WO2017220233A1

WO2017220233A1 - Motor vehicle electrical system with at least two energy stores, method for operating a motor vehicle electrical system and means of implementation

Info

Publication number: WO2017220233A1
Application number: PCT/EP2017/059469
Authority: WO
Inventors: Wolfgang Mueller
Original assignee: Robert Bosch Gmbh
Priority date: 2016-06-22
Filing date: 2017-04-21
Publication date: 2017-12-28
Also published as: DE102016211164A1

Abstract

A motor vehicle electrical system (100) is disclosed, which comprises an electric machine (EM) operable in motor-mode and generator mode, an active power converter (110) with DC voltage terminals (B+, B-) a first energy store (B₁) and a second energy store (B₂). Disclosed are means (S₁ -S₄, 120) which are configured to set all of the following specified circuit conditions a) to c) alternatively to each other: a) either only the first energy store (B₁) or only the second energy store (B₂) is connected to the DC voltage terminals (B+, B-) of the active power converter (110); b) the first energy store (B₁) and the second energy store (B₂) are connected to the DC voltage terminals (B+, B-) of the active power converter (110) in parallel; c) the first energy store (B₁) and the second energy store (B₂) are connected to the DC voltage terminals (B+, B-) of the active power converter (110) in series. A first capacitive element (C_z1) is permanently connected in parallel to the first energy store (B₁) and a second capacitive element (C_Z2) is permanently connected in parallel to the second energy store (B₂). A method for operating a corresponding motor vehicle electrical system and means of implementation are also objects of the invention.

Description

Description title

Motor vehicle electrical system with at least two energy stores, method for operating a motor vehicle electrical system and means for its implementation

The present invention relates to a motor vehicle electrical system with at least two energy stores, a method for operating such a motor vehicle electrical system and means for implementing the method.

State of the art

The recovery of braking energy in motor vehicles by recuperation is known. In recuperation systems mechanical energy is converted by an electric machine into electrical energy and stored in a battery or other energy storage during braking. If the braking power requested by the driver is less than or equal to the efficiency of the recuperation system, the braking is typically carried out exclusively by means of this. If the requested braking power is higher, a conventional braking system is additionally used. The potential fuel savings through recuperation is therefore not only dependent on the driving cycle and driver behavior, but also on the performance of the recuperation system. The latter is limited by the maximum regenerative power of the electric machine and the maximum electrical capacity of the energy store or stores in the motor vehicle electrical system.

Furthermore, as a fuel-saving measure the sailing operation is known. Here, the internal combustion engine is decoupled from the rest of the drive train during a so-called sailing phase. As a result, the engine drag does not affect the rest of the drive train and the coasting phase of the vehicle is significantly extended. In vehicles with a conventional drive train, the electrical consumers are supplied during the sailing phase only from the or the energy storage. Sailing and recuperation are not mutually exclusive. For example, the sailing operation can be initiated when the driver operates neither gas nor brake. The recuperation can be done when the driver operates the brake.

From DE 10 2013 204 894 A1, a motor vehicle electrical system is known which has an electrical machine, a first energy store and a second energy store. There are provided means by which either only one of the energy storage, both energy storage in parallel or the two energy storage can be connected in series to the electric machine. In such a motor vehicle electrical system, a more efficient recuperation can be achieved by increasing the voltage at the electric machine. This results in an increase of the regenerative power, which is available as braking power. Due to the variable interconnection of the energy storage results at the same time an increase in the absorption capacity of the motor vehicle electrical system. This makes it possible to feed in with higher generator power and thus an overall increase in the fuel savings in recuperation. The energy storage are also redundant for the sailing operation available, which increases the reliability of the vehicle electrical system in total.

It is desirable in certain cases, in a motor vehicle electrical system, as disclosed for example in DE 10 2013 204 894 A1, the electrical

Machine also to operate motor or to use a corresponding electric machine. In this way, mechanical power can be generated, for example for starting or assisting the internal combustion engine (for example to compensate for the turbo lag or in the so-called boost mode).

A corresponding electric machine is typically connected to the motor vehicle electrical system via an active power converter that can be operated as a DC and inverter, as also explained with reference to FIG. In a motorized operation of the electric machine, the active power converter is (pulse) alternating Inverter operated in a regenerative operation as a rectifier. A corresponding active power converter is typically designed as a bridge rectifier with a number of half-bridges corresponding to the number of phases of the electrical machine. An active power converter differs from a passive power converter in that controllable semiconductor switching elements, for example MOS field-effect transistors, are used as rectifying elements which are designed as diodes in a passive rectifier.

To compensate for voltage and current peaks and to smooth the voltage applied to the DC terminals (usually with B + and B-) of the active power converter in the generator and motor operation of the electric machine voltage is in a corresponding arrangement of electrical machine and active power converter between these DC voltage terminals According to the prior art, a capacitive element, the so-called intermediate circuit capacitor, is integrated.

Furthermore, it is desirable to be able to operate the electric machine in motor operation with different voltages in a motor vehicle electrical system, as disclosed, for example, in DE 10 2013 204 894 A1, which can be provided by the described interconnections of the energy stores. As a result, different mechanical services can be delivered by the electric machine as needed. However, when switching between these different voltages in a corresponding vehicle electrical system with a DC link capacitor, very high charge or discharge currents occur at the latter. The DC link capacitor can be damaged by these high charge or discharge currents.

There is therefore a need for corresponding on-board motor vehicle networks, which make it possible to operate the motor or generator-operated electric machine with different voltages without causing it

Damage due to switching between these different voltages or due to this caused high current flows can come. In particular, the present invention relates to the improvement of a vehicle electrical system, as disclosed in DE 10 2013 204 894 A1. Disclosure of the invention

Against this background, the present invention proposes a motor vehicle electrical system with at least two energy stores, a method for operating such a motor vehicle electrical system, and means for implementing the method having the features of the independent patent claims. Preferred embodiments are subject of the dependent claims and the following description.

Advantages of the invention

According to the invention, it has been recognized that a vehicle electrical system which has a motor and generator electric machine, a first energy storage and a second energy storage, and are provided in the means by which either only the first energy storage or only the second energy storage, the first Energy storage and the second energy storage in parallel or the first energy storage and the second energy storage can be connected in series to the electrical machine or its power converter can be improved, that instead of a capacitive element, ie the DC link capacitor, between the DC voltage terminals of the converter used several capacitive elements are used. These are permanently, ie not switched on or off, in each case arranged parallel to the energy storage, as also explained below.

In this case, a motor vehicle electrical system is proposed that has a generator and motor-operated electric machine, an active converter with DC voltage connections, a first energy store and a second energy store. The energy stores can be, for example, regular motor vehicle batteries, for example 12V lead batteries each. However, alternatively, other electrical storage technologies can be used. The electrical machine is connected with its phase connections to corresponding AC voltage connections of the power converter. concluded, as generally known. Furthermore, in the context of the present invention, means are provided which are set up to set all of the following switching states a) to c) as an alternative: a) either only the first energy store or only the second energy store is connected to the DC voltage terminals of the active converter; b) the first energy store and the second energy store are connected in parallel to the DC voltage terminals of the active power converter; and c) the first energy store and the second energy store are connected in series to the DC voltage connections of the active power converter.

According to the invention, a first capacitive element is connected to the first energy store and a second capacitive element is permanently connected in parallel to the second energy store. The capacitive elements may, for example, be commercially available capacitors.

Is here the term that corresponding capacitive elements "permanently connected in parallel" with the respective energy storage, it is understood that a corresponding capacitive element with one of two terminals-electrically conductive and permanently connected to a first pole of the energy storage, such as positive battery terminal, and with the second of the two terminals conductively and permanently connected to the second pole of the energy storage, for example, the negative battery terminal. Such a parallel connection is, of course, also present when the first of the two terminals of the capacitive element is connected to the positive battery terminal and the second of the two terminals and the "negative" battery terminal are connected to a common ground which is a conductive connection manufactures.

A corresponding connection is durable if it does not take place via a switching element but via cables and corresponding connections, for example solder joints, terminals or plugs, which are not interrupted in normal operation of a corresponding motor vehicle electrical system or whose elements are not separated or isolated from one another. With regard to the basic advantages of the vehicle electrical system proposed according to the invention with regard to recuperation, reference is made to the above explanations. This motor vehicle electrical system also offers fundamental advantages in sailing operation because, during a sailing phase, the motor vehicle electrical system can optionally be supplied by one of the energy stores. If one fails due to an error, you can switch to the other.

Due to the variable connection of the at least two energy storage devices, either one or both energy stores can feed the electric machine when the internal combustion engine is started, which then, powered by a motor, can start the internal combustion engine accordingly.

If only one of the energy storage devices is used for this purpose, the other energy storage device can be connected to the electrical consumers independently of this. This creates a starter circuit and a consumer circuit. At the

Starting the internal combustion engine, for example by means of a not shown in the attached Figure 2 for reasons of clarity pinion starter or with the electric machine already described, then results due to the decoupled starter circuit no disturbing voltage dip in the load circuit. This is particularly advantageous in the known start-stop systems in the warm start with the aid of a pinion starter or with the described electric machine (for example after a traffic light stop in city traffic).

In a cold start, in which voltage dips are typically perceived as less disturbing and a higher mechanical torque is required, however, the available power and thus the torque of the electric starting device can be increased by connecting at least two energy storage in parallel. The available voltage and thus the producible mechanical power can also be increased by a series connection of the energy storage.

The invention also makes it possible to encapsulate faults in the electric machine, ie to propagate a fault from the electric machine to the electrical machine. The supply network is prevented. This increases the reliability. In addition, a defective battery can be decoupled from the electrical system and thus the reliability can be further increased.

The advantages of the inventively provided, provided parallel to the energy storage capacitive elements is that at a desired switching between supply voltages for the motor or generator operated electric machine it can not come to high Umlade- currents in and out of the DC link capacitor and corresponding damage, because this is replaced by the capacitive elements.

An arithmetic unit according to the invention, e.g. a control device for a vehicle electrical system, as means for implementing the method according to the invention, in particular programmatically, adapted to carry out a method according to the invention.

The implementation of the method in the form of software is also advantageous, since this causes particularly low costs, in particular if an executing control device is still used for further tasks and therefore exists in any case. Such a control device is equipped with a suitable data carrier for storing a corresponding computer program, for example a hard disk and / or a flash memory.

Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination indicated, but also in other combinations or in isolation, without departing from the scope of the present invention.

The invention is illustrated schematically by means of exemplary embodiments in the drawing and will be described in detail below with reference to the drawing. Brief description of the drawings

Figure 1 illustrates a plot of maximum output power of a 14V claw pole generator over a generator speed for output voltages of 14V and 28V.

Figure 2 shows a motor vehicle electrical system according to an embodiment of the present invention in a simplified schematic representation.

FIG. 3 shows the motor vehicle electrical system according to FIG. 2 in accordance with FIG

Embodiment of the invention provided switching state.

FIG. 4 shows the motor vehicle electrical system according to FIG. 2 in accordance with FIG

Embodiment of the invention provided switching state.

FIG. 5 shows the motor vehicle electrical system according to FIG. 2 in accordance with FIG

Embodiment of the invention provided switching state.

FIG. 6 shows the motor vehicle electrical system according to FIG. 2 in accordance with FIG

Embodiment of the invention provided switching state.

Embodiment (s) of the invention

In FIG. 1, the typical profile of the maximum output power P of a generator-operated electric machine, for example a 14V claw-pole generator connected to a passive B6 bridge rectifier, is on the ordinate over the generator speed n on the abscissa for the exemplary output voltages Ui = 14V and U2 = 28V. The electric machine feeds a 14V vehicle electrical system, this can deliver electrical power from a speed n ₀ i. On the other hand, the same electric machine can only supply electric power when it is fed into a 28V vehicle electrical system at a speed of n ₀ 2 and above. At an output voltage of the electric machine of U2 = 28V, a speed of n> ni2 is significantly higher higher maximum output power than when supplying a 14V vehicle electrical system. This effect can be used to increase the output of the electric machine during recuperation. However, the invention particularly relates to the motor operation of a corresponding electric machine which is connected via a then operated as an inverter active power converter to a motor vehicle electrical system. The electrical machine can be supplied with different voltages depending on the operating state of the corresponding vehicle. Even with motorized operation of the electric machine results in a dining

Voltage of 28V a higher maximum output power than with a supply of 14V.

In FIG. 2, a motor vehicle electrical system according to an embodiment of the present invention in the form of a circuit diagram is shown schematically in simplified form. and 100 in total. This has a first energy store Bi and a second energy store B2, which may be formed, for example, in the form of identical or different motor vehicle batteries, for example in the form of two 12V motor vehicle batteries. With the first energy storage Bi a first capacitive element C _z i and the second energy storage B2, a second capacitive element C _Z 2 in the above-mentioned sense permanently connected in parallel. The energy storage Bi and B2 can by means of the switching elements S1, S2, S3 and S ₄ are connected variable. The switching elements S1 to S ₄ may be formed, for example, by MOS field-effect transistors or other electronic components. Electrical 14V consumers are collectively illustrated in the form of a resistor R1.

An electric machine EM can be operated both as a generator and as a motor. It can thus feed electrical power into the vehicle electrical system 100 or be supplied from this with electrical power. For this purpose, an active power converter 110 is provided, which can be operated in a regenerative operation of the electric machine, for example during recuperation, as a rectifier, and during a motor operation, for example to support the internal combustion engine, as an inverter. The active power converter 110 has controllable semiconductor switching elements 11 1. At DC voltage terminals B + and B- is a DC voltage. The DC voltage connection B- can be grounded. The controllable semiconductor switching elements 11 1 are illustrated as diodes with parallel switches. They can be designed, for example, as MOS field-effect transistors.

A control device 120 is designed to control the switching elements Si, S ₂ , S3 and S and possibly also the semiconductor switching elements 11 1.

The capacitive elements C _z i and C _z2 are used in the illustrated _{embodiment of} the invention instead of a single _{link capacitor} , which is included in the prior art between the DC voltage terminals B + and B-. In this way, as already explained above, achieved that when changing between supply voltages for the electric machine EM in motor and generator operation by a corresponding control of the switching elements S1, S ₂ , S3 and S no or at least no excessively high Umladeströme may occur ,

In the following, S _n = 0 denotes an open switching element S 1 to S, with S _n = 1 a closed switching element. The switching states of the motor vehicle electrical system 100 illustrated in FIGS. 3 to 6 and explained further below are set by means of the switching elements S1 to S.

With S1 = 0, S ₂ = 1, S3 = 0 and S = 1, a switching state according to FIG. 3 results. Both energy stores B1 and B ₂ are connected in series as shown in FIG. Thus, the capacitive elements C _z i and C _z2 permanently connected in parallel with the energy stores B1 and B ₂ are also connected in series. The electric machine EM feeds here either electrical power with an output voltage of about 28V in the vehicle electrical system 100 or is supplied from the motor vehicle electrical system 100 with a corresponding voltage. The second

Energy storage B ₂ supplies the consumers R1 with a voltage of 14V.

The switching state according to FIG. 3 can then be selected if both energy stores Bi and B ₂ are to be loaded simultaneously. He is preferred Recuperation selected. The higher output voltage of the electric machine EM also yields a higher output power for generator speeds n> ni 2 (see FIG. By the series connection of the two energy storage Bi and B2, the possible power consumption of the motor vehicle electrical system is doubled compared to a single energy storage Bi or B2.

During recuperation, the first energy store Bi is charged more than the second energy store B2, since the consumers R1 are connected in parallel to the second energy store B2. If the first energy store Bi has reached the upper limit of its state of charge, the first energy store must

Bi be discharged again to allow further Rekuperationsvorgänge. The discharge of the first energy store Bi is preferably carried out by setting the switching state shown in FIG. By supplying the electric machine EM in motor operation with a correspondingly high voltage, the maximum deliverable mechanical power is also increased. This can be used, for example, to assist the internal combustion engine in certain operating phases when necessary with a correspondingly high mechanical power.

With S1 = 1, S2 = 0, S3 = 1 and S = 0, a switching state according to FIG. 4 results.

The electric machine EM or the DC voltage connections B + and B- are connected according to FIG. 4 only with the first energy store Bi. Thus, only the capacitive element C _z i permanently connected in parallel with the first energy store Bi is connected between the DC voltage terminals B + and B-. The electric machine EM feeds an electric power with a correspondingly set voltage of approximately 14V into the motor vehicle electrical system 100 or is supplied with it accordingly. If the power fed in by the electric machine is smaller than the consumer power of the load R1, the first energy store Bi is discharged. If the power is greater than the consumer power R1, the first energy store Bi is charged. In the switching state shown, the first energy Memory Bi are charged without the state of charge of the second energy storage B2 is affected.

The switching state according to FIG. 4 is advantageously used when, for example, due to recuperation processes with the switching state shown in FIG. 3, the first energy store B1 has reached its upper permissible charging state. In this case, the power fed into the motor vehicle electrical system 100 by the electric machine is reduced so much that the first energy store B1 is discharged by the consumers R1. In the switching state shown in FIG. 4, the first energy store B1 can again be discharged to such an extent that, with the switching state shown in FIG. 3, further recuperation processes are subsequently possible again.

The switching state shown in Figure 4 can also be used for recuperation at low speed n <ni2 of the electric machine EM (see Figure 1).

In this case, electrical recuperation power can be stored in the first energy storage B1 even at low generator speed.

Optionally, the electric machine EM can also be supplied with only the first energy storage B1 in motor operation. This can be used, for example, to discharge a fully charged energy store B1 in order, as already described, to be able to store further recuperation energy in the energy store Bi. With S1 = 1, S2 = 0, S3 = 0 and S = 1, a switching state according to FIG. 5 results.

The electric machine EM is connected according to Figure 5 only with the second energy storage B2. Thus, only the capacitive element C _Z 2 permanently permanently connected in parallel with the second energy store B2 is connected between the DC voltage terminals B + and B-. The electric machine EM feeds at an output voltage of about 14V electrical power in the motor vehicle electrical system or is supplied from this with appropriate power. If the power fed in is smaller than the power of the consumers R1, the second energy store B2 is discharged. Is the fed-in power greater than the power of the consumer Ri, the second energy storage B2 is charged. The illustrated switching state can also be used for recuperation at low speed of the electric machine EM n <ni2 (see FIG. In this case, electrical recuperation power can be stored in the second energy store B2 even at low speed.

The electric machine EM can also be operated in the switching state according to Figure 5 optionally in the motor. With S1 = 1, S2 = 0, S3 = 1 and S = 1, a switching state according to FIG. 6 results.

Both energy stores B1 and B2 are connected in parallel according to FIG. This also applies to the capacitive elements C _z i and C _Z 2 permanently connected in parallel with the energy stores B1 and B2. This switching state can advantageously be used when both energy stores B1 and B2 are to be simultaneously charged or discharged. Another possible application arises when, during engine operation, the electric machine EM requires a high electrical current in order to generate the desired mechanical torque. This application can occur, for example, during a cold start after a very cold night.

Another application is sailing with the internal combustion engine switched off. In this case, as explained, the electrical consumers R1 are supplied only by the energy stores B1 and B2 because the electric machine EM is not driven. In vehicles with high-power electrical consumers R1, an excessively high voltage dip can be prevented by connecting the energy stores B1 and B2 in parallel.

Another application is the recuperation at low speeds of the electric machine EM n <ni2 (see Figure 1). In this case, even at a low speed, electric recuperation power can be stored in both energy stores Bi and B2. Due to the parallel connection of the two energy stores Bi and B2, as in the case of serial operation, the maximum possible charging power is doubled compared to only one energy store Bi or B2. In the however, equal to the serial operation of the energy storage Bi and B2 now the maximum possible charging current is doubled.

The invention can be implemented in a large number of variants and configurations, as illustrated, for example, in FIGS. 8 to 11 of DE 10 2013 204 894 A1, with the proviso that capacitive elements are also permanently connected in parallel with the energy stores B1 and B2 ,

An electric supply redundant with respect to the battery function, eg during a sailing phase, can be ensured according to an embodiment of the invention by switching between the switching states according to FIGS. 4 and 5 and the energy stores B1 and B2 if one of the energy stores Bi or B2 fails. In the case of faults in the electric machine EM or faults in the active power converter, which lead, for example, to an excessively high output voltage, the electrical supply of the consumers R1 can be decoupled from the electric machine, so that they are not damaged. This is possible with S1 = 0, S2 = 0, S3 = 1 and S ₄ = 1 or with S1 = 0, S2 = 0, S3 = 0 and S ₄ = 1 according to FIG.

Depending on the respective operating point of the electric machine EM, such as speed and output power, an improvement in the efficiency of the electric machine EM can be achieved both in regenerative and in motor operation by switching to another voltage.

A further embodiment of the motor vehicle electrical system of the invention results when, in the embodiment according to FIG. 2, the second switching element S2 is replaced by a diode. In this embodiment, advantageously only the three switching elements S1, S3 and S _{4 are} required. With the difference that the switching element S2 does not have to be controlled, this embodiment corresponds mutatis mutandis to the embodiment according to the figure 2 (ie motor vehicle electrical system 100). Again, however, the capaci- integrated elements in the illustrated arrangement. In a correspondingly modified embodiment according to FIG. 2, however, motor operation is no longer possible with serial shading of the batteries (FIG. 3).

Claims

claims

1. motor vehicle electrical system (100) having a motor and generator operable electric machine (EM), an active power converter (110) with DC voltage terminals (B +, B-), a first energy storage (Bi) and a second energy storage (B2) and in which means (S1-S, 120) are provided which are set up to set all of the following switching states a) to c) as an alternative to one another: a) either only the first energy store (B1) or only the second energy store Giespeicher (B2) is connected to the DC voltage connections (Β +, B-) of the active

Power converter (1 10) connected; b) the first energy store (B1) and the second energy store (B2) are connected in parallel to the DC voltage terminals (Β +, B-) of the active power converter (110); c) the first energy store (B1) and the second energy store (B2) are connected in series to the DC voltage connections (Β +, B-) of the active power converter (110); wherein a first capacitive element (Czi) is connected to the first energy store (B1) and a second capacitive element (C _Z 2) is permanently connected in parallel to the second energy store (B2). 2. Motor vehicle electrical system (100) according to claim 1, which is adapted for operation in a first and in a second operating mode, wherein the electric machine (EM) in the first and second operating modes motor or generator operated and in the first operating mode by adjusting of the switching state a) or b) with a first, lower Voltage and in the second mode of operation by setting the switching state c) with a second, higher voltage over the powered as an inverter or rectifier active power converter (1 10) is fed.

Motor vehicle electrical system (100) according to claim 1 or 2, having at least one electrical load (Ri), optionally by means of an output voltage of the first energy store (Bi), by means of an output voltage of the second energy store (B2) or by means of an output voltage of both parallel-connected energy storage ( Bi, B2) is food-free.

Motor vehicle electrical system (100) according to one of the preceding claims, wherein the means (S1, S2, S3, S, 120), by means of which the switching states a) to c) are adjustable, a first switching element (S1), a second switching element or a Diode (S2,), a third switching element (S3) and a fourth switching element (S) each having first and second voltage terminals and a control device (120), and wherein, when a diode (S2) is provided, this a current between her through the first and its second voltage connection and vice versa blocks.

Motor vehicle electrical system (100) according to claim 4, wherein in each case the following voltage terminals are conductively connected to each other: a first DC voltage terminal (B +) of the active power converter (110), the first voltage terminal of the first energy store (Bi), a first voltage terminal of the first capacitive element ( Czi) and the first voltage terminal of the first switching element (S1); a second DC voltage terminal (B-) of the active power converter (110), the second voltage terminal of the second energy storage device (B2), a second voltage terminal of the second capacitive element (Cz2), and the second voltage terminal of the third switching element (S3); the second voltage terminal of the first energy store (Bi), the first voltage terminal of the second switching element or the diode (S2) of first voltage terminal of the third switching element (S3) and a second voltage terminal of the first capacitive element (Czi); the second voltage terminal of the first switching element (S1), the second voltage terminal of the second switching element or the diode (S2), and the first voltage terminal of the fourth switching element (S); the second voltage terminal of the fourth switching element (S), a first voltage terminal of the second capacitive element (Cz2) and the first voltage terminal of the second energy storage device (B2).

Motor vehicle electrical system according to claim 4, wherein each of the following voltage terminals are conductively connected to each other: a first DC voltage terminal (B +) of the active power converter (1 10), the first voltage terminal of the first energy storage device (B1), a first voltage terminal of the first capacitive element (Czi) and the first voltage terminal of the first switching element (S1); a second DC voltage terminal (B-) of the active power converter (110), the second voltage terminal of the third switching element (S3), and the second voltage terminal of the fourth switching element (S); the second voltage terminal of the first energy storage device (B1), the first voltage terminal of the second switching element or the diode (S2), a second voltage terminal of the first capacitive element (Czi), and the first voltage terminal of the third switching element (S3); the second voltage terminal of the first switching element (S1), the second voltage terminal of the second switching element or the diode (S2), a first voltage terminal of the second capacitive element (Cz2) and the first voltage terminal of the second energy storage device (B2); the first voltage terminal of the fourth switching element (S), a second Voltage terminal of the second capacitive element (Cz2) and the second voltage terminal of the second energy storage device (B2).

Motor vehicle electrical system according to one of claims 1 to 3, wherein the means (Si, S2, S3, S ₄ , 120), by means of which the switching states a) to c) are adjustable, at least a first switching element (S1), a second switching element ( S2), a third switching element or a diode (S3) and a fourth switching element (S ₄ ) each having first and second voltage terminals and a control device (120), and wherein, when a diode (S3) is provided, this a current between the first and the second voltage connection is read and reversed blocks.

Motor vehicle electrical system according to claim 7, wherein each of the following voltage terminals are conductively connected to each other: a first DC voltage terminal (B +) of the active power converter (1 10), the first voltage terminal of the first energy storage device (B1), a first voltage terminal of the first capacitive element (Czi) and the first voltage terminal of the first switching element (S1); a second DC voltage terminal (B-) of the active power converter (110), the second voltage terminal of the second energy storage device (B2), a second voltage terminal of the second capacitive element (Cz2), and the second voltage terminal of the fourth switching element (S ₄ ); the second voltage terminal of the first energy storage device (B1), the first voltage terminal of the third switching element or the diode (S3), a second voltage terminal of the first capacitive element (Czi), and the first voltage terminal of the fourth switching element (S ₄ ); the first voltage terminal of the second energy storage device (B2), the second voltage terminal of the third switching element or the diode (S3), a first voltage terminal of the second capacitive element (Cz2), and the first voltage terminal of the second switching element (S2); the second voltage terminal of the first switching element (Si) and the second voltage terminal of the second switching element (S2).

9. motor vehicle electrical system (100) according to one of claims 4 to 8, wherein the switching elements (S1 - S) by means of the control device (120) for adjusting the switching states a) to c) are adjustable.

10. A method for operating a motor vehicle electrical system (100) according to any one of the preceding claims, comprising, optionally, the switching states a) to adjust c).

1 1. The method of claim 10, wherein the electric machine (EM) is operated in a first and in a second operating mode, wherein the electric machine (EM) in the first and second operating modes motor or generator operated and in the first operating mode by controlling the switching state a) or b) with a first, lower voltage and in the second operating mode by setting the switching state c) with a second, higher voltage via the active as inverter or rectifier active power converter (1 10) is driven.

12. Control device (120) for a motor vehicle electrical system (100) according to any one of the preceding claims, which is adapted to carry out a method according to claim 10 or 11.

13. Computer program with program code means which cause a computing unit to perform a method according to claim 10 or 11, when executed on the computing unit, in particular on the control device (10) according to claim 12.

14. A machine-readable storage medium with a computer program stored thereon according to claim 13.