage (OCV) from a pre-established map of the OCV as a function of the battery's SOC, temperature, and SOH. It should be noted that other methods for determining the OCV could be considered. Figure 3 is a graph with the battery's SOH, expressed as a percentage of the battery's initial maximum capacity (Cjnjtjaie), on the vertical axis and the DC resistance (DCR), expressed in milliohms (mΩ), on the horizontal axis. The graph in [Fig.Figure 3 shows an example of the relationship obtained between the battery's state of health (SOH) and the DC resistance (DCR) calculated for three time intervals of 1 second, 10 seconds, and 30 seconds.

[0061] A fifth step 150 of the process 100 includes determining the SOH state of the battery by providing the model obtained in step 140 with the indicator obtained in step 130. For example, if the model includes a function defining the SOH state of the battery as a function of the battery indicator, the SOH state of the battery is obtained by evaluating the function f with the value obtained in step 130 of the battery indicator.

[0062] An optional sixth step 160 of the method 100 includes a supplementary charge of the battery until a second predetermined voltage Umax and / or a predetermined state of charge is reached. The second predetermined voltage Umax is then higher than the first predetermined voltage Umax. Alternatively or in combination, the supplementary charge 160 is carried out for a predetermined duration, for example, between 1 and 60 seconds.

[0063] In one example, consistent with the previous examples, the second predetermined maximum voltage U max is less than the maximum permissible voltage that can be supplied by the battery manufacturer.

[0064] In an example consistent with the preceding examples, method 100 is used in a battery charging protocol. This charging protocol can be useful from a logistical point of view. For example, when the battery is used to power an eVTOL (electric Vertical Take-Off and Landing) aircraft, the aircraft may perform several missions in a single day, interspersed with relatively rapid partial charges, and remain idle for a few hours overnight. To prevent premature battery aging, the ideal is to store the battery at a state of charge (SOC) of approximately 30%, or even 50%. A state of charge that is too low or too high can accelerate the Battery degradation. Thus, with this charging protocol, the battery maintains an average state of charge (SOC) for most of its rest time and is only fully charged a few minutes or even hours before its first mission of the day.

[0065] Figure 4 is a graph illustrating an example of the battery current and voltage profile during this charging protocol. In Figure 4, the horizontal axes represent time, in hours, and the vertical axis of graph 401 represents the current intensity, expressed in amperes, applied to the battery, while the vertical axis of graph 402 represents the battery voltage, expressed in volts. In Figure 4, the aircraft performs six missions between the 24th and 30th hours and six more missions between the 48th and 54th hours. The initial charging step 110, represented by rectangle 403 in Figure 4, is carried out at the 32nd hour until the first predetermined stop voltage U, denoted 405 in Figure 4, is obtained. The additional charging step 160 is carried out at the 46th hour, materialized by rectangle 404 on [Fig.4].

[0066] The optional seventh step 170 involves modifying the battery's operating conditions based on the state of health (SOH) determined in step 150. For example, when the determined SOH 150 is below a predetermined SOH value, for example, 70% or 80%, modifying the battery's operating conditions may involve replacing the battery with a new one. The battery with a determined SOH below a predetermined SOH value may, for example, be used in so-called second-life applications. In a second example, modifying the battery's operating conditions may involve charging the battery, for example, in step 160, at a current less than or equal to a maximum current, the maximum current being determined from the determined SOH.

[0067] It is also possible to note that the method for determining the state of health SOH of an electric cell can be used as an input for methods of determining other parameters of the electric cell such as, for example, the state of charge SOC of the electric cell, the state of energy SOE of the electric cell or the state of power SOP of the electric cell.

[0068] Unless otherwise specified, the same element appearing on different figures has a unique reference.

Claims

Demands

1. A method (100) for monitoring the state of health (SOH) of a battery, comprising the steps of: - initial charging (110) of the battery until a predetermined first stop voltage (U) of between 3.5 and 4.1 volts is reached, the initial charging of the battery being carried out by applying a constant current, - resting (120) of the battery for a suitable duration such that at the end of the resting period: • the battery voltage is equal to, or sufficiently close to, the open-circuit voltage (OCV) of the battery, and • the battery temperature is stable, - obtaining (130) a battery indicator from among: • the open-circuit voltage (OCV) of the battery, • a DC resistance (DCR) of the battery, - obtaining (140) a battery model taking as input the battery indicator and providing as output the state of health (SOH) of the battery,and - determination (150) of the SOH state of the battery by providing the obtained model (140) with the indicator obtained (130).

2. Method (100) according to claim 1, further comprising an additional charge (160) of the battery: - for a predetermined time, and / or - until a second predetermined voltage U max is reached, and / or - a predetermined state of charge.

3. A method (100) according to claim 1 or 2 further comprising a modification (170) of the battery operating conditions based on the determined SOH state of health (150), the modification (170) of the battery operating conditions comprising: - when the determined SOH health status (150) is below a predetermined health status threshold value, replace the battery with a new battery, and / or - charge the battery, during additional charging (160) at a current intensity less than or equal to a maximum current intensity, the maximum current intensity being determined from the determined SOH health status (150).

4. A method (100) according to any one of the preceding claims wherein the rest period (120) of the battery is equal to or greater than one hour.

5. Method (100) according to any one of the preceding claims wherein the battery indicator is the open circuit voltage OCV of the battery.

6. Method (100) according to any one of claims 2 to 4 wherein: - the battery indicator is the DC resistance DCR of the battery, and - obtaining (130) the DC resistance DCR of the battery is carried out at the beginning of the additional charging (160) of the battery.

7. An electrical cell management system comprising means for implementing the method according to one of the preceding claims.

8. Aircraft comprising a battery and a battery management system according to the preceding claim.

9. A computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out a method according to any one of claims 1 to 6.

10. A computer-readable data carrier on which the computer program according to claim 9 is recorded.