Multi-voltage battery device and on-board power system for a motor vehicle

Abstract

Multi-voltage battery device (MB) for a motor vehicle, comprising: - a first output current terminal (AA1) and a ground current terminal (MA) for providing a first nominal voltage (U1); - a second output current terminal (AA2) and the ground current terminal (MA) for providing a second nominal voltage (U2); - a first battery cell group (ZG1) and a first controllable switch (RL1) electrically connected to each other in series between the first output current terminal (AA1) and the ground current terminal (MA); - a second battery cell group (ZG2) which is electrically connected between the second output current terminal (AA2) and the first output current terminal (AA1) and is switchably connected in series with the first battery cell group (ZG1); - a battery management arrangement (BM) for operating the multi-voltage battery device (MB); - a unidirectional or bidirectional DC-DC converter (GW) which is electrically connected on the input voltage side to the second output current connection (AA2) and on the output voltage side to the first output current connection (AA1) and is configured to charge the first battery cell group (ZG1) with current from the second battery cell group (ZG2) and / or the second battery cell group (ZG2) with current from the first battery cell group (ZG1), characterized in that - the battery management arrangement (BM) is electrically connected via supply current connections (VA1, VA2) between the first output current connection (AA1) and the ground current connection (MA) and is designed to monitor a charge state of the first battery cell group (ZG1) and to open the first switch (RL1) in a controlled manner in the event of a critical charge state of the first battery cell group (ZG1); - wherein the battery management arrangement (BM) is further configured to operate the DC-DC converter (GW) for charging the first battery cell group (ZG1) with the current of the second battery cell group (ZG2) when the first battery cell group (ZG1) is in a critical state of charge.

Description

Technical area:

[0001] The present invention relates to a multi-voltage battery device and an on-board power system for a motor vehicle, in particular a hybrid electric / electric vehicle, having a multi-voltage battery device. State of the art and task of the invention:

[0002] Multi-voltage battery devices for providing different nominal or operating voltages for on-board electrical systems of motor vehicles, in particular hybrid electric / electric vehicles, are known.

[0003] Due to the limited available charging capacity, multi-voltage battery devices can discharge below their critical charge level (deep discharge), for example, during extended periods of inactivity, which can damage individual battery cells. This, in turn, can lead to malfunctions in the vehicle's electrical systems.

[0004] DE 10 2016 208 893 A1 describes a motor vehicle electrical system which has an electrical machine which is connected to an active power converter with a positive and a negative DC voltage connection, a first energy storage device with a positive and a negative voltage connection, a second energy storage device with a positive and a negative voltage connection and a DC-DC converter with a first and a second connection, wherein switching means are also provided which are designed to set a first switching state in which the first energy storage device and the second energy storage device are connected in parallel to the DC voltage connections of the power converter, and a second switching state in which the first energy storage device and the second energy storage device are connected in series to the DC voltage connections of the power converter.

[0005] DE 10 2015 219 589 A1 describes a vehicle battery device with a first rechargeable battery having an intermediate tap, a second rechargeable battery having a positive pole, and with a first and a second positive terminal. The positive pole of the second rechargeable battery is connected to the intermediate tap of the first rechargeable battery. The second rechargeable battery has a nominal voltage that is lower than the nominal voltage of the first rechargeable battery. The first positive terminal is connected to a positive pole of the first rechargeable battery. The second positive terminal is connected to the positive pole of the second rechargeable battery. A ground terminal of the vehicle battery device is connected to a negative pole of the first rechargeable battery via a first switch. The ground terminal is further connected to a negative pole of the second rechargeable battery via a second switch.

[0006] US 2014 / 0 152 262 A1 describes a power supply device that has a plurality of secondary batteries connected in series, outputs a composite voltage of all the secondary batteries, and outputs an output of a part of the secondary batteries as a partial voltage, the power supply device comprising: a detection unit that detects states of the secondary batteries; and a switching unit that changes an order of series connection of the plurality of secondary batteries based on a detection result of the detection unit such that the partial voltage is output from a secondary battery that is in a relatively good condition.

[0007] US 2004 / 0 222 771 A1 describes a battery circuit comprising: a first battery group connected to a load; a second battery group connected in series with the first battery group; and a bidirectional DC / DC converter for electrical power migration between the first battery group and the second battery group, wherein a charge capacity of the first battery group differs from a charge capacity of the second battery group.

[0008] The object of the invention is therefore to provide a possibility with which the multi-voltage battery devices and thus also the on-board electrical systems of motor vehicles can be reliably protected against malfunctions caused by damage to the battery cells. Description of the invention:

[0009] This problem is solved by the subject matter of the independent claims. Advantageous embodiments are the subject matter of the dependent claims.

[0010] According to a first aspect of the invention, a multi-voltage battery device for a motor vehicle, in particular a hybrid electric / electric vehicle, is provided.

[0011] The multi-voltage battery device comprises a first output current terminal and a ground current terminal for providing a first nominal voltage, for example a first vehicle electrical system voltage for a vehicle electrical system of a motor vehicle.

[0012] The multi-voltage battery device further comprises a second output current terminal which, together with the ground current terminal, serves to provide a second nominal voltage, for example a second vehicle electrical system voltage for the vehicle electrical system.

[0013] The multi-voltage battery device further comprises a first battery cell group and a first controllable switch electrically connected to each other in series between the first output current terminal and the ground current terminal.

[0014] The multi-voltage battery device further comprises a second battery cell group electrically connected between the second output current terminal and the first output current terminal and switchably connected in series with the first battery cell group.

[0015] The multi-voltage battery device further comprises a battery management arrangement for operating the multi-voltage battery device, which is electrically connected via its supply current terminals between the first output current terminal and the ground current terminal (and thus in parallel with the series connection of the first battery cell group and the first switch), or between an emergency power supply terminal of the multi-voltage battery device and the ground current terminal.

[0016] The battery management arrangement is configured to monitor the charge state of the first battery cell group and, in the event of a critical charge state of the first battery cell group, to open the first switch in a controlled manner and thus to interrupt the flow of current from the first battery cell group via the first power connection to external power consumers and to the battery management arrangement.

[0017] The invention is based on the idea that a multi-voltage battery device, which serves to provide different nominal voltages or on-board network voltages for different on-board network branches of a motor vehicle, must provide a minimum current, even during a standby mode in which the vehicle is parked (after the end of driving operation) and in which only safety-relevant and other relevant vehicle functions are performed, in order to be able to maintain these relevant functions. For this purpose, one (or the first) battery cell group of the multi-voltage battery device is provided, which supplies systems or components (the aforementioned external power consumers) with power during the standby mode so that these external power consumers can perform the relevant functions.

[0018] Since the (first) battery cell group has a limited charging capacity and thus a limited amount of electrical energy, and is generally not charged during standby mode, there is a risk of deep discharge due to the energy consumption of external power consumers. Deep discharge can lead to irreversible damage to the battery cells and thus to malfunctions or even failure of the (first) battery cell group.

[0019] In order to prevent or delay deep discharge, a solution is required in which the affected battery cell group is monitored (during the vehicle's idle mode) and can be completely electrically disconnected from external power consumers if necessary, or, if possible, charged with power from another power source.

[0020] Based on this idea, a multi-voltage battery device is provided with a battery management arrangement which is electrically connected via its supply current terminals between the first output current terminal and the ground current terminal and thus parallel to the series connection of the first battery cell group and the first switch.

[0021] Because the battery management arrangement is electrically connected in parallel to the series circuit of the first battery cell group and the first switch, a power supply circuit is formed between the first battery cell group as a power source and the battery management arrangement as a current sink, which can be interrupted by the first switch (in the open switching state).

[0022] The battery management arrangement is also reliably supplied with power by the first battery cell group during the motor vehicle's idle mode, in which the first battery cell group supplies power to the external power consumers (which perform the relevant functions) connected via the first power connection via the closed first switch.

[0023] This ensures that the battery management arrangement remains fully functional during this time and reliably monitors the first battery cell group with regard to its charge level.

[0024] If the first battery cell group is at risk of deep discharge, the battery management arrangement can safely electrically disconnect the first battery cell group from the external power consumers by opening the first switch and can thus protect the first battery cell group from the impending deep discharge.

[0025] The fact that the battery management arrangement is also electrically separated from the first battery cell group and thus from the power supply by opening the first switch is not disadvantageous in this case, since the battery management arrangement has already successfully fulfilled its core function (while the motor vehicle is stationary), namely to protect the first battery cell group from the impending deep discharge in the motor vehicle's idle mode, by opening the first switch.

[0026] Rather, the electrical separation of the battery management arrangement from the first battery cell group has the advantage that the battery management arrangement does not further discharge the first battery cell group through its power consumption and thus additionally protects the first battery cell group from the threat of deep discharge.

[0027] In the version with the battery management arrangement that is electrically connected via supply current connections between an emergency power supply connection of the multi-voltage battery device and the ground current connection, the battery management arrangement can be supplied with power from an external power source, if applicable, even after the first switch has been opened and can thus continue its monitoring function.

[0028] This provides a means of reliably protecting a multi-voltage battery device and thus also a motor vehicle electrical system from faults caused by a deep discharge of a battery cell group.

[0029] The multi-voltage battery device further comprises a unidirectional or bidirectional DC-DC converter, which is electrically connected on the input voltage side to the second output current terminal and on the output voltage side to the first output current terminal. The DC-DC converter is configured, in particular depending on requirements or the charge states of the two battery cell groups, to charge the first battery cell group with current from the second battery cell group and / or the second battery cell group with current from the first battery cell group. The battery management arrangement is further configured to operate the DC-DC converter with the current from the second battery cell group to charge the first battery cell group when the charge state of the first battery cell group is critical.

[0030] For example, the multi-voltage battery device further comprises a second controllable switch that is electrically connected between the second output current terminal and the first output current terminal and in series with the second battery cell group, thus switchably electrically connecting the second battery cell group to the first battery cell group. The battery management arrangement is further configured, for example, to monitor the charge state of the second battery cell group and, in the event of a critical charge state of the second battery cell group, to close the second switch to charge the second battery cell group or both battery cell groups.

[0031] For example, the first and / or the second switch are each designed as a relay.

[0032] The critical state of charge of the first and / or second battery cell group is, for example, at or below 30%, 20%, 15%, 10%, 8%, 5%, or 3%. The critical state of charge depends, among other things, on the materials used and cell chemistry, as well as on cell temperatures and other physical conditions of the battery cells in the first and second battery cell groups.

[0033] For example, the first nominal voltage is 12 volts and / or the second nominal voltage is 48 volts.

[0034] According to a further aspect of the invention, an on-board electrical system for a motor vehicle, in particular a hybrid electric / electric vehicle, is provided.

[0035] The vehicle electrical system comprises a first vehicle electrical system branch with a first vehicle electrical system voltage and a second vehicle electrical system branch with a second vehicle electrical system voltage. The vehicle electrical system further comprises a previously described multi-voltage battery device, which is electrically connected to the first vehicle electrical system branch via the first output current connection and to the second vehicle electrical system branch via the second output current connection.

[0036] Advantageous embodiments of the multi-voltage battery device described above are, insofar as they are otherwise transferable to the above-mentioned on-board electrical system, also to be regarded as advantageous embodiments of the on-board electrical system. Short description of the drawings:

[0037] In the following, exemplary embodiments of the invention are explained in more detail with reference to the accompanying drawings. In the drawings: Fig. 1 shows a schematic representation of an on-board electrical system of a hybrid electric vehicle with a multi-voltage battery device according to an exemplary embodiment of the invention; and Fig. 2 shows a further schematic representation of a further electrical system of a hybrid electric vehicle with a further multi-voltage battery device according to a further exemplary embodiment of the invention. Detailed description of the drawings:

[0038] The on-board network BN in Fig. 1 comprises a first electrical system branch BZ1, in which a first electrical system voltage U1 of, for example, 12 volts is present. The electrical system BN further comprises a second electrical system branch BZ2, in which a second electrical system voltage U2 of, for example, 48 volts is present. The electrical system BN is installed in a hybrid electric vehicle with a 48-volt mild hybrid drive.

[0039] The on-board network BN also includes a multi-voltage battery device MB for providing or maintaining the two on-board network voltages U1, U2.

[0040] The multi-voltage battery device MB is built as a so-called AES battery, i.e. a 48 volt battery with a 12 volt tap and a DC-DC converter GW.

[0041] The multi-voltage battery device MB comprises, on the current output side, a first output current terminal AA1 and a ground current terminal MA, via which the multi-voltage battery device MB is electrically connected to the first vehicle electrical system branch BZ1. The multi-voltage battery device MB provides the first vehicle electrical system voltage U1 as a first nominal voltage, which is present between the first output current terminal AA1 and the ground current terminal MA.

[0042] The multi-voltage battery device MB further comprises a second output current terminal AA2 on the current output side and is electrically connected to the second vehicle electrical system branch BZ2 via the second output current terminal AA2 and the ground current terminal MA. The multi-voltage battery device MB provides the second vehicle electrical system voltage U2 as a second nominal voltage, which is present between the second output current terminal AA2 and the ground current terminal MA.

[0043] The multi-voltage battery device MB comprises, between the first output current terminal AA1 and the ground current terminal MA, a first series circuit of a first battery cell group ZG1, a first fuse FS1, and a first relay RL1 as a first controllable switch. The first series circuit with the first battery cell group ZG1 thus forms a first current / voltage source for the first on-board electrical system branch BZ1. The first battery cell group ZG1 has a nominal voltage of 12 volts and thus provides the first on-board electrical system voltage U1. The first battery cell group ZG1 is electrically connected to the first output current terminal AA1 via its positive pole PP1 and to the ground current terminal MA via its negative pole NP1 (and via the first fuse FS1 and the first relay RL1).

[0044] The multi-voltage battery device MB further comprises a second series circuit of a second battery cell group ZG2, a second fuse FS2, and a second relay RL2 as a first controllable switch between the first output current terminal AA1 and the second output current terminal AA2. The second battery cell group ZG2 is electrically connected to the second output current terminal AA2 via its positive terminal PP2 (and via the second relay RL2 and the second fuse FS2) and to the first output current terminal AA1 via its negative terminal NP2.

[0045] Between the second output current connection AA2 and the ground current connection MA, the first and the second series circuits thus form a larger series circuit of the first battery cell group ZG1, the first fuse FS1 and the first relay RL1, as well as the second battery cell group ZG2, the second fuse FS2 and the second relay RL2, wherein the two battery cell groups ZG1, ZG2 are connected in series to one another via the second relay RL2.

[0046] The second series connection with the second battery cell group ZG2 forms a second current / voltage source for the second on-board electrical system branch BZ2 with the first series connection with the first battery cell group ZG1. The second battery cell group ZG2 has a nominal voltage of 36 volts and, together with the first battery cell group ZG1 connected in series with a nominal voltage of 12 volts, provides the second on-board electrical system voltage U2 of 48 volts.

[0047] The two battery cell groups ZG1, ZG2 are made of Li-ion cells.

[0048] The multi-voltage battery device MB also comprises a bidirectional DC-DC converter GW, which is electrically connected on the input voltage side to the second output current terminal AA2 and on the output voltage side to the first output current terminal AA1, and thus to the positive pole PP1 of the first battery cell group ZG1. The DC-DC converter GW is configured to charge the two battery cell groups ZG1, ZG2, as needed, with a current provided by a power source connected to the second vehicle electrical system branch BZ2. The DC-DC converter GW is further configured to charge the first battery cell group ZG1, as needed, with a current provided by another power source connected to the first vehicle electrical system branch BZ1.The DC-DC converter GW is further configured to shift charges between the two battery cell groups ZG1, ZG2 as needed and thus to charge the first battery cell group ZG1 with current from the second battery cell group ZG2 or the second battery cell group ZG2 with current from the first battery cell group ZG1.

[0049] The multi-voltage battery device MB also includes a battery management arrangement BM for operating or controlling the multi-voltage battery device MB. The battery management arrangement BM is electrically connected via its supply current connections VA1, VA2 between the first output current connection AA1 and the ground current connection MA, and thus in parallel with the first series circuit or the first battery cell group ZG1 and the first relay RL1. Thus, the battery management arrangement BM is supplied with power by the first battery cell group ZG1 (provided the first relay RL1 is closed).

[0050] The battery management arrangement BM is connected on the signal input side to the two battery cell groups ZG1, ZG2 via two measuring signal connections MA1, MA2 and monitors the charge states (in English "State of Charge, SoC"), the aging states (in English "State of Health, SoH") and other properties, such as temperatures, of the two battery cell groups ZG1, ZG2 via these measuring signal connections MA1, MA2 in a manner known to those skilled in the art.

[0051] The battery management arrangement BM is connected on the signal output side via three control signal connections SA1, SA2, SA3 to the DC-DC converter GW, the first and second relays RL1, RL2 or to the respective control signal connections of these components.

[0052] The battery management arrangement BM is designed to monitor the charge states of the two battery cell groups ZG1, ZG2 in a manner known to those skilled in the art and, in the event of critical charge states of the respective battery cell groups ZG1, ZG2, to control or regulate the DC-DC converter GW and the two relays RL1, RL2 in such a way that the two battery cell groups ZG1, ZG2 are each charged with the current from the respective other battery cell group ZG1, ZG2 and are thus protected from damage caused by deep discharge.

[0053] If the vehicle is switched off after a driving session and enters sleep mode, only safety-relevant and other relevant vehicle functions continue to operate. Accordingly, electrical systems and components, such as sensors, bus systems, and control units, that perform these relevant functions continue to consume power.

[0054] As a rule, these systems or components have a nominal voltage of 12 volts (12 volt consumers) and are therefore connected to the first on-board network branch BZ1 and supplied with power by the first battery cell group ZG1 - even during the vehicle's idle mode.

[0055] In the vehicle's idle mode, the second relay RL2 is usually opened under the control of the battery management arrangement BM and the second battery cell group ZG2 is electrically separated from the on-board network BN and thus from power consumers in the second on-board network branch BZ2 with a nominal voltage of 48 volts.

[0056] However, the first relay RL1 remains closed under the control of the battery management arrangement BM in order to ensure a continuous supply of the previously mentioned 12 volt consumers, i.e. the systems or components that perform the safety-relevant and other relevant functions during the vehicle's standby mode. Most of these 12 volt consumers are also in sleep mode and therefore have a significantly reduced power consumption.

[0057] If the first battery cell group ZG1 is sufficiently charged at the time the vehicle enters sleep mode, it can continue to supply the 12 volt consumers with power for a certain period of several days or weeks.

[0058] For this purpose, the battery management arrangement BM closes the first relay RL1 or keeps it in the closed switching state, so that the power supply for the 12 volt consumers is guaranteed by the first battery cell group ZG1 for a certain period of time.

[0059] If the Li-ion cells of the first battery cell group ZG1 are discharged to such an extent after a certain time, the cells are at risk of irreversible damage due to deep discharge.

[0060] This impending deep discharge is detected by the battery management system BM by monitoring the state of charge of the first battery cell group ZG1 and by comparing it with a specified minimum state of charge of, for example, 5%.

[0061] The battery management arrangement BM then opens the first relay RL1 and thus interrupts the power supply to the 12 volt consumers that perform the safety-relevant and other relevant functions during the vehicle's sleep mode.

[0062] Because the battery management arrangement BM is also supplied with power from the first battery cell group ZG1, the power supply for the battery management arrangement BM is also interrupted when the first relay RL1 opens.

[0063] Due to the loss of power supply, the battery management assembly (BM) is now inactive, and the first battery cell group (ZG1) is no longer monitored. Since the first battery cell group (ZG1) is now electrically isolated from the 12-volt loads, including the battery management assembly (BM), it is no longer discharged except for a comparatively slow self-discharge. This prevents or significantly delays deep discharge.

[0064] Alternatively, in the event of imminent deep discharge, the battery management arrangement BM checks the charge state of the second battery cell group ZG2.

[0065] If the second battery cell group ZG2 has a sufficient charge level, the battery management arrangement BM keeps the first relay RL1 closed and additionally closes the second relay RL2, which was opened under the control of the battery management arrangement BM when the motor vehicle went into standby mode.

[0066] Furthermore, the battery management system BM controls or regulates the DC-DC converter GW such that it charges the first battery cell group ZG1 with the current from the second battery cell group ZG2. This also protects the first battery cell group ZG1 from the threat of deep discharge. In addition, the 12-volt loads continue to be supplied with power from the first battery cell group ZG1, thus keeping them in operation.

[0067] The on-board network BN in Fig. 2 differs from that in Fig. 1 in that its multi-voltage battery device MB has, in addition to the first and second output current terminals AA1, AA2, an emergency power supply terminal NA.

[0068] The battery management system BM is electrically connected via its positive-voltage supply connection VA1 not to the first output power connection AA1, but rather to the emergency power supply connection NA. An external power source SQ can be connected to the emergency power supply connection NA.

[0069] Such an embodiment has advantages over the Fig. 1 has the advantage that the battery management arrangement BM can continue to be supplied with power from the external power source when the first relay RL1 opens in the event of an impending deep discharge of the first battery cell group ZG1 and can thus continue its monitoring function.

Claims

[1] Multi-voltage battery device (MB) for a motor vehicle, comprising: - a first output current terminal (AA1) and a ground current terminal (MA) for providing a first nominal voltage (U1); - a second output current terminal (AA2) and the ground current terminal (MA) for providing a second nominal voltage (U2); - a first battery cell group (ZG1) and a first controllable switch (RL1) electrically connected to each other in series between the first output current terminal (AA1) and the ground current terminal (MA); - a second battery cell group (ZG2) which is electrically connected between the second output current terminal (AA2) and the first output current terminal (AA1) and is switchably connected in series with the first battery cell group (ZG1); - a battery management arrangement (BM) for operating the multi-voltage battery device (MB); - a unidirectional or bidirectional DC-DC converter (GW) which is electrically connected on the input voltage side to the second output current connection (AA2) and on the output voltage side to the first output current connection (AA1) and is configured to charge the first battery cell group (ZG1) with current from the second battery cell group (ZG2) and / or the second battery cell group (ZG2) with current from the first battery cell group (ZG1), characterized by , that - the battery management arrangement (BM) is electrically connected via supply current connections (VA1, VA2) between the first output current connection (AA1) and the ground current connection (MA) and is designed to monitor a charge state of the first battery cell group (ZG1) and to open the first switch (RL1) in a controlled manner in the event of a critical charge state of the first battery cell group (ZG1); - wherein the battery management arrangement (BM) is further configured to operate the DC-DC converter (GW) for charging the first battery cell group (ZG1) with the current of the second battery cell group (ZG2) when the first battery cell group (ZG1) is in a critical state of charge. [2] Multi-voltage battery device (MB) for a motor vehicle comprising: - a first output current terminal (AA1) and a ground current terminal (MA) for providing a first nominal voltage (U1); - a second output current terminal (AA2) and the ground current terminal (MA) for providing a second nominal voltage (U2); - a first battery cell group (ZG1) and a first controllable switch (RL1) electrically connected to each other in series between the first output current terminal (AA1) and the ground current terminal (MA); - a second battery cell group (ZG2) which is electrically connected between the second output current terminal (AA2) and the first output current terminal (AA1) and is switchably connected in series with the first battery cell group (ZG1); - a battery management arrangement (BM) for operating the multi-voltage battery device (MB); - a unidirectional or bidirectional DC-DC converter (GW) which is electrically connected on the input voltage side to the second output current connection (AA2) and on the output voltage side to the first output current connection (AA1) and is configured to charge the first battery cell group (ZG1) with current from the second battery cell group (ZG2) and / or the second battery cell group (ZG2) with current from the first battery cell group (ZG1), characterized by , that - the battery management arrangement (BM) is electrically connected via supply current connections (VA1, VA2) between an emergency power supply connection (NA) of the multi-voltage battery device (MB) and the ground current connection (MA) and is configured to monitor a charge state of the first battery cell group (ZG1) and to open the first switch (RL1) in a controlled manner in the event of a critical charge state of the first battery cell group (ZG1); - wherein the battery management arrangement (BM) is further configured to operate the DC-DC converter (GW) for charging the first battery cell group (ZG1) with the current of the second battery cell group (ZG2) when the first battery cell group (ZG1) is in a critical state of charge. [3] Multi-voltage battery device (MB) according to claim 1 or 2, further comprising: - a second controllable switch (RL2) which is electrically connected between the second output current terminal (AA2) and the first output current terminal (AA1) and in series with the second battery cell group (ZG2) and thus switchably electrically connects the second battery cell group (ZG2) to the first battery cell group (ZG1); - wherein the battery management arrangement (BM) is further configured to monitor a charge state of the second battery cell group (ZG2) and, in the event of a critical charge state of the second battery cell group (ZG2), to close the second switch (RL2) for charging the second battery cell group (ZG2) or the two battery cell groups (ZG1, ZG2). [4] Multi-voltage battery device (MB) according to claim 1, 2 and / or 3, wherein the first (RL1) and / or the second (RL2) switch are each designed as a relay. [5] Multi-voltage battery device (MB) according to one of the preceding claims, wherein the critical state of charge of the first (ZG1) and / or the second (ZG2) battery cell group is 30%, 20%, 15%, 10%, 8%, 5% or 3%. [6] Multi-voltage battery device (MB) according to one of the preceding claims, wherein the first nominal voltage (U1) is 12 volts and / or the second nominal voltage (U2) is 48 volts. [7] On-board network (BN) for a motor vehicle, comprising: - a first vehicle electrical system branch (BZ1) with a first vehicle electrical system voltage (U1); - a second on-board network branch (BZ2) with a second on-board network voltage (U2); - a multi-voltage battery device (MB) according to one of the preceding claims, which is electrically connected to the first vehicle electrical system branch (BZ1) via the first output current connection (AA1) and to the second vehicle electrical system branch (BZ2) via the second output current connection (AA2).

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