CN105934860A - Method for operation of an onboard power supply - Google Patents

Abstract

The invention relates to a method for operating an onboard electrical system (1) for a motor vehicle, wherein the onboard electrical system (1) has a low-voltage subsystem (21) for at least one low-voltage load (29), for A high-voltage subsystem (20) of at least one high-voltage load (25) and a starter-generator (30), wherein the high-voltage subsystem (20) is connected to the low-voltage subsystem (21) via a coupling unit (33) The connection, the coupling unit is arranged to extract energy from the high voltage subsystem (20) and deliver it to the low voltage subsystem (21), wherein the high voltage subsystem (20) has an accumulator (40) arranged to generate a high voltage And output to the high-voltage sub-grid (20), and the battery has at least two battery cells (41) with a single voltage tap (80), the single voltage tap is led to the coupling unit (33), wherein the coupling unit (33) is It is provided that the battery unit (41) is selectively connected to the low-voltage sub-grid (21), characterized in that, in the following steps, from the first battery unit connected to the low-voltage sub-grid (21) (41) Switching to the second battery unit (41) to be connected to the low-voltage subsystem (21): a) separate the first, connected and second battery unit (41) to be connected ) between the wires; b) connect the second battery unit (41) to be connected to the low-voltage subsystem (21); c) cut off the first connected battery unit (21) from the low-voltage subsystem (21) battery unit (41); d) connected to the first battery unit (41) that has been disconnected from the low-voltage subsystem (21) and the second battery unit (41) that has been connected to the low-voltage subsystem (21) between the wires. Furthermore, the invention relates to a battery management system and a computer program, which are provided for carrying out the method, as well as to an on-board electrical system and a motor vehicle, on which the method can be carried out.

Description

Method for operating a vehicle electrical system

technical field

The invention relates to a method for operating an onboard electrical system of a motor vehicle.

Furthermore, the invention relates to a battery management system and a computer program, which are provided for carrying out the method, as well as to an on-board electrical system and a motor vehicle, on which the method can be carried out.

Background technique

In motor vehicles with an internal combustion engine, an on-board electrical system is provided for supplying an electric starter or starter for the internal combustion engine as well as other electric devices of the motor vehicle, which are operated as standard at 12 volts. During starting of the internal combustion engine, the starter battery supplies voltage via the vehicle electrical system to the starter, which starts the internal combustion engine when the starter is switched on, for example via a corresponding start signal. If the internal combustion engine is started, it drives a generator, which then generates a voltage of approximately 12 volts and supplies the various electrical loads in the vehicle via the on-board electrical system. In this case, the generator in turn charges the starter battery charged by the starting process. If the battery is charged via the vehicle electrical system, the actual voltage can be a rated voltage, for example 14V or 14.4V. A vehicle electrical system with a voltage of 12 V or 14 V is also referred to as a low-voltage vehicle electrical system within the scope of the present invention.

In electric and hybrid vehicles it is known to use a further on-board electrical system with a nominal voltage of 48 V, which is also referred to as a high-voltage on-board electrical system within the scope of the present invention.

Contents of the invention

The method according to the invention relates to an onboard electrical system for a motor vehicle, wherein the onboard electrical system has a low-voltage subsystem for at least one low-voltage load, a high-voltage subsystem for at least one high-voltage load, and a starter - a generator, wherein the high-voltage subsystem is connected to the low-voltage subsystem via a coupling unit configured to extract energy from the high-voltage subsystem and supply it to the low-voltage subsystem , wherein the high-voltage subsystem has a battery configured to generate a high voltage and output to the high-voltage subsystem, and the battery has at least two battery cells with a single voltage tap, the A single voltage tap leads to the coupling unit, wherein the coupling unit is configured to selectively connect the battery unit (41) to the low-voltage subsystem (21). Switching between a first battery unit connected to the low-voltage subsystem and a second battery unit to be connected to the low-voltage subsystem takes place in the following steps:

a) disconnect the wires between the first, connected battery unit and the second, to-be-connected battery unit;

b) connecting the second, to-be-connected battery unit to the low-voltage subsystem;

c) disconnecting said first, connected battery unit from said low voltage subsystem;

d) A line connected between a first battery unit disconnected from the low-voltage subsystem and a second battery unit connected to the low-voltage subsystem.

The invention has the advantage that electrical loads can be operated via the low-voltage subsystem, which are set at a low first voltage, and the high-voltage subsystem supplies high-power loads, i.e. with a higher voltage than the first voltage. sub-vehicle power grid. The supply of the low-voltage subsystem overlaps with the charging and discharging processes in the high-voltage subsystem. The supply of the low-voltage subsystem via the high-voltage subsystem takes place unidirectionally, ie the coupling unit provides energy conversion, preferably only in one direction.

The method provides an advantageous switching configuration which allows the low-voltage subsystem to be supplied without interruption, ie the low-voltage subsystem is also supplied by at least one battery unit during the switchover. As a result, voltage interruptions in the low-voltage subsystem can be avoided without additional buffer systems. During the switching process, the battery is also available as storage for the high-voltage subsystem. In this case, the voltage can fall below the nominal value for a short time, but an energy flow in both directions, ie charging and discharging of the accumulator, is possible.

The terms "accumulator" and "accumulator unit" have been used above for accumulators or accumulator units in accordance with the usual language conventions. A battery includes one or more battery cells, which can represent battery cells, battery modules, module lines or battery packs. The battery cells are preferably spatially combined and electrically connected to one another, for example connected in series or in parallel to form a module. A plurality of modules can form a so-called battery direct converter (BDC: battery direct converter), and a plurality of battery direct converters can form a battery direct inverter (BDI: battery direct inverter).

Advantageous developments and optimizations of the method given in the subclaims are possible by means of the solutions mentioned in the subclaims.

It is therefore advantageous if the selectively switchable battery cells are each provided for supplying a low voltage. This can therefore alternately require the battery cell, which provides a low voltage, for example to support a start-stop system, which leads to an increased service life of the battery cell.

According to a preferred embodiment, the coupling unit has a reverse blocking switch. Advantageously, the reverse blocking switch is suitable for switching on and off the selectively switchable battery cells of the low-voltage subsystem. The switch has the property that, in the "on" state, a current flow in only one direction is possible, and in the "off" state, it is able to absorb a blocking voltage with both polarities.

During switching on of the second battery cell to be switched on in step b), preferably at least one counter-blocking switch, particularly preferably two counter-blocking switches, is actuated. During the switching of the first, switched-on battery cell in step c), at least one counter-blocking switch, particularly preferably two counter-blocking switches, is also preferably actuated.

According to a preferred embodiment, the coupling unit has a forward-blocking switch. Advantageously, the positively blocked switch is suitable for the series connection of selectively switchable battery cells. It is preferably provided that at least one positively blocked switch is actuated during the disconnection of the line between the first, switched-on and the second, to-be-connected battery cell in step a). It is also preferably provided that at least one forward direction cut-off switch.

According to a preferred embodiment, the first, connected battery unit and the second, to-be-connected battery unit in step b) connect the second, to-be-connected battery unit After the low-voltage subsystem and before the disconnection of the first, connected battery unit from the low-voltage subsystem in step c), a parallel connection is made with respect to the low-voltage subsystem. This makes it possible to supply the low-voltage sub-grid from a battery unit that has a higher state of charge or provides a higher voltage. In the same or similar states of charge of the battery cells, the low-voltage subsystem is supplied by the two battery cells.

According to a preferred embodiment, the first, switched-on battery unit and the second, to-be-connected battery unit, or the first, switched-off battery unit and the second, switched-on The battery cells are connected in series with respect to the high-voltage subsystem with conductors connected between them. It is particularly advantageous if the first and second battery cells are connected in series with respect to the high-voltage subsystem and are adjacent to each other with a conductor connected between them. If switching is required on battery cells that are not directly adjacent, several switching processes are carried out successively in short order, so that adjacent battery cells are involved in each switching process.

Furthermore, it can be provided that the low-voltage subsystem has at least one capacitor. The capacitor is preferably provided to further stabilize the low voltage during the alternation of the connected battery cells. The capacitor is preferably sized here according to

$\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; h</span>}\mathrm{<span\; class="notranslate">\; a</span>}\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; t</span>}}}{{\mathrm{<span\; class="notranslate">\; \&Delta;\; U</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}}$

A selection is made where I _max is the maximum on-board grid current that should flow in the low-voltage subsystem during the switching process, t _umschalt is the duration during which no battery cells are provided for power supply, and ΔU _max is the The maximum permissible variation of the onboard grid voltage during the process. Furthermore, the capacitor is preferably also suitable as an energy store, which is designed to generate a low voltage at least briefly and output it to the low-voltage subsystem.

Voltage interruptions in the low-voltage subsystem can be further advantageously reduced if the switching takes place at a point in time in which the vehicle electrical system current is as low as possible. This can be achieved, for example, by evaluating the signal for the vehicle electrical system current and controlling the switches of the coupling unit depending thereon. In addition, synchronization can be achieved by the load management system to switch off high-power loads, such as heating systems, for a short time without loss of comfort, so that switching operations of the battery cells without a nominal voltage interruption are possible.

A computer program is also proposed according to the invention, according to which the method described here is carried out when said computer program is executed on a programmable computer device. The computer program can be, for example, a module for implementing a device for operating an on-board electrical system or a module for implementing a battery management system of a vehicle. The computer program can be stored on a machine-readable storage medium, such as a permanent or rewritable storage medium, or in an assortment of computer devices, for example in a portable memory such as a CD-ROM, DVD, Blu-ray Disc, USB memory or memory card. Additionally and alternatively, the computer program can be made available for download on a computer device such as a server or cloud server, for example via a data network such as the Internet or a communication link such as a telephone line or a wireless connection.

According to the invention, a battery management system (BMS) is also provided, which has a device for carrying out the described method for operating a vehicle electrical system. In particular, the battery management system has a unit which is designed to control the coupling unit in such a way that the battery unit is connected to the low-voltage subsystem or disconnected from the low-voltage subsystem.

According to the invention, a vehicle electrical system is also provided on which the described method can be carried out, wherein the coupling unit is provided to couple the battery cells to one another in series with respect to the high-voltage subsystem and in parallel with respect to the low-voltage subsystem.

The vehicle electrical system can be used in stationary applications, such as wind power plants, or in vehicles, such as hybrid and electric vehicles. In particular, the onboard electrical system is used in vehicles with a start-stop system.

The proposed system, namely the on-board electrical system and the battery management system, is particularly suitable for use in vehicles with a 48-volt generator and a 14-volt starter, wherein the 14-volt starter is preferably provided for a start-stop system.

The presented system is particularly suitable for application in vehicles having a so-called propulsion recovery system (BRS). In the case of a recuperation system (BRS), electrical energy is harvested during braking, downhill or coasting in order to thereby supply electrical loads. The BRS increases system efficiency, enabling fuel savings or lower emissions. Batteries in the high-voltage sub-grid both support internal gas, known as a booster, and are also used at low speeds for short distances and even for electric driving, for example in electric parking and parking.

A motor vehicle is also provided according to the invention, which has an internal combustion engine and the above-described on-board electrical system.

Advantages of the invention

The invention provides an inexpensive on-board electrical system with a lithium-ion battery system for a vehicle, which has, for example, a high-voltage subsystem with a 48-volt generator, a low-voltage subsystem and a unidirectional system with a low-voltage subsystem Powered by the propulsion recovery system. In this case, a potential-separated DC/DC converter and a lead-acid battery can be dispensed with compared to known systems. Furthermore, no separate starters are required in the low-voltage subsystem. With a suitable design, this propulsion recuperation system can save significantly more energy than the current BRS system in development, and thus recover more electrical energy in the system during longer braking procedures or downhill slopes .

The invention according to the invention described above comprises an operating strategy which enables the supply of the low-voltage sub-grid without interruption.

Description of drawings

Various embodiments of the invention are shown in the drawings and further described below. In the attached picture,

Figure 1 shows a low voltage subsystem according to the prior art;

FIG. 2 shows a vehicle electrical system with a high-voltage subsystem, a low-voltage subsystem and a unidirectional, potential-separated DC/DC converter;

FIG. 3 shows a vehicle electrical system with a high-voltage subsystem, a low-voltage subsystem and a bidirectional, potential-separated DC/DC converter;

FIG. 4 shows a vehicle electrical system with a high-voltage subsystem, a low-voltage subsystem and a unidirectional, current-independent DC/DC converter;

Figure 5 shows the coupling unit;

Figure 6 shows the coupling unit in Figure 5 in an exemplary operating state; and

Figure 7 shows a switch with reverse and forward cutoff.

detailed description

FIG. 1 shows a vehicle electrical system 1 according to the prior art. During starting of the internal combustion engine, starter battery 10 supplies voltage via vehicle electrical system 1 to starter 11 , which starts internal combustion engine (not shown) when switch 12 is switched on, for example via a corresponding start signal. When the internal combustion engine starts, it drives a generator 13 which then generates a voltage of approximately 12 volts and supplies it to various electrical loads 14 in the vehicle via the vehicle electrical system 1 . The generator 13 in turn charges the starter battery 10 , which is loaded by the starting process.

FIG. 2 shows a vehicle electrical system 1 with a high-voltage subsystem 20 , a low-voltage subsystem 21 and a unidirectional, potential-separated DC/DC converter 22 formed between the high-voltage subsystem 20 and the low-voltage subsystem 21 between coupling units. Onboard electrical system 1 can be an onboard electrical system of a vehicle, in particular a motor vehicle, a transport vehicle or a forklift.

The high-voltage subsystem 20 is, for example, a 48-volt vehicle electrical system with a generator 23 , which is driven by an internal combustion engine (not shown). The generator 23 is designed in the exemplary embodiment to generate electrical energy as a function of the rotational movement of the electric motor of the vehicle and to feed it into the high-voltage subsystem 20 . The high-voltage subsystem 20 also includes a battery 24 , which can be designed, for example, as a lithium-ion battery and which is designed to output the required operating voltage to the high-voltage subsystem 20 . Arranged in the high-voltage subsystem 20 is a further load resistor 25 , which can be formed, for example, by at least one, preferably a plurality of electric loads of the motor vehicle, which are driven by the high voltage.

In the low-voltage subsystem 21 there is a starter 26, which is arranged at the output of the DC/DC converter 22, which starter 26 is arranged to switch on a switch 27 for starting the internal combustion engine, and an energy store 28, which It is set to provide the low-voltage subsystem 21 with a rated voltage of 12V. Further loads 29 are accommodated in the low-voltage subsystem 21 , which are driven by the low voltage. The energy store 28 includes, for example, a plurality of electrical cells, such as lead-acid accumulators, which typically have a voltage of 12.8 volts in a fully charged state (state of charge, SOC=100%). In a discharged battery (state of charge, SOC=0%), the energy store 28 has an unloaded terminal voltage of typically 10.8 volts. The vehicle electrical system voltage in low-voltage subsystem 21 is in the range between approximately 10.8 volts and 15 volts during driving operation, depending on the temperature and state of charge of energy store 28 , respectively.

The DC/DC converter 22 is connected on the input side to the high-voltage subsystem 20 and to a generator 23 . The DC/DC converter 22 is connected on the output side to a low-voltage subsystem 21 . The DC/DC converter 22 is configured to receive a DC voltage, for example between 12 and 48 volts, received on the input side, for example driving a high-voltage subsystem, and to generate an output voltage which corresponds to the input voltage received on the input side. The voltages are different, in particular to produce an output voltage which is lower than the voltage received on the input side, for example 12V or 14V.

FIG. 3 shows a vehicle electrical system 1 with a high-voltage subsystem 20 and a low-voltage subsystem 21 , which are connected via a bidirectional, potential-separated DC/DC converter 31 . The vehicle electrical system 1 shown is designed essentially as the vehicle electrical system shown in FIG. 2 , the generator being connected to the high-voltage sub-system and using a DC/DC conversion for energy transmission between the sub-systems 20 , 21 . device 31, which is implemented as potential-separated. Batteries 24 , 28 and loads 25 , 29 are also accommodated in the two sub-vehicle electrical systems 20 , 21 , as described with reference to FIG. 2 . The systems shown in FIG. 3 differ essentially in the way the starters are connected. In the system shown in FIG. 2 , the starter 26 is arranged in the low-voltage subsystem 21 and thus the DC/DC converter 22 can be arranged unidirectionally in the low-voltage subsystem 20 for the energy transmission of the high-voltage subsystem. grid 21 , whereas in the structure of the starter-generator 30 shown in FIG. 3 it is inserted into the high-voltage subsystem grid 20 . In this case, the DC/DC converter 31 is implemented as bidirectional, so that the lithium-ion accumulator 24 can (if necessary) be charged via the low-voltage subsystem 21 . The starting assistance of the low-voltage vehicle is then realized via the low-voltage interface and the DC/DC converter 31 .

FIG. 4 shows an onboard electrical system 1 , for example an onboard electrical system 1 of a vehicle, in particular a motor vehicle, a transport vehicle or a forklift, with a high-voltage subsystem 20 and a low-voltage subsystem 21 . The vehicle electrical system 1 is particularly suitable for use in vehicles with a 48-volt generator, a 14-volt starter and a recuperation system.

The high-voltage subsystem 20 includes a starter generator 30 , which is capable of starting an internal combustion engine (not shown) and is driven thereby. The starter-generator 30 is designed to generate electrical energy as a function of the rotational movement of the electric motor of the vehicle and to feed it into the high-voltage subsystem 20 . Arranged in the high-voltage subsystem 20 is a further load resistor 25 , which can be formed, for example, by at least one, preferably a plurality of electric loads of the motor vehicle, which are driven by the high voltage.

The high-voltage subsystem 20 also includes a battery 40 , which can be designed, for example, as a lithium-ion battery and which is designed to output an operating voltage of 48 volts to the high-voltage subsystem. The lithium-ion accumulator 40 preferably has a minimum capacity of approximately 15 Ah at a nominal voltage of 48 volts in order to be able to store the required electrical energy.

The battery 40 has a plurality of battery cells 41 - 1 , 41 - 2 . Reach the required power and energy figures. The individual battery cells are, for example, lithium-ion batteries with a voltage range of 2.8 to 4.2 volts.

The battery units 41-1, 41-2 ... 41-n are assigned to single voltage taps 80-11, 80-12, 80-21, 80-22, ... 80-n1, 80-n2, through which the coupling unit 33 voltage. The coupling unit 33 has the task of connecting at least one battery cell 41 of the battery 40 to the low-voltage subsystem 21 for its operation or support.

The coupling unit 33 couples the high-voltage subsystem 20 to the low-voltage subsystem 21 and supplies the low-voltage subsystem 21 with the required operating voltage, for example 12V or 14V, at its output. The structure and function of this coupling unit 33 will be described with reference to FIGS. 5 and 6 .

The low-voltage subsystem 21 includes low-voltage loads 29 , which are set for operation with a voltage of 14V, for example. According to one specific embodiment, it is provided that the lithium-ion battery 40 supplies the static current loads shown as loads 25 , 29 in a parked vehicle. It can be provided, for example, that the requirements of the so-called airport test are met here, wherein the vehicle is still startable after a six-week shutdown period, and wherein during the shutdown period the quiescent current of the low-voltage load 29 is supplied to the low-voltage sub The grid 21 thus supplies, for example, the anti-theft alarm device.

In the low-voltage subsystem 21 , a high-power store 28 or a buffer store can optionally be arranged, which can provide very high power for a short time, ie is optimized for high power. The high-power store 28 fulfills the purpose of further avoiding overvoltages during switching of the battery cells 41 . If capacitors are used as high power storage 28, their dimensions are preferably:

$\mathrm{<span\; class="notranslate">\; C</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}<span\; class="notranslate">\; \&Center\; Dot;</span>{\mathrm{<span\; class="notranslate">\; t</span>}}_{\mathrm{<span\; class="notranslate">\; u</span>}\mathrm{<span\; class="notranslate">\; m</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}\mathrm{<span\; class="notranslate">\; c</span>}\mathrm{<span\; class="notranslate">\; h</span>}\mathrm{<span\; class="notranslate">\; a</span>}\mathrm{<span\; class="notranslate">\; l</span>}\mathrm{<span\; class="notranslate">\; t</span>}}}{{\mathrm{<span\; class="notranslate">\; \&Delta;\; U</span>}}_{\mathrm{<span\; class="notranslate">\; max</span>}}}$

where I _max is the maximum on-board current that should flow in the on-board system during the switching process, t _umschalt is the time duration during which no battery cells are provided for power supply, and ΔU _max is the on-board network during the switching process The maximum allowable change in voltage.

The vehicle electrical system shown in FIG. 4 can further include a battery management system (BMS) (not shown). The battery management system includes a controller arranged to acquire, process measurement data about the temperature of the battery 40 or battery cells 41 , the supplied voltage, the supplied current and the state of charge and predict the state of health of the battery 40 therefrom. The battery management system here includes a unit which is designed to adjust the coupling unit 33 in such a way that it can selectively connect the battery cells 41 into the low-voltage subsystem 21 .

FIG. 5 shows the coupling unit 33 , which is implemented as a unidirectional, current-insensitive DC voltage converter (DC/DC converter). The coupling unit 33 comprises reverse-blocking switches 44, 45 which have the property that in the state "ON" a current flow in only one direction is possible and in the second state "OFF" enable The cut-off voltage is absorbed with two polarities. It differs substantially from a simple semiconductor switch like an IGBT switch because it cannot absorb the cut-off voltage in the reverse direction due to its internal diode. Depending on the direction of current flow, two different switch types are shown in FIG. 5 , RSS_I 45 and RSS_r44 , which do not differ in their technology, but are only constructed with different polarity. An example of a further configuration of the reverse-blocking switches 44 , 45 is described with reference to FIG. 7 .

In the coupling unit 33 , the individual taps 80 of the battery cells 41 are each supplied to one of the different reverse blocking switches RSS_I 45 and RSS_r 44 . The reverse blocking switch RSS_I 45 is connected on the output side of the coupling unit 33 to the positive pole 52 , and the reverse blocking switch RSS_r 44 is connected on the output side of the coupling unit 33 to the negative pole 51 .

The coupling unit 33 includes a forward-blocking switch 90 which can be, for example, a standard semiconductor switch. An example of a further structure of the forward-blocking switch 90 is described with reference to FIG. 7 . In the coupling unit 33 , the single taps 80 of the battery cells 41 are branched off and fed in parallel to the reverse-blocking switch to the forward-blocking switch VSS 90 . If the switch 90 is turned on, the positively blocked switch VSS 90 connects the battery cells 41 to each other in parallel. Here, a positive cutoff switch 90 is arranged between every two battery cells 41, so that n−1 forward cutoff switches VSS 90-1, VSS 90-2, . . . are provided in n battery cells 41 VSS 90-n-1.

The voltage level of the high-voltage subsystem 20 is referred to the ground of the low-voltage subsystem 21 depending on where one or more battery cells 41 are connected. Not in the operating state, however, one of the potentials has an amount that exceeds the voltage limit by the magnitude of the sum of the high voltage and the low voltage, ie 62 volts in the case of a 48-volt network and a 14-volt network. However, negative potentials with respect to the ground of the low-voltage subsystem can also occur.

The operation of the starter-generator 30 depends on the operation of the coupling unit 33 and the supply of the low-voltage subsystem. The charging current fed into the entire battery 40 via the low-voltage subsystem and (if necessary) the starter-generator 30 (generator operation) or via it from the entire battery unit 41 is drawn from the connected battery unit The discharge current (motor operation) is drawn from the battery 40 , which supplies the low-voltage subsystem 21 . As long as the permissible limits of the battery cells, for example the maximum permissible discharge current of the cells, are not exceeded, the processes can be observed independently of one another. Since at least one battery cell 41 is always connected to the coupling unit 33 via the aforementioned switches 44 , 45 , 90 , the low-voltage subsystem 21 is reliably supplied. Due to the multiple redundant power supply of the low-voltage subsystem 21 , it is possible to construct a system with the proposed structure which has a very high availability of electrical energy in the low-voltage subsystem.

FIG. 6 shows the low-voltage sub-grid 21 connected by the battery cells 41 - 1 , 41 - 2 via reverse blocking switches RSS_I 45 - i , RSS_I 45 - j , RSS_r 44 - i , RSS_r 44 - j and An open positive feeds off switch VSS 90-1, which is located between battery cells 44-1, 44-2. A first current path 71 is conducted from the positive pole 52 to the negative pole 51 via the reverse-blocked switch RSS_I 45 - i , via the first battery cell 41 - 1 and via the further reverse-blocked switch RSS_r 44 - j . Furthermore, a second current path 72 is conducted from the positive pole 52 to the negative pole 51 via the reversely blocked switch RSS_I 45 - j , via the second battery cell 41 - 2 and via a further reversely blocked switch RSS_r 44 - i . Since the switch 90-1 is open, the first battery unit 41-1 and the second battery unit 41-2 are connected in parallel with respect to the low voltage subsystem. The positive pole of the first battery cell 41 - 1 is electrically high-resistance connected.

For the uninterrupted power supply of the low-voltage subsystem 21, the present invention proposes a switching method, wherein in step a), by means of the forward cut-off switch VSS90-1 arranged in the line, the first, already A line between the connected battery cell (here, for example, battery cell 41 - 1 ) and the second battery cell to be connected (here, for example, battery cell 41 - 2 ). After step a), the accumulator 40 has a total voltage of 36 volts, which supplies the high-voltage subsystem 20 such that a bidirectional energy flow is provided for the high-voltage subsystem 20 . The further battery cells 41 - 2 , . . . 41 -n here form a series circuit of n−1 battery cells.

In the following second step b), the second battery cell 41-2 to be switched on is switched to the low voltage sub Grid 21. FIG. 6 shows the state after step b), in which the two battery cells 41-2 and 41-1 are connected in parallel.

A delay between switching off and switching on is necessary because otherwise the voltage in the low-voltage subsystem 21 would increase to impermissibly high values during the transition phases of all switching operations, which is shown in FIG. 6 The case where the voltage is raised to the sum of the voltages of the sub-battery batteries 41-1 and 41-2, that is, doubled. If the coupling device 33 is switched on with a delay time, this however means that the supply of the low-voltage subsystem 21 is briefly interrupted. In order to avoid impermissible voltage interruptions, buffering can be implemented according to some embodiments by means of capacitors 28 , as described with reference to FIG. 4 .

In a third step c), the first, connected battery unit 41 - 1 is disconnected from the low-voltage subsystem 21 , if a switchover of the connected battery unit 41 from the low-voltage subsystem 21 is provided. In a fourth step d), the connection between the first, disconnected battery unit 41-1 from the low-voltage subsystem 21 and the second, connected to the low-voltage subsystem is established again via the positively blocked switch VSS 90-1. The wire connection between the storage battery unit 41-2 of 21. After the connection has been established again, the switchover from the first to the second battery unit is ended without interrupting the supply of the low-voltage subsystem 21 .

According to a further embodiment of the method, it can be provided that in step a) all forward-blocking switches 90 are switched off. The starter-generator 30 does not feed energy into the high-voltage subsystem during the switching phase and operates in boost mode. The switches 44 , 45 which are closed in the opposite direction switch on the battery cell or cells 41 to be switched on with a short delay, the duration of which depends on the properties of the switches used. This switching can therefore also take place between battery cells 41 that are not directly adjacent.

FIG. 7 shows a possible configuration of the reversely blocked switches 44 , 45 and the forwardly blocked switch 90 . The direction of flow is given by I here. The reverse-blocking switch RSS_r 44 includes, for example, an IGBT, a MOSFET or a bipolar transistor 101 and a diode 103 connected in series therewith. FIG. 8 shows a MOSFET 101 with an internal diode 102 shown together. The diode 103 connected in series to the MOSFET 101 is polarized opposite to the direction of the diode 102 inside the MOSFET 101 . The reverse-blocked switch RSS_r 44 allows the current to flow in the opposite direction to the flow direction I or blocks it. The reverse blocking switch RSS_I 45 corresponds to RSS_r 44 and is only designed with opposite polarity so that the flow and blocking directions are reversed. The forward-blocking switch 90 comprises a MOSFET, an IGBT or a bipolar transistor 101, with its internal diode 102 being shown together. In particular, the switches RSS_I 45 , RSS_r 44 and VSS 90 are also distinguished during the switching process by an almost imperceptible delay, ie very short switching time periods are allowed. By means of suitable control circuits, the time delay between switching off and switching on of the switch can be adjusted very precisely.

The invention is not limited to the exemplary embodiments described here and the aspects emphasized here. Rather, various modifications are possible within the scope given by the claims, which are within the purview of a person skilled in the art.

Claims (11)

1. A method for operating an onboard electrical system (1) for a motor vehicle, wherein the onboard electrical system (1) has a low-voltage subsystem (21) for at least one low-voltage load (29), with A high-voltage sub-grid (20) and a starter-generator (30) for at least one high-voltage load (25), wherein the high-voltage sub-grid (20) is connected to the low-voltage sub-grid via a coupling unit (33) The power grid (21) is connected, and the coupling unit is configured to extract energy from the high-voltage sub-grid (20) and transmit it to the low-voltage sub-grid (21), wherein the high-voltage sub-grid (20) There is an accumulator (40) arranged to generate a high voltage and output to the high voltage subsystem (20), and the accumulator has at least two accumulator cells (41) with single voltage taps (80) , the single voltage tap is led to the coupling unit (33), wherein the coupling unit (33) is configured to selectively connect the battery unit (41) to the low voltage subsystem (21), characterized in that, in the following steps, from the first battery unit (41) that has been connected to the low-voltage subsystem (21) to the second battery unit (41) that is to be connected to the low-voltage subsystem The switching of the accumulator unit (41) of (21): a) disconnecting the wires between the first, connected and the second, to-be-connected battery unit (41); b) connecting the second, to-be-connected battery unit (41) to the low-voltage subsystem (21); c) disconnecting said first, connected battery unit (41) from said low voltage subsystem (21); d) connected between a first battery unit (41) that has been disconnected from the low-voltage subsystem (21) and a second battery unit (41) that has been connected to the low-voltage subsystem (21) of wires. 2 . The method according to claim 1 , characterized in that the battery cells ( 41 ) are each provided for supplying a low voltage. 3 . 3. The method according to any one of the preceding claims, characterized in that the coupling unit (33) has a switch (44, 45) which is blocked in reverse, and in step b) the second, At least one reverse-blocking switch (44, 45) is actuated during the switching-on of the battery cell (41) to be switched on. 4. The method according to any one of the preceding claims, characterized in that the coupling unit (33) has switches (44, 45) that are reversely blocked, and that in step b) the first, At least one reverse-blocking switch (44, 45) is actuated during the disconnection of a connected battery cell (41). 5. The method according to any one of the preceding claims, characterized in that the coupling unit (33) has a forward-blocking switch (90), and in step a) in the first, already At least one positively blocked switch (90) is actuated during separation of the line between the connected and the second battery cell (41) to be connected. 6. The method according to any one of the preceding claims, characterized in that the first, connected battery cell (41) and the second, to-be-connected battery cell (41) are in step b) after connecting the second, to-be-connected battery unit (41) to the low-voltage subsystem and disconnecting the second battery unit (41) from the low-voltage subsystem (21) in step c); 1. Connected in parallel with respect to the low-voltage sub-network (21) ahead of the switched-on battery unit (41). 7. The method as claimed in any one of the preceding claims, characterized in that the first, switched-on battery cell (41) and the second, to-be-connected battery cell (41) are In the case of connecting wires therebetween, they are connected in series with respect to the high-voltage sub-grid (20) and adjacent to each other. 8. A battery management system for carrying out the method as claimed in claim 1, having a unit for controlling a coupling unit (33) for connecting a battery unit (41). 9. A computer program arranged to carry out the method according to any one of claims 1 to 7, when said computer program is implemented on a programmable computer device. 10. A vehicle electrical system (1) on which the method according to any one of claims 1 to 7 can be carried out, wherein the coupling unit (33) is configured to connect the battery unit (41) with respect to the high voltage The subsystems ( 20 ) are coupled to one another in series and in parallel with respect to the low-voltage subsystem ( 21 ). 11. A motor vehicle having an internal combustion engine and an onboard electrical system (1) according to claim 10.