CIRCUIT ARRANGEMENT FOR A VEHICLE ELECTRICAL SYSTEM OF AN ELECTRICALLY DRIVEN MOTOR VEHICLE AND METHOD FOR OPERATING A CIRCUIT ARRANGEMENT OF THIS TYPE

Abstract

The invention relates to a circuit arrangement (24) for a vehicle electrical system of an electrically driven motor vehicle (42), comprising: a high-voltage battery (26) for supplying power to an electrical drive machine (28) of the motor vehicle (42); a range extender (22), which is designed to charge the high-voltage battery (26) and which has a plurality of identical fuel-cell base modules (10) having interfaces for supplying reactants in the form of hydrogen and air; a switching device (32) for connecting the range extender (22), the circuit arrangement (42) having no DC-to-DC converter; a control device (36) which is designed to carry out the following steps after the control device has received an activation signal regarding the range extender (22): determining an operating point of the range extender (22) in dependence on at least one variable of the high-voltage battery (26); defining a setpoint value regarding the supply of the reactants to the fuel-cell base modules (10) and/or a setpoint value regarding an operating temperature of the fuel cells on the basis of the determined operating point; controlling a system (40), which is designed to provide the reactants to and/or to control the temperature of the fuel cells, in accordance with each defined setpoint value; controlling the switching device (34) so as to connect the range extender (22), only after the setpoint value regarding the supply of the reactants and/or the setpoint value regarding the operating temperature has been reached. The invention further relates to an electrically driven motor vehicle (42) having the circuit arrangement (24), and to a method for operating the circuit arrangement (24).

Claims

1.-19. (canceled)

20. A circuit arrangement (24) for an on-board network of an electrically driven motor vehicle (42), comprising a high-voltage battery (26) for supplying energy to an electrical engine (28) of the motor vehicle (42); a range extender (22) designed for charging the high-voltage battery (26), having a plurality of identical fuel cell base modules (10) each with a plurality of fuel cells connected in series and interfaces for supplying reactants in the form of hydrogen and air; a switching device (32) for connecting the range extender (22), which is designed to interconnect the high-voltage battery (26) and the range extender (22) in an electrically conductive manner without a direct current voltage converter in the form of a parallel circuit; a control device (36) which is arranged to carry out the following steps after it has received an activation signal relating to the range extender (22): determining an operating point of the range extender (22) as a function of at least one quantity of the high-voltage battery (26); defining a setpoint value relating to a supply of the reactants to the fuel cell base modules (10) and/or an operating temperature of the fuel cells based on the determined operating point; controlling a system (40) designed for providing the reactants corresponding to the defined setpoint value, if the setpoint value relates to the supply of the reactants, and controlling a system (40) designed to control the temperature of the fuel cells in accordance with the defined setpoint value, if the set-point value relates to the operating temperature; controlling the switching device (32) for connecting the range extender (22), only after the setpoint value relating to the supply of the reactants has been reached, if the setpoint value relates only to the supply of the reactants; controlling the switching device (32) for connecting the range extender (22), only after the setpoint value relating to the operating temperature has been reached, if the setpoint value only relates to the operating temperature; controlling the switching device (32) for connecting the range extender (22), only after the setpoint value relating to the supply of the reactants and/or the operating temperature has been reached, if the setpoint value relates to the supply of the reactants and the operating temperature.

21. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged to monitor the respective volume flows of the hydrogen and the air to the fuel cell base modules (10) and only then to control the switching device (32) for connecting the range extender (22) when the volume flows correspond to the specified setpoint values.

22. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged to determine a respective difference between an actual temperature of the fuel cells and the defined setpoint value relating to the operating temperature of the fuel cells and, as a function of the determined difference, to set a temperature and/or a volume flow of a fluid of a temperature regulation device of the range-extender (22).

23. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged, as a function of a voltage of the high-voltage battery (26), to determine the operating point of the range extender (22).

24. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged, as a function of a charging state of the high-voltage battery (26), to determine the operating point of the range extender (22).

25. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged, as a function of an operating state of the high-voltage battery (26), to determine the operating point of the range extender (22) and/or to define the setpoint values.

26. The circuit arrangement (24) according to claim 25, wherein the control device (36) is arranged, regarding the operating state of the high-voltage battery (26), to determine whether and how much the high-voltage battery (26) is being discharged or charged.

27. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged, as a function of data characterising a route profile of the motor vehicle (42) and/or a fleet profile of a plurality of motor vehicles (42), to determine the operating point of the range extender (22) and/or to define the setpoint values.

28. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged, as a function of data characterising the driving behaviour of a driver of the motor vehicle (42), to determine the operating point of the range extender (22) and/or to define the setpoint values.

29. The circuit arrangement (24) according to claim 29, wherein the control device (36) is arranged to set the setpoint values relating to the reactants by a specified amount higher than is necessary for the determined operating point of the range extender (22).

30. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged, after the range extender (22) has been connected, to compare the previously determined operating point with an actually setting operating point of the range extender (22) and, as a function thereof, to adjust the supply of hydrogen and air matched to the actual operating point that is being set.

31. The circuit arrangement (24) according to claim 30, wherein the control device (36) is arranged, based on a current intensity, which characterises a current flowing from the range extender (22) to the high-voltage battery (26) to determine the actual operating point of the range extender (22) that is being set.

32. The circuit arrangement (24) according to claim 20, wherein the control device (36) is arranged to determine a state of the fuel cell base modules (10) and to specify different setpoint values for respective groups of fuel cell base modules (10) connected in parallel, depending on the state of their fuel cell base modules (10).

33. The circuit arrangement (24) according to claim 32, wherein the state comprises a respective temperature and/or a respective aging state of the fuel cell base modules (10).

34. The circuit arrangement (24) according to claim 20, wherein the range extender (22) is designed according to the following formula: $Z_{\mathrm{BZ},R}=\frac{Z_{\mathrm{Bar},R}*U_{Z,\mathrm{Bat},\min }}{U_{Z,\mathrm{BZ},\mathrm{VL}}},$ wherein Z.sub.BZ,R denotes the number of all in series connected fuel cells of the range extender (22), Z.sub.Bat,R denotes the number of all in series connected battery cells of the high-voltage battery (26), U.sub.Z,Bat,min denotes a minimum permissible cell voltage of the high-voltage battery (26) during operation, U.sub.Z,BZ,VL denotes a fuel cell operating voltage at cell level at full load.

35. The circuit arrangement (24) according to claim 20, wherein the range extender (22) is designed according to the following formula: $A_{\mathrm{BZ}}=\frac{P_M}{Z_{\mathrm{BZ},R}}*J_{\mathrm{BZ},\mathrm{MEA}},$ wherein A.sub.ABZ denotes a reactive area of all fuel cells of the range extender (22), P.sub.M denotes an average specified power requirement of the motor vehicle (42), J.sub.BZ,MEA denotes a maximum permissible current density of the respective membrane-electrode units of the range extender (22).

36. The circuit arrangement (24) according to claim 35, wherein the average specified power requirement (PM) of the motor vehicle (42) comprises an average power requirement of the engine (28) of the motor vehicle (42) for a specified driving profile of the motor vehicle (42) and at least one auxiliary consumer of the motor vehicle (42), in particular an air conditioning device of the motor vehicle (42).

37. An electrically driven motor vehicle (42) with a circuit arrangement (24) according to claim 20.

38. A method for operating a circuit arrangement (24) according to claim 20, in which the control device (36) of the circuit arrangement (24) carries out the following steps after it has received an activation signal relating to the range extender (22): determining an operating point of the range extender (22) as a function of at least one quantity of the high-voltage battery (26); defining a setpoint value relating to a supply of the reactants to the fuel cell base modules (10) and/or an operating temperature of the fuel cells based on the determined operating point; controlling a system (40) designed for providing the reactants corresponding to the defined setpoint value, if the setpoint value relates to the supply of the reactants, and controlling a system (40) designed to control the temperature of the fuel cells in accordance with the defined setpoint value, if the setpoint value relates to the operating temperature; controlling the switching device (32) for connecting the range extender (22), only after the setpoint value relating to the supply of the reactants has been reached, if the setpoint value relates only to the supply of the reactants; controlling the switching device (32) for connecting the range extender (22), only after the setpoint value relating to the operating temperature has been reached, if the setpoint value only relates to the operating temperature; controlling the switching device (32) for connecting the range extender (22), only after the setpoint value relating to the supply of the reactants and/or the operating temperature has been reached, if the setpoint value relates to the supply of the reactants and the operating temperature.

Description

[0063] The drawing shows in:

[0064] FIG. 1 a schematic perspective view of a fuel cell base module which has a plurality of fuel cells connected in series and to which a compressor for supplying air is assigned;

[0065] FIG. 2 a schematic representation of a plurality of fuel cell base modules, which combined in groups are connected in series, some of these groups in turn being interconnected in parallel;

[0066] FIG. 3 a schematic representation of a circuit arrangement for an on-board network of an electrically driven motor vehicle, the circuit arrangement comprising a parallel connection of a variant of a range extender formed from the fuel cell base modules and a high-voltage battery;

[0067] FIG. 4 a schematic representation of an electrically driven motor vehicle having the circuit arrangement;

[0068] FIG. 5 a diagram in which an instantaneous power requirement of the motor vehicle over time and an average power requirement of the motor vehicle are schematically shown;

[0069] FIG. 6 a diagram in which a course of the charging state of the high-voltage battery of the motor vehicle is shown;

[0070] FIG. 7 a diagram in which respective curves of a voltage and a power output of the range extender are schematically shown for the range extender.

[0071] In the figures, identical or functionally identical elements have been provided with the same reference numerals.

[0072] In FIG. 1, a fuel cell base module 10 is shown in a perspective view in a highly schematic manner. The fuel cell base module 10 comprises a pile 12, also called a stack, of a plurality of fuel cells connected in series and not designated in any more detail. The fuel cells comprise respective bipolar plates and respective so-called membrane electrode units. The fuel cell base module 10 also comprises a first end plate 14 and a second end plate 16, between which the stack 12 is arranged.

[0073] In addition, the fuel cell base module 10 comprises a plurality of interfaces, not shown here, for supplying hydrogen and air and for removing water and residual gas. It may be provided that only the first end plate 14 forms a kind of plug-and-play front end which has all interfaces. The fuel cell base module 10 is also assigned an air compressor 18, which serves to convey air and thus oxygen to the individual fuel cells. Contrary to the present illustration, however, it is not necessary for the air compressor 18 to be arranged directly on the basic fuel cell module 10. Instead, it can also be provided that the air compressor 18 is arranged at a completely different point during the installation in the motor vehicle in question than the fuel cell base module 10. It must only be assured that the air compressor 18 can convey air and thus oxygen to the basic fuel cell module 10 over an appropriate conduit or piping.

[0074] In FIG. 2, a plurality of the fuel cell base modules 10 are shown. To provide different powers and/or voltages, different numbers of the fuel cell base modules 10 can be electrically interconnected in different series and/or parallel circuits and configured with a media supply device, not shown here, to form respective variants of a range extender. Said media supply device, not shown here, is designed to supply air and hydrogen via the interfaces mentioned to the respective fuel cell base modules 10 and to discharge water and residual gas from the respective fuel cell base modules 10 via the interfaces.

[0075] In the present case, a plurality of groups 19 of fuel cell base modules 10 interconnected in series are shown schematically. For example, for each group 19, so many of the fuel cell base modules 10 can be interconnected in series that they can provide a voltage of, for example, 480 V and a power of 24 kW. Another interconnection is of course also possible. By connecting the individual groups 19 in parallel, it is possible to increase the power that can be provided while the voltage remains the same. In principle, any scaling of the power is possible for each application by means of a corresponding interconnection of the individual fuel cell base modules 10.

[0076] The individual fuel cell base modules 10 can have, for example, a reactive area of approximately 100 cm.sup.2 and 80 individual fuel cells. Other areas and numbers are also possible. For example, it is possible that the respective fuel cell base modules 10 can provide an open circuit voltage of 80 V and a voltage of 48 V under full load, wherein the fuel cell base modules 10 can be designed, for example, to provide a power in the range of 2 to 8 kW. Other voltages and powers are also possible, in particular depending on the selected or installed membrane electrode units in the individual fuel cell base modules 10.

[0077] Above all, it can be provided that the fuel cell base modules 10 all have the same structure regarding their components. Hence, the fuel cell base modules 10 form highly standardized units in which the same components are installed everywhere. This enables high economies of scale to be achieved with correspondingly low purchasing and production costs. Hence, the individual fuel cell base modules 10, as well as the media supply device mentioned, together form a modular range extender system 20, wherein, depending on the boundary conditions or application, the standardized fuel cell base modules 10 are able to be interconnected in a wide variety of configurations. The fuel cell base modules 10 can each have their own controller, wherein, for example, these can be run on common hardware for a specific configured variant of a range extender.

[0078] In FIG. 3, a circuit arrangement 24 for an on-board network, not designated in any more detail, of an electrically driven motor vehicle is shown in a highly schematic manner. The circuit arrangement 24 comprises a predefined variant of a range extender 22 based on said modular range extender system 20. Furthermore, the circuit arrangement 24 comprises a high-voltage battery 26 for supplying energy to an electric engine 28 of the relevant motor vehicle. A frequency converter 30, which is assigned to the electric engine 28, is also shown schematically. The high-voltage battery 26 and the range extender 22 are interconnected without a direct current voltage converter in the form of a parallel circuit, wherein the range extender 22 is designed to charge the high-voltage battery 26.

[0079] A switching device 32 for establishing and separating an electrically conductive connection between the range extender 22 and the high-voltage battery 26 is also part of the circuit arrangement 24. The switching device 32 can be, for example, a MOSFET, a transistor or also a mechanical relay. One or more freewheeling diodes 34 can also be provided as safety elements. Because the circuit arrangement 24 does not have a direct current voltage converter, installation space and corresponding costs can be saved. By a correspondingly suitable predictive regulation or control can be ensured that when the range extender 22 is connected, it is nevertheless not damaged.

[0080] The circuit arrangement 24 also comprises a control device 36. The control device 36 is arranged and designed to operate the switching device 32 in order to connect the range extender 22 such that the range extender 22 is connected to the high-voltage battery 26 in an electrically conductive manner. In addition, the control device 36 can also actuate the switching device 32 in such a way that the electrically conductive connection between the range extender 22 and the high-voltage battery 26 is disconnected. In addition, a sensor device 38 and a system 40 for providing reactants and for regulating the temperature of the fuel cells of the range extender 22 are indicated schematically. The sensor device 38 and the system 40 do not have to belong to the circuit arrangement 24, but can partially or entirely be part of the circuit arrangement 24.

[0081] The control device 36 is arranged to carry out the following steps after it has received an activation signal relating to the range extender 22:

[0082] determining an operating point of the range extender 22 as a function of at least one quantity of the high-voltage battery 26;

[0083] defining a setpoint value relating to a supply of the reactants, that is, the hydrogen and the air, to the fuel cell base modules 10 (not shown here) of the range extender 22 and/or an operating temperature of the fuel cells of the range extender 22 based on the determined operating point the range extender 22;

[0084] controlling the system 40 designed for providing the reactants and/or for regulating the temperature of the fuel cells corresponding the respective defined setpoint value;

[0085] controlling the switching device 32 for connecting the range extender 22, only after the respective setpoint value relating to the supply of the reactants and/or the operating temperature has been reached.

[0086] Regarding the reactants, for example, respective volume flows of the hydrogen and the air to the fuel cell base modules 10 of the range extender 22 can be specified. The control device 36 can monitor these volume flows and only then switches the switching device 32 accordingly to connect the range extender 22; when the volume flows correspond to the specified setpoint values. The system 40 can, for example, have a plurality of sensors, not shown here, which are designed to monitor said volume flows of the hydrogen and the air and to transmit the relevant data to the control device 36. Based on these data, the control device 36 can determine a respective difference between an actual temperature of the fuel cells and the defined setpoint value relating to the operating temperature of the fuel cells and, depending on the determined difference, may set a temperature and/or a volume flow of a fluid of a temperature regulation device (not shown here) of the range extender 22. If the control device 36 determines, for example, that the difference between the respective actual temperature of the fuel cells and the defined setpoint value for the operating temperature of the fuel cells is particularly high, the control device 36 can set, for example, a particularly high volume flow regarding the fluid of the temperature regulation device and/or heat the fluid to a particularly high temperature. This makes it possible to produce the setpoint value relating to the operating temperature of the fuel cells particularly quickly.

[0087] The control device 36 can, for example, as a function of a voltage of the high-voltage battery 26, determine the operating point of the range extender 22. In addition, the control device 36 can, for example, also as a function of a charging state of the high-voltage battery 26, determine the operating point of the range extender 22. Knowing the voltage of the high-voltage battery 26 before the range extender 22 is connected is very important, since the circuit arrangement 24 does not have a direct current voltage converter that could compensate for any voltage differences between the range extender 22 and the high-voltage battery 26. The sensor device 38 can, for example, measure the voltage of the high-voltage battery 26 and, based on this, determine a current charging state of the high-voltage battery 26 and transmit the relevant data to the control device 36.

[0088] To determine the operating point of the range extender 22 before the range extender 22 is connected, the control device 36 can also determine an operating state of the high-voltage battery 26. Regarding the operating state of the high-voltage battery 26, the control device 36 can determine, for example, whether and how much the high-voltage battery 26 is currently being discharged or charged. The sensor device 38 can, for example, provide the control device 36 with corresponding data or information. If the high-voltage battery 26 is currently being charged, for example due to recuperation, the voltage applied to the high-voltage battery 26 differs from the case in which the high-voltage battery 26 is being discharged to provide power. Before connecting the range extender 22, it can therefore also be important to know the respective operating state of the high-voltage battery 26, in particular regarding a instantaneous discharge or charge. Based on this, the control device 36 can, still before the range extender 22 is connected, matched to the operating state of the high-voltage battery 26, determine the operating point of the range extender 22 and/or define said setpoint values.

[0089] The control device 36 can also be provided with data which characterise a route profile of a motor vehicle in which the circuit arrangement 24 is installed and/or a fleet profile of a plurality of motor vehicles. Based on this data, the control device 36 can evaluate how the high-voltage battery 26 has been used up to now and/or how it will be used in the future, in particular when it is likely to be charged or discharged. Knowing this data, the control device 36 can likewise determine the matching operating point of the range extender 22 and/or define said setpoint values. Data relating to the route profile of the motor vehicle can be provided to the control device 36, for example, by a navigation device of the motor vehicle, not shown here. Knowing this route profile, for example, further fleet profiles of vehicles that have travelled the same route can be retrieved. Based on the data from the fleet profiles, the control device 36 can estimate even more precisely when and how the high-voltage battery 26 is likely to be charged and discharged.

[0090] In addition, the control device 36 can also be provided with data which characterise a driving behaviour of a driver of the relevant motor vehicle in which the circuit arrangement 24 is installed. On the basis of these data which characterise the driving behaviour of the driver, the control device 36 can likewise determine the operating point of the range extender 22 and/or define said setpoint values. Knowing the driving behaviour of the driver, it is in particular possible to estimate his future driving behaviour, on which may largely depend how much the high-voltage battery 26 is discharged and/or recuperated.

[0091] The control device 36 can also set said setpoint values regarding the reactants to be provided and/or the operating temperature of the fuel cells higher by a specified amount than is necessary for the determined operating point of the range extender 22. Regarding the reactants, the control device 36 can therefore select an overstoechiometric specification for the point in time at which the range extender 22 is connected, such that after the range extender 22 is connected, more reactants are made available to it than is necessary for the determined operating point. The operating temperature of the fuel cells can also be selected to be somewhat higher, such that the resulting reactions within the fuel cells can take place faster than would be necessary for the determined operating point. This creates a type of buffer such that the range extender 22 can, if necessary, provide ad hoc more power for charging the high-voltage battery 26, if this should be necessary.

[0092] After the range extender 22 has been connected by appropriate activation of the switching device 32, the control device 36 can compare the previously determined operating point with an actually established operating point of the range extender 22 and, depending thereon, adjust the supply of the reactants to match the actual operating point. For example, the control device 36 can determine, based on a current intensity that characterises a current flowing from the range extender 22 to the high-voltage battery 26, the actual operating point of the range extender 22. After the range extender 22 has been connected, the control device 36 can also adjust or set, in particular the supply of the reactants, very quickly such that the actually setting operating point of the range extender 22 can be maintained, and without too much of the reactants being supplied.

[0093] The control device 36 can also determine a state of the fuel cell base modules 10 and specify different setpoint values for respective groups 19 (see FIG. 2) of fuel cell base modules 10 connected in parallel, depending on the state of their fuel cell base modules 10. This again takes place before the range extender 22 is connected. Regarding said state of the fuel cell base modules 10, the control device 36 can take into account, for example, a respective temperature and/or a respective aging state of the fuel cell base modules 10 of the range extender 22. In particular, the respective temperature of the fuel cells has a very strong influence on the reactions within the fuel cells and thus on their voltage development.

[0094] In FIG. 4, an electrically driven motor vehicle 42 is schematically shown. Part of the circuit arrangement 24 is also schematically shown. A plurality of the aforementioned air compressors 18 are also schematically shown. The air compressors 18 are assigned to the individual fuel cell base modules 10 (not shown) of the relevant or configured variant of the range extender 22. Via corresponding channels or conduits, the air compressors 18 can convey aspirated air to the individual fuel cell base modules 10. As already mentioned, the air compressors 18 do not have to be arranged directly on the fuel cell base modules 10. Instead, the air compressors 18 can be arranged at other suitable locations in the motor vehicle 42, in particular where the space conditions permit it particularly well and at the same time ambient air can be aspirated particularly well by means of the air compressor 18.

[0095] The individual fuel cell base modules 10 can in turn be arranged elsewhere in the motor vehicle 42, in view of a favourable vehicle centre of gravity and favourable packaging. The individual air compressors 18 can be combined, for example, in the form of a compressor module. Depending on the configuration of the range extender 22 and, above all, depending on the number of fuel cell base modules 10 installed therein, the number of air compressors 18 to be used may vary. The number of air compressors 18 corresponds exactly to the number of fuel cell base modules 10.

[0096] In FIG. 5 a diagram is shown in which an instantaneous power requirement P of the motor vehicle 42 over the time t, as well as an average power requirement P.sub.M of the motor vehicle 42 are schematically shown. The instantaneous power requirement P of the motor vehicle 42 may comprise, for example, a power requirement of the electric engine 28, as well as one or more auxiliary consumers of the motor vehicle 42. The secondary consumers may be, for example, an air conditioning device of the motor vehicle, i.e., for example, an air conditioning system, an infotainment system or any other consumers that are not directly used to drive the motor vehicle 42. Therefore, the average power requirement P.sub.M not only includes the pure power requirement of the electric engine 28 but also, for example, all the power requirements of all secondary consumers of the motor vehicle 42. Knowing the average power requirement P.sub.M can be used to design the range extender 22. The range extender 22 can be designed according to the following formula.

[00003]$A_{\mathrm{BZ}}=\frac{P_M}{Z_{\mathrm{BZ},R}}*J_{\mathrm{BZ},\mathrm{MEA}},$

[0097] wherein

[0098] A.sub.BZ denotes a reactive area of all fuel cells of the range extender 22,

[0099] P.sub.M denotes an average specified power requirement of the motor vehicle 42;

[0100] J.sub.BZ,MEA denotes a maximum permissible current density of the respective membrane-electrode units of the range extender 22.

[0101] The reactive area A.sub.BZ of all fuel cells of the range extender 22 can therefore be designed taking into account the average power requirement P.sub.M and the specified maximum permitted current density J.sub.BZ,MEA of the respective membrane electrode units of the range extender 22.

[0102] Additionally or alternatively, the range-extender 22 may also be designed according to the following formula:

[00004]$Z_{\mathrm{BZ},R}=\frac{Z_{\mathrm{Bar},R}*U_{Z,\mathrm{Bat},\min }}{U_{Z,\mathrm{BZ},\mathrm{VL}}},$

[0103] wherein

[0104] Z.sub.BZ,R denoted the number of all in series connected fuel cells of the range extender 22,

[0105] Z.sub.Bat,R denotes the number of all in series connected battery cells of the high-voltage battery 26,

[0106] U.sub.Z,BZ,VL denotes a minimum permissible cell voltage of the high-voltage battery 26 during operation,

[0107] U.sub.Z,BZ,VL denotes a fuel cell operating voltage at cell level at full load.

[0108] The number Z.sub.BZ,R of all in series connected fuel cells of the range extender 22 is calculated according to the formula depending on the number Z.sub.Bat,R of all in series connected battery cells of the high-voltage battery 26, the minimum permissible cell voltage U.sub.Z,Bat,min of the high-voltage battery 26 in operation, as well as the fuel cell operating voltage U.sub.Z,BZ,VL set at cell level. As a result, the range extender 22 can be designed to be particularly well matched to the high-voltage battery 26. Among other things, this can ensure that, when the range extender 22 is connected, it can reliably provide at least approximately the same voltage level as the high-voltage battery 26 and can also charge it to a sufficient extent.

[0109] By designing the range extender 22 in accordance with the two above-mentioned formulas, it can also be ensured that the range extender 22 is neither oversized nor undersized.

[0110] In FIG. 6, a diagram is shown in which a profile of the charging state SoC of the high-voltage battery 26 is shown over time t. In the case schematically indicated here, it can be assumed, for example, that the high-voltage battery 26 is discharged more or less continuously. Of course, the charging state SoC can also increase again during operation, for example when the high-voltage battery 26 is charged by recuperation and/or by connecting the range extender 22. In addition, a possible area B, which can be suitable for connecting the range extender 22, is indicated schematically.

[0111] In FIG. 7, a diagram is shown in which, when the range extender 22 is connected, the respective curves of a voltage U.sub.REX of the range extender 22 and a power output P.sub.REX of the range extender 22 to the high-voltage battery 26 via a current I.sub.REX flowing from the range extender 22 to the high-voltage battery 26, are plotted. The area B for connecting the range extender 22 is again schematically shown. Due to the parallel connection of the range extender 22 to the high-voltage battery 26 without a direct current voltage converter, the voltage U.sub.REX of the range extender 22 and the voltage U of the high-voltage battery 26 automatically adapt to one another when the range extender 22 is connected.

[0112] On the basis of the present diagram, it can be seen how the power output P.sub.REX of the range extender 22 to the high-voltage battery 26 can vary, depending on when the range extender 22 is connected. In other words, the diagram shown here depicts different possible operating points of the range extender 22. This operating point of the range extender 22 in turn depends—as already mentioned before—significantly on the different quantities of the high-voltage battery 26. Therefore, before connecting the range extender 22, it is important that the control device 36, in the manner already explained, accurately determines or estimates the operating point of the range extender 22 as a function of as many quantities of the high-voltage battery 26 as possible. As a function of the determined operating point, said setpoint values relating to the supply of the reactants to the fuel cell base modules 10 of the range extender 22 and/or an operating temperature of the fuel cells of the range extender 22 are defined. By appropriately controlling the system 40 by the control device 36, it can be ensured that at the point in time the range extender 22 is connected, it is also provided with sufficient reactants and/or has the required operating temperature, such that the range extender 22 can be operated immediately in the previously determined operating point. Because the switching device 32 is only then activated to connect the range extender 22 by means of the control device 36, when the respective setpoint value relating to the supply of the reactants and/or the operating temperature of the fuel cells has been reached.

[0113] Since there is no direct current voltage converter between the range extender 22 and the high-voltage battery 26, is important, at the point in time the range extender 22 is connected, that the range extender 22 has sufficient reactants available to be able to operate at the operating point, previously determined as precisely as possible. For example, after the range extender 22 has been connected, it can be prevented that hotspots within the range extender 22 arise and/or that the range extender 22 cannot provide sufficient power to charge the high-voltage battery 26.

[0114] Because the control device 36 estimates or determines the operating point of the range extender 22 before it is connected particularly precisely in the manner described, it can be operated particularly gently after it has been connected. Without the explained procedure of the control device 36, it would be very likely that the range extender 22 would be damaged after only a few connection operations—due to the absence of the direct current voltage converter and the parallel connection to the high-voltage battery 26. This can be prevented by the aforementioned procedure. It is thus possible to use the range extender 22 to extend the range of the motor vehicle 42 even without a direct current voltage converter.

LIST OF REFERENCE NUMERALS

[0115] 10 fuel cell basic module

[0116] 12 stack of fuel cells connected in series

[0117] 14 first end plate

[0118] 16 second end plate

[0119] 18 air compressor

[0120] 19 groups of fuel cell base modules connected in series

[0121] 20 modular range extender system

[0122] 22 variant of a range extender

[0123] 24 circuit arrangement

[0124] 26 high-voltage battery

[0125] 28 electric engine

[0126] 30 frequency converter

[0127] 32 switching device

[0128] 34 freewheeling diode

[0129] 36 control device

[0130] 38 sensor device

[0131] 40 system for providing reactants and for controlling the temperature of the fuel cells

[0132] 42 motor vehicle

[0133] A.sub.BZ reactive area of all fuel cells of the range extender

[0134] B range for connecting the range extender

[0135] I.sub.REX current flowing from the range extender to the high-voltage battery

[0136] J.sub.BZ,MEA maximum permissible current density of the respective membrane electrode units of the range extender

[0137] P instantaneous power requirement of the motor vehicle

[0138] P.sub.M average specified power requirement of the motor vehicle

[0139] P.sub.REX power output of the range extender to the high-voltage battery

[0140] SoC charging state of the high-voltage battery

[0141] t time

[0142] U voltage of the high-voltage battery

[0143] U.sub.REX voltage of the range extender

[0144] U.sub.Z,Bat,min minimum permissible cell voltage of the high-voltage battery during operation

[0145] U.sub.Z,BZ,VL fuel cell operating voltage at cell level at full load

[0146] Z.sub.Bat,R number of all in series connected battery cells of the high-voltage battery

[0147] Z.sub.BZ,R number of all in series connected fuel cells of the range extender