Method for identifying the onset of accelerated state-of-health degradation in a rechargeable battery pack

Abstract

A method (1) for identifying the onset of accelerated degradation of the state of health of a rechargeable battery pack, the battery pack being charged using a charging method comprising a dynamic charging phase and a passive charging phase, wherein during the passive charging phase the onset of accelerated degradation of the state of health of the rechargeable battery pack is identified (13) based on a linear function representing the progression of increasing degrees of duration of the passive charging phase, these degrees being calculated based on the accumulated discharge energy of the battery pack.

Description

Technical Field

[0001] The invention relates to a method for identifying the onset of accelerated degradation of the health state of a battery, in particular a Li-ion battery.

[0002] More particularly, the present invention relates to the management of batteries for electric or hybrid motor vehicles. Background Art

[0003] One of the main challenges in the development of electric vehicles is the durability of lithium-ion batteries, as users must be guaranteed a minimum range throughout the vehicle's use.

[0004] Typically, the degradation of a lithium-ion battery's state of health (SOH) occurs gradually. However, certain usage conditions can cause this degradation to accelerate unexpectedly. This type of degradation, also known as a sudden battery drain, limits the vehicle's range in ways that are detrimental to the user.

[0005] However, this type of degradation is not considered in empirical models for estimating battery life, which are often implemented on motor vehicles and calculate SOH based on capacity loss and / or other performance indicators using linear or power laws.

[0006] Therefore, the inability to predict sudden battery degradation in real time, especially in electric vehicles, is a known problem.

[0007] The prior art includes in particular patent document FR 2966250 A1, which describes a method for estimating the state of health of a battery by means of environmental measurements and the actual capacity of the battery. However, this document does not allow the onset of accelerated degradation of the state of health to be identified.

[0008] Therefore, a solution is needed that allows for faster identification of the onset of accelerated battery state of health degradation. Summary of the Invention

[0009] To this end, a method is provided for identifying the onset of accelerated degradation of the state of health of a battery that is charged in a dynamic charging phase and subsequently in a passive charging phase, in particular at a constant voltage or at a low current or even at a low power.

[0010] The identification method implements the following steps:

[0011] -For each charging of the battery, obtaining the accumulated discharge energy of the battery in the previous discharge stage;

[0012] - during each charging of the battery, measuring the duration of the passive charging phase to reach a predetermined state of charge value;

[0013] - after each charge of the battery, calculating the extent to which the measured duration of the passive charging phase to reach said predetermined state of charge value increases relative to a reference duration of the passive charging phase to reach said predetermined state of charge value; and

[0014] - determining a function representative of the progression of said increasing degree of passive charging duration as a function of the accumulated discharged energy of the battery; and

[0015] The onset of an accelerated degradation of the battery's state of health is detected when the calculated degree of increase no longer progresses according to the function.

[0016] Therefore, the starting point of accelerated degradation can be identified in a relatively simple and robust manner. This indicator may be used for diagnostic purposes to better manage electrified vehicles. The estimation is based on charging data characteristics that are relatively easy to collect by the battery's computer, making this method suitable for implementation in motor vehicles.

[0017] Advantageously and without limitation, the function representing the progression of the degree of increase in the passive charging duration as a function of the accumulated discharged energy is linear. Thus, the onset of accelerated degradation of the battery's state of health can be identified when the calculated progression of the degree of increase no longer follows the linear progression of the determined linear function. This is a relatively reliable and relatively easy criterion to determine, in particular by an onboard computer.

[0018] Advantageously and without limitation, the identifying step includes determining a limit increase degree and deriving a limit cumulative discharge energy of the battery corresponding to the limit increase degree using the function. Furthermore, when the cumulative discharge energy of the battery during the previous discharge reaches or exceeds the limit cumulative amount, the onset of accelerated degradation of the battery's health state is identified. Thus, the onset of accelerated degradation can be identified in advance, or in other words, predicted. This may be particularly useful for preventive maintenance.

[0019] Advantageously and without limitation, the reference duration of the passive charging phase is calculated based on the average duration of the passive charging phases of multiple charging at the beginning of the battery life. Therefore, the reference duration can be defined relatively simply and with particularly low computational effort.

[0020] Advantageously and without limitation, the linear function is obtained by fitting a straight line to a plurality of calculated increasing degrees as a function of the accumulated discharge energy of the battery. Thus, a particularly accurate linear function can be obtained that is relatively easy to calculate.

[0021] According to an advantageous alternative, the linear function is a pre-calculated function. This significantly reduces the number of calculations to be performed by the on-board device.

[0022] The present invention also relates to a method for charging a battery, comprising the identification method described above.

[0023] The invention also relates to a device for identifying the onset of accelerated degradation of the state of health of a battery, said battery being charged in a dynamic charging phase and subsequently in a passive charging phase.

[0024] The identification device includes:

[0025] - means for obtaining the accumulated discharge energy of the battery during these discharge phases;

[0026] - means for measuring the duration of each passive charging phase;

[0027] - means for calculating the extent to which the measured duration of each passive charging phase increases relative to a reference duration; and

[0028] - means for determining a function representative of the progression of the increasing degree of passive charging duration in relation to the accumulated discharged energy of the battery; and

[0029] - means for identifying the onset of accelerated degradation of the battery's state of health when the calculated degree of increase no longer progresses according to said function.

[0030] The invention also relates to an electrical component comprising: a battery; a device for managing the charging of said battery, adapted to control the recharging of the battery according to a method comprising a dynamic charging phase and a passive charging phase; and a device as described above for identifying the onset of accelerated deterioration of the battery's state of health.

[0031] The invention also relates to a motor vehicle comprising an electrical assembly as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Other characteristics and advantages of the invention will become apparent on reading the two particular embodiments of the invention given below, given by way of indication and without limitation, with reference to the accompanying drawings, in which:

[0033] [ Figure 1 ] is a flow chart of a method of the present invention according to a first embodiment of the present invention;

[0034] [ Figure 2 ] is a flow chart of a method of the present invention according to a second embodiment of the present invention;

[0035] [ Figure 3 ] is a graphical representation of various manifestations of the battery state of health and its internal resistance (DCR) in relation to the total cumulative energy exchanged during cycling of a Li-ion battery cell designed for electric vehicle applications;

[0036] [ Figure 4 ] is a representation of the percentage of charge time elapsed during the dynamic phase and the passive phase, and the total charge time percentage, versus the cumulative discharge energy during cycling of a Li-ion battery cell designed for electric vehicle applications; and

[0037] [ Figure 5 ] is a graphical representation of the relationship between the increase in time spent in the passive charging phase and the cumulative discharge energy during cycling of a Li-ion battery cell designed for electric vehicle applications. DETAILED DESCRIPTION

[0038] According to the first embodiment of the present invention, referring to Figure 1 and Figures 3 to 5 , a method for identifying the onset of accelerated degradation of the state of health (abbreviated as SOH in the remainder of this description) of a battery of a motor vehicle is implemented.

[0039] In a motor vehicle, the method 1 , 1 ′ according to the invention is carried out by a computer, for example an on-board computer when the identification is carried out automatically, or an external computer when the identification is carried out, for example during a maintenance phase.

[0040] For this purpose, the on-board computer communicates with or is identical to the computer used to manage the charging of the motor vehicle battery.

[0041] The computer can be selected from computers known to those skilled in the art, as long as the computer includes a computing device, a storage device, and a communication device required to implement the method.

[0042] The state of health of a battery can be defined in two different main ways.

[0043] On the other hand, the state of health can be defined based on the remaining capacity (abbreviated as SOHQ) of the battery cell according to the following equation:

[0044] [Math1]

[0045]

[0046] in,

[0047] Q MOLx : Capacity (Ah) when discharged from 100% SOC to 0% SOC at a given C-rate at the middle of the battery's life (MOLx).

[0048] Q BOL : Capacity (Ah) when discharged from 100% SOC to 0% SOC at a given C-rate at the beginning of the battery's life (BOL).

[0049] C rate (in h-1 The C-rate is a value well known to those skilled in the art and is a measure of the discharge rate of a battery. The C-rate is defined as the ratio between the discharge current and the theoretical current draw, according to which the battery would deliver its nominal capacity in one hour.

[0050] The state of health can also be defined based on the remaining energy (abbreviated as SOHE) of the battery according to the following equation:

[0051] [Math2]

[0052]

[0053] in,

[0054] E MOLx : Energy (Wh) discharged from 100% SOC to 0% SOC at a given C-rate at MOLx.

[0055] E BOL : The energy (Wh) discharged from 100% SOC to 0% SOC at a given C-rate at the beginning of the battery's life.

[0056] However, reference Figure 3 It will be noted that the variations of these two SOHs: SOHQ and SOHE are essentially similar during battery health degradation, and in this regard, the onset of such degradation can be well identified by calculating either of these two SOHs.

[0057] Reference again Figure 3 It will be further noted that the accelerated degradation of the battery SOH is accompanied by a large increase in resistance.

[0058] Although resistance is a likely relevant indicator of a sudden loss of power, reliable resistance measurements are not always possible in practice.

[0059] Therefore, the method 1 according to the present invention aims to estimate the onset of accelerated SOH degradation based on changes in battery charging characteristics.

[0060] The term "start" or "starting point" is understood to refer to the time at which the onset of accelerated battery state of health degradation is identified.

[0061] Therefore, the term "start" should not be understood as the precise time when a specific event causes battery damage, but rather a relatively short time or period within the life of the battery at which a significant acceleration in battery health degradation is identified or detected.

[0062] Typically, for Li-ion batteries designed for electrified vehicles (electric vehicles (EV) or plug-in hybrid electric vehicles (PHEV)), charging can be divided into two consecutive phases:

[0063] a dynamic phase, also called phase 1, in which one or more power peaks and / or current peaks are applied until a limiting state of charge (SOC) is reached, which forms a dynamic phase stop condition, and / or until a limiting voltage is reached;

[0064] The passive phase, also known as phase 2, is where passive charging is applied until a high energy level is reached. In the passive phase, a constant voltage (CV) is applied for a defined time, or multiple low current peaks and / or low power peaks may be applied.

[0065] The dynamic phase is affected by the above reference Figure 3 Describes the effect of increased battery resistance after degradation.

[0066] refer to Figure 4 This increase will generate polarization during charging and thus reach the limit state of charge and / or limit voltage at which the dynamic phase stops more quickly. In contrast, the passive phase becomes longer.

[0067] In the case of batteries used in electrified vehicles, it is relatively rare for a vehicle user to fully discharge the battery, so when the battery begins to charge, the state of charge is typically non-zero but unpredictable because it depends on the vehicle's usage prior to the current charge.

[0068] In this sense, it is difficult to track the evolution of the dynamic phase during battery aging, since the time spent in the dynamic phase depends on the starting value of the state of charge.

[0069] However, users generally tend to charge the battery to its maximum allowed state of charge. Therefore, the passive phase seems more likely to be completed more regularly.

[0070] The method 1 according to the invention therefore consists in identifying the onset of accelerated battery degradation on the basis of a change in the characteristic charge value.

[0071] As mentioned above, these values ​​may advantageously be the time taken to charge during the passive phase, or the capacity and / or energy recorded for the passive phase.

[0072] The onset may be identified based on just one of these parameters or a combination of these parameters.

[0073] Thus, in a first step of method 1 , for each charge of the battery, the duration of the constant voltage passive charging phase is measured 10 .

[0074] Next, for each charge of the battery, the extent to which the duration of the charging phase has increased relative to the previous charge is calculated 11 .

[0075] By way of example, refer to Figure 5, it can be seen that as the battery ages, the constant voltage charging time in the passive charging phase increases (here expressed as a percentage). Initially, even if the SOH of the battery cell decreases, the constant voltage duration in the passive phase does not increase (see Figure 3 ).

[0076] Next, a linear function representing the progression of the calculated degree of increase is determined 11 .

[0077] The onset of an accelerated degradation of the battery state of health is detected 13 when the calculated progression of the degree of increase no longer follows a linear progression 12 .

[0078] refer to Figure 5 , it can be seen that the percentage of constant voltage time increases linearly with 41.

[0079] In this example, the linear growth function for increasing degrees of constant voltage time is in the form of f(x)=0.006x-0.907; where x is the accumulated discharge energy of the battery (in kWh).

[0080] The linear function is obtained by fitting a straight line to at least two, preferably more than two, calculated increments.

[0081] There are many methods for fitting a straight line to a data set, such as the method of least squares, Mayer's method, or even linear regression.

[0082] The straight line obtained by fitting (often called a trend line) can then be recalculated at each charge, especially during the first recharges where incremental growth is observed, for example, the first 2 to 10 recharges after the incremental growth begins, as this is likely the stage of linear growth.

[0083] According to an alternative embodiment, the linear function may be pre-calculated based on the expected performance of the battery.

[0084] Any other method of defining linear functions can be implemented, such as machine learning algorithms.

[0085] Thus, when the percentage increase in constant voltage time deviates from its linear growth 41 region, degradation is accelerating 42. Thus, when the percentage increase in constant voltage time deviates from the linear growth region, an indication of accelerated battery state of health degradation is defined.

[0086] A departure from the linear growth region is detected when the value deviates from the calculated linear function by more than a predefined limit.

[0087] In this example, the degree of increase in the constant voltage time during the passive phase is expressed as a percentage increase.

[0088] This percentage increase corresponds to the ratio between the duration of the passive phase in the previous charge and a reference duration of the passive phase (e.g., the average duration of the passive phase in the first recharge of the battery), or corresponds to a predetermined value corresponding to a theoretical or nominal duration. However, the present invention is by no means limited to this specific definition, and any other relevant degree unit can be specified and measured.

[0089] According to the second embodiment of the present invention, referring to Figure 2 as well as Figures 3 to 5 , the prediction of the onset of accelerated degradation is calculated based on the characteristic changes during the passive charging stage.

[0090] In a step preceding the identification step 23 of the method 1 ′ according to this second embodiment, a limit increase degree is determined 21 , for example in experiments or by simulations, beyond which it is likely that degradation is accelerating.

[0091] Thus, by way of example, in Figure 5 , it was observed that the accelerated degradation started after the constant voltage time increased by substantially 150%.

[0092] Using this limit increase value, the starting point of acceleration of degradation can be estimated based on the slope of the linear function 41 calculated according to the first embodiment.

[0093] Currently, in electric vehicles or plug-in hybrid electric vehicles, a cumulative discharge energy value as a function of operating time (also referred to as the cumulative discharge energy of the battery) is regularly recorded, which is correlated with the vehicle usage time.

[0094] In the identification step 23, since the linear function 41 is known, a solution 231 can be found to calculate to which accumulated discharge energy the limit increase value corresponds, so that the limit accumulated discharge energy can be determined 232 in advance and the user is warned in advance of the risk of accelerated battery degradation starting.

[0095] exist Figure 5 In the example of , this cumulative value is reached when the following conditions are met (the value of x is calculated using the straight line function defined in the first embodiment):

[0096] x = (1.5 + 0.907) / 0.006.

[0097] In other words, the cumulative value is basically 401.16 kWh.

[0098] Furthermore, in the method according to the present invention, the identification of the first embodiment and the identification of the second embodiment may be combined, so that the start of a possible acceleration may be identified in advance using the method according to the second embodiment, but real-time identification is still performed in order to provide a warning to the user in the event that acceleration occurs earlier than predicted.

[0099] Furthermore, although less advantageous because dynamic phases are more often incomplete, it is conceivable to implement the invention by applying the described method to dynamic phases rather than passive phases.

Claims

1. A method (1, 1') for identifying the onset of accelerated degradation of a battery's state of health, The battery is charged during a dynamic charging phase and subsequently during a passive charging phase; It is characterized by: The identification method (1, 1') implements the following steps: - for each charge of the battery, obtaining (9) the accumulated discharge energy of the battery in the previous discharge stage; - during each charging of the battery, measuring (10) the duration of the passive charging phase to reach a predetermined state of charge value; - after each charge of the battery, calculating (11) the extent to which the measured duration of the passive charging phase to reach said predetermined state of charge value increases relative to a reference duration of the passive charging phase to reach said predetermined state of charge value; and - determining (12) a function (41) representing the progression of said increasing degree of passive charging duration as a function of the accumulated discharged energy of the battery; and - when the calculated degree of increase no longer progresses according to the function, it is recognized that (13, 23) The battery health state begins to deteriorate at an accelerated rate.

2. The identification method (1) according to claim 1, characterized in that The function representing the progression of increasing degrees of passive charging duration as a function of accumulated discharged energy is linear.

3. The identification method (1') according to claim 1 or 2, characterized in that The identification step (23) includes determining (21) a limit increase degree, and deriving therefrom a limit cumulative discharge energy of the battery corresponding to the limit increase degree using the function; and is characterized in that when the cumulative discharge energy of the battery during the previous discharge stage reaches or exceeds the said limit cumulative discharge energy, the beginning of accelerated degradation of the battery health state is identified (232).

4. The identification method (1, 1') according to claim 1 or 2, characterized in that The reference duration of the passive charging phase is calculated based on an average of durations of passive charging phases of multiple charging operations at the beginning of the battery life.

5. The identification method (1, 1') according to claim 2, characterized in that The linear function is obtained by fitting a straight line to a plurality of calculated increasing degrees as a function of the accumulated discharge energy of the battery.

6. The identification method (1, 1') according to claim 2, characterized in that The linear function is a pre-computed function.

7. A method for charging a battery, comprising the identification method according to any one of the preceding claims.

8. A device for identifying the onset of accelerated degradation of the state of health of a battery, the battery being charged in a dynamic charging phase and subsequently in a passive charging phase; It is characterized by: The identification device includes: - a device for obtaining the accumulated discharge energy of the battery during the discharge phase; - means for measuring the duration of each passive charging phase; - means for calculating the extent to which the measured duration of each passive charging phase increases relative to a reference duration; and - means for determining a function (41) representing the progression of the increasing degree of passive charging duration in relation to the accumulated discharged energy of the battery; and - means for identifying the onset of accelerated degradation of the battery's state of health when the calculated degree of increase no longer progresses according to said function.

9. An electrical component comprising: batteries; a device for managing the charging of said battery, the device being suitable for controlling the recharging of the battery according to a method comprising a dynamic charging phase and a passive charging phase; And the identification device as claimed in claim 8.

10. A motor vehicle comprising the electrical assembly of claim 9.

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