WO2017202537A1

WO2017202537A1 - Motor vehicle on-board system having at least two energy stores, method for operating a motor vehicle on-board system; and means for the implementation thereof

Info

Publication number: WO2017202537A1
Application number: PCT/EP2017/058539
Authority: WO
Inventors: Wolfgang Mueller
Original assignee: Robert Bosch Gmbh
Priority date: 2016-05-23
Filing date: 2017-04-10
Publication date: 2017-11-30
Also published as: DE102016208893A1

Abstract

The invention relates to a motor vehicle on-board system (100, 200, 300, 400), comprising an electric machine (EM) connected to an active converter (10) having a positive and a negative d.c. voltage connection (Β+, B-), a first energy store (B₁) having a positive and a negative voltage connection, a second energy store (B₂) having a positive and a negative voltage connection, and a d.c. voltage converter (DC/DC) having a first and a second connection, wherein furthermore switching means (S₁ - S_4, 20) are provided, which are configured for setting a first switching state, in which the first energy store (B1) and the second energy store (B₂) are connected in parallel to the d.c. voltage connections (B+, B-) of the converter (10), and a second switching state, in which the first energy store (B₁) and the second energy store (B₂) are connected in series to the d.c. voltage connections (Β+, B-) of the converter, wherein in the first and the second switching state the d.c. voltage converter (DC/DC) is permanently connected or can be temporarily connected between the positive voltage connection of the first energy store and the positive voltage connection of the second energy store. The invention further relates to means for the implementation of the method.

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 fuel-saving during recuperation. The energy storage are also redundant for the sailing operation available, which increases the reliability of the vehicle electrical system in total.

Against this background, the object of the present invention is to further improve a corresponding motor vehicle electrical system.

Disclosure of the invention

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 of the invention are the subject of the dependent claims and the following description. Advantages of the invention

If in a motor vehicle electrical system, as disclosed in DE 10 2013 204 894 A1, two energy stores are connected in series to the electric machine or its power converter, the supply voltage which can be fed into the motor vehicle electrical system by the electric machine can be increased. If, for example, two 12V energy storage devices are used, a supply voltage of 28V can be fed into the motor vehicle electrical system when connected in series. In contrast, the feedable

Supply voltage with parallel connection of corresponding energy storage, similar to conventional motor vehicle electrical systems, limited to 14V.

Due to the higher output power of the electric machine, for example a claw-pole machine of a known type, with a corresponding interconnection, there is also an improved recuperation of braking energy. However, the prerequisite is that the energy storage devices can absorb the recuperation power. This can be ensured by choosing suitable high-performance batteries. These must have both a sufficient charge acceptance and a sufficient capacity. Under the charge acceptance thereby the ability of an energy storage to take up current, under the capacity the storage capacity in Ah or Wh understood.

However, corresponding high-performance batteries are cost-intensive and require a preferred installation location with regard to the ambient temperature. If these do not have the same charge acceptance or capacity for reasons of cost and / or space, the energy store with the lower charge acceptance or the lower capacity limits the energy storage during the recuperation in the series connection described. This represents a disadvantage of the arrangement described in DE 10 2013 204 894 A1, since in order to achieve a high recuperation power, both energy stores must be designed as high-performance batteries of the same design. If the electrical machine of a corresponding vehicle electrical system can be operated both as a generator and as a motor, two high-performance batteries are also required in order to achieve efficient motor operation according to DE 10 2013 204 894 A1. Again, these are connected in series.

A further disadvantage of the arrangement described in DE 10 2013 204 894 A1 consists in the fact that the energy store designated there by Bi is typically charged more strongly in a corresponding series connection, eg during recuperation, than the energy store designated there by B2 , When the electric machine is operated by a motor, the energy store B2 is discharged more strongly than the energy store Bi. The reason for this behavior is that a part of the current to supply to be used there with R 1 to R _n identified consumer and thereby the energy storage B2 is less charged or discharged more.

For this reason, the typically increased state of charge of the energy store Bi or the typically lower state of charge of the energy store B2 must be matched to one another, since overcharging of the energy store Bi and substantial discharge of the energy store B2 must be avoided. A fully charged energy storage Bi can no longer absorb recuperation energy. In the case of an excessively discharged energy store B2, motor operation can no longer take place. This can be prevented, for example, by the switching states shown in FIG. 4 and FIG. 5 of DE 10 2013 204 894 A1. However, this results in additional switching cycles of the switches to change the operating mode and therefore limits the degree of freedom of the operating strategy. For example, during the approximation of the state of charge according to FIG. 4 or FIG. 5 of DE 10 2013 204 894 A1, there can be no recuperation with serial connection of the energy stores.

A third disadvantage according to DE 10 2013 204 894 A1 results in the parallel connection of the energy store according to the local FIG. Depending on the different states of charge of the energy storage devices, aging conditions and temperatures, even with identically designed energy stores Bi and B2 equalizing currents flow between them. These equalizing currents cause considerable electrical losses, possibly overloading the energy storage devices and may also cause unwanted transient voltage spikes in the vehicle electrical system.

The disadvantages just described of the vehicle electrical system described in DE 10 2013 204 894 A1 are remedied in the context of the present invention with the aid of a DC voltage or DC / DC converter. This is according to the invention permanently integrated between the positive terminals of the two energy storage or temporarily einbindbar, as shown in the accompanying figures 2 to 7. One advantage of the solutions shown in these figures is that the energy store B1 can have a smaller charge or discharge acceptance than the energy store B2. The DC-DC converter passes part of the current past the energy store B1 and to the energy store B2. The currents of the electric machine therefore do not have to be adapted to the lower current carrying capacity of the energy store B1. The energy storage device B1 can therefore be realized by a low-cost battery technology and / or a battery with a small capacity, for example by a standard 12V lead-acid battery with low capacity.

Even with high generator or motor output power of the electric machine so in the present invention advantageously only one of the energy storage, for example, in the electrical system of the accompanying figures 2 to 7 of the energy storage B2, be designed for high charging and discharging. This results in a cost savings.

If the state of charge of one of the energy stores, for example the energy store Bi according to the appended FIGS. 2 to 7, is higher than the state of charge of the energy store B2, the state of charge can be compensated with the aid of the DC-DC converter within the scope of the present invention. Thus, energy is transported from energy storage B1 to energy storage B2. An adaptation of the charge state by means of a changed switch position as shown in Figure 4 or 5 of DE 10 2013 204 894 A1 is no longer necessary. This results in an increase in the degrees of freedom of the operating strategy. If the state of charge of the energy store B2 according to attached FIGS. 2 to 7 is higher than the state of charge of the energy store B1, the state of charge can be compensated with the aid of the DC-DC converter in the context of the present invention. Thus, there is an energy transport of energy storage B2 to energy storage Bi. The already mentioned advantages over DE 10 2013 204 894 A1 apply correspondingly.

In the context of the present invention, the states of charge of the energy stores can be adjusted with the aid of the DC-DC converter so that no or only minimal transient equalizing currents occur between the energy stores, in contrast to a direct galvanic parallel connection of the energy store according to FIG. 6 of DE 10 2013 204 894 A1. Such a direct galvanic coupling of the batteries is advantageous, for example, during a cold start after a long parking time.

For the energy stores B1 and B2 of the attached Figures 2 to 7 different technologies with different output voltages in the range of e.g. 12 to 15.5V used, so would in the parallel galvanic coupling according to Figure 6 of DE 10 2013 204 894 A1 possibly high compensation currents. These can be prevented in the context of the present invention by the energy stores are coupled in parallel by means of the DC-DC converter, as illustrated in the accompanying Figure 7. A direct galvanic coupling then no longer takes place.

Furthermore, the cyclicization, that is to say the energy or charge throughput of the less powerful energy store, is reduced by the DC-DC converter used in the context of the present invention. In this way, the requirements for the lower-power energy storage in the context of the present invention are further reduced.

In order to achieve the advantages explained, the present invention proposes a motor vehicle electrical system which has an electric machine which can be operated by a motor and a generator and which is connected to a converter with a positive and a nem negative DC voltage connection is connected. The motor vehicle electrical system also has a first energy store with a positive and a negative voltage connection, a second energy store with a positive and a negative voltage connection, and a DC voltage converter with a first and a second connection. Further are

Switch means are provided which are adapted to a first switching state, in which the first energy storage and the second energy storage are connected in parallel to the DC voltage terminals of the power converter, and a second switching state, in which the first energy storage and the second energy storage in series connection to the DC voltage connections of the power converter are connected, wherein the DC-DC converter in the first and the second switching state between the positive voltage terminal of the first energy storage and the positive voltage terminal of the second energy storage permanently integrated or temporarily einbindbar.

The present invention can be used, in particular, in motor vehicle on-board networks, in which an electric machine which can be operated both as a motor and as a generator can be used. This can be designed, for example, as a claw-pole machine. The electric machine is then over as

DC and inverter operated active power converter connected to the vehicle electrical system. In a motorized operation of the electric machine, the active converter is operated as a (pulse) inverter, in a generator mode 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. However, the present invention can also be used in on-board networks, in which the electric machine can only be operated as a generator and can therefore be used for recuperation. In this case, a separate starter is provided and the power converter can be designed as a passive rectifier.

If, in the following, it is mentioned that a power converter has a positive and a negative DC voltage connection, this may be understood as the DC voltage connections connected to the electrical system. The negative DC voltage connection has a lower voltage potential than the positive DC voltage connection. He can also be grounded.

If it is mentioned here that the energy stores are connected in series connection or parallel to the DC voltage connections of the power converter, this does not preclude the interposition of further elements. As illustrated, for example, with reference to the attached FIG. 7 and already mentioned above, in the context of the present invention, for example, the energy stores are not directly connected in parallel in parallel but connected to one another via the DC-DC converter.

The vehicle electrical system is advantageously set up for operation in a first and in a second operating mode, wherein the electric machine is operated in the first and the second operating mode by motor or generator, wherein in the first operating mode by adjusting the first

Switching a first, lower voltage and in the second operating mode by setting the second switching state, a second, higher voltage is provided. As mentioned, for example, in a cold start by the parallel connection of the energy storage advantages can be achieved. By means of the series connection, the recuperation power can be increased.

The switching means, which are provided in the motor vehicle electrical system according to the invention, may comprise a different number of switching elements and a control unit that drives these switching elements. Details are explained with reference to the attached figures.

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.

Also, the implementation of the method in the form of software is advantageous because this causes very low costs, especially if an executing control device is still used for other tasks and therefore anyway is available. 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 invention in a simplified schematic representation.

Figure 3 shows a motor vehicle electrical system according to an embodiment of the invention in a simplified schematic representation.

FIG. 4 illustrates power flows in a motor vehicle electrical system according to an embodiment of the invention.

FIG. 5 shows a motor vehicle electrical system according to an embodiment of the invention in a simplified schematic representation. Figure 6 shows a motor vehicle electrical system according to an embodiment of the invention in a simplified schematic representation.

FIG. 7 illustrates a parallel connection of energy stores in one

Motor vehicle electrical system according to an embodiment of the invention.

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. With an output voltage of the electric machine of U2 = 28V, a significantly higher maximum output power results from a speed n> ni2 than when supplying a 14V motor vehicle electrical system. This effect can be used to increase the output of the electric machine during recuperation.

The invention also relates, in embodiments, to the motorized operation of a corresponding electric machine which is connected to a motor vehicle electrical system via an active power converter which is then operated as an inverter. The electrical machine can be supplied with different voltages depending on the operating state of the corresponding vehicle. Even with motor operation of the electric machine results in a supply voltage of 28V, a higher maximum output power than with a supply of only 14V.

In Figure 2, a motor vehicle electrical system according to an embodiment of the present invention in the form of a schematic diagram is shown schematically simplified, and generally designated 100. This has a first energy storage B1 and a second energy storage B2, which may be formed as explained above, on. The energy storage devices B1 and B2 can be variably connected in the motor vehicle electrical system 100 of FIG. 2 by means of the switching elements S1, S2, S3 and S. The switching elements S1 to S may be formed, for example, by MOS field-effect transistors or other electronic components. These each have a first and a second voltage connection, by means of which they are integrated into the electrical system. In a MOS field effect transistor, these voltage terminals correspond to the drain and source terminals. For controlling the switching elements S1, S2, S3 and S, a control device 20 is provided. Electrical consumers with an operating voltage of 14V, for example, are collectively illustrated in the form of resistors R1 to R _n.

In the electrical system 100, an electric machine EM is provided, which is connected to a power converter 10. The latter can, as already mentioned, depending on the operating strategy and design of the electrical machine be designed as a passive or active power converter. The electric machine EM can be set up for a purely regenerative but also for a motor and a generator operation. In the former case, the power converter 10 is operated as a rectifier, in the latter case, the power converter 10 is operated as a DC or inverter. It has a positive DC voltage connection B + and a negative DC voltage connection B-, whereby the DC voltage connection B- can be at ground or the same reference potential as the electric machine EM. This is illustrated in FIG. 2 and the following figures. S is a starter, such as a pinion starter, referred to, which is provided separately when the electric machine EM is operable only as a generator or when the internal combustion engine, for example, during cold start, not to be started with the electric machine. In the manner explained above and illustrated in more detail below, a DC-DC converter, denoted by DC / DC, is integrated into the motor vehicle electrical system 100.

In the motor vehicle electrical system 100 according to FIG. 2, the positive DC voltage connection B + of the power converter 10, the positive voltage connection of the first energy storage Bi, the first voltage terminal of the first switching element Si and the first voltage terminal of the DC-DC converter DC / DC each permanently connected to each other conductive. Is in the context of this application of the speech that elements are permanently connected, it is understood that such a connection does not take place via a switching element, but via cables and corresponding permanent cable connections, such as solder joints, terminals or connectors in a normal operation of Motor vehicle electrical system not interrupted or their elements are not separated or isolated. The same applies to a permanent integration of a DC-DC converter. On the other hand, a temporary integration is realized by switching elements.

In the motor vehicle electrical system 100 according to FIG. 2, the negative voltage terminal of the first energy store Bi, the first voltage terminal of the second switching element S2 and the first voltage terminal of the third switching element S3, as well as the second voltage terminal of the first switching element Si, the second voltage terminal of the second switching element S2 and the first voltage terminal of the fourth switching element S, the second voltage terminal of the fourth switching element S, the second voltage terminal of the DC-DC converter DC / DC and the positive voltage terminal of the second energy storage B2, and finally the negative DC voltage terminal B- of the power converter 10, the negative voltage terminal of the second Energy storage B2 and the second voltage terminal of the third switching element S3 permanently conductive connected to each other.

The implemented DC-DC converter DC / DC leads to the described advantages of the invention. With its help, the charge can be discharged by the charge-discharging current flowing through the first energy store Bi which can be embodied as a low-cost battery in accordance with the lower efficiency of this energy store

Bi sufficiently reduced.

In Figure 3, a motor vehicle electrical system according to a further embodiment of the present invention in the form of a schematic simplified schematic represented and indicated generally at 200. The essential difference to the motor vehicle electrical system 100 according to FIG. 2 is the different connection of the illustrated elements.

Thus, in the motor vehicle electrical system 200 according to FIG. 3, the positive DC voltage connection B + of the converter 10, the positive voltage connection of the first energy store Bi, the first voltage connection of the first switching element Si and the first voltage connection of the DC-DC converter DC / DC, furthermore the negative voltage connection of the first Energy storage Bi, the first voltage terminal of the third switching element S3 and the first voltage terminal of the fourth switching element S, further the second voltage terminal of the first switching element S1 and the second voltage terminal of the second switching element S2, also the second voltage terminal of the third switching element S3, the first voltage terminal of the second Switching element S2, the second voltage terminal of the DC-DC converter DC / DC and the positive voltage terminal of the second energy storage B2, and finally the negative DC voltage terminal B- of the power converter 10, the negative S voltage terminal of the second energy storage B2 and the second voltage terminal of the fourth switching element S permanently conductively connected to each other. FIG. 4 shows the currents during recuperation and motor operation of the electric machine EM when the energy stores Bi and B2 are connected in series. In the case of recuperation (IEM> 0), part of the recuperation current IEM of the electric machine EM is fed directly from a network node 102 to a network node 101 with the aid of the DC-DC converter DC / DC. If the input current of the DC-DC converter IDCDC = k * IEM, with 0 <k <1, the energy store Bi is only charged with the current IB2 = (1-k) * IEM. The energy store Bi can therefore have a lower charge acceptance and / or capacity than the energy store B2. In the case of a motor operation of the electrical machine (IEM <0), if a bidirectional DC / DC converter DC / DC is used, the lower performance of the energy accumulator Bi can also be compensated with the aid of the DC / DC converter DC / DC. During the engine operation takes place by the DC converter DC / DC a feedback of the current from network node 101 to network node 102. Thus, the current flow through the energy store Bi during engine operation according to the performance, the discharge acceptance and / or the capacity of the energy store Bi can be reduced to a suitable value. In FIG. 5, a motor vehicle electrical system in accordance with a further embodiment of the present invention in the form of a circuit diagram is shown schematically in simplified form and indicated as a whole by 300. The essential difference to the vehicle electrical system 100 according to Figure 2 and the vehicle electrical system

200 according to Figure 3 consists in the reduced number of switching elements. For better comparability with FIGS. 2 and 3, a first switching element with S2, a second switching element with S3 and a third switching element with S _{4 are} designated in FIG.

In the motor vehicle electrical system 300 according to FIG. 5, the positive DC voltage connection B + of the converter 10, the positive voltage connection of the first energy store Bi and the first voltage connection of the DC-DC converter DC / DC, furthermore the negative voltage connection of the first energy store Bi, are the first voltage connection of the first switching element

S2 and the first voltage terminal of the second switching element S3, further the second voltage terminal of the first switching element S2, the second voltage terminal of the DC-DC converter DC / DC and the first voltage terminal of the third switching element S ₄ , also the positive voltage connection of the second energy storage B2 and the second one Voltage terminal of the third switching element S ₄ , and finally the negative DC voltage connection B- of the power converter 10, the negative voltage terminal of the second energy storage B2 and the second voltage terminal of the second switching element S3 each conductively connected together. In FIG. 6, a motor vehicle electrical system according to a further embodiment of the present invention in the form of a circuit diagram is shown schematically in simplified form and designated as a whole by 400. Similar to the motor vehicle electrical system 300 according to FIG. 5, the number of switching elements is reduced here. A first switching element is also denoted here by S2, a second switching element by S3 and a third switching element by S.

In the motor vehicle electrical systems 300 and 400 according to FIGS. 5 and 6, the switching element S1 present in the motor vehicle electrical systems 100 and 200 according to FIGS. 2 and 3 is omitted and is replaced by the DC voltage converter DC / DC. This results in additional cost savings.

In the motor vehicle electrical system 400 according to FIG. 6, the positive DC voltage connection B + of the converter 10, the positive voltage connection of the first energy store B1 and the first voltage connection of the DC-DC converter DC / DC, furthermore the negative voltage connection of the first energy store B1, are the first voltage connection of the second switching element S3 and the first voltage terminal of the third switching element S, further the second voltage terminal of the second switching element S3, the first voltage terminal of the first switching element S2 and the positive voltage terminal of the first energy storage B2, also the second voltage terminal of the first switching element S2 and the second voltage terminal of the DC-DC converter DC / DC, and finally the negative DC voltage connection B- of the power converter 10, the negative voltage terminal of the second energy storage B2 and the second voltage terminal of the third switching element S dau respectively electrically connected.

The parallel coupling of the energy storage takes place, as illustrated in FIG. 7, by means of a DC-DC converter DC / DC. Another advantage of such a parallel coupling results when different battery technologies are used, for example, a standard lead-acid battery for the energy storage Bi and a powerful Li-ion battery for the energy storage B2. In the case of a parallel galvanic coupling of batteries with different technologies, depending on the difference in the battery voltages, high, undesired balancing currents between the batteries and, if applicable, voltage peaks in the vehicle electrical system may result. A coupling using the DC / DC converter DC / DC prevents this, since the compensation current between the batteries can be specified by the DC / DC converter DC / DC. By using the present invention, the voltage ranges are freely selectable and not limited to 28 / 14V. The energy stores Bi and B2 can optionally be implemented in different battery technologies. The electric machine can be designed as a generator or as an electric machine with both regenerative and motor operation. The machine type is freely selectable. The starter S can optionally be omitted, then the starter of the burner is carried out only with the help of the electric machine.

In the on-board networks 300 and 400, optionally, the energy stores Bi and B2 may also have significantly different voltages, e.g. Energy storage Bi 24V and energy storage B2 12V. As an energy storage in addition to batteries, other electrical storage, such as double-layer capacitors, are used. The switching elements S1 to S may optionally be powered by power electronics or by other electrical switching means, e.g. Relays, be realized. The DC / DC converter DC / DC can be designed both unidirectionally and bidirectionally.

Claims

claims

1. motor vehicle electrical system (100, 200, 300, 400), which an electric machine (EM), which is connected to an active power converter (10) with a positive and a negative DC voltage connection (Β +, B-), a first energy storage ( Bi) having a positive and a negative voltage terminal, a second energy store (B2) having a positive and a negative voltage terminal, and a DC / DC converter having a first and a second terminal, further comprising switching means (S1-S, 20) are provided, which are adapted to a first switching state, in which the first energy store (Bi) and the second energy store (B2) are connected in parallel to the DC voltage terminals (B +, B-) of the power converter (10), and a second switching state, in which the first energy store (Bi) and the second energy store (B2) connected in series to the DC voltage terminals (B +, B-) of the power converter sin d, wherein the DC-DC converter (DC / DC) in the first and the second switching state between the positive voltage terminal of the first energy storage and the positive voltage terminal of the second energy storage permanently integrated or temporarily einbindbar.

2. Motor vehicle electrical system (100, 200, 300, 400) according to claim 1, which is adapted for operation in a first and a second operating mode, wherein the electric machine (EM) in the first and second operating mode is operated by motor or generator, wherein in the first operating mode by setting the first switching state, a first, lower voltage and in the second operating mode by setting the second switching state, a second, higher voltage is provided.

Vehicle electrical system (100, 200) according to any one of the preceding claims, wherein the switching means (Si - S _4, 20) a first switching element (Si), a second switching element (S2), a third switching element (S3) and a fourth switching element (S ₄ ) each having a first and a second voltage terminal and a control device (20).

Motor vehicle electrical system (100) according to claim 3, are connected to the each permanently conductive: the positive DC voltage terminal (B +) of the power converter (10), the positive voltage terminal of the first energy storage device (B1), the first voltage terminal of the first switching element (Si) and the first voltage terminal of the DC-DC converter (DC / DC), the negative voltage terminal of the first energy storage device (B1), the first voltage terminal of the second switching element (S2) and the first voltage terminal of the third switching element (S3), the second voltage terminal of the first switching element (Si), the second voltage terminal of the second switching element (S2) and the first voltage terminal of the fourth switching element (S ₄ ), the second voltage terminal of the fourth switching element (S ₄ ), the second voltage terminal of the DC-DC converter (DC / DC) and the positive voltage terminal of the second energy storage ( B2), the negative DC voltage connection (B-) the power converter (10), the negative voltage terminal of the second energy store (B2) and the second voltage terminal of the third switching element (S3).

Motor vehicle electrical system (200) according to claim 3, are connected to the each permanently conductive: the positive DC voltage terminal (B +) of the power converter (10), the positive voltage terminal of the first energy store (Bi), the first voltage terminal of the first switching element (Si) and the first voltage terminal of the DC-DC converter (DC / DC), the negative voltage terminal of the first energy store ( Bi), the first voltage terminal of the third switching element (S3) and the first voltage terminal of the fourth switching element (S), the second voltage terminal of the first switching element (S1) and the second voltage terminal of the second switching element (S2), the second voltage terminal of the third switching element ( S3), the first voltage terminal of the second switching element (S2), the second voltage terminal of the DC-DC converter (DC / DC) and the positive voltage terminal of the second energy store (B2), the negative DC voltage terminal (B-) of the power converter (10), the negative voltage terminal the second energy store (B2) and the second Spannu ngsanschluss the fourth switching element (S).

Motor vehicle electrical system (300, 400) according to claim 1 or claim 2, wherein the switching means (S1 - S, 20) comprises a first switching element (S2), a second switching element (S3) and a third switching element (S) having a first and second voltage terminal and a control device (20).

Motor vehicle electrical system (300) according to claim 6, are connected to the each permanently conductive: the positive DC voltage terminal (B +) of the power converter (10), the positive voltage terminal of the first energy storage (Bi) and the first voltage terminal of the DC-DC converter (DC / DC), the negative voltage terminal of the first energy store (Bi), the first voltage terminal of the first switching element (S2) and the first voltage terminal of the second switching element (S3), the second voltage terminal of the first switching element (S2), the second voltage terminal of the DC-DC converter (DC / DC) and the first voltage terminal of the third switching element (S), the positive voltage terminal of the second energy store (B2), the second voltage terminal of the third switching element (S), the negative DC voltage terminal (B-) of the power converter (10), the negative voltage terminal of the second energy storage (B2) and the second voltage terminal of the second switching element (S3).

Motor vehicle electrical system (400) according to claim 6, in which are permanently connected to one another in each case: the positive DC voltage terminal (B +) of the power converter (10), the positive voltage terminal of the first energy store (B1) and the first voltage terminal of the DC-DC converter (DC / DC), the negative voltage terminal of the first energy storage device (B1), the first voltage terminal of the second switching element (S3) and the first voltage terminal of the third switching element (S), the second voltage terminal of the second switching element (S3), the first voltage terminal of the first switching element (S2) and the positive voltage terminal of the second energy store (B2), the second voltage terminal of the first switching element (S2) and the second voltage terminal of the DC-DC converter (DC / DC), the negative DC voltage terminal (B-) of the power converter (10), the negative voltage terminal of the second energy store (B2) and the second voltage terminal of the third switching element (S).

A method for operating a vehicle electrical system (100, 200, 300, 400) according to one of the preceding claims, comprising setting the first and the second switching state, wherein the DC-DC converter (DC / DC) in the first and the second switching state between the positive voltage terminal the first energy storage and the positive voltage connection of the second energy storage is or is integrated.

10. Control device (20) for a motor vehicle electrical system (100, 200, 300, 400) according to one of claims 1 to 8, which is adapted to perform a method according to claim 9.

1 1. Computer program with program code means which cause a computing unit to perform a method according to claim 9, when they are executed on the computing unit, in particular on the control device (20) according to claim 10.

Machine-readable storage medium with a computer program stored thereon according to claim 11.