Electrical system for an electrically powered motor vehicle

Abstract

Method for controlling an electrical system (12) of a hybrid vehicle (10), wherein a first DC electrical voltage is provided by means of a first electrical energy storage device (18) of a first on-board electrical system (14) and a second DC electrical voltage is provided by means of a second electrical energy storage device (20) of a second on-board electrical system (16), wherein the first and the second electrical energy storage devices (18, 20) are designed for a rated electrical voltage which is a low voltage within the meaning of the standard, wherein the two on-board electrical systems (14, 16) are electrically coupled to each other by means of a DC / DC converter (22), wherein the hybrid vehicle (10) is driven in intended driving operation by means of an electric machine (24) connected to the first on-board electrical system (14) and which can be coupled to an internal combustion engine used to drive the hybrid vehicle (10), wherein at a charge state of the first electrical energy storage device (18),which is smaller than a predefinable minimum charge state, an activation signal is provided for the internal combustion engine coupled to the electric machine (24) to assume and / or maintain an activated state of the internal combustion engine, wherein the DC / DC converter (22) is controlled depending on the charge state of the first electrical energy storage device (18) and on a charge state of the second electrical energy storage device (20) and the activation signal is provided taking into additional consideration the charge state of the second electrical energy storage device (20), wherein the energy storage capacities of the first and the second electrical energy storage device differ by a factor of at most 5, wherein even when the charge state of the first electrical energy storage device (18) is greater than or equal to the predefinable minimum charge state,electrical energy is transferred from the second to the first electrical network (14, 16) by means of the DC / DC converter (22) as long as the charge level of the second electrical energy storage device (20) is greater than or equal to the predefinable minimum charge level, wherein the activation signal is a stop prohibition signal for the internal combustion engine when the internal combustion engine is in the activated state, and that if the charge level of the first electrical energy storage device (18) is less than or equal to the predefinable minimum charge level, the charge level of the second electrical energy storage device (20) is determined, wherein if the charge level of the second electrical energy storage device (20) is greater than the predefinable minimum charge level, a comparison value (L, SE) for the charge state of the first electrical energy storage device (18), which is determined according to a proportional charge surplus of the second electrical energy storage device (20) above the predefinable minimum charge state, and the stop prohibition signal is only triggered when the charge state of the first electrical energy storage device (18) is smaller than the reference value (L SE ) is provided.

Description

[0001] The present invention relates to an electrical system for an electrically driven motor vehicle, comprising a first on-board electrical system having a first electrical energy storage device for a first electrical direct voltage, a second on-board electrical system having a second electrical energy storage device for a second electrical direct voltage, an electrical machine connected to the first on-board electrical system and capable of being coupled to an internal combustion engine for driving the motor vehicle in a normal driving operation, a DC / DC converter connected to the first and second on-board electrical systems for electrically coupling the two on-board electrical systems, and a control unit for providing an activation signal for the internal combustion engine, which can be coupled to the electrical machine, for assuming and / or maintaining an activated state of the internal combustion engine, wherein the control unit is designedto provide the activation signal for the internal combustion engine when the charge state of the first electrical energy storage device is less than a predeterminable minimum charge state. Furthermore, the invention also relates to a motor vehicle with an electrical system. Finally, the invention also relates to a method for controlling an electrical system of an electrically driven motor vehicle, wherein a first electrical direct voltage is provided by means of a first electrical energy storage device of a first on-board electrical system and a second electrical direct voltage is provided by means of a second electrical energy storage device of a second on-board electrical system, wherein the two on-board electrical systems are electrically coupled to one another by means of a DC / DC converter, wherein the motor vehicle is driven in intended driving mode by means of an electrical machine connected to the first on-board electrical system and coupleable to an internal combustion engine,wherein, when the charge state of the first electrical energy storage device is less than a predeterminable minimum charge state, an activation signal is provided for the internal combustion engine that can be coupled to the electric machine to assume and / or maintain an activated state of the internal combustion engine.

[0002] Electrical systems of the generic type, motor vehicles therewith, and corresponding methods are extensively known in the prior art, so that separate written proof is not required. The electrical system serves to distribute electrical energy between electrical units comprised by the electrical system, for example electric drives, electrical energy storage devices, vehicle lights, in particular vehicle headlights, and / or the like. Such units can, in addition to another electrical energy storage device, an energy source such as a solar module or the like, also be the electric drive unit that serves to drive the electrically driven motor vehicle. The electric drive unit generally comprises a rotating electrical machine that is preferably connected to the first on-board electrical system by means of an energy converter.In addition, a number of further electrical units, for example a lighting unit, headlights, interior lighting and / or the like, but also functional units such as a car radio, a navigation device, an electrically driven air conditioning compressor, electric windows and / or the like can be provided.

[0003] An electrically driven motor vehicle is preferably an electric vehicle, a hybrid vehicle that can be driven by both an electric motor and an internal combustion engine, and / or the like. The motor vehicle is preferably designed as a motor vehicle, in particular as a passenger car.

[0004] In today's electrically powered motor vehicles, the electrical system usually comprises two on-board electrical systems, namely the first and the second on-board electrical systems, which operate with different direct current voltages. One of the direct current voltages, provided by one of the two on-board electrical systems, is usually 12 V and is used to operate particularly low-power consumers such as the navigation system, the vehicle's interior lighting and / or the like. This is usually intended for the second on-board electrical system. A second of the electrical direct current voltages, preferably that of the second on-board electrical system, is usually 48 V. Units with a high power consumption are connected to this second on-board electrical system, for example the motor vehicle's electric machine, which serves to drive the vehicle and which provides the drive unit or part of it.For each of these two on-board electrical systems, namely the first and second energy storage units, a separate energy storage unit is provided, preferably in the form of an accumulator, which determines the respective direct current voltage. Furthermore, the two on-board electrical systems are electrically coupled to each other by a clocked energy converter in the form of a DC / DC converter, whereby an energy flow is possible at least unidirectionally from the second on-board electrical system to the first on-board electrical system, but preferably bidirectionally.

[0005] The presence of an internal combustion engine and an electric motor to drive the motor vehicle makes it possible to optimize the operation of the internal combustion engine so that, on the one hand, the internal combustion engine only needs to be operated when the available electrical energy of the first electrical energy storage device falls below a predetermined minimum charge level of the first electrical energy storage device. This can significantly reduce the consumption of fossil fuels by the internal combustion engine. At the same time, the emission of harmful exhaust gases can be reduced. The state of charge, also known as SOC (state of charge), thus influences the start-stop strategy for the operation of the internal combustion engine. If the state of charge is lower than the predetermined minimum charge level, a stop prohibition signal can be used as an activation signal to prevent the internal combustion engine from being switched off.This makes it possible to avoid a situation in which the supply of electrical energy by the first on-board electrical system, and preferably also by the second on-board electrical system, can no longer be guaranteed. If the internal combustion engine is in an activated state, a coupling with the electrical machine can cause a generation of electrical energy, by means of which at least the first electrical energy storage device, but preferably also the second electrical energy storage device, can be charged. However, if the internal combustion engine is in a deactivated state, a forced start signal can be generated if the charge state of the first electrical energy storage device falls below the predetermined minimum charge state, which triggers an immediate start of the internal combustion engine.

[0006] Typically, a compromise is determined between a comfort function for the driver, for example, by avoiding incomprehensible starts of the internal combustion engine after short stop phases, functional availability, for example, the presence of a start-stop function, and efficiency, for example, the lowest possible predefined minimum charge level. A method for providing a start-stop control is disclosed, for example, in DE 10 2009 029 227 A1. According to this document, an operating state of an electrical energy source is determined, and a stop enable is activated if the operating state of the electrical energy source is sufficient for starting the internal combustion engine with a starting device. Thus, it is determined whether the internal combustion engine can be restarted so that the motor vehicle can continue driving.DE 10 2009 034 004 A1 develops this idea further and attempts to enable a start-stop operation of the motor vehicle by predicting the expected voltage of a vehicle battery. This avoids undesirable undervoltage at on-board power supply consumers, preventing the undervoltage from disrupting the reliable operation of the on-board power supply consumers. Finally, DE 10 2007 037 937 A1 discloses that, in a motor vehicle's start-stop function, a second on-board power supply is available, decoupled from the first on-board power supply, which is intended to reduce voltage fluctuations for sensitive consumers. Consequently, no charge equalization is provided between the two on-board power supplies in relation to the start-stop function.

[0007] Furthermore, DE 10 2013 225 097 A1 discloses an energy management method for operating an electrical system of a motor vehicle, and DE 10 2004 008 817 A1 discloses a method and a device for charging a battery with dual-voltage electrical systems. Furthermore, DE 10 2010 046 616 A1 discloses a system and a method for supplying electrically operated consumers and motor vehicles. Furthermore, DE 10 2016 105 042 A1 discloses a method for controlling a drive train of a vehicle. Finally, DE 10 2015 006 454 A1 discloses a method and a control device for energy recuperation in a hybrid vehicle.

[0008] Even though this state-of-the-art technology has proven itself, it still has its drawbacks. Although the two electrical systems are electrically coupled, the current state of the art generally does not allow for the two electrical systems to interact for the reasons mentioned above. This prevents the start-stop function from being used optimally.

[0009] The object of the invention is to improve the start-stop function.

[0010] As a solution, the invention proposes an electrical system, a hybrid motor vehicle and a method according to the independent claims.

[0011] Further advantageous embodiments result from features of the dependent claims.

[0012] The invention is based on the finding that - contrary to the prior art - the additional use of the second on-board network for the start-stop function is possible without serious effects occurring, as discussed in the prior art.

[0013] In terms of the method, it is therefore proposed in particular in a generic method that the DC / DC converter is controlled as a function of the charge state of the first electrical energy storage device and of a charge state of the second electrical energy storage device, and that the activation signal is provided with additional consideration of the charge state of the second electrical energy storage device.

[0014] With regard to a generic electrical system, it is proposed in particular that the control unit for controlling the DC / DC converter is connected to the DC / DC converter and is designed to control the DC / DC converter depending on the charge state of the first electrical energy store and on a charge state of the second electrical energy store and to provide the activation signal with additional consideration of the charge state of the second electrical energy store.

[0015] On the motor vehicle side, it is particularly proposed that the motor vehicle has an electrical system of the invention.

[0016] The invention therefore proposes that the second electrical energy storage device of the second vehicle electrical system is used to improve the start-stop function.

[0017] For the purposes of the invention, it is irrelevant whether the current generated by the operation of the electric motor is positive or negative. Drive, within the meaning of the invention, therefore means not only increasing or maintaining the speed of the motor vehicle during normal driving operation, but also reducing the speed of the motor vehicle.

[0018] The first electrical energy storage device is preferably an accumulator, in particular a lithium-ion battery. The second electrical energy storage device is also an accumulator, preferably a lead-acid battery.

[0019] The control unit can be included in the electrical system. However, it can also be included in a higher-level vehicle control system of the motor vehicle or be designed as a separate control unit. So that the control unit can control the DC / DC converter, it is connected to it in a suitable manner. The control unit supplies a suitable control signal to the DC / DC converter so that it carries out the appropriate energy conversion. Depending on the operating state, the DC / DC converter can cause an energy flow from the second to the first on-board electrical system or, preferably, also vice versa. The DC / DC converter can preferably also be used to adjust the power exchanged between the two on-board electrical systems.

[0020] The control unit is also connected to a control device for the internal combustion engine. It can also be formed integrally with the internal combustion engine. The control unit provides the activation signal so that the internal combustion engine either maintains its active state if it is already active, or assumes the active state if it is deactivated. In the latter case, this means that the internal combustion engine is started. The internal combustion engine can be started using the electric motor or a separate starter unit.

[0021] The control unit can be embodied as an electronic circuit, in particular as a hardware circuit. Furthermore, it can also comprise a computer unit having a processor, for example, a microprocessor or the like. The computer unit further comprises a program memory in which an executable program for the computer unit is stored, wherein the computer unit is configured to provide a functionality corresponding to the executable program. The computer unit can, of course, also be combined with an electronic circuit in order to be able to provide the desired function of the control unit.

[0022] In addition to the units already mentioned that are connected to the vehicle electrical systems, it can be provided, for example, that an electric turbocharger, an electric roll stabilization system, an electric heater, and / or the like can be connected to the first vehicle electrical system. Furthermore, a sound system, various heating devices such as seat heating, window heating, and / or the like, an air conditioning system, and / or the like can be connected to the second vehicle electrical system.

[0023] The invention is particularly suitable for use in electrical energy storage devices that have approximately comparable energy storage capacity, wherein the energy storage capacities of the first and second electrical energy storage devices differ by a factor of approximately at most 5, preferably 2. Particularly preferably, the first and second electrical energy storage devices are energy storage devices designed for a rated electrical voltage that is a low voltage as defined by the standard and is preferably not greater than approximately 60 V.

[0024] The predeterminable minimum charge, for example, for providing the activation signal, is preferably determined by the control unit. The control unit determines this by taking into account additional parameters, which in particular take into account the on-board electrical system protection of the first and second on-board electrical systems. To provide a forced start signal as an activation signal, the control unit can determine a minimum charge level of, for example, 10%. To provide a stop signal as an activation signal, the control unit can determine a minimum charge level of, for example, 16.7%. The minimum charge level can therefore vary depending on the type of activation signal.

[0025] The minimum charge level and the other charge levels are relative values ​​in this case, which in this case are related to the available respective energy storage capacity. For example, the control unit can specify a minimum charge level of 10% to provide the forced start signal. Furthermore, the control unit can, for example, take into account a charge level difference in order to determine a minimum time for a stop phase, for example 60 s. This means that the internal combustion engine remains in the deactivated state for at least this period and can only be started again after this minimum time has elapsed. The minimum time should not be chosen too short so that the driver can avoid as frequent starts of the internal combustion engine as possible, which are not immediately comprehensible, especially when waiting at a traffic light.The charge state difference is additionally determined based on the average consumption of the other units connected to the respective on-board electrical systems as well as the respective current charge states of the first and second electrical energy storage devices. The average power that can be consumed by the other units in the first and second on-board electrical systems can be, for example, 2 kW. Taking this into account, a minimum charge state of 16.7% can be determined by the control unit for a no-stop signal, for example. If this charge state is reached during operation of the first electrical energy storage device or the second electrical energy storage device, the no-stop signal can be used to maintain the activated state of the internal combustion engine. This prevents the internal combustion engine from automatically shutting down.

[0026] The invention is therefore based on the realization that the charge state of the two electrical energy storage devices should be considered jointly and not separately. Therefore, unlike the prior art, the provision of the activation signal no longer depends solely on the charge state of the first energy storage device. In particular, the invention takes into account the fact that the DC / DC converter enables energy to be transferred from one of the two electrical energy storage devices to the other, so that, for example, the point in time until the minimum charge state is reached can be postponed. Furthermore, the minimum charge state itself can be determined to be smaller because, for example, a charge state control provided by the control unit no longer needs to provide as much energy reserve, resulting in better energy efficiency.Finally, if the performance of other units, especially consumers, changes, the forced start of the internal combustion engine can be postponed.

[0027] This proves particularly advantageous when the two electrical energy storage devices provide a comparable charge capacity. In this case, the effect of the invention is particularly advantageous. Thus, the invention makes it possible to support the first electrical energy storage device with the second electrical energy storage device, and vice versa, for the purpose of an improved start-stop function.

[0028] Furthermore, it is proposed that the activation signal be provided taking into account the electrical power consumption of electrical devices connected to the first and / or second on-board electrical system. The electrical devices or units can, for example, result in electrical energy being drawn from the first and / or second electrical energy storage device. This can result in a permanent discharge at the first and / or second electrical energy storage device. The control unit can take this charge withdrawal into account in order to provide the activation signal. For example, the control unit can determine, as part of a forecast, when the activation signal should be provided so that, during current operation of the two on-board electrical systems, the electrical energy storage devices can avoid reaching or falling below the predetermined minimum charge level.

[0029] According to a first aspect, it is provided that electrical energy is supplied from the second to the first vehicle electrical system by means of the DC / DC converter as soon as the charge state of the first electrical energy storage device is greater than or equal to the predeterminable minimum charge state, as long as the charge state of the second electrical energy storage device is greater than or equal to the predeterminable minimum charge state. This makes it possible to delay the first electrical energy storage device reaching the minimum charge state during a discharge by units connected to the first electrical energy storage device. This allows the activation signal to be provided with a corresponding delay, so that the start-stop function can be improved.

[0030] It is further proposed that the activation signal be provided only when both the charge level of the first electrical energy storage device and the charge level of the second electrical energy storage device are less than the predeterminable minimum charge level. The control unit therefore considers the discharge of both electrical energy storage devices jointly, and the activation signal is only provided when both electrical energy storage devices have reached or fallen below the minimum charge level.

[0031] It is provided that the activation signal is a stop prohibition signal for the internal combustion engine when the internal combustion engine is in the activated state, and that if the charge state of the first electrical energy store is less than or equal to the predeterminable minimum charge state, the charge state of the second electrical energy store is determined, wherein if the charge state of the second electrical energy store is greater than the predeterminable minimum charge state, a comparison value for the charge state of the first electrical energy store is determined, which is smaller than the predeterminable minimum charge state in accordance with a proportional charge surplus of the second electrical energy store above the predeterminable minimum charge state, and the stop prohibition signal is only provided when the charge state of the first electrical energy store is lower than the comparison value.This embodiment allows for a further improvement in the start-stop function with regard to the stop-stop signal as an activation signal. This embodiment makes it possible to ensure that the stop-stop signal only needs to be provided when a charge level is reached that is significantly below the predeterminable minimum charge level and is determined by the comparison value for the charge level of the first electrical energy storage device. This allows the stop-stop signal to be provided considerably later. The control unit determines a proportional excess charge of the second electrical energy storage device, which can be made available to the first electrical energy storage device by means of the DC / DC converter. For this reason, the first electrical energy storage device can be discharged more deeply than would otherwise be appropriate.Because the first electrical energy storage device can receive a corresponding proportion of charge from the second electrical energy storage device in a deeply discharged state, reliable operation can be guaranteed even when the charge level of the first electrical energy storage device is lower than the minimum charge level. The proportion of available electrical energy is determined by the respective loads and charge levels of the first and second electrical energy storage devices. The control unit takes these into account to determine the comparison value. For example, it can be provided that with an approximately equal load from electrical consumers as units or electrical devices, approximately half of the excess charge of the second electrical energy storage device can be made available to the first electrical energy storage device.On this basis, the control unit can then determine the comparison value.

[0032] According to a second aspect, a predeterminable minimum time period for a deactivated state of the internal combustion engine is taken into account to provide the stop prohibition signal. The predeterminable minimum time period can be, for example, 60 seconds or more. It is preferably also determined by the control unit, taking into account the charge states of the two electrical energy storage devices as well as the energy consumption by electrical devices or units of the first and second vehicle electrical systems.

[0033] The control unit is preferably also connected to the electrical devices or units of the first and second electrical systems in order to be able to determine a respective energy contribution for the respective electrical system. For example, it can be provided that the respective devices or units transmit their energy consumption or energy supply to the control unit.

[0034] It is further proposed that the activation signal be a forced start signal for the internal combustion engine when the internal combustion engine is deactivated. This makes it possible to reliably start the internal combustion engine while simultaneously ensuring reliable operation of the first and second electrical systems. This makes it possible to provide the forced start signal in such a way that starting the internal combustion engine can largely prevent effects on the first and second electrical systems.

[0035] A further development of the invention provides that the first electrical direct voltage is greater than the second electrical direct voltage. This proves to be particularly advantageous in that the electrical machine is connected to the first electrical on-board network. As a rule, the electrical machine requires a high electrical output, which is why it is advisable to use it at the highest possible electrical voltage in order to keep the current applied comparatively low. This means that cable cross-sections can be reduced accordingly because, due to the high voltage, only a correspondingly reduced current is required for the same output. In principle, however, it is of course also possible for the first and second electrical direct voltages to be of the same magnitude.

[0036] Further advantages and features will become apparent from the following description of embodiments with reference to the attached figures.

[0037] They show: Fig. 1 is a schematic representation of an electrical system in a motor vehicle according to the invention, Fig. 2 is a first schematic diagram in which a state of charge, denoted by SOC, is represented on an ordinate, whereas an abscissa represents time t, Fig. 3 a second schematic diagram as Fig. 2, whereby a reduced minimum charge is determined, and Fig. 4 a third schematic diagram as Fig. 2, which represents an extended time until a forced start signal is provided by the control unit.

[0038] It shows Fig. 1 shows an electrical system 12 for an electrically driven hybrid vehicle 10, which in this case is a passenger car. The electrical system 12 has a first electrical system 14, which has a first electrical energy storage device in the form of a lithium-ion accumulator 18 for a first electrical direct voltage, which in this case is approximately 48 V. Furthermore, the electrical system 12 includes a second electrical system 16, which has a second electrical energy storage device in the form of a conventional lead-acid accumulator 20 for a second electrical direct voltage, which in this case is approximately 12 V.

[0039] An electric drive unit 24 for driving the hybrid vehicle 10 during normal driving operation is connected to the electrical system 14. The electric drive unit 24 comprises an electric machine (not shown in detail), which in this case is a synchronous machine. Furthermore, the drive unit 24 comprises an inverter (also not shown in detail), to which the synchronous machine is connected. On the DC side, the inverter is connected to the first electrical system 14, thereby providing the connection of the electric machine to the first electrical system 14.

[0040] The electrical system 12 further comprises a DC / DC converter 22 connected to the first and second on-board electrical systems 14, 16 for electrically coupling the two on-board electrical systems 14, 16. The DC / DC converter 22 is designed in this case for a bidirectional energy exchange between the on-board electrical systems 14, 16. The DC / DC converter 22 is further connected to a control unit 28, which in this case is comprised of a motor vehicle-side vehicle control system (not shown in detail). Alternatively or additionally, the control unit can also be at least partially comprised by the electrical system 12. The energy transfer between the first and second on-board electrical systems 14, 16 can be controlled by means of the control unit 28 using the DC / DC converter 22.

[0041] Furthermore, the electrical system 12 has a current sensor 30 connected to the control unit 28 for detecting an electrical energy storage current of the first accumulator 18. Accordingly, a further current sensor is provided for the second accumulator 20, which is also connected to the control unit 28, but this is Fig. 1 is not shown.

[0042] Furthermore, it is provided that the electrical system 12 uses a common electrical reference potential 32 for the first and second electrical systems 14, 16. In this case, the reference potential 32 is the vehicle ground of the hybrid vehicle 10. Thus, the first electrical DC voltage is greater than the second electrical DC voltage.

[0043] For the exemplary embodiments described below, it is provided that the first and second rechargeable batteries 18, 20 have the same energy storage capacity. For the following exemplary embodiments, it is further provided that the electric machine does not provide any electrical power. For both rechargeable batteries 18, 20, the control unit 28 has determined a forced start charge state of 10% as the minimum charge state. For the following exemplary embodiments, it is further provided that consumers 34 connected to both the first on-board electrical system and the second on-board electrical system 14, 16 draw a total power of approximately 2 kW each from the first and second rechargeable batteries 18, 20. In the present case, 500 Wh is provided as the energy content for the first and second rechargeable batteries 18, 20.

[0044] A first design is based on Fig. 2, which shows a first diagram in which a state of charge, designated SOC, is shown on an ordinate, whereas an abscissa represents time t. This embodiment relates to an extended time until a stop prohibition signal is provided as an activation signal by the control unit 28. The first accumulator 18 has a state of charge of 18% at time t = 0, whereas the second accumulator 20 has a state of charge of 22% at this time. Both accumulators 18, 20 are discharged with a power of approximately 2 kW, which is indicated by the graphs 40, 46 in the diagram of Fig. 2. Graph 46 represents the discharge of accumulator 18, whereas graph 40 represents the discharge of accumulator 20.

[0045] The control unit 28 has already determined a preset minimum charge of 16.7 percent. In the diagram in Fig. 2, this charge state level is designated by the reference symbol L s If the first accumulator 18 reaches this charge level L s , in the prior art, the control unit 28 would provide the stop prohibition signal, as shown by the pointer 58 in Fig. 2. This would provide the no-stop signal at time t1.

[0046] According to the invention, the DC / DC converter 22 is now controlled by the control unit 28 in such a way that the accumulator 20 supports the accumulator 18 in such a way that both accumulators 18, 20 have the charge state L at the same time. s reach. This can be seen in Fig. 2, that this state is reached at time t2, which is later than time t1. This is indicated by the pointer 56 in Fig. 2. This also changes the discharge curves of the accumulators 18, 20, as shown in graphs 42 for the accumulator 20 and 44 for the accumulator 18.

[0047] Out of Fig. 2 it can be seen that the charge state of the accumulator 20 already at time t1 has reached the charge state of 16.7%, that is L s , is achieved. By means of the control according to the invention, it is now possible to postpone the time for providing the stop prohibition signal to time t2. The achievement of the predeterminable minimum charge state L s and the associated stop prohibition for the internal combustion engine would therefore have occurred earlier without the support of the accumulator 18 by the accumulator 20.

[0048] A second embodiment of the invention is based on Fig. 3, which shows a diagram like Fig. 1. Here, too, the state of charge SOC is shown on the ordinate against a time axis t as the abscissa. The initial charge states of the accumulators 18, 20 are again shown here as in the previous example for Fig. 2. Here too, the internal combustion engine is in the activated state.

[0049] This configuration takes into account that the accumulator 20 can support the accumulator 18 as soon as necessary. First, both accumulators 18, 20 are discharged again with 2 kW each, which is shown by the graphs 40, 46. Fig. 3 it can be seen that the charge state 10% is a forced start threshold L Zas an activation signal. In the prior art, reaching this charge level by the accumulator 18 would result in the control unit 28 issuing a forced start signal. However, this is not necessary in the present case because, in this embodiment, the internal combustion engine is already in the active state.

[0050] From the Fig. 3 it is also evident that the control unit 28 has already specified the minimum charge state, which is again indicated by L s Immediately before reaching the charge state L z by the accumulator 18 at time t5, the charge level of the accumulator 20 is still about 14%. This means that there is a 4% reserve until the accumulator 20 also reaches the 10% threshold L z would reach. Upon reaching the charge state L zthe accumulator 18 is no longer discharged. At this point in time, the accumulator 20 additionally takes over the energy supply for the accumulator 18, namely for an amount of energy corresponding to approximately 2% of the charge stroke, and thus supplies the first on-board network 14 of the accumulator 18. Fig. 3, the graphs 48, 50 show the predicted charge curve when the accumulator 18 has the charge state L z would achieve.

[0051] Out of Fig. 3 further shows that at time t3, a 60-second signal is provided by the control unit 28, which allows the internal combustion engine to stop, i.e., the internal combustion engine can be switched off during this period, designated Δt. This period ends at time t5. At time t5, the first accumulator 18 reaches the charge level L z, so that the control unit 28 would have to provide a corresponding forced start signal.

[0052] By supplying the first on-board network 14 by the second accumulator 20, the time for reaching the charge state L z for both accumulators 18, 20 can be shifted to time t6. This also allows the 60-second signal Δt to be shifted accordingly, so that the beginning of the period Δt falls at time t4. The period Δt, in which a stop of the internal combustion engine is possible while adhering to the 60-second period, is thus shifted back in time. As a result, the stop prohibition signal, which would otherwise be generated by the accumulator 18 upon reaching the charge level L s provided by the control unit 28, to the charge state L sewhich in this case is 16.7 - 2 = 14.7%. By proportionally taking into account the charge reserve of the second accumulator 20, the minimum charge level can thus be determined to be lower.

[0053] A third configuration is shown in a diagram according to Fig. 4, which relates to an extended time until a forced start signal is provided as an activation signal by the control unit 28. From the diagram according to Fig. 4, which, like the diagrams according to the Fig. 2 and Fig. 3, it follows that the time until a forced start can be delayed.

[0054] In this embodiment, an actual charge level of 12% is provided for the accumulator 18 as the initial charge level, whereas the accumulator 20 has an actual charge level of 16% as the initial charge level. In this embodiment, the internal combustion engine is in the deactivated state. Graphs 40, 46 again represent the discharge curves in the prior art. At time t7, the accumulator 18 would have the charge level L Z and the control unit 28 provides the forced start signal. The accumulator 18 would consequently reach the charge state L z already at time t7.

[0055] In the present case, however, the DC / DC converter 22 is controlled by the control unit 28 in such a way that the accumulator 20 supports the accumulator 18. This changes the discharge curves, which is shown by the graphs 52 and 54 in Fig. 4. As a result, both accumulators 18, 20 reach the specified minimum charge level for the forced start threshold L z at the same time t8. Both accumulators 18, 20 are discharged - as in the previous embodiments - each with a power of 2 kW, which is shown by the graphs 40, 46. This makes it possible to ensure that the charge state of both accumulators 18, 20 is only reached at time t8, so that an extended period is available until which the control unit 28 must provide the forced start signal. In the diagram of the Fig. 4 this is shown by graphs 52 and 54. Reaching the charge state L z and the associated start request for the internal combustion engine would therefore have occurred earlier in the prior art due to the sole operation of the accumulator 18, which would not have been supported by the accumulator 20.

[0056] The exemplary embodiments described above serve merely to explain the invention and are not limiting thereof. Thus, virtually any values ​​can be realized for the DC voltages without departing from the spirit of the invention. Other types of accumulators can also be provided for the electrical energy storage devices, for example, nickel-cadmium accumulators, lithium titanate-based accumulators, and / or the like. Even though the exemplary embodiments assume a constant discharge, it is self-evident to those skilled in the art that varying discharges can also be taken into account accordingly. For example, when such variations occur, the control unit can redetermine the minimum charge level, and the like.

[0057] The advantages and configurations described for the electrical system according to the invention apply equally to the motor vehicle equipped with the electrical system according to the invention and to the method according to the invention. Accordingly, method features can also be provided for device features, and vice versa.

Claims

[1] Method for controlling an electrical system (12) of a hybrid vehicle (10), wherein a first electrical direct voltage is provided by means of a first electrical energy store (18) of a first on-board electrical system (14) and a second electrical direct voltage is provided by means of a second electrical energy store (20) of a second on-board electrical system (16), wherein the first and the second electrical energy store (18, 20) are designed for an electrical rated voltage which is a low voltage in the sense of the standardization, wherein the two on-board electrical systems (14, 16) are electrically coupled to one another by means of a DC / DC converter (22), wherein the hybrid vehicle (10) is driven in a normal driving operation by means of an electrical machine (24) connected to the first on-board electrical system (14) and which can be coupled to an internal combustion engine serving to drive the hybrid vehicle (10), wherein in a charge state of the first electrical energy store (18),which is smaller than a predeterminable minimum charge state, an activation signal is provided for the internal combustion engine that can be coupled to the electric machine (24) to assume and / or maintain an activated state of the internal combustion engine, wherein the DC / DC converter (22) is controlled as a function of the charge state of the first electrical energy store (18) and of a charge state of the second electrical energy store (20), and the activation signal is provided with additional consideration of the charge state of the second electrical energy store (20), wherein the energy storage capacities of the first and second electrical energy stores differ by a factor of at most 5, wherein even when the charge state of the first electrical energy store (18) is greater than or equal to the predeterminable minimum charge state,by means of the DC / DC converter (22) electrical energy is supplied from the second to the first vehicle electrical system (14, 16) as long as the charge state of the second electrical energy store (20) is greater than or equal to the predeterminable minimum charge state, wherein the activation signal is a stop prohibition signal for the internal combustion engine when the internal combustion engine is in the activated state, and that if the charge state of the first electrical energy store (18) is less than or equal to the predeterminable minimum charge state, the charge state of the second electrical energy store (20) is determined, wherein if the charge state of the second electrical energy store (20) is greater than the predeterminable minimum charge state, a comparison value (L, SE) is determined for the charge state of the first electrical energy store (18), which is smaller than the predeterminable minimum charge state in accordance with a proportional charge surplus of the second electrical energy store (20) above the predeterminable minimum charge state, and the stop prohibition signal is only triggered when the charge state of the first electrical energy store (18) is smaller than the comparison value (L SE ) is provided. [2] Method according to claim 1, characterized by that a predeterminable minimum time period (Δt) for a deactivated state of the internal combustion engine is taken into account in order to provide the stop prohibition signal. [3] Method for controlling an electrical system (12) of a hybrid vehicle (10), wherein a first electrical direct voltage is provided by means of a first electrical energy store (18) of a first on-board electrical system (14) and a second electrical direct voltage is provided by means of a second electrical energy store (20) of a second on-board electrical system (16), wherein the first and the second electrical energy store (18, 20) are designed for an electrical rated voltage which is a low voltage in the sense of the standardization, wherein the two on-board electrical systems (14, 16) are electrically coupled to one another by means of a DC / DC converter (22), wherein the hybrid vehicle (10) is driven in a normal driving operation by means of an electrical machine (24) connected to the first on-board electrical system (14) and which can be coupled to an internal combustion engine serving to drive the hybrid vehicle (10), wherein in a charge state of the first electrical energy store (18),which is smaller than a predeterminable minimum charge state, an activation signal is provided for the internal combustion engine that can be coupled to the electric machine (24) to assume and / or maintain an activated state of the internal combustion engine, wherein the DC / DC converter (22) is controlled as a function of the charge state of the first electrical energy store (18) and of a charge state of the second electrical energy store (20), and the activation signal is provided with additional consideration of the charge state of the second electrical energy store (20), wherein the energy storage capacities of the first and second electrical energy stores differ by a factor of at most 5, wherein the activation signal is a stop prohibition signal for the internal combustion engine when the internal combustion engine is in the activated state, and that,if the charge state of the first electrical energy store (18) is less than or equal to the predeterminable minimum charge state, the charge state of the second electrical energy store (20) is determined, wherein, if the charge state of the second electrical energy store (20) is greater than the predeterminable minimum charge state, a comparison value (L, SE ) is determined for the charge state of the first electrical energy store (18), which is smaller than the predeterminable minimum charge state in accordance with a proportional charge surplus of the second electrical energy store (20) above the predeterminable minimum charge state, and the stop prohibition signal is only triggered when the charge state of the first electrical energy store (18) is smaller than the comparison value (L SE), wherein a predeterminable minimum time period (Δt) for a deactivated state of the internal combustion engine is taken into account for providing the stop prohibition signal. [4] Method according to claim 3, characterized by that, even when the charge state of the first electrical energy store (18) is greater than or equal to the predeterminable minimum charge state, electrical energy is conveyed from the second to the first on-board network (14, 16) by means of the DC / DC converter (22), as long as the charge state of the second electrical energy store (20) is greater than or equal to the predeterminable minimum charge state. [5] Method according to one of the preceding claims, characterized by that the activation signal is provided taking into account an electrical power consumption of electrical devices connected to the first and / or the second on-board network (14, 16). [6] Method according to one of the preceding claims, characterized by that the activation signal is only provided when both the charge state of the first electrical energy store (18) and the charge state of the second electrical energy store (20) are less than the predeterminable minimum charge state. [7] Method according to one of the preceding claims, characterized by that the activation signal is a forced start signal for the internal combustion engine when the internal combustion engine is in the deactivated state. [8] Method according to one of the preceding claims, characterized by that the first electrical DC voltage is greater than the second electrical DC voltage. [9] Electrical system (12) for a hybrid vehicle (10), comprising: - a first on-board network (14) having a first electrical energy storage device (18) for a first electrical direct voltage, - a second on-board network (16) having a second electrical energy store (20) for a second electrical direct voltage, wherein the first and the second electrical energy store (18, 20) are designed for an electrical rated voltage which is a low voltage in the sense of the standard, - an electric machine (24) connected to the first vehicle electrical system (14) and capable of being coupled to an internal combustion engine used to drive the hybrid vehicle (10) for driving the hybrid vehicle (10) in a normal driving mode, - a DC / DC converter (22) connected to the first and second vehicle electrical systems (14, 16) for electrically coupling the two vehicle electrical systems (14, 16), and - a control unit (28) for providing an activation signal for the internal combustion engine that can be coupled to the electric machine (24) for assuming and / or maintaining an activated state of the internal combustion engine, wherein the control unit (28) is designed to provide the activation signal for the internal combustion engine when the charge state of the first electrical energy store (18) is less than a predeterminable minimum charge state, wherein the energy storage capacities of the first and second electrical energy stores differ by a factor of at most 5 and the control unit (28) is connected to the DC / DC converter (22) and designed to control the DC / DC converter (22),to control the DC / DC converter (22) depending on the charge state of the first electrical energy store (18) and on a charge state of the second electrical energy store (20) and to provide the activation signal with additional consideration of the charge state of the second electrical energy store (20), wherein, even when the charge state of the first electrical energy store (18) is greater than or equal to the predeterminable minimum charge state, electrical energy is conveyed from the second to the first vehicle electrical system (14, 16) by means of the DC / DC converter (22) as long as the charge state of the second electrical energy store (20) is greater than or equal to the predeterminable minimum charge state, wherein the activation signal is a stop prohibition signal for the internal combustion engine when the internal combustion engine is in the activated state, and wherein,if the charge state of the first electrical energy store (18) is less than or equal to the predeterminable minimum charge state, the charge state of the second electrical energy store (20) is determined, wherein, if the charge state of the second electrical energy store (20) is greater than the predeterminable minimum charge state, a comparison value (L, SE ) is determined for the charge state of the first electrical energy store (18), which is smaller than the predeterminable minimum charge state in accordance with a proportional charge surplus of the second electrical energy store (20) above the predeterminable minimum charge state, and the stop prohibition signal is only triggered when the charge state of the first electrical energy store (18) is smaller than the comparison value (L SE ) is provided. [10] Electrical system (12) for a hybrid vehicle (10), comprising: - a first on-board network (14) having a first electrical energy storage device (18) for a first electrical direct voltage, - a second on-board network (16) having a second electrical energy store (20) for a second electrical direct voltage, wherein the first and the second electrical energy store (18, 20) are designed for an electrical rated voltage which is a low voltage in the sense of the standard, - an electric machine (24) connected to the first vehicle electrical system (14) and capable of being coupled to an internal combustion engine used to drive the hybrid vehicle (10) for driving the hybrid vehicle (10) in a normal driving mode, - a DC / DC converter (22) connected to the first and second vehicle electrical systems (14, 16) for electrically coupling the two vehicle electrical systems (14, 16), and - a control unit (28) for providing an activation signal for the internal combustion engine that can be coupled to the electric machine (24) for assuming and / or maintaining an activated state of the internal combustion engine, wherein the control unit (28) is designed to provide the activation signal for the internal combustion engine when the charge state of the first electrical energy store (18) is less than a predeterminable minimum charge state, wherein the energy storage capacities of the first and second electrical energy stores differ by a factor of at most 5 and the control unit (28) is connected to the DC / DC converter (22) and designed to control the DC / DC converter (22),to control the DC / DC converter (22) depending on the charge state of the first electrical energy store (18) and on a charge state of the second electrical energy store (20) and to provide the activation signal with additional consideration of the charge state of the second electrical energy store (20), wherein the activation signal is a stop prohibition signal for the internal combustion engine when the internal combustion engine is in the activated state, and that if the charge state of the first electrical energy store (18) is less than or equal to the predeterminable minimum charge state, the charge state of the second electrical energy store (20) is determined, wherein if the charge state of the second electrical energy store (20) is greater than the predeterminable minimum charge state, a comparison value (L, SE) is determined for the charge state of the first electrical energy store (18), which is smaller than the predeterminable minimum charge state in accordance with a proportional charge surplus of the second electrical energy store (20) above the predeterminable minimum charge state, and the stop prohibition signal is only triggered when the charge state of the first electrical energy store (18) is smaller than the comparison value (L SE ), wherein a predeterminable minimum time period (Δt) for a deactivated state of the internal combustion engine is taken into account for providing the stop prohibition signal. [11] Hybrid vehicle (10), characterized by an electrical system (12) according to claim 9 or 10.

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