Method for operating an on-board electrical system for a motor vehicle, corresponding on-board electrical system for a motor vehicle, and computer program product

Abstract

The invention relates to a method for operating an electrical system (1) for a motor vehicle, wherein the electrical system (1) comprises a first sub-network (2) with a first electrical voltage and a second sub-network (3) electrically connected to the first sub-network (2) via a voltage converter arrangement (16) with a second electrical voltage different from the first electrical voltage, and the voltage converter arrangement (16) is operated at least temporarily for the purpose of transferring electrical energy between the first sub-network (2) and the second sub-network (3), wherein a traction battery (4) is electrically connected to the first sub-network (2) via a switch arrangement (13) and a drive unit is electrically connected to the first sub-network (2) via an inverter arrangement, and the inverter arrangement is operated at least temporarily to supply electrical energy from the traction battery (4) to the drive unit for the purpose of providing drive torque.It is provided that, in order to match the first electrical voltage of the first subnetwork (2) to a battery voltage of the traction battery (4), the voltage converter arrangement (16) for converting electrical energy into heat is operated at least temporarily with a conversion efficiency that is lower than a transmission efficiency present when transferring electrical energy between the first subnetwork (2) and the second subnetwork (3).

Description

[0001] The invention relates to a method for operating an electrical system for a motor vehicle, wherein the electrical system comprises a first sub-network with a first electrical voltage and a second sub-network electrically connected to the first sub-network via a voltage converter arrangement with a second electrical voltage different from the first electrical voltage, and the voltage converter arrangement is operated at least temporarily for the transmission of electrical energy between the first sub-network and the second sub-network, wherein a traction battery is electrically connected to the first sub-network via a switch arrangement and a drive unit is electrically connected to the first sub-network via an inverter arrangement, and the inverter arrangement is operated at least temporarily to supply electrical energy from the traction battery to the drive unit for the provision of a drive torque.The invention further relates to an on-board electrical system for a motor vehicle and a computer program product.

[0002] For example, the prior art includes the publication DE 10 2022 208 288 B3.This describes a method for operating a motor vehicle comprising a high-voltage electrical system and a low-voltage electrical system connected by means of a DC-DC converter, wherein the high-voltage electrical system is connected via a switching unit to a battery unit which has two sub-sections that can be electrically connected in series and electrically in parallel by means of a switching unit, in which the switching unit is opened, the sub-sections are electrically connected in parallel by means of the control unit, electrical energy is passed from the high-voltage electrical system to the low-voltage electrical system by means of the DC-DC converter so that the electrical voltage of the high-voltage electrical system is reduced, and when the electrical voltage of the high-voltage electrical system differs from the electrical voltage applied to the battery unit by no more than a tolerance value, the switching unit is closed.

[0003] German patent application DE 10 2022 212 442 A1 relates to a DC-DC converter with a first converter stage and a second converter stage, wherein the first converter stage is, in particular, galvanically isolated, wherein the first converter stage and the second converter stage are connected to each other via an internal DC link capacitor, wherein the first converter stage is configured for connection to a low-voltage source and has a positive low-voltage terminal, and wherein the first converter stage has a positive intermediate terminal and is connected to the internal DC link capacitor via this terminal, wherein the second converter stage is configured for connection to a high-voltage side and has a first positive terminal, a second positive terminal and a third positive terminal, which can be used as a first positive high-voltage terminal, as a second positive high-voltage terminal and as a positive intermediate terminal.wherein the second converter stage is connected to the internal intermediate circuit capacitor via the positive intermediate voltage terminal, wherein the DC-DC converter is configured to connect the third positive terminal of the second converter stage in a path, in particular via a second control switch and an inductor, either to the first positive terminal or to the second positive terminal.

[0004] The prior art also includes the publication DE 10 2022 210 118 A1.

[0005] The object of the invention is to propose a method for operating an on-board electrical system for a motor vehicle which has advantages over known methods, in particular always enabling a reliable adjustment of the electrical voltage of the on-board electrical system, especially the first electrical voltage of the first sub-network, to a battery voltage of the traction battery.

[0006] According to the invention, this is achieved by a method for operating an on-board network for a motor vehicle with the features of claim 1.It is provided that, in order to equalize the first electrical voltage of the first subnetwork with a battery voltage of the traction battery, the voltage converter arrangement is operated at least temporarily to convert electrical energy into heat with a conversion efficiency that is lower than the transmission efficiency present when transferring electrical energy between the first subnetwork and the second subnetwork, whereby, in order to equalize the first electrical voltage with the battery voltage, the first electrical voltage is reduced by converting electrical energy into heat, and after equalization, the first electrical voltage is maintained at least temporarily by transferring electrical energy from the second subnetwork to the first subnetwork using the voltage converter arrangement with the transmission efficiency.

[0007] Advantageous embodiments with expedient further developments of the invention are specified in the dependent claims. It should be noted that the exemplary embodiments described in the description are not limiting; rather, any variations of the features disclosed in the description, the claims, and the figures are possible.

[0008] The method serves to operate the vehicle's electrical system. The electrical system is preferably an integral part of the vehicle, but can, of course, also exist separately from the vehicle, particularly until the electrical system is installed in or on the vehicle. The electrical system comprises several sub-networks, namely at least the first sub-network and the second sub-network. The sub-networks differ in their nominal voltage, such that the first sub-network operates at the first voltage and the second sub-network at the second voltage. The first sub-network is, in particular, a high-voltage system, with the first voltage preferably being at least 100 V, at least 200 V, at least 400 V, or at least 800 V. The second sub-network, on the other hand, is designed as a low-voltage system. Accordingly, the second voltage is less than 100 V, in particular at most 48 V, at most 24 V, or at most 12 V.

[0009] At least temporarily, electrical energy is exchanged between the two subnetworks, i.e., the first subnetwork and the second subnetwork. For this purpose, the voltage converter arrangement serves, which at least temporarily extracts electrical energy from the first subnetwork at the first voltage, converts it to the second voltage, and supplies it to the second subnetwork, and / or at least temporarily extracts electrical energy from the second subnetwork at the second voltage, converts it to the first voltage, and supplies it to the first subnetwork.

[0010] A voltage converter arrangement is an arrangement with any number of voltage converters. The use of the term "voltage converter" in the plural does not imply a quantification; rather, the voltage converter arrangement can contain just one voltage converter or multiple voltage converters. In other words, the voltage converter arrangement contains at least one voltage converter. The voltage converter is designed as a DC-DC converter, so the voltage converter arrangement can also be referred to as a voltage converter array.

[0011] Whenever this description refers to the voltage transformer or at least one voltage transformer, the explanations are always equivalent. Explanations concerning the voltage transformer apply to at least one voltage transformer, and vice versa. If the voltage transformer arrangement comprises multiple voltage transformers, the explanations for the voltage transformer or at least one voltage transformer preferably apply to each of the multiple voltage transformers, unless otherwise indicated.

[0012] The traction battery and the drive unit are electrically connected to the first subnetwork. More precisely, the traction battery is electrically connected to the first subnetwork via the switchgear, and the drive unit is electrically connected to the first subnetwork via the inverter. The traction battery is an electrical energy storage device in which electrical energy is temporarily stored. The drive unit serves to propel the vehicle and thus to provide the drive torque for propelling the vehicle. To provide the drive torque, the drive unit has a drive unit, which in this case is designed as an electric traction motor.

[0013] The inverter arrangement is operated, at least temporarily, in such a way that electrical energy from the traction battery is supplied to the drive unit or drive system. The inverter arrangement comprises any number of inverters, particularly pulse inverters. For example, the inverter arrangement may have only a single inverter, but it can also include several inverters. Whenever the inverter arrangement is referred to in this description, it is assumed that it has at least one inverter. The inverter arrangement is electrically connected to the first subnetwork on one side and to the drive unit on the other.This means that the drive unit is preferably connected to the first subnetwork exclusively via the inverter arrangement and consequently is electrically connected to the traction battery exclusively via the first subnetwork.

[0014] The traction battery is electrically connected to the first subnetwork via the switch assembly. Thus, the switch assembly is electrically connected to both the first subnetwork and the traction battery, preferably permanently. The switch assembly can exist in different switching states, specifically a first switching state and a second switching state. In the first switching state, the electrical connection between the traction battery and the first subnetwork is interrupted, and in the second switching state, it is established. The switch assembly comprises a single switch or multiple switches. Each switch is electrically located between the traction battery and the first subnetwork, particularly at different electrical potentials.In the case of multiple switches, for example, a first switch is used to switch a positive pole and a second switch is used to switch a negative pole.

[0015] As long as the traction battery is electrically connected to the first subnetwork via the switch assembly, the same voltages are present in both; the first electrical voltage therefore corresponds to the battery voltage of the traction battery. However, if the first subnetwork and the traction battery are electrically isolated from each other by the switch assembly being in the first switching state, the first electrical voltage and the battery voltage can differ. Therefore, before establishing the electrical connection between the traction battery and the first subnetwork, i.e., before setting the switch assembly to the second switching state, the first electrical voltage and the battery voltage must be equalized.

[0016] To equalize the electrical voltages, an inverter arrangement can be used, for example. However, this has the disadvantage that precise adjustment of the first electrical voltage is hardly possible, so that, for example, the first electrical voltage is often reduced too much. This means that before electrically connecting the traction battery and the first subnetwork, electrical energy must first be fed back into the first subnetwork after reducing the first electrical voltage using the inverter arrangement, for example from the second subnetwork, particularly using the voltage converter arrangement.This results, on the one hand, in a large amount of time required to adjust the first electrical voltage to the battery voltage of the traction battery, and on the other hand, in high energy consumption, since initially too much energy is drawn from the first subnetwork, so that electrical energy must subsequently be supplied to it again in order to adjust the first electrical voltage to the desired voltage, in particular the battery voltage.

[0017] For this reason, the invention provides that the voltage converter arrangement, via which the two subnetworks are electrically connected, is used to adjust the first electrical voltage to the battery voltage. For example, it can be provided that electrical energy from the first subnetwork is supplied to the second subnetwork by means of the voltage converter arrangement. In the second subnetwork, the electrical energy is consumed by a load connected to the second subnetwork and / or temporarily stored in a battery connected to the second subnetwork. However, this is only possible if the amount of energy to be transferred from the first subnetwork to the second subnetwork can be consumed by the load and / or temporarily stored in the battery.If this is not the case, it is not possible to adjust the first electrical voltage to the battery voltage, as the electrical energy cannot be drawn from the first subnetwork.

[0018] Therefore, according to the invention, the voltage converter arrangement is operated in such a way that electrical energy is drawn from the first subnetwork but not supplied to the second subnetwork; instead, it is converted into heat. For this purpose, the voltage converter arrangement is operated with a conversion efficiency selected such that the electrical energy drawn from the first subnetwork is converted into heat. It is particularly preferred that the voltage converter arrangement is operated in such a way that the electrical energy drawn from the first subnetwork is completely converted into heat, for example, in a transformer of the voltage converter arrangement.

[0019] The conversion efficiency is always lower than the transmission efficiency, which is used to transfer electrical energy between the first and second subnetworks. In other words, the voltage converter arrangement is operated at the transmission efficiency at certain times to exchange electrical energy between the first and second subnetworks, for example, to transfer energy from the first to the second subnetwork or vice versa. However, if the first voltage needs to be adjusted to the battery voltage, the voltage converter arrangement is operated at least temporarily not at the transmission efficiency, but at the conversion efficiency, which is lower than the transmission efficiency. For example, the conversion efficiency is at most 10%, at most 5%, or at most 1% of the transmission efficiency.

[0020] It is particularly preferred that, in order to match the first electrical voltage of the first subnetwork to the battery voltage of the traction battery, the voltage converter arrangement for transferring electrical energy from the first subnetwork to the second subnetwork be operated at a certain transmission efficiency, at least temporarily, namely for as long as the energy demand of the second subnetwork is greater than zero. The voltage converter arrangement is thus operated at this transmission efficiency for as long as the electrical energy drawn from the first subnetwork is needed in the second subnetwork, for example, as long as the battery connected to the second subnetwork is not fully charged.If, on the other hand, the energy demand of the second subnetwork is zero, for example if the battery is fully charged, the voltage converter arrangement is operated with the conversion efficiency, so that the electrical energy taken from the first subnetwork is no longer supplied to the second subnetwork, but is converted into heat, in particular completely.

[0021] However, it is also possible to operate the voltage converter arrangement primarily for adjusting the first electrical voltage to the battery voltage using the conversion efficiency. This means that the electrical energy drawn from the first subnetwork is converted into heat even if it could be absorbed by the second subnetwork, for example, if the battery is only partially charged. In any case, it is ensured that the first electrical voltage is reliably adjusted to the battery voltage. The voltage converter arrangement allows for extremely precise adjustment of the first electrical voltage, thus preventing a significant drop in the first electrical voltage. Accordingly, despite the use of the lower conversion efficiency, high energy efficiency of the process is ensured.

[0022] A further development of the invention provides that the traction battery is electrically disconnected from the first subnetwork by means of the switch arrangement and its voltage is adjusted from a higher first voltage value to a lower second voltage value. Subsequently, the first voltage is adjusted to the second voltage value, and the traction battery is electrically connected to the first subnetwork by means of the switch arrangement. The traction battery is adjustable to different voltage values, namely at least to the first voltage value and the second voltage value. The first voltage value is, for example, greater than the second voltage value by a factor of at least 1.5, at least 2, or at least 2.5.

[0023] To change the battery voltage, it is necessary to first electrically disconnect the traction battery from the first subnetwork. For this purpose, the switch assembly is adjusted accordingly, specifically to its first switching position. After setting the second voltage value at the traction battery, the first electrical voltage is adjusted to match the now present battery voltage, i.e., to the second voltage value. For this, the voltage converter assembly is operated, at least temporarily, to convert the electrical energy from the first subnetwork into heat. Subsequently, the traction battery is electrically reconnected to the first subnetwork using the switch assembly. The described procedure ensures a rapid and reliable adjustment of the battery voltage.

[0024] The invention provides that, to adjust the first electrical voltage to the battery voltage, the first electrical voltage is reduced by converting electrical energy into heat. After adjustment, the first electrical voltage is maintained, at least temporarily, by transferring electrical energy from the second subnetwork to the first subnetwork with a certain transfer efficiency using the voltage converter arrangement. The conversion of electrical energy into heat by the voltage converter arrangement is carried out temporarily with a certain conversion efficiency to reduce the first electrical voltage. Subsequently, the first electrical voltage is maintained, at least temporarily, by transferring electrical energy from the second subnetwork to the first subnetwork using the voltage converter arrangement with a certain transfer efficiency.

[0025] Matching the first electrical voltage to the battery voltage initially comprises converting electrical energy into heat, with the voltage converter arrangement operating at its conversion efficiency, and transferring electrical energy from the second subnetwork to the first subnetwork, with the voltage converter arrangement operating at its transmission efficiency. The conversion efficiency and the transmission efficiency differ from each other; in particular, the conversion efficiency is lower than the transmission efficiency. Preferably, the electrical energy drawn from the first subnetwork is first converted into heat until the first electrical voltage equals the battery voltage. Subsequently, the first electrical voltage is maintained at the battery voltage until the traction battery is electrically connected to the first subnetwork, namely by appropriately adjusting the switch arrangement.

[0026] Maintaining the initial electrical voltage is achieved by supplying electrical energy from the second subnetwork to the first subnetwork. Both the conversion of electrical energy from the first subnetwork into heat and the supply of electrical energy from the second subnetwork to the first subnetwork are accomplished using the voltage converter arrangement, which operates at varying efficiencies. This allows for particularly efficient adjustment of the initial electrical voltage to the battery voltage.

[0027] A further development of the invention provides that the traction battery comprises several battery cells which are connected in a first circuit topology to provide the battery voltage with the first voltage value and in a second circuit topology to provide the battery voltage with the second voltage value. The circuit topologies are understood to be different configurations of the battery cells. For example, the battery cells are connected in series to provide the battery voltage with the first voltage value and in parallel to provide the battery voltage with the second voltage value.

[0028] Preferably, the traction battery is operated at the first voltage value during vehicle operation, i.e., while electrical energy is being drawn from the traction battery to power the drive system, and at the second voltage value during charging using externally supplied electrical energy. This ensures both compatibility of the vehicle with existing charging stations and a rapid charging process.

[0029] A further development of the invention provides that the voltage converter arrangement is connected to a cooling device which is operated to cool the voltage converter arrangement during the equalization of the first electrical voltage to the battery voltage of the traction battery, wherein the cooling capacity of the cooling device is set to the electrical conversion power of the voltage converter arrangement used during the equalization process. The cooling device has, for example, a coolant circuit in which a coolant is circulated at least temporarily.

[0030] The coolant is supplied to the voltage converter assembly or to a heat exchanger associated with the voltage converter assembly, so that heat is transferred from the voltage converter assembly to the coolant and dissipated from the voltage converter assembly. Preferably, the coolant is then supplied to a coolant radiator, in which the coolant is cooled, for example, by dissipating the heat it contains to the outside environment of the vehicle. The coolant radiator is, for example, the main radiator of the vehicle.

[0031] While the voltage converter assembly is operating to adjust the initial electrical voltage to the battery voltage, it is actively cooled using the cooling device, for example, by circulating the coolant in the coolant circuit. The cooling capacity is adjusted to be sufficient to dissipate the heat generated by the voltage converter assembly, in particular to limit its temperature to an operating temperature. The cooling capacity is selected such that the temperature of the voltage converter assembly does not exceed the operating temperature or is always lower than it.

[0032] For this purpose, the cooling capacity is adjusted depending on the conversion power, which describes the electrical power with which the voltage converter arrangement converts the electrical energy drawn from the first subnetwork into heat. The described procedure ensures a reliable adjustment of the first electrical voltage to the battery voltage.

[0033] A further development of the invention provides that the voltage converter arrangement comprises a first voltage converter and a second voltage converter, wherein the first voltage converter is electrically connected between the first subnetwork and a charger designed and configured for charging the traction battery, and the second voltage converter is electrically connected between the first subnetwork and the second subnetwork, wherein the first voltage converter is operated to reduce the first electrical voltage and the second voltage converter is operated to maintain the first electrical voltage. The voltage converter arrangement thus comprises several voltage converters, namely at least the first voltage converter and the second voltage converter. The charger is electrically connected to the first subnetwork via the first voltage converter, and the second voltage converter connects the second subnetwork to the first subnetwork.

[0034] The first voltage converter is used to charge the traction battery using externally supplied electrical energy. For example, a vehicle's charging current connection is electrically connected to the first subnetwork or the first voltage converter via the charger. The charger is typically an AC charger or a three-phase charger. In this respect, the charger has a rectifier that is electrically connected between the charging connection and the first voltage converter.

[0035] With the aid of the rectifier, which is preferably designed as an active rectifier, the alternating current or three-phase current is converted into direct current, the voltage of which is then adjusted to the first electrical voltage of the first subnetwork or the battery voltage of the traction battery using the first voltage converter. The rectifier is preferably a controllable rectifier or a synchronous rectifier.

[0036] To adjust the first electrical voltage to the battery voltage, both voltage converters, i.e., both the first and the second voltage converter, are operated intermittently, specifically alternately. Using the first voltage converter, the electrical energy drawn from the first subnetwork is converted into heat, whereas using the second voltage converter, electrical energy is drawn from the second subnetwork and supplied to the first subnetwork. The former reduces the first electrical voltage, and the latter maintains the first electrical voltage. In either case, the use of both voltage converters ensures reliable adjustment of the first electrical voltage to the battery voltage and subsequent maintenance of a constant first electrical voltage.

[0037] A further development of the invention provides that voltage converters with different converter topologies, in particular with galvanic isolation by means of a transformer, are used as the first and second voltage converters. The converter topologies are understood to be the electrical circuits of the voltage converters. The first voltage converter has a first electrical circuit and the second voltage converter has a second electrical circuit, the electrical circuits being different from each other. Preferably, however, both voltage converters are designed with galvanic isolation, so that the charger and the first subnetwork are galvanically isolated from each other via the first voltage converter, and the first subnetwork and the second subnetwork are galvanically isolated from each other via the second voltage converter. The galvanic isolation is preferably implemented (in each case) using a transformer.

[0038] The first voltage converter, for example, has one or more full bridges electrically connected to each other via a transformer. Particularly preferably, the first voltage converter is configured as a dual active bridge and thus has two full bridges electrically connected to each other via the transformer. The full bridge(s) are preferably each configured as a four-quadrant converter. Additionally, an inductor, for example in the form of a choke, can be provided between the full bridges.

[0039] The second voltage converter has a different converter topology than the first voltage converter; for example, it is a resonant converter or similar. Preferably, the rated power of the first voltage converter is greater than the rated power of the second voltage converter; for example, the rated power of the first voltage converter is greater than the rated power of the second voltage converter by a factor of at least 2, at least 4, or at least 8. The described configuration of the electrical system and the explained procedure achieve the advantages already mentioned.

[0040] A further development of the invention provides that the first voltage converter and the second voltage converter are integrated and controlled by a common control unit, at which the battery voltage is set. The two voltage converters are, for example, arranged in a common housing and / or are located on a common circuit board. They are controlled by the common control unit, which is preferably also located in the housing and / or is implemented on the circuit board. The control unit controls the two voltage converters according to the procedure described. For this purpose, the battery voltage is first set at the control unit, for example, by a higher-level control unit.The integrated design of the voltage converters and the use of the common control unit has the advantage that the control of the voltage converters is carried out directly by the control unit and not by different control units that are connected to each other via a data bus, for example, and therefore need to be coordinated.

[0041] The invention further relates to an electrical system for a motor vehicle, in particular for carrying out the method according to the explanations in this description, wherein the electrical system comprises a first sub-network with a first electrical voltage and a second sub-network electrically connected to the first sub-network via a voltage converter arrangement with a second electrical voltage different from the first electrical voltage, and is designed and configured to operate the voltage converter arrangement at least temporarily for the transmission of electrical energy between the first sub-network and the second sub-network, wherein a traction battery is electrically connected to the first sub-network via a switch arrangement and a drive unit is electrically connected to the first sub-network via an inverter arrangement, and the inverter arrangement is operated at least temporarily.to supply electrical energy from the traction battery to the drive unit to provide drive torque.

[0042] The vehicle electrical system is also designed and configured to operate the voltage converter arrangement for converting electrical energy into heat at least temporarily, with a conversion efficiency that is lower than the transmission efficiency present when transferring electrical energy between the first sub-network and the second sub-network, in order to match the first electrical voltage of the first sub-network to a battery voltage of the traction battery. In this process, the first electrical voltage is reduced by converting electrical energy into heat, and after matching, the first electrical voltage is maintained at least temporarily by transferring electrical energy from the second sub-network to the first sub-network using the voltage converter arrangement with the transmission efficiency.

[0043] The advantages of such a design of the electrical system and such a procedure have already been mentioned. Both the electrical system and the method for operating it may be further developed as explained in this description, and reference is made to those explanations.

[0044] Furthermore, the invention relates to a computer program product comprising commands that cause the vehicle electrical system to execute the described method as outlined in this description. For the advantages and possible advantageous developments, reference is made to the description in its entirety.

[0045] The features and combinations of features described in the description, in particular those described in the following figure description and / or shown in the figures, are not only usable in the combinations specified, but also in other combinations or individually, provided that this does not exceed the scope defined by the claims. Thus, embodiments that are not explicitly shown or explained in the description and / or the figures, but which emerge from or can be derived from the explained embodiments, are also to be considered as encompassed by the invention, provided that these embodiments are within the scope of the claims.

[0046] The invention is explained in more detail below with reference to the exemplary embodiments shown in the drawing, without limiting the invention. The only embodiment shown is... Fig. 1 A schematic representation of an area of ​​a vehicle's electrical system.

[0047] The Fig. Figure 1 shows a schematic representation of an electrical system 1 for a motor vehicle. The electrical system 1 has a first subnetwork 2 and a second subnetwork 3, which are operated at different electrical voltages. The electrical voltage present in the first subnetwork 2 is referred to as the first electrical voltage and the electrical voltage present in the second subnetwork 3 as the second electrical voltage.

[0048] Furthermore, the vehicle electrical system 1 includes a traction battery 4, which has several cell modules 5 and 6, each containing one or more battery cells. Using a connection unit 7, the cell modules 5 and 6 can be connected to each other in different configurations, namely in different circuit topologies. The connection unit 7 has several switches 8, 9, and 10 for this purpose. In the first switching position of the connection unit 7, the cell modules 5 and 6 are electrically connected in series between battery terminals 11 and 12 of the traction battery 4, and in the second switching position, they are electrically connected in parallel.

[0049] The traction battery 4 is electrically connected to the first subnetwork 2 via a switch assembly 13. The switch assembly 13 has several switches 14 and 15, with switch 14 being electrically connected between battery terminal 11 and the first subnetwork 2, and switch 15 being electrically connected between battery terminal 12 and the first subnetwork 2. A drive unit and an inverter assembly, via which the drive unit is electrically connected to the first subnetwork, are not shown. It is intended that the inverter assembly be operated, at least intermittently, to supply the drive unit with electrical energy from the first subnetwork, which is drawn in particular from the traction battery 4.

[0050] The vehicle electrical system 1 also has a voltage converter arrangement 16, which in the embodiment shown here comprises a first voltage converter 17 and a second voltage converter 18. The first subnetwork 2 is electrically connected to a charger 19, which is specifically designed as an AC charger, via the first voltage converter 17. In the illustrated embodiment, the first voltage converter 17 is a dual active bridge. It has two four-quadrant converters 20 and 21, each with four switches 22, which are galvanically isolated from one another via a transformer 23. Preferably, several first voltage converters 17 are connected in parallel, so that the first subnetwork 2 and the charger 19 are each electrically connected to one another via several parallel-connected first voltage converters 17.

[0051] The second subnetwork 3 is electrically connected to the first subnetwork 2 via the second voltage converter 18. The second voltage converter 18 also features galvanic isolation and therefore also includes a transformer 24. Furthermore, the second voltage converter 18 includes one or more of the following components: switch 25, resistor 26, capacitor 27, and inductor 28. A battery 29 may also be present in the second subnetwork 3. Multiple second voltage converters 18 can also be present, with the multiple second voltage converters 18 connected electrically in parallel. In this case as well, the first subnetwork 2 and the second subnetwork 3 are each electrically connected to each other via the parallel-connected voltage converters 18.

[0052] To switch the traction battery 4 between the circuit topologies, it is first necessary to disconnect the traction battery 4 from the first subnetwork 2 using the switch arrangement 13. The switching unit 7 is then adjusted to achieve the desired circuit topology. Finally, the voltage present in the first subnetwork 2 is adjusted to the battery voltage between battery terminals 11 and 12 using the voltage converter arrangement 16.

[0053] For this purpose, electrical energy is drawn from the first subnetwork 2 using the first voltage converter 17 and converted into heat, in particular completely. This continues until the first electrical voltage corresponds to the battery voltage. Subsequently, the first electrical voltage is kept constant by supplying electrical energy to the first subnetwork 2 via the second voltage converter 18, which was drawn from the second subnetwork 3. This continues until the traction battery 4 is reconnected to the first subnetwork 2 using the switch arrangement 13. This results in a particularly rapid and reliable adjustment of the first electrical voltage to the battery voltage of the traction battery 4. REFERENCE MARK LIST: 1 On-board electrical system 2 1. Subnetwork 3 2nd subnetwork 4 traction batteries 5-cell module 6-cell module 7 Interconnection unit 8 switches 9 switches 10 switches 11 Battery connection 12 Battery connection 13 Switch arrangement 14 switches 15 switches 16 Voltage converter arrangement 17 1. Voltage converter 18 2. Voltage converter 19 charger 20 four-quadrant plates 21 four-quadrant plates 22 switches 23 Transformer 24 Transformer 25 switches 26 Resistance 27 Capacitor 28 coil 29 Battery

Claims

[1] Method for operating an electrical system (1) for a motor vehicle, wherein the electrical system (1) comprises a first sub-network (2) with a first electrical voltage and a second sub-network (3) electrically connected to the first sub-network (2) via a voltage converter arrangement (16) with a second electrical voltage different from the first electrical voltage, and the voltage converter arrangement (16) is operated at least temporarily to transfer electrical energy between the first sub-network (2) and the second sub-network (3), wherein a traction battery (4) is electrically connected to the first sub-network (2) via a switch arrangement (13) and a drive unit is electrically connected to the first sub-network (2) via an inverter arrangement, and the inverter arrangement is operated at least temporarily to supply electrical energy from the traction battery (4) to the drive unit to provide a drive torque, characterized by, that in order to equalize the first electrical voltage of the first subnetwork (2) to a battery voltage of the traction battery (4), the voltage converter arrangement (16) is operated at least temporarily to convert electrical energy into heat with a conversion efficiency that is lower than the transmission efficiency present when transferring electrical energy between the first subnetwork (2) and the second subnetwork (3), wherein, in order to equalize the first electrical voltage to the battery voltage, the first electrical voltage is reduced by converting electrical energy into heat, and after equalization, the first electrical voltage is maintained at least temporarily by transferring electrical energy from the second subnetwork (3) to the first subnetwork (2) by means of the voltage converter arrangement (16) with the transmission efficiency [2] Method according to claim 1, characterized by, that the traction battery (4) is electrically disconnected from the first subnetwork (2) by means of the switch arrangement (13) and adjusted from a higher first voltage value of the battery voltage to a lower second voltage value of the battery voltage, and subsequently the first electrical voltage is adjusted to the second voltage value of the battery voltage and the traction battery (4) is electrically connected to the first subnetwork (2) by means of the switch arrangement (13). [3] Method according to any one of the preceding claims, characterized by , that the traction battery (4) has several battery cells which are connected to provide the battery voltage with the first voltage value in a first circuit topology and to provide the battery voltage with the second voltage value in a second circuit topology. [4] Method according to any one of the preceding claims, characterized by, that the voltage converter arrangement (16) is connected to a cooling device which is operated to cool the voltage converter arrangement (16) during the adjustment of the first electrical voltage to the battery voltage of the traction battery (4), wherein a cooling power of the cooling device is set to an electrical conversion power of the voltage converter arrangement (16) used during the adjustment. [5] Method according to any one of the preceding claims, characterized by, that the voltage converter arrangement (16) comprises a first voltage converter (17) and a second voltage converter (18), wherein the first voltage converter (17) is electrically arranged between the first subnetwork (2) and a charger (19) provided and configured for charging the traction battery (4), and the second voltage converter (18) is electrically arranged between the first subnetwork (2) and the second subnetwork (3), wherein the first voltage converter (17) is operated to reduce the first electrical voltage and the second voltage converter (18) is operated to maintain the first electrical voltage. [6] Method according to claim 5, characterized by , that voltage converters with different converter topologies are used as the first voltage converter (17) and as the second voltage converter (18). [7] Method according to claim 5 or 6, characterized by, that the first voltage converter (17) and the second voltage converter (18) are designed as an integrated unit and are controlled by a common control unit, on which the battery voltage is set. [8] On-board electrical system (1) for a motor vehicle for carrying out the method according to one or more of the preceding claims, wherein the on-board electrical system (1) comprises a first sub-network (2) with a first electrical voltage and a second sub-network (3) electrically connected to the first sub-network (2) via a voltage converter arrangement (16) with a second electrical voltage different from the first electrical voltage and is provided and configured to operate the voltage converter arrangement (16) at least temporarily for the transmission of electrical energy between the first sub-network (2) and the second sub-network (3), wherein a traction battery (4) is electrically connected to the first sub-network (2) via a switch arrangement (13) and a drive unit is electrically connected to the first sub-network (2) via an inverter arrangement and the inverter arrangement is operated at least temporarily,to supply electrical energy from the traction battery (4) to the drive unit to provide drive torque, characterized by, that the on-board network (1) is also designed and configured to operate the voltage converter arrangement (16) at least temporarily for converting electrical energy into heat with a conversion efficiency that is lower than the transmission efficiency present when transferring electrical energy between the first subnetwork (2) and the second subnetwork (3), in order to match the first electrical voltage to the battery voltage, the first electrical voltage is reduced by converting electrical energy into heat and, after matching, the first electrical voltage is maintained at least temporarily by transferring electrical energy from the second subnetwork (3) to the first subnetwork (2) using the voltage converter arrangement (16) with the transmission efficiency. [9] Computer program product comprising instructions that cause the on-board network (1) according to claim 8 to execute the method according to one or more of claims 1 to 7.

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