Method and device for controlling the charge level of a traction battery of an electric vehicle

Abstract

The invention relates to a method for controlling the charge level of a traction battery (2) of an electric vehicle (1) connected to an electricity distribution network (31) via a charger (3) during a stop phase of the vehicle, characterised in that the method comprises: a first step of forced discharge of the battery, carried out in a discharge circuit (5) associated with the battery, at a discharge current calibrated in intensity relative to the nominal capacity of the battery, so as to discharge the battery slowly and completely to its minimum voltage; a step of normal charging of the battery at a set point charging current prescribed by the charger or the battery, so as to charge the battery to its useful charge level.

Description

Technical Field

[0001] The present invention relates to a method and apparatus for regulating the charge level of a traction battery of an electric vehicle. The invention has particular, but not exclusive, application in the field of managing the charge level of lithium-ion (Li-ion) batteries of rechargeable hybrid and electric vehicles. Background Art

[0002] One of the main limiting factors to date for the growth in the use of electric vehicles is the charging time of the battery, especially compared to the time required to fill a fuel tank with an internal combustion vehicle. Therefore, new "fast" or "ultra-fast" charging Li-ion battery technologies have been developed for the automotive industry, allowing charging times to be greatly reduced. However, it has been observed that batteries that allow such fast or ultra-fast charging, i.e. batteries that are able to accept large charging currents, are more susceptible to experiencing a degradation of their characteristics over time. In other words, the ability of the battery to accept fast or ultra-fast charging leads to an increased probability of occurrence of battery aging mechanisms of physico-chemical origin. These mechanisms are promoted at high currents below a certain charging time threshold, resulting in a reduction in battery capacity, which can seriously affect the vehicle mileage. Therefore, fast or ultra-fast charging has a negative impact on the aging of Li-ion batteries and, therefore, on vehicle performance.

[0003] However, in order to solve the challenge of fast charging of batteries while limiting their tendency to age more rapidly, a known solution consists in regulating the level of the charging current as a function of the battery's state of charge and temperature. However, beyond a certain need to reduce charging times, it has been observed that this strategy does not prevent an increased degradation of the battery's capacity compared to slow charging.

[0004] In addition, it has been demonstrated that certain intermediate charge levels or excessive temperatures may be conditions that negatively affect the battery's service life. Patent document FR 2992779 filed by the applicant discloses a method for regulating the charge level of a traction battery of an electric vehicle connected to a distribution network via a bidirectional charger, thereby allowing the battery's service life to be maximized. The method described in this document is based on the principle of limiting the time the battery spends at a too high charge level when it is not in use. Thus, during phases of non-use of the vehicle, when the vehicle's battery is being charged, as much energy as possible is put back into the distribution network, which will maximize the battery's service life, and this energy is then recovered shortly before the vehicle is actually used again. Thus, this method first comprises a step of discharging the battery into the network through a bidirectional charger, reaching an optimal charge level, thereby allowing the battery's capacity loss to be minimized according to the estimated time of non-use of the battery; and then comprises a step of charging the battery to the charge level required for the trip known in advance.

[0005] However, while this approach increases the battery's resistance to calendar aging (i.e., limits the capacity loss when the battery is at rest), it does not address the problem of vehicle range loss, as described above, due to battery capacity degradation caused by successive rapid charging operations, particularly charging operations whose duration is below a critical threshold of the battery's chemical processes.

[0006] Furthermore, in the aforementioned document, the implementation of a step to modify the state of charge level of the battery during periods of non-use, in order to maximize its useful life, requires the use of a bidirectional charger in order to be able to send energy back into the distribution network. This is a relatively complex and expensive device, which constitutes a limitation to the deployment of such a solution. Summary of the invention

[0007] The object of the present invention is therefore to provide a method and a device that are easy to implement and that make it possible to at least partially overcome the problem of capacity loss due to cycling of rapidly charged Li-ion batteries, in particular used as traction batteries for electric vehicles.

[0008] To this end, the invention relates to a method for regulating the charge level of a traction battery of an electric vehicle connected to an electricity distribution network via a charger during a stop phase of the vehicle, characterized in that the method comprises:

[0009] A first step of forced discharging the battery, carried out in a discharge circuit associated with the battery, at a discharge current calibrated in intensity relative to the nominal capacity of the battery, so as to discharge the battery slowly and completely to its minimum voltage;

[0010] The step of normally charging the battery at a set point charging current specified by the charger or the battery to charge the battery to a useful charge level of the battery.

[0011] According to one embodiment, the step of normally charging the battery is preceded by at least one intermediate sequence of charging and discharging the battery, the intermediate sequence sequentially comprising:

[0012] The step of performing limited charging on the battery at a limited charging current less than the set point charging current so as to charge the battery slowly to its maximum voltage; and

[0013] The second step is a forced discharge of the battery, performed in a discharge circuit associated with the battery, at the calibrated discharge current, to again discharge the battery slowly and completely to its minimum voltage.

[0014] Advantageously, the first and second steps of forced discharging the battery are performed in a discharge resistor, the discharge resistor being connected to the terminals of the battery during the first and second steps of forced discharging and being disconnected from the terminals of the battery during the steps of normal charging and constrained charging, the discharge resistor setting the calibrated discharge current.

[0015] Advantageously, said calibrated discharge current is at most equal to one tenth of the value of the nominal capacity of the battery.

[0016] Advantageously, said limiting charging current is at most equal to one tenth of the value of the nominal capacity of the battery.

[0017] Preferably, the set point charging current is suitable for fast charging the battery.

[0018] The invention also relates to a device for regulating the charge level of a traction battery of an electric vehicle connected to an electricity distribution network via a charger during a stop phase of the vehicle, characterised in that the device comprises: a discharge circuit for discharging the battery, electrically connectable to said battery; and a control module designed to selectively connect said discharge circuit to said battery in order to discharge said battery and designed to implement the above-mentioned method.

[0019] Advantageously, the discharge circuit comprises a discharge resistor, each terminal of which is connected to a corresponding terminal of the battery via a switch, the switch comprising a movable contact driven by said control module between two positions for connecting and disconnecting the terminals of the resistor with the terminals of the battery.

[0020] Advantageously, the discharge resistor is integrated into the housing of the battery.

[0021] The invention also relates to an electric vehicle comprising a device as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Other characteristics and advantages of the invention will become apparent when reading the description provided hereinafter of a particular embodiment of the invention given by way of non-limiting indication, with reference to the accompanying drawings, in which:

[0023] - Figure 1 A circuit diagram of an exemplary electric vehicle equipped with the device for regulating the charge level of a battery according to the present invention is shown;

[0024] - Figure 2 are block diagrams illustrating two embodiments of the method of the present invention. DETAILED DESCRIPTION

[0025] Figure 1 A circuit diagram of an exemplary embodiment of the invention is shown in an electric vehicle 1 provided with an electric motor (not shown). The electric motor of the vehicle according to this example is powered by a traction battery 2 of the lithium-ion type, which comprises, in a manner known per se, a battery housing 21 in which a plurality of electrochemical accumulators 22 or cells are arranged in series, comprising a rechargeable electrochemical system designed to provide a nominal voltage.

[0026] The vehicle of this example also includes a battery charger 3. The battery charger 3 is designed to be connected to an electricity distribution network 31 in order to charge the vehicle battery. The battery charger 3 is driven by a control module 4, which includes a computer designed to control the charging of the battery 2 via the electrical network 31 of the charger 3 when the vehicle is stationary and connected to the network 31 for charging.

[0027] In this context, in which the vehicle is stationary and connected to the distribution network 31 for charging, the invention proposes the implementation of a protocol that enables recovery of a portion of the battery capacity lost in particular at the end of a rapid charging phase previously undergone by the battery.

[0028] In particular, it has been observed that this loss of capacity of Li-ion batteries due to successive rapid charging operations may be attributed to parasitic electrochemical reactions (which are exacerbated by the large charging currents), in particular causing lithium metal deposits on the surface of the negative electrode ("Li plating") reacting with the electrolyte in which this negative electrode is immersed. The principle of the present invention is to provide a simple device associated with a specific protocol in order to recover the reversible part of the temporary loss of battery capacity associated with this phenomenon, thereby increasing the service life of the electric vehicle despite the implementation of rapid charging.

[0029] To this end, the device of the invention comprises a discharge circuit 5 associated with the battery 2. This discharge circuit comprises a single discharge resistor 51, which can therefore be easily integrated into the battery housing 21 in which the battery cells 22 are arranged. More precisely, one terminal 51a of the discharge resistor 51 is connected to the positive terminal of the battery 2 via a movable contact 52 of a first switch 54 (such as a relay), and the other terminal 51b of the discharge resistor 51 is connected to the negative terminal of the battery via a movable contact 52 of a second switch 54 (such as a relay). The movable contacts 52 of the first and second switches 54 can adopt two positions, respectively a first “open” position, in which the terminals 51a and 51b of the discharge resistor 51 are not connected to the positive and negative terminals of the battery (such as a relay). Figure 1 In this second "closed" position, the terminals 51a and 51b of the discharge resistor 51 are connected to the positive and negative terminals of the battery.

[0030] The positions of the first and second switches 54 of the discharge system associated with the battery are controlled by a control module 4 designed to determine the positions of the movable contacts 52 of the first and second switches from the first and second positions according to different charging strategies that the invention is able to offer the driver when the vehicle is stationary and connected to the network for charging, as will be described in more detail below.

[0031] The method of the present invention enables to provide at least two strategies, Figure 2 denoted Mode_1 and Mode_2, thereby allowing recovery of the reversible part of the battery capacity lost in particular at the end of the fast charging phase of the battery.

[0032] For both strategies, firstly, in a first step E1, a forced discharge of the battery is controlled. Therefore, in this first step, the discharge resistor 51 is connected between the terminals of the battery by driving the two movable contacts 52 of the relay 54 via the control module 4 so that the two movable contacts are in a closed position. The discharge resistor 51 connected to the battery in this way allows the effective discharge current in the discharge step E1 to be set. According to the invention, the size of the discharge resistor is determined to allow only a very small discharge current to flow, so that the battery can be slowly discharged to its minimum voltage. In order to give a preferred exemplary embodiment, the size of the discharge resistor 51 is determined to adjust the intensity of the discharge current to at most one tenth of the nominal capacity of the battery. This is called discharging at 0.1C, where C is the nominal energy capacity (in Ah) of the battery given by the total discharge in 10 hours. This means that in this discharge step E1, the battery will deliver a current with an intensity of one tenth of the battery capacity over a period of 10 hours. In other words, according to this exemplary embodiment, the battery will be slowly discharged to its minimum voltage over a period of 10 hours.

[0033] Therefore, the first step E1 of slowly and completely discharging the battery makes it possible to recover part of the battery capacity by making partially reversible the "Li plating" phenomenon that occurs during charging, which is an important source of battery degradation and is promoted by the high currents used during fast charging.

[0034] The minimum voltage or stop voltage allows the total discharge threshold of the battery to be determined and allows the battery to be protected. Thus, once the control module 4 detects the minimum voltage by means of voltage sensors arranged at the terminals of the battery cells forming the battery, it disconnects the discharge resistor 51 from the battery by driving the two movable contacts 52 of the relay 54 so as to be in the open position, which allows avoiding excessive discharge of the battery and avoiding negative effects on the battery's service life.

[0035] At the end of the first step E1 of slowly and completely discharging the battery, the method of the invention can be continued in two different sequences corresponding to Figure 2 There are two strategies represented by Mode_1 and Mode_2, which correspond to a first "fast" mode and a second "slow" mode respectively.

[0036] In fast mode, after the first step E1, a control step E2 is performed, which is a normal charging of the battery via the charger 3 connected to the power distribution network. Therefore, during this step, the discharge resistor 51 is disconnected from the battery as described above. This normal charging step is performed at a set point charging current, which is specified by the charger or the battery. In other words, normal charging is charging at a power level limited by the charger capacity or the battery capacity.

[0037] The normal charging step E2 can also be performed using a fast charging scheme (if available, in other words, if a high charging power is available through the network and the charger). In this case, the value of the setpoint charging current is set so as to allow fast charging of the battery, so that the battery can be charged in order to obtain an acceptable charge level as quickly as possible. For example, this charging current can be set to at least 40% of the battery capacity.

[0038] The battery is charged to a desired charge level, such as corresponding to a minimum charge level required to perform a known trip.

[0039] By applying a strategy according to the fast mode, it is possible to charge the battery to the desired charge level immediately after the first slow discharge step E1, including using a fast charging scheme, without compromising the battery life. In particular, in the next phase of charging the vehicle, the application of a preliminary step E1 of slow discharge of the battery according to the invention will make it possible to recover part of the lost battery capacity, as described above.

[0040] As a variant, at the end of the first step E1 of slowly and completely discharging the battery, the method of the invention can continue according to a second mode, called slow mode.

[0041] In this slow mode, step E2 of normal charging of the battery as described above is preceded by an intermediate sequence of charging and discharging the battery.

[0042] Therefore, during this intermediate sequence, first a step E12 is controlled, which is a limited charging of the battery via the charger 3 connected to the distribution network. Therefore, during this step E12, as in the normal charging step E2, the discharge resistor 51 is also disconnected from the battery according to the principle explained above. However, this limited charging step E12 is performed at a limited charging current lower than the set point charging current corresponding to the maximum charging current specified by the charger or the battery, so that the battery is slowly charged to its maximum voltage. According to a preferred exemplary embodiment, the control module 4 adjusts the intensity of the charging current to a level equal to at most one tenth of the nominal capacity of the battery. This means that the charging is limited to 0.1C. This means that, in this limited charging step E12, the battery will receive a current with an intensity of one tenth of the battery capacity over a period of 10 hours. In other words, according to this exemplary embodiment, the battery will be slowly and completely charged to its maximum voltage over a period of 10 hours. When the control module 4 detects the maximum voltage, the slow charging is stopped.

[0043] Next, still in this intermediate sequence, once the battery is fully charged, a second step E12 of forced discharge of the battery is controlled under the same implementation conditions as the first discharge step E1. Thus, the discharge resistor 51 is again connected to the battery so that the battery is slowly and completely discharged at 0.1C until the minimum voltage of the battery is reached.

[0044] At the end of this second slow and complete discharge of the battery carried out in step E12, the discharge resistor 51 is disconnected from the battery and a step E2 of normal charging of the battery is carried out, as described above with reference to the fast mode Mode_1. Thus, during this step E2, charging to the required charge level is provided at a power level limited by the capacity of the charger or the capacity of the battery, possibly using a fast charging scheme if available.

[0045] The slow mode Mode_2 produces the same beneficial effect as the fast mode Mode_1 in at least partially reversing the "Li plating" phenomenon by means of a first step E1 of slowly and completely discharging the battery, which is implemented in both modes. In addition, the intermediate sequence consisting of a step E11 of slowly and completely charging the battery and then a step E12 of slowly and completely discharging the battery after step E1 will re-homogenize the internal charge state of the Li-ion cells of the battery, which avoids excessive stress on certain active areas of the cells and therefore avoids premature degradation of the cells of the battery, thereby obtaining the benefit of recovering most of the battery capacity that may have been lost during the fast charging phase.

[0046] The slow mode Mode_2 is called slow relative to the fast mode Mode_1 because, according to an exemplary embodiment, the slow mode involves an additional 20 hours of immobilization of the vehicle due to the implementation of an intermediate sequence of steps E11 and E12 of slow charging and discharging at 0.1 C between steps E1 and E2. Therefore, the fast mode is particularly useful in the case of charging via charging points deployed in public places where charging and / or parking times may be limited.

[0047] Advantageously, before starting charging, once the vehicle has been stopped and connected to the network for charging, if the driver selects the option provided by the invention to restore the battery capacity according to one or other of the two modes described above, what may be provided is to inform the driver of the time required before his vehicle is usable again. This time required to implement one or other of the two modes will also depend on the state of charge of the vehicle.

[0048] The driver may also choose not to charge the battery according to one or the other of the two modes of the invention that allow recovery of battery capacity if the time required to implement these two modes is incompatible with the driver's constraints. In this case, standard charging is implemented.

[0049] It should also be noted that, during normal use of the vehicle, the battery cannot be discharged slowly and completely as in steps E1 and E12. In particular, while driving, the braking phases cause intermittent charging, which interrupts the discharge and prevents the implementation of a continuous slow and complete discharge. However, such a discharge of the battery is necessary, at least before step E1, in order to allow the implementation of the different strategies for restoring the battery capacity as described above during the charging of the vehicle battery. Therefore, the association of the discharge circuit 5 controlled by the control module 4 with the battery makes possible these slow and complete discharges of the battery required according to the invention.

Claims

1. A method for regulating the charge level of a traction battery (2) of an electric vehicle (1) which is connected to an electricity distribution network (31) via a charger (3) during a stop phase of the vehicle, characterized in that The method includes: a first step (E1): forced discharge of the battery, carried out in a discharge circuit (5) associated with the battery, at a discharge current calibrated in intensity relative to the nominal capacity of the battery to be at most one tenth of a value equal to said nominal capacity, so as to discharge the battery slowly and completely to its minimum voltage, Normal charging step (E2): charging the battery normally at a set point charging current specified by the charger or the battery to charge the battery to a useful charge level of the battery, Wherein, after the first step and before the normal charging step (E2) is at least one intermediate sequence of charging and discharging the battery, and the intermediate sequence sequentially includes: a limited charging step (E11): performing limited charging on the battery at a limited charging current less than the set point charging current so as to charge the battery slowly to its maximum voltage; and Second step ( E12 ): forced discharge of the battery, carried out in a discharge circuit associated with the battery, at said discharge current calibrated to be at most equal to one tenth of a value of said nominal capacity, so as to discharge the battery slowly and completely again to its minimum voltage.

2. The method according to claim 1, characterized in that The first step (E1) and the second step (E12) of forcibly discharging the battery are performed in a discharge resistor (51), which is connected to the terminals of the battery during the first step (E1) and the second step (E12) and is disconnected from the terminals of the battery during the normal charging step (E2) and the limited charging step (E11), and the discharge resistor sets the calibrated discharge current.

3. The method according to claim 1, characterized in that The limiting charging current is at most equal to one tenth of the value of the nominal capacity of the battery.

4. A method according to any one of the preceding claims, characterised in that The set point charging current is suitable for fast charging the battery.

5. A device for regulating the charge level of a traction battery (2) of an electric vehicle (1) which is connected to an electricity distribution network (31) via a charger (3) during a stop phase of the vehicle, characterized in that The device comprises: a discharge circuit (5) for discharging the battery and capable of being electrically connected to the battery; and a control module (4) designed to selectively connect the discharge circuit (5) to the battery so as to discharge the battery and designed to implement the method according to any one of the preceding claims.

6. The device according to claim 5, characterized in that The discharge circuit (5) comprises a discharge resistor (51), each terminal (51a, 51b) of which is connected to a corresponding terminal of the battery via a switch (54), the switch comprising a movable contact (52) driven by the control module (4) between two positions for connecting and disconnecting the terminals of the resistor with the terminals of the battery.

7. The device according to claim 6, characterized in that The discharge resistor (51) is integrated into the housing (21) of the battery.

8. An electric vehicle (1) comprising a device as claimed in any one of claims 5 to 7.

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