D E S C R I P T I O N

The present invention relates to a power supply system, in particular for a motor vehicle with electric drive.

In the operation of an electrically driven motor vehicle via a rechargeable battery, care must be taken, as described in DE 10 2017 111 573 A1 , that the battery is not discharged too deeply, as this can lead to chemical reactions which damage the battery irreversibly. If the battery is the sole source for drive energy, this can lead to the vehicle having to be stopped when the state of charge of the battery reaches a critically low level. Before that happens, the power of the battery decreases continuously, so that a driver, even without having to consult measuring instruments, notices a low state of charge of the battery and can consequently avoid energy- intensive driving manoeuvres and can seek a charging station promptly.

A way to increase the range of an electrically driven vehicle beyond the value corresponding to the charging capacity of its battery is the installation of an electrochemical generator, such as for instance a fuel cell, which during travel converts a fuel into electrical energy. This can be used for recharging the battery, but also directly for supplying a drive motor. However, compared to the motor output, its output is only slowly changeable; the fuel cell can therefore not be switched on for a short time in order for instance to supply the power necessary for an overtaking procedure, and to be switched off again thereafter.

When the battery is fully or almost fully charged and can not store the electric power generated by the fuel cell, the fuel cell must be switched off or must run at a minimal output, and only the output which the battery is able to deliver, if applicable plus the minimal output of the fuel cell, is available as drive power for the motor. When the battery is partially discharged and therefore its maximum output is less than in the fully charged state, at the same time, however, the fuel cell is in operation, in order to supply additional power, the battery and the fuel cell can deliver together the same power to the motor as the fully charged battery alone, so that no change occurs in the behaviour of the vehicle. Ideally, in this operating state, the output of the fuel cell should correspond to the medium output of the motor, so that it can operate continuously and efficiently and great differences in the state of charge of the batter are avoided. If, however, under these conditions, more power is continuously consumed than the fuel cell delivers, sooner or later a situation occurs, however, in which the battery is discharged so far that its output power must be restricted for its protection, and consequently the motor can no longer react to a performance requirement by the driver in the manner which the latter expects. Such situations should be avoided as far as possible.

In order to achieve this aim, it is proposed according to the invention that in a power supply system comprising a fuel cell, a battery which is chargeable by the fuel cell, a consumer connection, and a control unit for controlling the output of the fuel cell with the aid of the state of charge of the battery, the control unit is set up to detect a power flow in the power supply system to the consumer connection, and to control the output of the fuel cell further with the aid of the detected power flow. This enables an adaptation of the output of the fuel cell to the actual requirement with higher temporal resolution than when the output of the fuel cell is controlled solely with the aid of the state of charge of the battery, and above all, in a situation such as for instance when travelling uphill, in which a high motor output is required over a longer time span than for instance during an overtaking procedure, it enables a prompt adaptation of the output of the fuel cell, so that a state in which the performance requirement by the driver can not be fulfilled is delayed and, ideally, if the continuously high performance requirement ends in a timely manner, can be avoided entirely.

The detected power flow to the consumer connection is typically the entire power flow to the consumer connection composed if applicable from contributions by the battery and the fuel cell. However, it is also conceivable to use only one of these contributions. The power flow can be measured as amperage or as any desired measurement which is linked to the power flow, such as for instance the electrical output. The control unit should be set up to select the output of the fuel cell at a given state of charge of the battery to be all the higher, the higher the power flow is.

In particular, the control unit can establish the output of the fuel cell as an increasing function both of the relative discharge of the battery and also of the power flow.

According to a practical configuration, the control unit establishes the output of the fuel cell as an increasing function of the relative discharge of the battery multiplied by a scale factor, wherein the scale factor is an increasing function of the power flow.

In order to avoid all too frequent changes to the output of the fuel cell, establishing should take place with the aid of a temporal mean value of the power flow. The timespan over which averaging takes place can be between 10 s and 5 min, preferably approximately 1 min.

In order to establish the output of the fuel cell, the control unit can select from several predetermined characteristics of the output as a function of the state of charge respectively one with the aid of the power flow.

Here, the characteristics are to be constant and non-intersecting functions of the state of charge, i.e. one of these functions, which at a given value of the power flow is greater (or smaller) than another, is also greater (or smaller) at any other value.

In order to prevent changes to the output of the fuel cell which are too quick, provision can be made that at any time the choice of a new characteristic with the aid of the power flow by the control unit is restricted to those characteristics which are separated from a currently selected characteristic by no more than n further characteristics, wherein n is a small integer including zero. This means that if n=0, only a change from the currently selected characteristic to the next higher or next lower is possible; if n=1 , the respectively next but one can also be selected.

The selection of a new characteristic with the aid of the power flow should take place in regular time intervals and preferably at a mean value of the power flow in the time interval since the last selection. The mean value can take into consideration the power flows in the entire time interval or else only in a section of these. A further subject of the invention are a motor vehicle with a power supply system as described above, optionally in which a consumer, connected to the consumer connection, is an electric motor driving the motor vehicle, and a computer program with commands which are to be executed by a computer, the execution of which by a computer enables it to operate as a control unit in a power supply system as described above.

Further features and advantages of the invention will emerge from the following description of example embodiments with reference to the enclosed figures. There are shown:

Fig. 1 a block diagram of a motor vehicle with a power supply system according to the invention;

Fig. 2 a diagram with a set of characteristics of the output of the fuel cell as a function of the state of charge of the battery for respectively different power flows; and

Fig. 3 a diagram with an alternative set of characteristics.

Fig. 1 shows a block diagram of the drive system of a motor vehicle. An electric motor 1 drives one or more wheels, not illustrated, of the vehicle; a separate electric motor 1 can be assigned to each wheel. The electric motor 1 is supplied by a battery 2 and a fuel cell 3; a control unit 4 regulates via an arrangement of switches 5 the distribution of the electric output, provided by the fuel cell 3, to the motor 1 and the battery 2, and the contribution of fuel cell 3 and battery 2 to the present power requirement of the motor 1. This power requirement is determined by a performance requirement of the driver, typically via an accelerator pedal 6.

The control unit 4 is connected to various measuring instruments, known per se, for monitoring the state of charge of the battery 2, e.g. by measuring a charging current from the fuel cell 3 to the battery 2 via a measuring instrument 7-1 and of a discharge current from the battery 2 to the motor 1 , connected via a consumer connection 12, via a measuring instrument 7-2. The control unit 3 uses the information of the measuring instruments 7-1 and 7-2 in order to be able to put the fuel cell 3 into operation when the state of charge of the battery has fallen below a predetermined threshold, and then to be able to divide the output of the fuel cell 3 appropriately between the battery 2 and the motor 1. With a low state of charge of the battery 2, it may be necessary to give preference to this at the expense of the present energy supply of the motor 1 , in order to prevent the eventuality that a deep discharge state is reached, damaging the battery 2. With a sufficient state of charge, on the other hand, the control unit 4 can, if necessary, supply the entire or a portion of the output of the fuel cell 3 additionally to that of the battery 2 into the motor 1 , in order to also provide a specified motor performance when the battery 2 is discharged too far in order to be able to deliver this on its own. The information concerning the state of charge is provided furthermore at an output of the control unit 4, in order to be presented on a display instrument 8 in a manner which is visible for the driver.

A direct power flow from the fuel cell 3 to the motor 1 can be detected by a further measuring instrument 7-3; it is also conceivable, however, that the control unit 4 calculates this power flow with the aid of the known output of the fuel cell 3, controlled by it, and of the power flow from the fuel cell 3 to the battery 2.

With the aid of a stored characteristic, the control unit 4 is able to estimate the present output able to be delivered by the battery 2, with the aid of its state of charge. This information is likewise made available to the display instrument 8 at the output; it can also be drawn upon in order to establish the contributions of battery and fuel cell 3 to the present output provided to the motor 1.

The fuel cell 3 obtains required fuel, i.e. hydrogen, from a fuel tank 9. A valve 10 in a supply line from the fuel tank 9 to the fuel cell 3 is able to be controlled by the control unit 4, in order to interrupt or throttle the operation of the fuel cell 3, when the battery 2 is sufficiently charged or the motor does not require any energy.

The control unit 4 is furthermore connected to a sensor 11 for detecting the filling level of the fuel tank 9. This information is also made available to the display instrument 8 at the output.

Fig. 2 shows a family of characteristics Ao-A _n (here with n=4) of the output of the fuel cell 3 as a function of the state of charge of the battery 2, which are stored in the control unit 4 and are used optionally by the latter, in order to regulate the output of the fuel cell 3. A basic characteristic Ao can be optimized e.g. for motorway travel, i.e. for an operation with small speed fluctuations and low road gradients. With a fully or almost fully charged battery 2, the fuel cell 3 is operated at a minimal output, which in the case shown here amounts to few kW, which, however, can also be zero. In any case, this output is not sufficient in order to keep the state of charge of the battery in travelling operation; this therefore decreases in the course of travel, until a first threshold, here a state of charge of 90%, is reached, starting from which the output of the fuel cell 3 is regulated upwards with progressive discharging of the battery. If the vehicle were en route on a level roadway at a constant speed, the state of charge would thus level off in the course of time to a value at which the output of the fuel cell 3 is equal to the output necessary for maintaining this speed, wherein, however, this value is all the lower, the more energy-intensive the manner of driving is. The charge reserves with which the vehicle can sustain a section, for instance at a hill start or travelling on a loose or uneven substrate, in which the motor output exceeds the maximum output of the fuel cell of here just over 45 kW, are accordingly less.

In order to prevent this effect, the control unit 4 detects the power flow to the motor 1 by means of one or more of the sensors 7, and determines respectively after a predetermined timespan of e.g. one minute has elapsed, a mean value over this time span. Subsequently, it selects from the various characteristics Ao-A _n that at which the mean value comes closest to a desired state of charge of the battery, and regulates the output of the fuel cell with the aid of this characteristic until a new mean value is available. In so far as the mean value is not higher than the maximum output of the fuel cell, the state of charge can in this way be kept constantly close to the desired state of charge, and sufficient battery reserve is always available in order to supply the motor 1 for a time with a higher output than the maximum output of the fuel cell.

Proceeding form the base characteristic Ao, the further characteristics can be derived in a different manner. In Fig. 3 the characteristics AI’-A4 are derived from the basic characteristic Ao by spreading in the vertical (along the power axis P) and are respectively capped at the maximum output of the fuel cell. In Fig. 2 the characteristics A1-A4 are derived from the same basic characteristic Ao respectively by compressing in the horizontal (along the axis of the state of charge axis) and displacing towards the right. This means that when SOC designates the percentage state of charge of the battery, the relative discharge is d=100%-SOC, and the basic characteristic A corresponds to a function P(d), then the curve Aj, i=1 , , 4 corresponds to P(a*d), wherein ai is a scale factor >1 and ai+i >a,.

In order to prevent too intensive a change in the output of the fuel cell in reaction to a changed power flow to the motor 1 , provision can be made that, if the present mean value of the power flow would require a different characteristic Aj than the present adjusted characteristic Aj, the control unit does not immediately switch over to the characteristic Aj, but rather firstly only to the characteristic AM or Aj+i , depending on whether j<i or j>i, and thus gradually moves closer to the characteristic Aj in the course of several update cycles (provided that the power flow to the motor corresponding to the characteristic Aj persists so long).

Reference numbers electric motor battery fuel cell control unit switch accelerator pedal measuring instrument display instrument fuel tank valve sensor consumer connection