WO2019112505A1

WO2019112505A1 - A device for a motor vehicle electric system and a method for controlling such an electric system

Info

Publication number: WO2019112505A1
Application number: PCT/SE2018/051218
Authority: WO
Inventors: Fredrik Roos; Gunnar Ledfelt; Oscar Flärdh; Holger Dreher
Original assignee: Scania Cv Ab
Priority date: 2017-12-04
Filing date: 2018-11-27
Publication date: 2019-06-13
Also published as: DE112018005706T5; SE1751491A1; SE541350C2

Abstract

A device is configured to be arranged in a motor vehicle electric system comprising a power battery (2), a generator (5) as well as consumers (6) of electric energy. The device (9) is configured to check and determine the ability of the power battery (2) to deliver electric energy to said consumers (6) by during operation of the engine of the vehicle instantaneously lower a voltage reference value delivered for control of the generator for the system voltage to a limit value lower than an expected voltage level of the power battery and keep the voltage reference value at this limit value during a predetermined period of time. Means (11) will measure the system voltage during this period of time and means (13) will record the development of the system voltage measured during this period of time. The device (14) will evaluate this development and determine the status of the power battery based on the result of this evaluation.

Description

A device for a motor vehicle electric system and a method for controlling such an electric system

TECHNICAL FI ELD OF TH E INVENTION

The present invention relates to motor vehicles and specifically to electric systems of motor vehicles.

BACKGROUND ART

Constantly increasing considerations with respect to environmen- tal impact of the operation of motor vehicles and also a desire to save fuel have led to attempts to turn off the engine of motor ve- hicles during idling. However, this would then in a motor vehicle having an electric system with at least one power battery, a gen- erator driven by an engine of the vehicle as well as consumers of electric energy connected between opposite poles of the power battery on a consumer side thereof and a unit configured to control the generator to deliver a system voltage to the consumers ac- cording to a voltage reference value, mean that also the generator is shut off. The replacement of the idling by“engine off” is then supposed to occur during standstill but also when the vehicle is in motion, during so called“eco-roll” or freewheeling. Such shutting off of the generator will result in a vanishing of the in-built redun- dancy in the electric system provided by the combination of gen- erator and battery. This is particularly problematic in the case of shutting off the engine in a moving vehicle. It is in such a case of vital importance to know that the battery functions properly before a decision to shut off the engine may be taken. Shutting off the engine with a dead battery will not be either safe or very good for uptime, since it will most likely lead to a VoR (Vehicle off Road) situation. It would for instance be catastrophical to shut off the engine when the vehicle is moving in darkness if the battery would not be able to feed sufficient power to headlamps and other equip- ment for controlling the operation of the vehicle. This is the reason for providing such a motor vehicle electric system with a device configured to check and determine the ability of the power battery to deliver electric energy to the consumers.

DE 1020050041 74 discloses a device attempting to address this problem by checking the status of a power battery in an electric system of a vehicle by measuring a current drained from the power battery at different system voltages delivered by the generator of the electric system. However, the result of the current measuring may then also be influenced by the activity of said consumers of electric energy and not only by the status of the power battery.

SUMMARY OF TH E INVENTION

It is an object of the present invention to overcome said problem of said known device.

This object is obtained by providing a device configured to be ar- ranged in a motor vehicle electric system comprising at least one power battery, a generator driven by an engine of the vehicle as well as consumers of electric energy connected between opposite poles of the power battery on the consumer side thereof, a unit configured to control said generator to deliver a system voltage to said consumers according to a voltage reference value, and means configured to measure said system voltage, the device being con- figured to check and determine the ability of the power battery to deliver electric energy to said consumers, the device being char- acterized in that it is configured, during operation of the engine of said vehicle, to at a time of activation of the device instantaneously lower said voltage reference value delivered to said unit to a limit value lower than an expected voltage level of said power battery and keep the voltage reference value at this limit value during a predetermined period of time, configured to record the develop- ment of the system voltage measured during said period of time, and configured to evaluate said development of the system voltage during said period of time and determine the ability of said power battery to deliver electric energy to said consumers on the basis of the result of said evaluation .

Such a device provides for a reliable determination of the ability of the power battery to deliver electric energy to the consumers of a said electric system on the basis of the result of said evaluation. This is due to a recognition that the reaction of the change of sys- tem voltage upon an instantaneous lowering of the voltage refer- ence value includes excellent information about a power battery, since this reaction will be different for the case of no battery re- sponse at all, a bad battery response and a good battery response. Such a lowering of the voltage reference value may also be carried out during driving of the vehicle without jeopardizing the proper behavior of the vehicle.

The invention also relates to a method for controlling an electric system of a motor vehicle, said system comprising at least one power battery, a generator driven by an engine of the vehicle as well as consumers of electric energy connected between opposite poles of the power battery on a consumer’s side thereof, a unit configured to control said generator to deliver a system voltage to said consumers according to a voltage reference value and means configured to measure said system voltage, in which the method comprises the step of, during operation of the engine of said vehi- cle, check and determine the ability of the power battery to deliver electric energy to said consumers, the method being characterized in that said step of checking and determining comprises the fol- lowing sub-steps to at a time of starting said step instantaneously lower said voltage reference value delivered to said unit to a limit value lower than an expected voltage level of said power battery and keep the voltage reference value at this limit value during a predetermined period of time, record the development of the sys- tem voltage measured during said period of time, and evaluate said development of the system voltage during said period of time and determine the ability of said power battery to deliver electric energy to said consumers on the basis of the result of said evalu- ation.

By carrying out such a method the ability of a power battery to deliver electric energy to consumers of electric energy connected between opposite poles of the power battery can be reliably deter- mined.

The invention also relates to a computer program comprising com- puter program code for causing a computer to implement a method according to the invention when the computer program is executed in the computer. The invention further relates to a computer program product corn- prising a non-transitory data storage medium which can be read by a computer and on which the program code of a computer pro- gram just mentioned is stored.

The invention also relates to an electronic control unit of a motor vehicle comprising an execution means, a memory connected to the execution means and a non-transitory data storage medium which is connected to the execution means and on which the com- puter program code of a computer program defined above is stored. The invention also relates to a motor vehicle, especially a wheeled vehicle such as a truck or a bus, which comprises such an electronic control unit.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings, below follows a specific description of embodiments of the invention cited as examples.

In the drawings:

Fig. 1 illustrates very schematically an electric system ac- cording to an embodiment of the invention,

Fig. 2 is a graph illustrating a possible development over time of the system voltage for an electric system according to Fig 1 when methods according to embodiments of the present invention are carried out, Fig. 3 is a flow chart illustrating an example of steps carried out in a method according to an embodiment of the in vention, and

Fig. 4 is a schematic view illustrating an electronic control unit for implementing a method according to the invention .

DETAILED DESCRI PTION OF EMBODIMENTS OF THE INVENTION

Fig. 1 illustrates schematically the general structure of an electric system in which a device according to an embodiment of the in- vention is configured to be arranged and which electric system is arranged in a motor vehicle 1 schematically indicated and has two power or service batteries 2 connected in series. The power bat- teries are for example 1 2V-batteries, so that a service or operation voltage between two poles 3, 4 of the power batteries 2 is approx- imately 24V. A generator 5 and consumers 6 of electric energy are both connected to opposite poles 3, 4 of the power batteries 2 on a consumer side 7 thereof. The box 6 represents an amount of components of the motor vehicle to be supplied with electric en- ergy by the generator 5 and/or the power batteries 2.

An engine management system for controlling components of the electric system is schematically indicated by a box. This system 8 may provide functions to be carried out by different parts of said electric system, and such parts have for that sake been shown to be included in the drawing box and belong to a device 9 according to the present invention, which is configured to check and deter- mine the ability of the power batteries 2 to deliver electric energy to the consumers 6. One such part is a control unit 1 0 configured to control the generator 5 to deliver a system voltage U _sys to the consumers 6 according to a voltage reference value. Means 1 1 are configured to measure said system voltage. The device 9 is con- figured to provide the control unit 10 with said voltage reference value and then carry out a method for obtaining information about the ability of the power batteries to deliver electric energy to the consumers during operation of the motor vehicle 1 by instantane- ously lowering the voltage reference value delivered to the unit 10 to a limit value ( U n m ) lower than an expected voltage level ( U bex ) of the power batteries 2 and keep the voltage reference value at this limit value during a predetermined period of time. The device 9 is further configured to record the development of the system voltage measured by the means 1 1 during said predetermined pe- riod of time and to evaluate said development of the system volt- age during said predetermined period of time and then determine the ability of the power batteries 2 to deliver electric energy to the consumers 6 on the basis of the result of this evaluation. The func- tions provided by the device of providing the control unit with a reference voltage value, recording the development of the system voltage measured and evaluating said development of the system voltage are indicated in Fig . 1 by the reference numerals 1 2, 1 3 and 1 4, respectively. It is illustrated in Fig. 1 by arrows how the system current l _Sys for supplying the consumers 6 with electric en- ergy may be composed by the current l _b from the power batteries 2 and a currant I G from the generator 5. However, once the engine of the vehicle has been started the capacity of the generators are in general high enough for both charging the power batteries 2 and supplying said consumers 6 with electric energy. The power bat- teries will then only assist the generator to deliver electric energy to the system would the generator be overloaded or broken .

The device 9 is configured to keep the voltage reference value at said limit value Unm during a period of time with a length allowing the system voltage to fall below said expected voltage level U_bex of the power battery while at least one of a) falling to said limit value and b) arriving to and passing below a level as of which the time gradient of the system voltage drop is abruptly reduced, and the device is configured to determine that there is a malfunction of said power battery if case a) occurs without prior occurrence of case b). A length of said period of time in the order of seconds will be enough for obtaining a reliable evaluation of the development of the system voltage resulting in a reliable determination of the ability of the power battery to deliver electric energy to the con- sumers. This is due to the fact that if the power batteries are miss- ing or bad, the system voltage will nearly instantaneously reach said limit value Unm when the voltage reference value is lowered to this limit value. Would instead the power battery neither be missing nor bad, the system voltage will during such a short period of time arrive to and pass below a level as of which the time gra- dient of the system voltage drop is abruptly reduced and by that deliver information about the status of a good power battery. The device 9 is preferably configured to use information about the value of the system voltage level at a shift of time gradient of the system voltage according to b) to determine said ability and by that the status of the power battery 2. Thus, the level of the system voltage at which the time gradient thereof shifts will be dependent upon the status of the power battery. Furthermore, the device is according to an embodiment of the in- vention configured to carry out said evaluation by calculating or measuring the time needed for the system voltage to reach said limit value U nm . Thus, it may be possible to estimate said time needed for the system voltage to reach said limit value even if the limit value is not reached during said period of time, and the longer it takes to reach said limit value the better status of the battery. However, this time may also be influenced by the actual current consumption of the consumers 6 of electric energy and this may then also be considered when determining the status of the bat- tery. According to another embodiment of the invention the device is configured to calculate the integral between the system voltage level U_sys and said limit value Unm during a period of time starting by said activation of the device, such as said predetermined period of time or a time estimated to be necessary for the system voltage to drop the said limit value. A high integral value will then indicate a good battery. Also here will the actual consumption of current of the consumers of electric energy of the electric system have an influence, since if this consumption is high the system voltage will fall quicker and the integral value will be lower.

According to another embodiment of the invention, the device is configured to carry out said evaluation by estimating the develop- ment of the system voltage would said predetermined period of time be longer than determined by the device. By such estimations of the development it may be predicted how long it would take for the system voltage to reach said limit value, and that information will be a good indication of the status of the power battery. The device may be configured to control the control unit of the electric system to prevent operations, such as gear shifting, influencing the load of the electric system considerably during said predeter- mined period of time, which will result in an increased reliability of the determination of the status of the power battery. Furthermore, the device may be configured to lower said voltage reference value at a time being delayed enough from at start of said engine to allow the value of the system voltage to arrive to said voltage reference value. This will then also permit some recharging of the battery before checking thereof.

Furthermore, the device is according to another embodiment of the invention configured to lower said voltage reference value to a limit value being at least 5% lower than said expected voltage level of the power battery. This will ensure that the limit value will be below the actual voltage level of the power battery if the power battery is properly connected and in a fairly good status, so that the status of the power battery may be determined by said device.

According to another embodiment of the invention, the nominal voltage level of said power battery is 24V, a nominal system volt- age level is above 28V, and the device is configured to lower the voltage reference value delivered by the control unit to a limit value being at least 7% lower than the nominal system voltage value, 7- 20% or 7-15% lower than the nominal system voltage value. These are voltages normally used in the motor vehicles in question, and the 24V may then for instance be obtained by two 12V batteries connected in series. The limit value has then to be at least 2V below the nominal system voltage value for being lower than the expected voltage level of the battery, which normally is around 26V, but it would normally be suitable to select the limit value then being around 3V lower than the nominal system voltage value for ensuring that the status of the power battery may be reliably de- termined. However, the limit value should not be lower than 80% and mostly not lower than 85% of the nominal system voltage value, since would it be clearly below the nominal voltage level of the power battery (here 24V) , the lowering of the voltage reference value to said limit value could influence the function of said con- sumers of electric energy and result in dangerous situations if the vehicle is driven.

According to another embodiment of the invention, the device is configured to keep the voltage reference value at said limit value during a period of time of at least 2 s, 2 s - 1 0 s or 2 s - 5 s. It has been found that it will be enough to keep the voltage reference value at said limit value during such a short period of time for reli ably determining the status of the power battery so as to be able to for instance decide whether the engine may be temporarily shut down during operation of the motor vehicle. It would of course also be possible, but not necessary, to keep the voltage reference value at said limit value during a much longer period of time, such as for instance 2 minutes, sometimes for carrying out a very exact determination of the status of the power battery and how the de- velopment of the system voltage until reaching said limit value looks like for a good power battery. The result of this may then be used when later on carrying out determination based upon a shorter said predetermined period of time.

A method according to embodiments of the invention will now be described while making reference to the graph of Fig. 2. The method is carried out when the load of the generator is so that the power batteries 2 are not participating in providing the consumers with electric energy. The system voltage U _Sys is then higher than the expected battery voltage U_bex should the generator not be overloaded. U _sys is then typically 28-29V, whereas U bex is typically 26-27V. The method is started at the time to by instantaneously lowering the voltage reference value to a limit value U n m lower than said expected voltage level U_bex of the power battery. Unm is here typically around 25V. The nominal battery voltage level of 24V Un_om is also indicated in the graph. The voltage reference value is kept at said limit value during a predetermined period of time, which is here shown to be to ti , which may as shown in Fig. 2 be for instance 2 minutes after to. The development of the system voltage is during this period of time measured by the means 1 1 and recorded by the device 9, described with reference to Fig. 1 .

No power batteries or bad batteries will be felt by the consumers 6 of the electric system by a drop of the system voltage nearly instantaneously, i.e. within fractions of a second, to Un_m as soon as the voltage reference value has been changed thereto. This is indicated by a dashed line A. A good power battery will be felt by the consumers by a drop of the system voltage after to rapidly to the voltage level of the power batteries, which will be reached at the time

within fractions of a second, and the time gradient of the system voltage drop will then be abruptly reduced. The power batteries will as of this point define the system voltage and take over feeding of electric energy to the consumers and the system voltage will then fall slowly towards Unm as shown by the solid line B in Fig 2. The device will evaluate the development of the system voltage shown in Fig 2 and determine the status of the power bat- teries. This may be done by considering the value of the voltage at the point P where the time gradient of the system voltage changes. When this voltage is significantly higher than U n m this indicates that a good battery is there. The device may instead or in combination with considering the location of the point P deter- mine, or when said predetermined period of time is not chosen to be long enough calculate, how long time it will take for the system voltage to reach said limit value Un_m . The longer it takes the better status of the power battery would everything else be the same, but the inclination of the line B is also depending upon the actual cur- rent consumption of the consumers 6, and it is possible to also take this into account when carrying out said determination of the status of the power batteries.

Another option to be chosen by the device is to calculate the inte gral between the system voltage level U _Sys and the limit value U n m during a predetermined period of time starting at to . This integral is indicated by the area C and the size thereof will be dependent upon the status of the power batteries but also influenced by the current consumption of the consumers. The device is configured to control the control unit 10 to prevent operations, such as gear shifting , influencing the load of the electric system considerably during the predetermined period of time after to for obtaining de- velopments of the system voltage being comparable with such de- velopments obtained earlier by reducing the influence of the cur- rent consumption of the consumers.

It is pointed out that said predetermined time period may very well be ended after 10 s and a reliable evaluation of the status of the power batteries may even be achieved when ending said period of time already after 2 s as indicated by t3. Information about the status of the power battery obtained through the method according to the present invention may be used to de- cide whether it is advisable or not to replace idling of the engine with an“engine off” either when the vehicle is standing still or when it is moving during so-called“eco-roM” or freewheeling. The control system 1 0 may then gather information, for instance by means of a GPS device, and evaluate this information about the road ahead of the moving vehicle so as to determine whether there is any im- minent request to switch the engine off, and when this is the case the device may carry out said checking and determination of the ability of the power battery to deliver electric energy to the con- sumers would the engine be shut down. Fig . 3 illustrates a flow chart of a method according to an embod- iment of the present invention carried out for an electric system of the type shown in Fig . 1 . The method is started with a step S i of lowering the voltage reference value to Un_m and simultaneously in a step S2 measuring the system voltage. The voltage reference value is kept at U nm and the development of the system voltage is recorded during a predetermined period of time in a step S3. Eval- uation of the development of the system voltage is during said pre- determined period of time then carried out in a step S₄. Finally, the ability of the power battery to deliver electric energy to con- sumers is determined in a step S5.

Computer program code for implementing a method according to the invention is with advantage included in a computer program which can be read into the internal memory of a computer, e.g. the internal memory of an electronic control unit of a motor vehicle. Such a computer program is with advantage provided via a com- puter program product comprising a data storage medium which can be read by a computer and which has the computer program stored on it. Said data storage medium is for example an optical data storage medium in the form of a CD ROM disc, a DVD disc etc. , a magnetic data storage medium in the form of a hard disc, a diskette, a cassette tape etc. , or a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. Fig. 4 illustrates very schematically an electronic control unit 8 comprising an execution means 15, e.g. a central processor unit (CPU), for execution of computer software. The execution means 15 communicates with a memory 1 6, e.g . of the RAM type, via a data bus 17. The control unit 8 comprises also a non-transitory data storage medium 18, e.g . in the form of a flash memory or a memory of the ROM, PROM, EPROM or EEPROM type. The execution means 15 communicates with the data storage medium 1 8 via the data bus 17. A computer program comprising computer program code for implementing a method according to the invention, e.g. in accordance with the em- bodiment illustrated in Fig. 3 is stored on the data storage medium 18.

The invention is of course in no way restricted to the embodiments described above, since many possibilities for modifications thereof are likely to be obvious to one skilled in the art without having to deviate from the scope of invention defined in the appended claims.

It is pointed out that the invention is not restricted to devices for checking and determining the ability of a power battery to deliver electric energy to consumers in an electric system of any special type of motor vehicles, although the invention is particularly appli- cable to wheeled motor vehicles, especially utility vehicles, such as trucks or lorries and buses, and the invention has for that sake above primarily being discussed for that field od use for illuminat- ing the invention but accordingly not in any way restrict it thereto.

Furthermore, other reasons for checking and determining the abil- ity of the power battery to deliver electric energy to the consumers of the electric system of a motor vehicle than for deciding whether the engine may be shut off during operation of the vehicle or not do also exist, since there is always a desire to know that the power battery of a motor vehicle is in good order and that it may be relied on a good function thereof. The present invention described above is directed to checking and determining the ability of the power battery in a motor vehicle also for such reasons.

Claims

1 . A device configured to be arranged in a motor vehicle electric system comprising at least one power battery (2) , a generator (5) driven by an engine of the vehicle (1 ), as well as consum- ers (6) of electric energy connected between opposite poles (3, 4) of the power battery on the consumer side (7) thereof, a unit (1 0) configured to control said generator to deliver a system voltage to said consumers according to a voltage ref- erence value, and means (1 1 ) configured to measure said sys- tem voltage,

the device being configured to check and determine the ability of the power battery (2) to deliver electric energy to said con- sumers,

characterized in that said device is configured, during oper- ation of the engine of said vehicle, to at a time of activation of the device instantaneously lower said voltage reference value delivered to said unit (1 0) to a limit value ( U n m ) lower than an expected voltage level ( U bex ) of said power battery and keep the voltage reference value at this limit value during a prede- termined period of time,

configured to record the development of the system voltage ( U _Sys ) measured during said period of time, and

configured to evaluate said development of the system volt- age during said period of time and determine the ability of said power battery (2) to deliver electric energy to said consumers on the basis of the result of said evaluation.

2. A device according to claim 1 , characterized in that the de- vice is configured to keep the voltage reference value at said limit value ( U nm ) during a period of time with a length allowing the system voltage to fall below said expected voltage level ( U _bex) of the power battery while at least one of a) falling to said limit value and b) arriving to and passing below a level as of which the time gradient of the system voltage drop is abruptly reduced, and that the device is configured to deter- mine that there is a malfunction of said power battery if case a) occurs without prior occurrence of case b) .

3. A device according to claim 2, characterized in that the de- vice is configured to use information about the value of the system voltage level at a shift of time gradient of the system voltage according to b) to determine said ability and by that the status of the power battery (2).

4. A device according to claim 2 or 3, characterized in that the device is configured to carry out said evaluation by calculating or measuring the time needed for the system voltage to reach said limit value ( U nm ) .

5. A device according to any of claims 2-4, characterized in that the device is configured to calculate the integral between the system voltage level ( U _sys ) and said limit value ( U nm ) during a period of time starting by said activation of the device (9) , such as said predetermined period of time or a time estimated to be necessary for the system voltage to drop to said limit value.

6. A device according to any of claims 2-5, characterized in that the device is configured to carry out said evaluation by esti mating the development of the system voltage would said pre- determined period of time be longer than determined by the device.

7. A device according to any of the preceding claims, characterized in that the device is configured to control the control unit (10) to prevent operations, such as gear shifting , influencing the load of the electric system considerably during said pre- determined period of time.

8. A device according to any of the preceding claims, characterized in that the device is configured to lower said voltage ref erence value at a time (to) being delayed enough from a start of said engine to allow the value of the system voltage ( U _sys ) to arrive to said voltage reference value.

9. A device according to any of the preceding claims, characterized in that the device is configured to lower said voltage ref erence value to a limit value ( U n m ) being at least 5% lower than said expected voltage level ( U bex ) of the power battery (2) .

10. A device according to any of the preceding claims, characterized in that the nominal voltage level of said power battery (2) is 24V, a nominal system voltage value is above 28V, and the device ( 12) is configured to lower the voltage reference value delivered to the control unit ( 10) to a limit value being at least 7% lower than the nominal system voltage value, 7- 20% or 7-15% lower than the nominal system voltage value.

1 1 . A device according to any of the preceding claims, characterized in that the device is configured to keep the voltage ref- erence value at said limit value ( U nm ) during a period of time of at least 2 s, 2 s - 10 s or 2 s - 5 s.

12. A method for controlling an electric system of a motor vehicle (1 ), said system comprising at least one power battery (2) , a generator (5) driven by an engine of the vehicle as well as consumers (6) of electric energy connected between opposite poles (3, 4) of the power battery on a consumers’ side thereof, a unit ( 10) configured to control said generator (5) to deliver a system voltage ( U _Sys ) to said consumers according to a volt- age reference value and means (1 1 ) configured to measure said system voltage, in which the method comprises the step of, during operation of the engine of said vehicle,

• check and determine the ability of the power battery (2) to deliver electric energy to said consumers,

characterized in that said step of checking and determining comprises the following sub-steps to

• at a time of starting said step instantaneously lower said voltage reference value delivered to said unit (10) to a limit value ( U nm ) lower than an expected voltage level ( U _bex) of said power battery and keep the voltage refer- ence value at this limit value during a predetermined pe- riod of time,

• record the development of the system voltage ( U _Sys ) measured during said period of time, and • evaluate said development of the system voltage during said period of time and determine the ability of said power battery (2) to deliver electric energy to said con- sumers (6) on the basis of the result of said evaluation.

13. A method according to claim 12, characterized in that the voltage reference value is kept at said limit value ( U n m ) during a period of time with a length allowing the system voltage to fall below said expected voltage level ( U bex ) of the power bat- tery while at least one of a) falling to said limit value and b) arriving to and passing below a level as of which the time gra- dient of the system voltage drop is abruptly reduced, and that it is determined that there is a malfunction of said power bat- tery if case a) occurs without prior occurrence of case b).

14. A method according to claim 12 or 1 3, characterized in that it comprises the step of estimating whether an ordering of an engine off is approaching during driving of said vehicle ( 1 ), and that the checking and determining step is carried out so as to determine whether the power battery (2) is in a status allowing such an engine off to take place.

15. A method according to claim 14, characterized in that said estimation comprises gathering and evaluating information about the road ahead of the vehicle so as to determine whether there is any imminent request to switch the engine off.

16. A computer program comprising computer program code for causing a computer to implement a method according to any of claims 1 2- 15 when the computer program is executed in the computer.

17. A computer program product comprising a non-transitory data storage medium ( 18) which can be read by a computer and on which the program code of a computer program according to claim 16 is stored.

18. An electronic control unit (8) of a motor vehicle comprising an execution means (15) , a memory (1 6) connected to the exe- cution means and a non-transitory data storage medium (18) which is connected to the execution means and on which the computer program code of a computer program according to claim 16 is stored.

19. A motor vehicle, especially a wheeled vehicle such as a truck or a bus, characterized in that it comprises an electronic con- trol unit according to claim 1 8.