Electrical system and method for operating an electrical system

Abstract

The disclosure relates to an electrical system for a vehicle, comprising a low-voltage sub-network for at least one low-voltage load and comprising a high-voltage sub-network for at least one high-voltage load and an electric generator. The high-voltage sub-network has a battery which is designed to generate a high-voltage and output same to the high-voltage sub-network and which has at least two battery units with individual voltage taps. The high-voltage sub-network is connected to the low-voltage sub-network via a coupling unit which is designed to draw energy from the high-voltage sub-network and supply said energy to the low-voltage sub-network. The coupling unit is designed to selectively connect the battery units to the low-voltage sub-network. The disclosure further relates to a method for operating an electrical system, to a motor vehicle, and to a battery management system and a computer program which are designed to carry out the method.

Claims

1. An onboard electrical system for a motor vehicle, having a first subsystem having at least one first load configured to operate with a first voltage; a second subsystem having at least one second load configured to operate with a second voltage, the second voltage being higher than the first voltage, the second subsystem having a battery configured to produce the second voltage and to output the second voltage to the second subsystem, the battery having at least two battery units having individual voltage taps; an electrical generator; a coupling unit configured to connect the second subsystem to the first subsystem, the coupling unit being configured to draw power from the second subsystem and to supply the power to the first subsystem, the individual voltage taps of the at least two battery units being routed to the coupling unit, the coupling unit being configured to selectively connect individual ones of the at least two battery units to the first subsystem; and a controller operably connected to the coupling unit, the controller being configured to (i) ascertain a state of charge of each of the at least two battery units and (ii) operate the coupling unit to connect a battery unit of the at least two battery units that has a highest state of charge to the first subsystem.

2. The onboard electrical system as claimed in claim 1, wherein the at least two battery units are each configured to provide the first voltage.

3. The onboard electrical system as claimed in claim 1, wherein the coupling unit includes switches configured to, in an on state, allow current to flow in only one direction.

4. The onboard electrical system as claimed in claim 1, wherein the first subsystem includes at least one energy store configured to produce the first voltage and to output the first voltage to the first subsystem.

5. The onboard electrical system as claimed in claim 1, wherein the first subsystem includes a starter.

6. A method for operating an onboard electrical system for a motor vehicle, the onboard electrical system including (i) a first subsystem having at least one first load configured to operate with a first voltage, (ii) a second subsystem having at least one second load configured to operate with a second voltage, the second voltage being higher than the first voltage, the second subsystem having a battery configured to produce the second voltage and to output the second voltage to the second subsystem, the battery having at least two battery units having individual voltage taps, (iii) an electrical generator, and (iv) a coupling unit configured to connect the second subsystem to the first subsystem, the individual voltage taps of the at least two battery units being routed to the coupling unit, the coupling unit being configured to selectively connect individual ones of the at least two battery units to the first subsystem, the method comprising: ascertaining a state of charge of each of the at least two battery units; operating the coupling unit to connect a battery unit of the at least two battery units that has a highest state of charge to the first subsystem; drawing power from the second subsystem with the coupling unit; and supplying the power to the first subsystem with the coupling unit.

7. The method as claimed in claim 6, further comprising: operating the coupling unit to change which of the at least two battery units is connected to the first subsystem in response to a charge difference of the at least two battery units exceeding a threshold value.

8. The method as claimed in claim 6, further comprising: disconnecting a current-carrying battery unit of the at least two battery units; and connecting, thereafter, a selected further battery unit of the at least two battery units.

9. The method as claimed in claim 6, wherein the method is performed by a computer program that is executed on a programmable computer device.

10. A motor vehicle comprising: an internal combustion engine; and an onboard electrical system, the onboard electrical system comprising: a first subsystem having at least one first load configured to operate with a first voltage; a second subsystem having at least one second load configured to operate with a second voltage, the second voltage being higher than the first voltage, the second subsystem having a battery configured to produce the second voltage and to output the second voltage to the second subsystem, the battery having at least two battery units having individual voltage taps; an electrical generator; a coupling unit configured to connect the second subsystem to the first subsystem, the coupling unit being configured to draw power from the second subsystem and to supply the power to the first subsystem, the individual voltage taps of the at least two battery units being routed to the coupling unit, the coupling unit being configured to selectively connect individual ones of the at least two battery units to the first subsystem; and a controller operably connected to the coupling unit, the controller being configured to (i) ascertain a state of charge of each of the at least two battery units and (ii) operate the coupling unit to connect a battery unit of the at least two battery units that has a highest state of charge to the first subsystem.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the disclosure are presented in the drawings and explained in more detail in the description below. In the drawings,

(2) FIG. 1 shows a low voltage onboard electrical system according to the prior art,

(3) FIG. 2 shows an onboard electrical system with a high voltage subsystem and a low voltage subsystem and a unidirectional, potential-isolating DC/DC converter,

(4) FIG. 3 shows an onboard electrical system with a high voltage subsystem and a low voltage subsystem and a bidirectional, potential-isolating DC/DC converter,

(5) FIG. 4 shows an onboard electrical system with a high voltage subsystem and a low voltage subsystem and a unidirectional, electrochemical non-isolating DC/DC converter,

(6) FIG. 5 shows a coupling unit according to an embodiment of the disclosure,

(7) FIG. 6 shows the coupling unit from FIG. 5 in an exemplary operating state,

(8) FIG. 7 shows the coupling unit from FIG. 5 during an exemplary changeover process, and

(9) FIG. 8 shows switches with reverse blocking capability.

DETAILED DESCRIPTION

(10) FIG. 1 shows an onboard electrical system 1 according to the prior art. When starting an internal combustion engine, the onboard electrical system 1 is used to provide a voltage from a starter battery 10 for a starter 11 that starts the internal combustion engine (not shown) when a switch 12 is closed, for example by an appropriate starter signal. Once the internal combustion engine has been started, it drives an electrical generator 13 that then produces a voltage of approximately 12 volts and provides it for the various electrical loads 14 in the vehicle via the onboard electrical system 1. In so doing, the electrical generator 13 also recharges the starter battery 10 loaded by the starting process.

(11) FIG. 2 shows an onboard electrical system 1 with a high voltage subsystem 20 and a low voltage subsystem 21 and a unidirectional, potential-isolating DC/DC converter 22 that forms a coupling unit between the high voltage subsystem 20 and the low voltage subsystem 21. The onboard electrical system 1 may be an onboard electrical system of a vehicle, particularly a motor vehicle, transport vehicle or forklift truck.

(12) The high voltage subsystem 20 is, for example, a 48 volt onboard electrical system having an electrical generator 23 that can be operated by an internal combustion engine (not shown). In this exemplary embodiment, the generator 23 is designed to take a rotary movement of the engine of the vehicle as a basis for producing an electric power and supplying it to the high voltage subsystem 20. The high voltage subsystem additionally comprises a battery 24 that may be in the form of a lithium ion battery, for example, and that is set up to output the necessary operating voltage to the high voltage subsystem. The high voltage subsystem 20 contains further load resistors 25 that may be formed by at least one, preferably by a plurality of, electrical load(s) of the motor vehicle, for example, that are operated at the high voltage.

(13) The low voltage subsystem 21, which is arranged on the output side of the DC/DC converter 22, contains a starter 26 that is set up to close a switch 27 in order to start the internal combustion engine, and an energy store 28 that is set up to provide the low voltage at the level of 14 volts, for example, for the low voltage subsystem 21. The low voltage subsystem 21 contains further loads 29 that are operated at the low voltage. By way of example, the energy store 28 comprises electrochemical cells, particularly those of a lead acid battery, which usually has a voltage of 12.8 volts in the fully charged state (state of charge, SOC=100%). When the battery is discharged (state of charge, SOC=0%), the energy store 28 has a terminal voltage of typically 10.8 volts in the unloaded state. During driving mode, the onboard electrical system voltage in the low voltage subsystem 21 is approximately in the range between 10.8 volts and 15 volts, depending on the temperature and the state of charge of the energy store 28.

(14) The input side of the DC/DC converter 22 is connected to the high voltage subsystem 20 and to the generator 23. The output side of the DC/DC converter 22 is connected to the low voltage subsystem 21. The DC/DC converter 22 is designed to receive a DC voltage received on the input side, for example a DC voltage at which the high voltage subsystem is operated, for example between 12 and 48 volts, and to produce an output voltage that is different than the voltage received on the input side, particularly to produce an output voltage that is lower than the voltage received on the input side, for example 12 V or 14 V.

(15) FIG. 3 shows an onboard electrical system 1 with a high voltage subsystem 20 and a low voltage subsystem 21 that are connected by a bidirectional, potential-isolating DC/DC converter 31. The onboard electrical system 1 shown is essentially designed in the manner of the onboard electrical system shown in FIG. 2, wherein the generator is incorporated in the high voltage subsystem and a DC/DC converter 31, which is of potential-isolating design, is used for transferring power between the onboard electrical subsystems 20, 21. Both subsystems 20, 21 furthermore contain batteries 24, 28 and loads 25, 29, as described with reference to FIG. 2. Essentially, the system shown in FIG. 3 is distinguished by virtue of the incorporation of the starter. While the starter 26 is arranged in the low voltage subsystem 21 in the system shown in FIG. 2, and, as a result, the DC/DC converter 22 may be of unidirectional design for transporting power from the high voltage subsystem 20 to the low voltage subsystem 21, a starter generator 30 is used in the high voltage subsystem 20 in the case of the architecture shown in FIG. 3. In this case, the DC/DC converter 31 is of bidirectional design, so that the lithium ion battery 24 can be charged via the low voltage subsystem 21 if need be. The starting assistance for the low voltage vehicle is then provided by means of the low voltage interface and the DC/DC converter 31.

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

(17) The high voltage subsystem 20 comprises a generator 23 that can be operated by an internal combustion engine (not shown). The generator 23 is designed to take a rotary movement of the engine of the vehicle as a basis for producing electric power and supplying it to the high voltage subsystem 20. The high voltage subsystem 20 contains load resistors 25 that may be formed by at least one, preferably by a plurality of, electrical load(s) of the motor vehicle, for example, that are operated at the high voltage.

(18) The high voltage subsystem 20 furthermore comprises a battery 40 that may be in the form of a lithium ion battery, for example, and that is set up to output the operating voltage of 48 volts to the high voltage subsystem. At a rated voltage of 48 volts, the lithium ion battery 40 preferably has a minimum capacity of approximately 15 Ah in order to be able to store the required electric power.

(19) The battery 40 has multiple battery units 41-1, 41-2, . . . , 41-n, wherein the battery units 41 have multiple associated battery cells that are usually connected in series and in some cases are additionally connected in parallel with one another in order to obtain the requisite performance and power data with the battery 40. The individual battery cells are lithium ion batteries with a voltage range from 2.8 to 4.2 volts, for example.

(20) The battery units 41-1, 41-2, . . . , 41-n have associated individual voltage taps 42-1, 42-2, . . . , 42-n+1 via which the voltage is supplied to a coupling unit 33. When the battery units 41-1, 41-2, . . . , 41-n are connected in series, as shown in FIG. 4, the individual voltage taps 42 are arranged between the battery units 41, and one at each of the ends of the battery 40. When the number of battery units is n, this results in n+1 taps 42. The additional individual voltage taps 42 divide the lithium ion battery 40 into multiple battery units 41-1, 41-2, . . . 41-n, which can also be referred to as battery elements within the context of the disclosure. The individual voltage taps 42 are chosen such that the battery units 41 each have a voltage at which the low voltage subsystem 21, i.e. the 14 volt onboard electrical system, can be supplied with power. The individual voltage taps 42 of the battery units 41 are supplied to the coupling unit 33, as shown in FIG. 4. The coupling unit 33 has the task of connecting at least one of the battery units 41 of the battery 40 to the low voltage subsystem 21 for the purpose of operating or supporting the latter.

(21) The coupling unit 33 couples the high voltage subsystem 20 to the low voltage subsystem 21 and, on the output side, provides the low voltage subsystem 21 with the necessary operating voltage, for example 12 V or 14 V. The design and operation of the coupling unit 33 are described with reference to FIGS. 5 to 7.

(22) The low voltage subsystem 21 comprises the low voltage loads 29, which are designed for operation at 14 V voltage, for example. The low voltage subsystem 21 also has the starter 26, which is set up to operate the switch 27 in order to start the internal combustion engine.

(23) To supply power to the starter in the low voltage subsystem, specifically in the case of a cold start for the vehicle, for example, there is a further energy store 28 available in the low voltage subsystem 21. The energy store 28, for example a battery, can deliver very high currents for a short time and relieves the load on the lithium ion battery 40 in the starting phases. Specifically the effects of the known weaknesses of lithium ion batteries, that they cannot output high currents at low temperatures, are lessened through the use of the energy store 28 in the system shown in FIG. 4. If the energy store 28 is embodied as a double layer capacitor, then the starting currents, even in a high total number, can be provided over the entire life of the battery and even for individual starting processes, if need be also repeatedly in succession, i.e. in the event of an unsuccessful starting attempt, after the power store has recharged. This allows a system to be implemented that has very high availability for electric power in the low voltage subsystem 21 and can output very high power for a short time, i.e. is optimized for high performance. The high power store 28 furthermore fulfils the purpose of avoiding overvoltages when the battery units 41 are changed over. If the energy store 28 used is a capacitor, then the dimensioning of said capacitor is preferably:

(24) $C=\frac{I_{\max }.Math.t_{\mathrm{changeover}}}{U_{\max }},$
where I.sub.max is the maximum onboard electrical system current that can flow in the onboard electrical system during the changeover processes, t.sub.changeover is the period of time during which no battery unit 41 is available for the supply of power, and U.sub.max is the maximum permissible change in the onboard electrical system voltage during the changeover process.

(25) According to one embodiment, provision is made for the lithium ion battery 40 to undertake the supply of power to quiescent current loads, which are shown as loads 25, 29, when the vehicle is switched off. By way of example, provision may be made for the requirements of what is known as the airport test to be met in this case, wherein the vehicle can still be started after a standing time of 6 weeks and wherein, during the standing time, the battery provides the quiescent currents from the low voltage loads 29 in the low voltage subsystem 21 so that a theft warning system is supplied with power, for example.

(26) The onboard electrical system shown in FIG. 4 can additionally comprise a battery management system (BMS) (not shown). The battery management system comprises a controller that is set up to capture measurement data about temperatures, voltages provided, currents output and states of charge of the battery 40 or of the battery units 41, to process them and to make statements about the state of health of the battery 40, for example, therefrom. In this case, the battery management system comprises a unit that is set up to regulate the coupling unit 33 such that it can selectively connect the battery units 41 in the low voltage subsystem 21.

(27) FIG. 5 shows a coupling unit 33 that is embodied as a unidirectional, electrochemical non-isolating DC/DC voltage converter (DC/DC converter). The coupling unit 33 comprises switches with reverse blocking capability 44, 45 that have the property that they allow a flow of current only in one direction in an on state and can accept a blocking voltage having both polarities in a second, off state. This is a significant difference in relation to simple semiconductor switches, such as e.g. IGBT switches, since these cannot accept a blocking voltage in a reverse direction on account of their intrinsic diode. The dependence on the direction of current flow means that FIG. 5 shows two different switch types, namely RSS_l 45 and RSS_r 44, which do not differ in terms of their manufacture but rather are merely installed with different polarity. An example of the more detailed design of the switches with reverse blocking capability 44, 45 is described with reference to FIG. 8.

(28) In the coupling unit 33, the individual voltage taps 42 of the battery units 41 are each branched at branch points 43 and each supplied to one of the different switches with reverse blocking capability RSS_l 45 and RSS_r 44. The switches with reverse blocking capability RSS_l 45 are connected up to the positive pole 52 on the output side of the coupling unit 33, and the switches with reverse blocking capability RSS_r 44 are connected to the negative pole 51 on the output side of the coupling unit 33.

(29) FIG. 6 shows the supply of power to the low voltage subsystem 21 by way of example from the battery unit 41-2 via the associated taps 42-2 and 42-3. The current path 61 leads from the positive pole 52 via a switch with reverse blocking capability RSS_l 45-i, via a branch point 43-i, via the voltage tap 42-2, to the connected battery unit 41-2, and from there via the voltage tap 42-3 arranged downstream of the connected battery unit 41-2, via the branch point 43-j, via a further switch with reverse blocking capability RSS_r 44-i, to the negative pole 51. At the first branch point 43-i, a connection also leads to a further switch with reverse blocking capability RSS_r 44-j. Since the latter is designed to have reverse blocking capability, a current cannot flow at this location, however. In the case of an ordinary MOSFET switch, said switch would transmit in the reverse direction, which means that the current path would lead not via the battery unit 41-2 but rather via the switch RSS_r 44-j. The same applies to the second branch point 43-j, which again leads to a switch with reverse blocking capability RSS_l 45-j that is off, meaning that no flow of current is possible in this case either.

(30) The voltage of the high voltage subsystem 20 referenced to the ground of the low voltage subsystem 21 is dependent on which of the battery units 41 is connected. In none of the operating states does one of the potentials have an absolute value that exceeds a voltage limit amounting to the sum of the high voltage and the low voltage, however, i.e. approximately 62 volts in the case of a 48 volt system and a 14 volt system. Negative potentials relative to the ground of the low voltage subsystem can appear, however.

(31) The operation of the high voltage generator 23 is independent of the operation of the coupling unit 33 and of the supply of power to the low voltage subsystem. In the connected battery unit 41 that supplies power to the low voltage subsystem 21, an overlay results from the low voltage subsystem current and the charging current possibly supplied to the whole lithium ion battery 40 by the generator 23 (generator mode) or from the discharge current drawn from the whole lithium ion battery 40 (engine mode). So long as the permissible limits of the battery cells, e.g. the maximum permissible discharge current from the cells, are not exceeded, these processes can be considered independently of one another. So that the low voltage subsystem 21 is safely supplied with power, precisely one of the battery units 41 is connected by means of the associated switches 44, 45 of the coupling device 33. The supply of power to the low voltage subsystem 21 on the basis of multiple redundancy means that the presented architecture can be used to design a system that has a very high level of availability for the electric power in the low voltage subsystem.

(32) FIG. 7 shows a changeover process by means of the coupling unit 33 by way of example from the battery unit 41-1 to the battery unit 41-n. Prior to changeover, a first current path 71 leads via a first switch with reverse blocking capability RSS_l 45-i, via first voltage taps 42-1, 42-2, which are associated with the first battery unit 41-1, and via a second switch with reverse blocking capability RSS_r 44-i, to the negative pole 31. After changeover, the current path 72 leads via a second switch with reverse blocking capability RSS_l 45-k, via voltage taps 42-n, 42-n+1, which are associated with the n-th battery unit 41-n, and via a further switch with reverse blocking capability RSS_r 44-k, to the negative pole 51.

(33) Changing involves the switches with reverse blocking capability 45-i, 44-i being switched off and the other switches with reverse blocking capability 45-k, 44-k being switched on. Were the coupling unit 33 to receive the switching commands for the switches 45-i, 44-i, 45-k, 44-k in synch, then the operation of the switches with reverse blocking capability means that the positive pole 52 of the low voltage subsystem would be connected to the higher potential of the two battery elements during the switching phase of the power switches and the negative pole 51 would be connected to the lower potential of the two battery elements during the switching phase, i.e. in the example, to the negative pole of the battery unit 41-n. Hence, a much higher voltage would be applied to the low voltage subsystem for a short time than the specification of the low voltage subsystem allows. In the example shown in FIG. 6, the series-connected battery units 41 mean that the low voltage subsystem 21 would be provided with the sum of the partial voltages from the whole battery for a short time. In order to avoid these overvoltages, the procedure when changing over coupling unit 33 is as follows: changeover is effected such that the switches on the battery element currently carrying current, the battery unit 41-1 in the example shown, are switched off first, and after the switches on the battery element that has carried current hitherto no longer carry current, the switches on the battery element that are intended to undertake the supply of power to the low voltage subsystem are switched on. The principle described is also referred to as break before make.

(34) When looking at an optimized operating strategy for the onboard electrical system 1 with the series circuit shown for the battery units 41, the considerations that follow are employed. In this case, it is assumed that for uniformly aged cells, the internal resistance and the capacitance of the cells are approximately the same for the same reference conditions, i.e. essentially the same temperature and the same state of charge.

(35) For uniformly aged cells, the maximum outputable power is limited by that cell having the lowest state of charge.

(36) For uniformly aged cells, the maximum drawable power is limited by the cell having the lowest state of charge.

(37) For uniformly aged cells, the maximum permissible power for charging processes is limited by the cell having the highest state of charge.

(38) For uniformly aged cells, the maximum suppliable power is limited by the cell having the highest state of charge.

(39) Since the battery system in a boost recuperation system needs to be capable of storing as much power as possible during a braking process at any time, and at the same time needs to be capable of supporting a boost process as well as possible, it is possible to infer therefrom the requirement that the battery units 41 and the cells they contain must all have the same state of charge as far as possible in order to meet the stipulated requirements as well as possible.

(40) In addition to the requirements for the high voltage subsystem 20, requirements for the starting processes in the low voltage subsystem 21 are also stipulated for the system. So that these requirements are met as well as possible by means of a combination of the high power energy store 28 and the lithium ion battery 40, preferably that battery unit 41 that has the highest state of charge at a given time is used to supply power to the low voltage subsystem.

(41) The requirements for the selection of the switching states of the coupling unit 33 can be met using the following operating strategy: the low voltage subsystem 21 is always supplied with power from that battery element 41 that currently has the highest state of charge. Since the supply of power to the low voltage subsystem is overlaid on the charging and discharge processes in the high voltage subsystem and the supply of power to the low voltage subsystem takes place unidirectionally, this selection specification ensures that the battery element 41 having the highest state of charge is discharged more quickly or is charged more slowly than the other battery units 41. This results in the states of charge of the battery elements being balanced.

(42) So that, when the state of charge of the battery units 41 is the same, there is not a very rapid change from one battery unit 41 to the next, a threshold value for the difference SOC.sub.changeover between the states of charge is introduced, e.g. a difference SOC.sub.changeover having a defined value of between 0.5% and 20%, preferably between 1% and 5%, particularly preferably approximately 2%, that needs to be exceeded so that the supply of power to the low voltage subsystem 21 changes from one battery unit 41 to that battery unit 41 that has a correspondingly higher state of charge than the battery unit 41 that is currently used to supply power to the low voltage subsystem 21. The changeover for the supply of power is always made toward that battery unit 41 that currently has the highest state of charge, and the changeover is made when the battery unit 41 that is currently connected for the purpose of supplying power to the low voltage subsystem 21 has a state of charge that is lower by at least SOC.sub.changeover than the state of charge of that battery unit 41 having the highest state of charge.

(43) FIG. 8 shows a possible design for switches with reverse blocking capability 44, 45. In this case, the forward direction is indicated by I. A switch with reverse blocking capability RSS_r 44 comprises an IGBT, MOSFET or bipolar transistor 101 and a diode 103 connected in series therewith, for example. FIG. 8 shows a MOSFET 101 that has an intrinsic diode 102, which is also shown. The diode 103 connected in series with the MOSFET 101 is biased in the opposite direction to the intrinsic diode 102 of the MOSFET 101. The switch with reverse blocking capability RSS_r 44 allows the current to pass in the forward direction I and blocks in the opposite direction. The switch with reverse blocking capability RSS_l 45 corresponds to the RSS_r 44, is merely installed at the opposite polarity, so that the forward and reverse directions are interchanged. The switches RSS_l 45, RSS_r 44 are particularly also distinguished by a barely noticeable delay in the switching processes, i.e. allow a very short changeover period. A suitable actuating circuit can set the time delay between the switches being switched off and switched on very accurately.

(44) The disclosure is not limited to the exemplary embodiments described here and the aspects highlighted therein. Rather, a multiplicity of modifications that lie within the scope of action of a person skilled in the art are possible in the area specified by the disclosure.