BATTERY AGING MANAGEMENT

Abstract

The present invention relates to a charging strategy for controlling battery aging, wherein a charging current is reduced by a charging current coefficient (6), and said charging current coefficient (6) is defined according to a rule-based approach (9) for determining the battery's health status under actual battery usage. Figure for abstract: (Figure 3)

Description

Title of the invention: Battery aging management Technical field of the invention

[0001] The invention relates generally to the technical field of managing battery charging using an external electrical power source. The batteries in question can be used to supply electrical power to the powertrains of electric or hybrid vehicles.

[0002] The invention relates more specifically to the optimization of battery charging in order to reduce their aging without excessively hindering the performance of the battery charging process.

[0003] The invention relates particularly to the management of rapid charging cycles and regenerative braking.

[0004] In what follows, reference is made to batteries used in battery packs to power either electric vehicles, namely vehicles powered exclusively by electrical energy, or hybrid vehicles, namely vehicles powered by electrical energy and an internal combustion engine. In the remainder of this text, the term battery electric vehicles (BEVs) will be used to refer to both electric vehicles and hybrid vehicles. State of the art

[0005] Battery packs are used at all times, regardless of the battery's age. Battery performance decreases as the battery ages, which has a significant impact on its use, for example, on its ability to store electrical energy. The battery aging process results primarily from so-called "calendar" and "cycling" effects. Consequently, battery aging impacts battery charging and is also influenced by battery charging. Battery health is determined by a combination of cycling and calendar aging effects.

[0006] The calendar effect on battery aging refers to the effects of time on the state of health (SOH) of the battery. Batteries degrade over time, even when they are not being charged or discharged during use. This calendar aging primarily refers to the loss of capacity of a battery that occurs over time, regardless of battery usage. This type of aging is caused by factors such as chemical reactions within the battery, exposure to high temperatures, and gradual changes in the electrodes and electrolytes.

[0007] A charge cycle is the process of charging and discharging a battery. The lifespan of a battery is primarily defined by the number of charge cycles. This is known as the cycling effect on battery aging.

[0008] It has been observed that what is called "Trip and Daily cycling" has a significant impact on the positive electrode (increasing resistance), but no impact on the negative electrode. However, based on observations and tests, a realistic combination of cycling and calendar aging has an impact on both the positive and negative electrodes.

[0009] It is known that, taking into account the cycling effect, regenerative braking and fast charging have a major impact on the aging of battery cells. It is therefore necessary to focus on battery degradation and on a control strategy to optimize energy recovery during regenerative braking and fast charging applications for an aging cell.

[0010] Lithium plating, or Li plating, is one of the main causes of battery degradation. Li plating consists of the formation of metallic lithium on the graphite cathode, which occurs during the battery charging process when the temperature is low and the charging current is high. Li plating affects not only EV battery packs, but also those in other applications such as energy storage systems, mobility, etc.

[0011] Therefore, it is necessary to define an optimal charging strategy aimed at avoiding Li plating in order to ensure safe battery operation.

[0012] In EVs, fast charging and regenerative braking are two key features for battery performance. However, in this specific battery charging mode, the charging current density must be closely monitored to avoid Li plating. Brief description of the invention

[0013] The present invention aims to provide an improved fast-charging strategy for EVs by avoiding, or at least reducing, lithium plating, as well as during energy recovery in regenerative braking mode. Fast charging can be managed by MAP (maximum current as a function of battery temperature and state of charge, and as a function of temperature and voltage) or by an algorithm.

[0014] To achieve this objective, a charging strategy is proposed according to a first aspect of the invention, allowing control of the aging of a battery, a charging current being reduced by a charging current coefficient, and said charging current coefficient being defined according to a determination, based on rules, of the state of health of the battery, taking into account the actual use of the battery.

[0015] Preferably, this rule-based approach takes into account said actual use of the battery, including cycling aging and / or calendar aging.

[0016] Advantageously, the determination of the health status of the battery is carried out by continuous monitoring of the battery by a battery management system, for example by separating calendar aging and cycling aging to define said charging current coefficient.

[0017] Preferably, said battery management system takes into account a predefined percentage margin on the charging current to integrate regenerative braking and / or during fast charging; and / or a predefined potential margin on the potential of the negative electrode relative to the Li plating potential during fast charging.

[0018] Advantageously, said predefined percentage margin on the charging current for regenerative braking is 10%, said predefined potential margin on said negative electrode during fast charging is 10 mV relative to the Li plating potential; and / or said predefined current margin during fast charging is 10% with no potential margin relative to the Li plating at the negative electrode.

[0019] Preferably, said charging current coefficient is applied during fast charging and / or regenerative braking and said charging current coefficient is the same or different for fast charging and regenerative braking.

[0020] According to a second aspect of the invention, a computer program is proposed for the implementation of the charging strategy to control the aging of a battery, defined above.

[0021] According to a third aspect of the invention, a battery management system is proposed that is capable of controlling the charging of a battery at least in a fast charging mode and / or a regenerative braking mode, said battery management system being configured to reduce the charging current by a charging current coefficient defined according to a charging strategy allowing control of the aging of a battery, as defined above.

[0022] Furthermore, according to a fourth aspect of the invention, a battery pack is proposed comprising a battery having a plurality of battery cells and a battery management system, as defined above.

[0023] In addition, according to a fifth aspect of the invention, a vehicle is proposed that is powered at least partially by electrical energy stored in a battery pack, as defined above.

[0024] Other features and advantages of the invention are highlighted by the description below of non-limiting examples of embodiments of the invention. Brief description of the figures

[0025] The description refers to the accompanying figures, which are also provided as non-limiting examples of embodiments of the invention: - [Fig. 1]: [Fig. 1] illustrates a battery-powered electric vehicle, - [Fig.2]: [Fig.2] shows a table based on a simulation of a model electrochemical, and - [Fig.3]: [Fig.3] shows the interactions between the components of the battery management system.

[0026] For ease of reading, the same reference signs indicate similar or identical features in the figures. Detailed description of the invention

[0027] Figure 1 shows a vehicle electric vehicle (VEV) 1 comprising a powertrain 2 that propels the VEB, a battery pack 3 that stores electrical energy, and a motor control unit 4. The battery pack 3 comprises a battery with a plurality of assembled battery cells and a battery management system (BMS) controlling battery regeneration, namely the different battery charging modes. The battery pack 3 supplies the powertrain 2 with the necessary electrical energy, determined by the vehicle system 4, to enable the powertrain 2 to provide the engine torque required to achieve the desired acceleration. When the speed of the VEB 1 needs to be reduced, deceleration is desired, and the BMS switches to a regenerative braking mode to convert the mechanical energy of the VEB 1 into electrical energy to be stored in the battery pack 3.

[0028] As mentioned above, it is necessary to optimize the battery charging process to avoid Li plating. Li plating results primarily from a high charging current density. Therefore, it is necessary to control the charging current intensity. The standard for reducing the charging current in the case of aging batteries consists of using a reduction coefficient for a given state of health (SOH_C) of the battery and defining the charging current intensity, for example during fast charging, for a given state of charge (I_FC_SOC) as follows: I_FC_SOC = I_FC_BOL * SOH_C where SOH_C = 1 for a new battery and SOH_C = 0 for an old battery if no energy can be recovered from it, and I_FC_BOL represents the intensity of the charging current in fast charging mode at the beginning of the battery's life (BOL).

[0029] The invention defines an optimal current reduction strategy using a realistic scenario based on a rule-based approach, which separately considers the impact of cycling and calendar aging. In particular, it proposes an optimal current reduction strategy for regenerative braking and fast charging based on the combined cycling and calendar effects resulting from actual EV use and, consequently, is suitable for use by any driver. The strategy can vary between regenerative braking and fast charging, since regenerative braking mainly consists of short pulses of a few seconds, while fast charging generally lasts from a few minutes to several tens of minutes.

[0030] The methodology of the charging strategy is defined below according to values ​​which are provided only as non-limiting examples intended to improve understanding of the invention.

[0031] Calendar aging and cycling aging can be separated by an integrated open-loop model. Calendar aging is related to the battery's rest time at different temperatures and SOC (or voltage), and cycling aging is related to the Ah cycled and the number of cycles at different depths of discharge (DOD), as a function of temperature and possibly current.

[0032] During battery charging control, the BMS implements margins that are defined to preserve the battery's state of health (SOH) and control battery aging. For example, at the beginning of a battery's life (BOL), a margin on the battery charging current is selected to incorporate regenerative braking and / or fast charging to avoid the lithium plating effect. For example, this charging current margin can be set at 10% for regenerative braking and / or at 10% of the maximum rated current without a potential margin regarding lithium plating at the negative electrode during fast charging. Furthermore, during fast charging, a potential margin on the negative electrode of the cells is predefined relative to the lithium plating potential to lower the ceiling of the charging current density.For example, this potential margin can be set at 10 mV relative to the Li plating potential. The BMS takes these margins into account to allow the charging power to be delivered to the battery within the voltage limitations. The charging currents are known to those skilled in the art and are generally provided by battery manufacturers based on the battery's chemistry and structure. The Li plating also depends on the specific chemistry, structure, and characteristics of the battery. Therefore, those skilled in the art are informed of this once the battery is manufactured.

[0033] The BMS continuously monitors the various operating parameters of the battery pack, such as voltage, current, and temperature. This allows the BMS to assess the battery's state of health (SOH) in terms of increasing resistance and decreasing capacity. In fact, the BMS monitoring encompasses both the effects of calendar aging and those of cycling aging.

[0034] Next, based on the estimated SOH, the BMS defines the SOH_C, which is applied to the charging current during the battery charging phases, particularly during fast charging and regenerative braking. This SOH_C is unique because it is defined according to the actual usage by the driver and the battery's aging state or the resulting battery SOH. By effectively reducing the charging current, the SOH_C ensures, on the one hand, the absence of lithium deposition on the graphite cathode, thus preventing Li plating. On the other hand, by avoiding an excessively high charging current density based on the SOH, the SOH_C allows for optimal charging conditions at each battery age, which helps to preserve the battery's long-term performance.

[0035] The SOH_C is determined by compiling data from different aging tests, with particular emphasis on the cycling effect and the calendar effect, defining a correlation between the SOH_C and the SOH of the battery.

[0036] Aging tests were conducted according to different protocols, such as purely calendar aging, combined cycling and calendar aging, and what is called "Trip and Daily" cycling, which includes both travel and daily cycling. The same capacity loss was observed for both types of tests (purely calendar aging, combined cycling and calendar aging under partial DOD, also called "Trip and Daily" cycling).

[0037] Based on the results obtained during these tests, it was observed that the current density of the exchange current, which characterizes the insertion kinetics of Li ions inside the graphite of the negative electrode (iO neg), determines the level of Li plating. For different aging protocols, iO decreases approximately by a factor of 1.5 during "Trip and Daily" cycling aging and by a factor of 3 during purely calendar aging for a capacity loss of 5%.

[0038] The difference in battery degradation observed during these tests is mainly due to the different aging effects arising from the calendar effect and the cycling effect, the combination of which will vary depending on the use of the EV by different drivers.

[0039] The reduction in fast charging current therefore depends on calendar aging and specific cycling, and not on overall aging. Otherwise, one would have to consider the worst-case scenario of purely calendar aging (corresponding to a very low vehicle usage), which is detrimental to customers who frequently use their vehicle.

[0040] According to the invention, the generic formula - I_FC_SOC = I_FC_BOL * SOH_C is replaced by I_FC_SOC = I_FC_BOL * f(SOH_C_calendar) * g(SOH_C_cycling)

[0041] Where the functions f and g are evaluated by the 3E test measurements and the electrodes are aged respectively by calendar test and by cycling test.

[0042] The default values ​​are as follows:

[0043] g(SOH_C_cycling) = SOH_C_cycling (0.9 meaning a 10% capacity loss due to aging during cycling)

[0044] f(SOH_C_calendar) = 1 - 3 * (1 - SOH_C_calendar) (0.9 meaning 10% capacity loss due to calendar aging, leading to a reduction in fast charging current of 30% - factor of 0.7). The coefficient 3 is a variable between 1.1 and 5.

[0045] The invention thus constitutes an optimized solution in the case where calendar aging and cycling aging cannot be separated by integrating the significant contribution of calendar aging, which corresponds to the most frequent use of the VEB. See the values ​​in [Fig. 2] below.

[0046] The SOH_C determined according to the different aging effects resulting from cycling aging and calendar aging is applied for the calculation of the actual charging current (I_FC_SOC) at different SOHs of the battery.

[0047] Based on the methodology described above, an electrochemical simulation was also performed to define the SOH_C as a function of the SOC (state of charge). The table presented in [Fig. 2] primarily considers calendar-type tests. This type of test protocol generates additional penalties compared to the prior art, namely a greater reduction in the charging current. However, it better protects the battery from degradation due to Li plating, which would otherwise significantly reduce the battery's lifespan, as observed during the tests.

[0048] Figure 3 illustrates the vehicle system 4 sending data to the BMS 5 concerning the electrical energy required to move the vehicle EB 1. The BMS 5 continuously monitors the various operating parameters of the battery pack 3 to evaluate the battery's SOH. Furthermore, the BMS monitoring encompasses both the calendar aging effect 7 and the cycling aging effect 8. According to a rule-based approach 9, the BMS 5 determines, among other things, the SOH_C 6 to be applied to I_FC_BOL to obtain the I_FC_SOC to be supplied to the battery for charging.

[0049] As indicated at the beginning of the description above, the various aspects of the invention can be implemented in different ways depending on the context of operation. For example, the invention described above is implemented in the context of EVs, namely vehicles powered exclusively by electrical energy stored in batteries. However, it could be implemented similarly in hybrid vehicles, namely vehicles powered both by electrical energy stored in batteries and by an internal combustion engine.

[0050] Of course, the invention is described above as an example of an embodiment. It should be understood that a person skilled in the art may establish different ways of implementing specific aspects of the invention without departing from the scope of the invention as defined in the claims.

Claims

Demands

1. 1 Charging strategy for controlling the aging of a battery, wherein: - a charging current is reduced by a charging current coefficient (6), and - said charging current coefficient (6) is defined according to a rule-based approach (9) for determining the health status of the battery taking into account actual battery usage.

2. 2 Charging strategy for controlling the aging of a battery according to claim 1, characterized in that said rule-based approach (9) takes into account said actual battery use, including: - cycling aging (8), and / or - calendar aging (7).

3. A charging strategy for controlling the aging of a battery according to claim 1 or 2, characterized in that the determination of the health status of the battery is carried out by continuous monitoring of the battery by a battery management system (5), for example by separating calendar aging and cycling aging to define said charging current coefficient (6).

4. 4 Charging strategy enabling control of battery aging according to any one of the preceding claims, characterized in that said battery management system (5) takes into account: - a predefined percentage margin on the charging current to integrate regenerative braking and / or during a fast charge; and / or - a predefined potential margin on a negative electrode potential relative to a Li plating potential during a fast charge.

5. 5 Charging strategy for controlling the aging of a battery according to claim 5, wherein: - said predefined percentage margin on the charging current for regenerative braking is 10%; - said predefined potential margin on said negative electrode during a rapid charge is 10 mV relative to a Li plating potential; and / or - said predefined current margin during a rapid charge is 10% relative to the maximum current evaluated without potential margin relative to a lithium plating potential at a negative electrode.

6. 6 Charging strategy for controlling the aging of a battery according to any one of the preceding claims, characterized in that said charging current coefficient (6) is applied during fast charging and / or regenerative braking and said charging current coefficient (6) is the same or different for fast charging and regenerative braking.

7. 7 Computer program for implementing the charging strategy to control the aging of a battery, which strategy is defined in any one of the preceding claims.

8. 8 Battery management system capable of controlling the charging of a battery in at least one fast charging mode and / or a regenerative braking mode, characterized in that said battery management system (5) is configured to reduce the charging current by a charging current coefficient (6) defined according to a charging strategy enabling control of the aging of a battery according to any one of claims 1 to 6.

9. 9 Battery pack (3) comprising a battery having a plurality of battery cells and a battery management system according to claim 8.

10. 10 Vehicle powered at least partially by electrical energy stored in a battery pack (3) according to claim 9.

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