Method for braking a vehicle

Abstract

The present disclosure relates to a method for braking a vehicle, a vehicle comprising a control unit configured to perform such a method, and a computer program element for braking a vehicle. The method for braking a vehicle comprises estimating a potential braking energy to be regenerated during a braking event, determining a threshold value based on a regenerative capability of a first axle drive unit, comparing the potential braking energy to be regenerated with the threshold value, and switching a second axle drive unit to a regenerative mode to regenerate the braking energy if the potential braking energy to be regenerated is higher than the threshold value.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for braking a vehicle, a vehicle comprising a control unit configured to perform such a method and a computer program element for braking a vehicle. BACKGROUND

[0002] In motor vehicles, a braking system is actuated to reduce the travel speed of the vehicle. Typically, conventional braking systems comprise brake discs on the wheels, which generate friction to decelerate or stop the vehicle. The friction generated during braking can be converted into heat energy, which can cause the braking system to overheat. In contrast, battery electric or hybrid electric vehicles apply regenerative braking systems to reduce the thermal load on the brake discs and to efficiently use the battery system.

[0003] During regenerative braking, the electric motor acts as a generator and converts mechanical energy into electrical energy, which is fed into the battery system. However, it can require energy to bring the regenerative braking or engagement components of the braking system into the regenerative mode. SUMMARY

[0004] It can be desirable to provide an improved method for braking a vehicle, which allows for a more efficient regenerative braking of energy.

[0005] This problem is solved by the subject matter of the present disclosure. It should be noted that the aspects of the present disclosure described below apply to the method for braking a vehicle, the vehicle comprising a control unit configured to perform such a method and the computer program element for braking a vehicle.

[0006] According to the present disclosure, a method for braking a vehicle is proposed. The method comprises estimating a potential braking energy to be regenerated during a braking event, determining a threshold value based on a regenerative capability of a first axle drive unit, comparing the potential braking energy to be regenerated to the threshold value and switching a second axle drive unit into a regenerative mode to regenerate the braking energy in case the potential braking energy to be regenerated is higher than the threshold value.

[0007] The braking method according to the present disclosure can improve the energy efficiency of the vehicle by timely switching the axle drive units into the regenerative mode depending on the estimated potential braking energy. Thus, the travel potential of the vehicle and the sustainability of the vehicle can be increased.

[0008] The vehicle can comprise a first axle drive unit and a second axle drive unit, which are primarily configured to supply power to the wheels. The power supply in each axle drive unit can be realized by an electric machine and / or an internal combustion engine, which are coupled to the battery system. In the regenerative mode of the braking energy, the energy can flow in reverse, i.e. from the wheels via the electric machine to the battery system, wherein the electric machine can act as a generator.

[0009] The first axle drive unit can be arranged at a rear side of the vehicle and the second axle drive unit can be arranged at a front side of the vehicle. Alternatively, the first axle drive unit can be arranged at a front side of the vehicle and the second axle drive unit can be arranged at a rear side of the vehicle. The battery system can be a single battery system for both axle drive units or it can comprise two independent battery units, each of which can be connected to each axle drive unit.

[0010] Generally, in a battery electric vehicle or a hybrid electric vehicle, at least one axle drive unit is continuously connected during a braking event to regenerate braking energy. However, if the potential braking energy that can be generated during the braking event is higher than a threshold value determined based on the regenerating capacity of the connected axle drive unit, the excess braking energy can be lost. In other words, if the connected axle drive unit is not able to absorb or regenerate the full braking energy, the excess braking energy can be wasted in the environment. The term “regenerating capacity” can be understood as the ability of an axle drive unit to regenerate or recover braking energy during a braking event, in particular during a limited braking time. The regenerating capacity of an axle drive unit can be defined by the manufacturer specification.

[0011] In case the potential braking energy is higher than the threshold value, the second axle drive unit, which is generally disconnected from the wheels during the braking event, can also be switched to a regenerative mode to regenerate braking energy. In other words, the second axle drive unit can be connected to the wheels to transfer and recover braking energy and to power the battery system.

[0012] The threshold value can thus represent when it is worth switching the second axle drive unit to the regenerative mode. The moment when it is worth switching the second axle drive unit to the regenerative mode can be the moment when the potential braking energy to be regenerated exceeds the threshold value determined based on the regenerating capacity of the first axle drive unit.

[0013] In one embodiment, the second axle drive unit is only switched to the regenerative mode in case the potential braking energy to be regenerated is higher than the threshold value. Thus, only if the estimated potential braking energy that can be regenerated during the braking event is higher than the threshold value comprising the regenerating capacity of the first axle drive unit, the first axle drive unit can be connected to the wheels to regenerate braking energy.

[0014] In one embodiment, determining the threshold value based on the regenerative capability of the first axle drive unit comprises estimating a connection energy required to switch the second axle drive unit into the regenerative mode. The threshold value comprises the regenerative capability of the first axle drive unit and the connection energy. During the braking event, the second axle drive unit can consume the energy to be converted in the regenerative mode. This energy can be considered when estimating the threshold value to verify whether it is worth connecting the second axle drive unit to recover the braking energy. In other words, the threshold value can be determined by adding the connection energy of the second axle drive unit and the regenerative capability of the first axle drive unit. Thus, the second axle drive unit can be switched into the power generation mode only if the potential braking energy is higher than the sum of the connection energy of the second axle drive unit and the regenerative capability of the first axle drive unit.

[0015] In one embodiment, estimating the connection energy comprises estimating an energy consumption required to connect the electric motor unit arranged at the second axle drive unit to the wheel arranged at the second axle drive unit. Typically, drive energy can be supplied from the battery system to the electric motor unit to actuate the wheel connected to the axle drive unit. However, in the regenerative mode, the electric motor unit can generate energy caused by the reverse torque applied to the wheel to reduce the speed of the vehicle. The recovered energy can then be supplied to the battery system to store it.

[0016] Thus, to change the energy flow direction, at least the wheel, the second axle drive unit and the electric motor unit can be switched into the regenerative mode, which can require energy. For example, a clutch member arranged in the second axle drive unit can engage a rotating shaft to transfer kinetic energy to the electric motor unit, which can also require such connection energy. Thus, when estimating the connection energy required to switch the second axle drive unit into the regenerative mode, the energy consumption to connect the wheel, the second axle drive unit and the electric motor unit in the regenerative mode can also be considered.

[0017] In one embodiment, switching the second axle drive unit into the regenerative mode comprises connecting the electric motor unit arranged at the second axle drive unit to the wheel arranged at the second axle drive unit during the braking event. If the control unit of the vehicle verifies that the potential braking energy to be regenerated is higher than the threshold value, the wheel, the second axle drive unit and the electric motor unit can be switched into the regenerative mode to allow a reverse energy flow.

[0018] In one embodiment, switching the second axle drive unit into the regenerative mode comprises connecting a first electric motor and a second electric motor of an electric motor unit arranged at the second axle drive unit to each wheel arranged at the second axle drive unit. The electric motor unit arranged at the second axle drive unit can comprise a first electric motor and a second electric motor, each of which is assigned to each wheel arranged at the second axle drive unit. In other words, the wheels arranged at the second axle drive unit can comprise individual electric motors. A battery system can be connected to each electric motor for supplying power or recovering brake energy.

[0019] During a braking event, if the potential brake energy is above a threshold value, the second axle drive unit can be switched into the regenerative mode and the first electric motor and / or the second electric motor can be activated to regenerate the brake energy and to supply the regenerated electric power to the battery system. Thus, the energy efficiency of the vehicle can be improved.

[0020] In one embodiment, the method further comprises monitoring the vehicle environment which has an influence on the braking event. The potential brake energy which can be regenerated during the braking event can be influenced by several parameters, such as braking distance, vehicle speed, vehicle mass, etc. In order to estimate a reliable potential brake energy, the vehicle environment can be monitored in real time.

[0021] In one embodiment, monitoring the environment comprises estimating a distance between the vehicle and a reference object, a speed of the vehicle, a relative speed of the vehicle with respect to the reference object and / or a mass of the vehicle. The parameters which can influence the brake energy are mainly the speed of the vehicle and the mass of the vehicle, by which the kinetic energy can be determined. Furthermore, in order to determine the potential brake energy which can be regenerated during the braking event, the distance between the vehicle and the reference object, the relative speed of the vehicle with respect to the reference object, the driving path and / or the weather can be considered.

[0022] In one embodiment, the reference object is a preceding vehicle in the driving direction. In other words, the reference object is a vehicle which is driving ahead with respect to the vehicle to be braked. Additionally or alternatively, the reference object can also be any obstacle or light signal on the road. The potential brake energy can thus depend on the relative speed of the vehicle to be braked with respect to the preceding vehicle and the distance between the vehicle to be braked and the preceding vehicle.

[0023] For example, if the vehicle to be braked approaches the preceding vehicle or the reference object with a low overtake speed and a substantially long or sufficient braking distance, the first axle drive unit can not need an additional axle drive unit to regenerate the brake energy. In other words, if the vehicle to be braked drives substantially a little bit faster than the preceding vehicle at a substantially long braking distance, the potential brake energy can be below a threshold value and the second axle drive unit can not be switched into the regenerative mode.

[0024] However, if the vehicle to be braked approaches the preceding vehicle or reference object with a high overtake speed and a substantially long or sufficient braking distance, the first axle drive unit can require an additional axle drive unit to regenerate braking energy. In other words, if the vehicle to be braked is substantially much faster than the preceding vehicle at a sufficient braking distance, the potential braking energy can be above the threshold. Therefore, the second axle drive unit can be switched to a regeneration mode and the wheels arranged at the second axle drive unit can be connected to the electric motor unit to recover braking energy.

[0025] Likewise in the case that the vehicle to be braked approaches the preceding vehicle or reference object with a low overtake speed but at a short braking distance, the first axle drive unit can require an additional axle drive unit to regenerate the full braking energy. In other words, if the vehicle to be braked is substantially slightly faster than the preceding vehicle but at a short braking distance, the potential braking energy can be above the threshold. Therefore, the second axle drive unit can be switched to a regeneration mode and the wheels arranged at the second axle drive unit can be connected to the electric motor unit to recover braking energy.

[0026] In one embodiment, the monitoring is performed by a laser detection and ranging system or a radar system. A laser detection and ranging system (Lidar) comprises at least one light source and a receiver to measure the distance to a remote target. The light source emits light towards the target, which then scatters the light. Some of the scattered light is received back at the receiver. The system determines the distance to the target based on one or more features related to the returning light. While a radar system determines the distance to a remote target by measuring the reflection of high-frequency signals from the target. Additionally or alternatively, an ultrasonic sensor can also be used to measure the distance to a remote target. By applying such distance measuring systems, an accurate distance measurement between the vehicle to be braked and the reference object can be achieved and an accurate estimation of the potential braking energy can be performed.

[0027] In one embodiment, the method further comprises actuating a mechanical braking element in case the potential braking energy to be regenerated during the braking event is higher than the sum of the regeneration capabilities of the first axle drive unit and the second axle drive unit. In other words, even if both the first axle drive unit and the second axle drive unit can be operated in a regeneration mode, the potential braking energy can be even higher than the sum of their regeneration capabilities. In this case, a mechanical braking element such as a brake disc can be connected to the wheels to mechanically brake the vehicle. This can happen for example during an emergency braking event.

[0028] In one embodiment, the first axle drive unit is an electric rear axle drive (ERAD) unit and the second axle drive unit is an electric front axle drive (EFAD) unit. The electric axle drive units can comprise a motor unit, power electronics and a transmission unit to directly power each vehicle axle. Typically, the vehicle comprises one electric axle drive unit at the front side of the vehicle and one electric axle drive unit at the rear side of the vehicle, wherein the wheels arranged at the electric rear axle drive (ERAD) unit can be continuously connected during a braking event. In contrast, the wheels arranged at the electric front axle drive (EFAD) unit can be connected during a braking event in case the potential braking energy to be regenerated is above a threshold value of the regenerative capability of the electric rear axle drive (ERAD) unit and the connection energy of the electric front axle drive (EFAD) unit to the wheels.

[0029] According to the present disclosure, a vehicle is proposed. The vehicle comprises a control unit configured to perform

[0030] - estimate a potential braking energy to be regenerated during a braking event,

[0031] - determine a threshold value based on a regenerative capability of a first axle drive unit,

[0032] - compare the potential braking energy to be regenerated with the threshold value, and

[0033] - switch a second axle drive unit to a regenerative mode to regenerate the braking energy in case the potential braking energy to be regenerated is above the threshold value.

[0034] Thus, by timely switching the axle drive units to the regenerative mode depending on the estimated potential braking energy, the energy efficiency of the vehicle can be improved. In particular, the control unit can determine when it is worth switching the second axle drive unit to the regenerative mode. Thus, the driving potential of the vehicle and the sustainability of the vehicle can be enhanced.

[0035] In one embodiment, the vehicle is a battery electric vehicle (BEV) or a hybrid electric vehicle (HEV). The control unit can be an engine control unit (ECU) or a separate control unit. The vehicle can further comprise an electric rear axle drive (ERAD) unit and an electric front axle drive (EFAD) unit. Each electric axle drive unit can comprise an electric motor unit, power electronics and a transmission unit to directly power each axle. Typically, the wheels arranged at the electric rear axle drive (ERAD) unit can be continuously connected during a braking event. In contrast, the wheels arranged at the electric front axle drive (EFAD) unit can be connected during a braking event in case the potential braking energy to be regenerated is above a threshold value comprising the regenerative capability of the electric rear axle drive (ERAD) unit and the connection energy of the electric front axle drive (EFAD) unit to the wheels. Alternatively, the wheels arranged at the electric front axle drive (EFAD) unit can be continuously connected during a braking event and the wheels arranged at the electric rear axle drive (ERAD) unit can be connected during a braking event in case the braking energy to be regenerated is above a threshold value comprising the regenerative capability of the electric front axle drive (EFAD) unit and the connection energy of the electric rear axle drive (ERAD) unit to the wheels.

[0036] According to the present disclosure, a computer program element is also proposed. The computer program is configured to brake a vehicle as described above, which, when executed by a processing element, is adapted to perform the method as described above.

[0037] It should be noted that the above described embodiments can be combined with each other, independent of the aspects involved. Thus, the method can be combined with structural features and likewise the system can be combined with features described above in relation to the method.

[0038] These and other aspects of the present disclosure will become apparent from the embodiments described hereinafter and with reference to the accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0039] Exemplary embodiments of the present disclosure will be described hereinafter with reference to the accompanying drawings.

[0040] Figure 1 An embodiment of a vehicle according to the present disclosure is schematically and exemplarily shown.

[0041] Figs. 2a, 2b, 2c schematically and exemplarily show an embodiment of a method for braking a vehicle according to the present disclosure.

[0042] Figure 3 An embodiment of a method for braking a vehicle according to the present disclosure is schematically and exemplarily shown. DETAILED DESCRIPTION

[0043] Figure 1A vehicle 100 to be braked is shown. The vehicle 100 can be a battery electric vehicle (BEV) or a hybrid electric vehicle (HEV) comprising a control unit 70 configured to perform a braking method as shown. Figure 3

[0044] The vehicle 100 further comprises a first or electric rear axle drive (ERAD) unit 10 and a second or electric front axle drive (EFAD) unit 20. Each of the electric rear axle drive (ERAD) unit 10 and the electric front axle drive (EFAD) unit 20 comprises an electric motor unit 11, 21, power electronics (not shown) and a transmission unit (not shown) to allow rotation of the wheels 12, 22 arranged at each axle drive unit, wherein the electric motor unit 11, 21 can comprise two electric motors for each wheel arranged at the respective axle drive unit 10, 20. A battery system 80 is connected to each electric motor unit 11, 21 of the axle drive units 10, 20 for power supply. In addition to the battery system 80, the electric rear axle drive (ERAD) unit 10 or the electric front axle drive (EFAD) unit 20 can also be connected to an internal combustion engine (not shown).

[0045] Generally, the wheels 12 arranged at the electric rear axle drive (ERAD) unit 10 are continuously connected during a braking event to recover braking energy. In contrast, the wheels 22 arranged at the electric front axle drive (EFAD) unit 20 are connected during a braking event in case the potential braking energy E to be regenerated is above a threshold 40. In other words, if the potential braking energy E to be regenerated is below the threshold 40, braking energy can only be recovered by the electric rear axle drive (ERAD) unit 10.

[0046] Thus, the control unit 70 of the vehicle 100 is configured to determine the threshold 40, which represents when it is worth connecting the wheels 22 to the electric front axle drive (EFAD) unit 20. The threshold 40 is determined by summing up the regenerative capability 31 of the electric rear axle drive (ERAD) unit 10 and the connection energy 33 required to connect the wheels 22 to the electric motor unit 21 or to transform the electric front axle drive (EFAD) unit 20 into a regenerative mode.

[0047] The potential braking energy E can be estimated by monitoring the vehicle 100 environment influencing the braking event, for example the distance between the vehicle 100 and a reference object 200, the speed of the vehicle 100, the relative speed 100 of the vehicle with respect to the reference object 200 and / or the mass of the vehicle 100. Monitoring the environment can be performed by a laser detection and ranging system (Lidar) or a radar system or the like.

[0048] ​As shown in Fig. 2a, the reference object 200 can be a preceding vehicle 200 in the driving direction relative to the vehicle 100 to be braked. If the vehicle 100 to be braked approaches the preceding vehicle 200 at a low overtake speed at a substantially long or sufficient braking distance, the electric rear axle drive (ERAD) unit 10 can be able to regenerate the entire braking energy. In other words, the potential braking energy E is below the threshold 40, so that the electric rear axle drive (ERAD) unit 10 does not need any additional axle drive unit to regenerate the braking energy.

[0049] However, if the vehicle 100 to be braked approaches the preceding vehicle 200 or the reference object 200 (see Fig. 2b) at a high overtake speed at a sufficient braking distance, the electric rear axle drive (ERAD) unit 10 needs an additional axle drive unit, i.e. the electric front axle drive (EFAD) unit 20, to regenerate the entire braking energy. In other words, the estimated potential braking energy E is above the threshold 40. Therefore, the electric front axle drive (EFAD) unit 20 is switched to the regeneration mode and the wheels arranged at the electric front axle drive (EFAD) unit 20 are connected to the electric motor unit 21 to recover the braking energy.

[0050] Fig. 2c shows that, if the vehicle 100 to be braked approaches the preceding vehicle 200 at a low overtake speed but at a short braking distance, the potential braking energy E can also be above the threshold 40, so that the electric front axle drive (EFAD) unit 20 is switched to the regeneration mode to recover the braking energy.

[0051] However, in case the potential braking energy E to be regenerated is even above the sum of the regeneration capabilities 31, 32 of the electric rear axle drive (ERAD) unit 10 and the electric front axle drive (EFAD) unit 20, the mechanical brake element 13 of the vehicle 100 can be actuated to perform an emergency braking and the braking capability 35 of the conventional mechanical brake element 13 can be applied.

[0052] Figure 3 A method for braking a vehicle 100 is shown. The method comprises, but does not necessarily follow this order:

[0053] - monitoring S1 the vehicle 100 environment affecting a braking event,

[0054] - monitoring S2 the environment comprises estimating a distance between the vehicle 100 and a reference object 200, a speed of the vehicle 100, a relative speed of the vehicle 100 relative to the reference object 200 and / or a mass of the vehicle 100,

[0055] - estimating S3 a potential braking energy E to be regenerated during the braking event,

[0056] - determining S4 a threshold 40 based on a regeneration capability 31 of a first axle drive unit 10,

[0057] - comparing S5 the potential brake energy E to be regenerated with a threshold value 40, and

[0058] - switching S6 the second axle drive unit 20 to a regeneration mode to regenerate brake energy in case the potential brake energy E to be regenerated is higher than the threshold value 40.

[0059] The method further comprises, but not necessarily in this order:

[0060] - estimating S41 the connection energy 33 required to switch the second axle drive unit 20 to the regeneration mode, the threshold value 40 comprising the regeneration capability 31 of the first axle drive unit 10 and the connection energy 33,

[0061] - estimating S42 the energy consumption required to connect the electric motor unit 21 arranged at the second axle drive unit 20 to the wheel 22 arranged at the second axle drive unit 20,

[0062] - connecting S61 the electric motor unit 21 arranged at the second axle drive unit 20 to the wheel 22 arranged at the second axle drive unit 20 during the braking event,

[0063] - connecting S62 the first and second electric motor of the electric motor unit 21 arranged at the second axle drive unit 20 to each of the wheels 22 arranged at the second axle drive unit 20, and

[0064] - actuating S7 the mechanical brake element 13 during the braking event if the potential brake energy E to be regenerated is higher than the sum of the regeneration capabilities 31, 32 of the first axle drive unit 10 and the second axle drive unit 20.

[0065] It has to be noted that embodiments of the disclosure are described with reference to different subject matters. In particular, some embodiments are described with reference to method type claims whereas other embodiments are described with reference to device type claims. However, a person skilled in the art will gather from the above and the following description that, unless other notified, in addition to any combination of features belonging to one type of subject matter, also any combination between features

[0066] While the disclosure has been illustrated and described in the drawings and disclosure, such illustration and description is to be considered illustrative or exemplary only and not restrictive. The disclosure is not limited to the disclosed embodiments. Further variations of the disclosed embodiments can be understood and implemented by those skilled in the art upon

[0067] In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality or pluralitiy. A single processor or other unit can fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope of the claims.

Claims

1. A method for braking a vehicle (100), comprising: - estimating (S3) the potential braking energy (E) to be regenerated during the braking event, - determining (S4) a threshold value (40) based on the regeneration capability (31) of the first shaft drive unit (10), - comparing (S5) said potential braking energy to be regenerated (E) with said threshold value (40), and - switching (S6) the second axle drive unit (20) to a regeneration mode to regenerate braking energy if the potential braking energy (E) to be regenerated is higher than the threshold value (40), Wherein, determining (S4) a threshold value (40) based on the regeneration capability (31) of the first shaft drive unit (10) includes: estimating (S41) a connection energy (33) required to switch the second shaft drive unit (20) to the regeneration mode, wherein the threshold value (40) includes the regeneration capability (31) of the first shaft drive unit (10) and the connection energy (33).

2. The method according to claim 1, wherein the second axle drive unit (20) is switched to the regeneration mode only if the potential braking energy (E) to be regenerated is above the threshold value (40).

3. The method according to claim 1, estimating the connection energy (33) comprises: Energy consumption required to connect the motor unit (21) arranged at the second shaft drive unit (20) to the wheel (22) arranged at the second shaft drive unit (20) is estimated (S42).

4. The method according to claim 3, switching (S6) the second shaft drive unit (20) to the regeneration mode comprises: A motor unit (21) arranged at the second axle drive unit (20) is connected (S61) to a wheel (22) arranged at the second axle drive unit (20) during a braking event.

5. The method according to claim 4, switching (S6) the second shaft drive unit (20) to the regeneration mode comprises: The first motor and the second motor of the motor unit (21) arranged at the second shaft drive unit (20) are connected (S62) to each of the wheels (22) arranged at the second shaft drive unit (20).

6. The method according to any of the preceding claims 1 to 5, further comprising monitoring (S1) the vehicle (100) environment influencing a braking event.

7. The method according to claim 6, wherein monitoring the environment comprises: The method further comprises estimating (S2) a distance between the vehicle (100) and a reference object (200), a speed of the vehicle (100), a relative speed of the vehicle (100) relative to the reference object (200), and / or a mass of the vehicle (100).

8. The method of claim 6, wherein the monitoring is performed by a LiDAR system or a radar system.

9. The method of claim 7, wherein the monitoring is performed by a LiDAR system or a radar system.

10. The method according to claim 7, wherein the reference object (200) is a preceding vehicle (200) in the direction of travel.

11. The method according to any one of the preceding claims 1 to 5 further comprises actuating (S7) a mechanical brake element (13) when, during the braking event, the potential braking energy (E) to be regenerated is higher than the sum of the regeneration capabilities (31, 32) of the first shaft drive unit (10) and the second shaft drive unit (20).

12. The method according to any of the preceding claims 1 to 5, wherein the first axle drive unit (10) is an electric rear axle drive (ERAD) unit and the second axle drive unit (20) is an electric front axle drive (EFAD) unit (20).

13. A vehicle (100) comprising a control unit (70), the control unit (70) being configured to perform: - estimation of the potential braking energy (E) to be regenerated during a braking event, - determining a threshold value (40) based on the regeneration capability (31) of the first shaft drive unit (10), - comparing said potential braking energy to be regenerated (E) with said threshold value (40), and - switching the second axle drive unit (20) to a regeneration mode to regenerate braking energy if the potential braking energy (E) to be regenerated is higher than the threshold value (40), in, Determining (S4) a threshold value (40) based on the regeneration capability (31) of the first shaft drive unit (10) includes estimating (S41) a connection energy (33) required to switch the second shaft drive unit (20) to a regeneration mode, the threshold value (40) including the regeneration capability (31) of the first shaft drive unit (10) and the connection energy (33).

14. The vehicle (100) of claim 13, being a battery electric vehicle or a hybrid electric vehicle.

15. A computer program element for braking a vehicle (100) according to one of claims 13 and 14, which, when executed by a processing element, is adapted to perform the steps of the method according to claims 1 to 12.

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