DE102013008829B4 - motor vehicle - Google Patents

Abstract

Motor vehicle, comprising a battery (1) having a first pole and a second pole, a capacitor (4), at least one consumer (2), a DC / DC converter (5) and a control device (8), wherein the at least one consumer (2) is connected between the first pole of the battery (1) and a ground potential (3), whereby at the load (2) an operating voltage drops, and wherein a reference potential of an input (6) and an output (7) of the DC / DC converter (5) is the ground potential (3), wherein in at least a first operating mode of the control device (8), the capacitor (4) is connected in series with the battery, wherein the capacitor (4) having a first terminal to the second pole the battery (1) and with a second terminal to the ground potential (3) and the DC / DC converter (5) to the input (6) to the first pole of the battery (1) and the output (7) to the second Pol of the battery (1) is connected, characterized in that the control device (8) the DC / DC converter (5) in the first operating mode such that current flows between the input (6) and the output (7) of the DC / DC converter (5).

Description

The invention relates to a motor vehicle comprising a battery having a first pole and a second pole, a capacitor, at least one load, a DC / DC converter and a control device, wherein the at least one load is connected between the first pole of the battery and a ground potential, whereby an operating voltage drops at the consumer, and wherein a reference potential of an input and an output of the DC / DC converter is the ground potential, wherein in at least a first operating mode of the control device ( 8th ) the capacitor ( 4 ) is connected in series with the battery, wherein the capacitor ( 4 ) with a first connection to the second pole of the battery ( 1 ) and with a second connection to the ground potential ( 3 ) and the DC / DC converter ( 5 ) with the entrance ( 6 ) with the first pole of the battery ( 1 ) and with the output ( 7 ) with the second pole of the battery ( 1 ) connected is.

In current vehicles, the usable vehicle electrical system voltage range with the engine running is in the range between 13 and 15 volts. This voltage is provided by the battery voltage, which is normally provided by a lead-acid battery and which is charge state dependent in the unloaded state between about 11.5 and 13 volts, and an additional voltage provided by the generator, such as a starter generator, during engine run.

If the engine does not run, only a lower voltage is available in the electrical system. This voltage can continue to fall if one or more consumers are operated in the vehicle electrical system, which have a high power consumption. It is possible, for example, for automobiles with automatic start / stop to start high currents in the range of several 100 amperes when the motor restarts. Under these conditions, the battery voltage and thus the available voltage in the electrical system can fall to voltages of less than 6 volts.

To compensate for voltage dips in such short-term loads on a time scale of a few 100 milliseconds is in the document US 2010/0308650 A1 proposed a device in which during the starting process, a capacitor with the battery in series is switchable to raise the voltage of the electrical system. When driving, this capacitor is recharged via a DC / DC converter.

The publication FR 2964511 A1 describes an electrical system of a motor vehicle, wherein the motor vehicle is designed for automatically stopping and starting an internal combustion engine. The vehicle electrical system comprises two switches with which one pole of a battery can be connected either directly or via a supercapacitor to a ground potential. When starting the engine, the supercapacitor is connected in series with the battery. In a charge mode, this series connection is disconnected to charge the supercapacitor using a DC / DC converter.

The publication WO 2012/049387 A1 discloses a similar circuit in which a battery is selectively connectable to ground potential either directly or through a supercapacitor. To charge the capacitor, the battery is connected directly to the ground potential and the supercapacitor is charged via a voltage converter.

The publication DE 10 2009 028 417 A1 discloses a circuit arrangement for a vehicle electrical system, which comprises a base on-board network with an energy storage, a power electrical system with an additional energy storage and a DC / DC converter. The DC / DC converter is used for coupling between the base electrical system and the power electrical system.

An electrical operating circuit for a starter is from the document WO 02/46608 A1 known. Here, a series connection of a battery and a capacitor is used. The starter can be connected to the series connection so that the voltage applied to the starter is the sum of the voltage applied to the battery and the voltage applied to the capacitor.

In many applications, however, it is desirable to increase or stabilize the voltage of a vehicle electrical system for longer periods of time, in particular periods of more than one second. Thus, some consumers in modern motor vehicles, such as components of catalyst, steering and driver assistance systems, can only function reliably in a relatively narrow voltage range. In particular, when driving with the engine off, sufficient power can often not be made available for such systems since the onboard power supply is not supported by the generator. In these cases, it may be necessary to restart the engine early, resulting in a reduction in the efficiency of the motor vehicle. Alternatively, individual functions of the motor vehicle must be shut down, which leads at least to a deterioration of ride comfort. If this is to be avoided, a stabilization of the power supply of the individual components would be necessary, which is very expensive.

Also in the drive of a motor vehicle by an electric motor or in the support of an internal combustion engine by an electric motor, it may be advantageous to raise the voltage of the electrical system, since in this case greater voltages can be provided on the electric motor, which in particular a larger torque of the engine can be achieved.

The invention is thus based on the object of specifying a motor vehicle in which an increase in the vehicle electrical system voltage without operation of a generator over longer periods is possible.

The object is achieved in that the control device controls the DC / DC converter in the first operating mode such that current flows between the input and the output of the DC / DC converter.

The invention is based on the idea, during the series connection of the capacitor with the battery additionally a DC / DC converter, which is connected between the poles of the battery to operate. Depending on the output voltage of the DC / DC converter and the voltage drop across the capacitor, this leads to different current flows. If the capacitor is relatively far discharged, that is, it falls from a low voltage across the capacitor, the DC / DC converter may have an output voltage which is greater than the voltage dropping across the capacitor. In this case, despite series connection of the capacitor with the battery, the capacitor is charged by the DC / DC converter. After a certain transition time, which is necessary to charge the capacitor, the voltage drop across the capacitor and the output voltage of the DC / DC converter is the same. If, for example, the DC / DC converter is operated with a constant output voltage, then the voltage dropped across the capacitor and thus the state of charge of the capacitor is also constant.

In the known from the prior art method for short-term increase of a battery voltage by means of a series-connected capacitor, the capacitor, as soon as it is connected in series with the battery, discharged. With the discharge of the capacitor, however, at the same time decreases the voltage dropping across the capacitor, which is why only an increase of the vehicle electrical system voltage for a short time interval is possible, especially since the capacitance of the capacitor is usually far smaller than that of the battery. In the method according to the invention, however, the charge state of the capacitor remains constant with a uniform load on the battery and a fixed conversion ratio of the DC / DC converter. This allows an increase in the vehicle electrical system voltage for any times, as long as the battery provides power.

It should be noted that the description of the state of charge of the capacitor as constant, of course, only applies when averaging over at least one switching cycle of the DC / DC converter. DC / DC converters include one or more switches that generate a high frequency AC voltage. This is usually smoothed via inductors and capacitors in the DC / DC converter, whereby, in addition, depending on the type of converter, diodes are still used to rectify the voltage at certain points. The switching frequencies of DC / DC converters are often in the multi-digit kilohertz range. This in turn means that the capacitor is charged and discharged on a time scale of a few tens to a few 100 ms, but the average voltage remains constant.

Overall, the battery current is thus transported through both the capacitor and the DC / DC converter. It should be noted that while the coils and capacitors of the DC / DC converter and the capacitor, which is connected in series with the battery, although large currents flow and at the same time drop voltages, the voltage drop relative to the current is in each case phase-shifted, so that the coils or capacitors is dissipated substantially no power.

As a battery any kind of energy storage can be used in the process according to the invention. Supercapacitors, for example double-layer capacitors or pseudocapacitors, can be used in particular as a capacitor. As a DC / DC converter, for example, a buck converter can be used. The output voltage of a buck converter can be adjusted between the input voltage and 0 volt, depending on the pulse width ratio, ie the ratio of the times between the two switching states within a switching cycle of the DC / DC converter. The pulse width can be done either directly by a pulse width modulation by the controller or by the controller controlled components, but it can also be a current or voltage controlled DC / DC converter can be used, whereby a direct specification of the output voltage is possible.

Of course, it is also possible to regulate the DC / DC converter so that the sum of the DC / DC converter voltage and the battery voltage is constant.

For example, the DC / DC converter can provide a variable output voltage in the range of a few 100 mV to 3 to 4 volts, the DC / DC converter can be controlled so that the output voltage at both a low battery level and thus a lower Battery voltage is increased as well as a high battery load and thus a lowering of the battery voltage by the internal resistance of the battery.

In the motor vehicle according to the invention it is thus possible to vary the voltage range of the electrical system with little additional energy consumption and low additional volume and weight in a wide voltage range almost arbitrarily.

In a further development of the motor vehicle according to the invention, it is possible for the control device to be designed to drive a switch device which connects the second pole of the battery in the first operating mode with the DC / DC converter and the first terminal of the capacitor and in at least one second operating mode Earth potential connects. Such a circuit means that in the wide operating mode, the battery is connected directly between ground potential and electrical system, as in conventional vehicles. If, for example, when operating an internal combustion engine by operation of a generator sufficient additional voltage for the electrical system provided, with this circuit a current transport through the DC / DC converter can be avoided, whereby it is possible to reduce the total internal resistance of the power supply of the motor vehicle, thereby greater efficiency is achieved. At the same time, in the second operating mode, the DC / DC converter can be used to charge the capacitor. This is particularly advantageous when the capacitor has a particularly large capacity, for example when it is designed as a hybrid capacitor, in which case, for example, energy recovered during braking can be stored in the capacitor. If energy is already stored in the capacitor when switching to the first operating mode, less power must be provided by the DC / DC converter, which means less load on the battery and less power.

It is particularly advantageous if the control device is designed for operation in the first operating mode for an operating time of at least one second, in particular at least 5 seconds, in particular at least 30 seconds. As already explained, the essential advantage of the motor vehicle according to the invention is that an increase in the vehicle electrical system voltage is possible in principle for any desired times. While it is of course also possible to use the first mode of operation only for short-term heavy loads, such as starting the engine, it is possible for example by a longer operation in the first operating mode to reliably supply system components in an operation of the motor vehicle with the engine off that require a certain minimum voltage, or to increase the operating voltage of an electric motor during a longer acceleration interval. In principle, it is also possible to operate the motor vehicle during the entire operating time in the first operating mode. In particular, when using lead-acid batteries, however, it may be advantageous to operate the control device at least temporarily in the second operating mode, since due to the usually much larger capacity of the battery compared to the capacitor and the charge / discharge hysteresis of lead-acid batteries overcharging of the capacitor when charging the Battery would otherwise be avoided only with great effort.

In a particularly simple way, a permanent increase in the voltage in the electrical system can be achieved if the control device is designed to control the DC / DC converter, such that the voltage drop across the capacitor, averaged over a switching cycle of the DC / DC converter, over the operating time is essentially constant.

Usually, DC / DC converters are not directly controlled by a pulse width with a variable pulse width ratio whose pulse width ratio directly determines the ratio between input voltages and output voltages, but via a digital or analog predetermined reference voltage. For this control loop usually a current control or a voltage control is used, which differ in the type of feedback circuit. If, in this way, the output voltage of the DC / DC converter is regulated to a constant value, this leads to the fact that always the same voltage between the ground potential and the output of the DC / DC converter and thus at the capacitor drops. Thus, the charge of the capacitor is constant. When this capacitor is connected in series with the battery, this increases the vehicle electrical system voltage by the value that is output at the output of the DC / DC converter.

Alternatively, it is possible for the control device to be designed to control the DC / DC converter, such that in the first operating mode the operating voltage averaged over a switching cycle of the DC / DC converter is essentially independent of the consumer current flowing through the load, as long as the load current does not exceed a predetermined or specifiable current limit. In this case, for example, a control loop can be used, which receives the vehicle electrical system voltage or a voltage derived from the vehicle electrical system voltage as input voltage, compares this with the desired value, which is predetermined by a reference voltage, and as a control variable, either a voltage for a control circuit of the DC / DC converter or a pulse width ratio for the DC / DC converter specifies. Such Of course, control can also be digital. Thus, the vehicle electrical system voltage can be fed either directly or with the aid of a voltage divider to an analog-to-digital converter, the measured value are then internally compared with a digitally present setpoint for the vehicle electrical system voltage and then either via a digital-to-analog converter a target voltage for the DC / DC converter or the DC / DC converter is directly controlled by a digital output which outputs a pulse width modulated voltage. However, the skilled person will also be aware of numerous other embodiments for controlling the vehicle electrical system voltage or the voltage drop across the capacitor.

In an advantageous embodiment of the motor vehicle according to the invention, the control device comprises a regulator for controlling the voltage dropping across the capacitor or the operating voltage by controlling the DC / DC converter, which comprises a proportional and / or an integral and / or differential element. The controller in the control device can be both an analog controller and a digital controller. A particularly simple construction is possible if the control device comprises a digitally implemented control circuit and this directly serves as a source for the pulse wave switching the DC / DC converter. Of course, however, the control device can also provide digital or analog setpoints via a separate control circuit of the DC / DC converter, in particular for current or voltage control.

Particularly advantageous is the DC / DC converter is a buck converter or a synchronous converter. Since the on-board voltage is to be increased by only a few volts, but at the input of the DC / DC converter in the simplest case, the voltage of the electrical system itself is applied, which is typically in the two-digit voltage range, must in the motor vehicle according to the invention only a conversion of higher voltages to lower voltages respectively. This makes a particularly simple construction of the DC / DC converter possible. Synchronous converters have the particular advantage that they can have a higher efficiency compared to simple buck converters. Of course, a variety of other types of transducers can be used. In particular, when using capacitors with a large capacity, such as double-layer or hybrid capacitors, it may be advantageous to recharge them when parking the motor vehicle and to use the energy of the capacitor to recharge the battery. For this bidirectional DC / DC converters can be used.

The battery of the motor vehicle according to the invention may be, for example, a lead-acid battery. Lead-acid batteries are used in almost all vehicles with internal combustion engines as energy storage. They offer numerous advantages, such as high discharge rates, high availability and low price.

The motor vehicle according to the invention may be a hybrid or electric vehicle, the consumer or one of the consumers being an electric motor or a starter generator, the control device being in or switching to the first operating mode during acceleration or starting, the operating voltage being connected in series by capacitor and Battery is increased, whereby the maximum torque of the electric motor or the starter generator is increased. Often it is desired to increase the power of a motor vehicle in a short period of a few seconds, for example, to overtake another motor vehicle. In addition, it is often desired in hybrid vehicles that during acceleration processes, in particular from the state, the torque of an internal combustion engine is increased by an electric motor in order to achieve a better acceleration. It is therefore desired in numerous driving situations to increase the torque of an electric motor in the short term.

Many electric motors can be operated at least in the short term with a voltage that is significantly higher than the voltage available in the electrical system. An increase in the operating voltage of the electric motor, however, leads in particular in the middle speed range to a significant increase in the available torque. Thus, in some speed ranges, an increase of the voltage applied to the motor by 3-4 volts can already lead to a doubling of the torque. As explained above, it is easily possible in the motor vehicle according to the invention to increase the vehicle electrical system voltage for periods of several seconds. Thus, it is also easily possible in the motor vehicle according to the invention to increase the torque of an electric motor in order to achieve a better acceleration.

It is also possible for the control device or a monitoring device to monitor the battery voltage and to change the control device into the first operating mode when the voltage falls below a predetermined or predefinable minimum voltage. There are numerous reasons for lowering the battery voltage when operating a motor vehicle. There is a, typically substantially linear, relationship between the state of charge of the battery and the battery voltage. A relatively low state of charge of the battery can therefore lead to insufficient voltage being available for some of the vehicle components and not being able to do so work as expected. This can be avoided in the motor vehicle according to the invention in that the control device changes in a drop in battery voltage in the first operating mode in which the voltage in the electrical system is additionally increased by the series connection of a capacitor and the operation of the DC / DC converter.

A drop in the battery voltage can also be caused, for example, that individual consumers must be supplied with a very high power consumption of the battery. This is the case, for example, in the operation of a starter generator or an electric motor, however, other vehicle components, such as an electric spring adjustment or the like, can lead to a short-term increase in power consumption. While a motor vehicle according to the invention may also be configured such that when certain components are automatically switched to the first operating mode, it is nevertheless advantageous to use a monitoring device for the battery voltage, since a large number of situations can be absorbed without any special effort where the battery voltage and thus the vehicle electrical system voltage drops in a motor vehicle.

Furthermore, it is possible for the motor vehicle to have a function for driving with the engine stopped and for the control device to be in the first operating mode when driving with the engine stopped or to change into the same. Motor vehicles usually have a generator which is operated with the engine running and provides the motor vehicle with electrical energy available. Typically, this generator is connected in parallel to the battery. The generator can, for example, feed a voltage into the vehicle electrical system which is greater than the battery voltage. In this case, the vehicle components are completely powered by the generator and the battery is also charged. However, it is also possible that the generator provides a voltage that is approximately equal to the battery voltage. In this case, the parallel circuit of generator and battery effectively forms a common power source having a lower internal resistance than the battery. Thus, even with large consumption in the electrical system, the output voltage of this common power supply less than in a pure battery operation.

However, when driving with the engine off, the generator may not be supportive of the battery. In addition, additional vehicle systems that are otherwise powered by the engine may need to be electrically powered, such as brake boosters or heaters. As a result, the battery is more heavily loaded when driving with the engine stopped. In addition, a typical lead-acid battery in vehicles even in the unloaded state provides only voltages of about 12 volts. Therefore, when driving with the engine stopped, it is particularly desirable to stabilize or increase the voltage in the vehicle electrical system by operating the control device in the first operating mode.

While it is advantageously possible in the motor vehicle according to the invention, as described, for the motor vehicle to be operated in the first operating mode for longer periods of time, it is also possible for the first operating mode to be used to supply the voltage in the vehicle electrical system during short and heavy loads on the battery increase or stabilize. In particular, it is possible for the control device to be in the first operating mode when starting the motor vehicle or to change over to it.

The further advantages and details of the invention will become apparent from the embodiments described below and the accompanying drawings. Showing:

1 1 schematically the electrical system of a first exemplary embodiment of a motor vehicle according to the invention,

2 a representation of the influence of the internal resistance on the output voltage of a battery,

3 schematically the electrical system of another embodiment of a motor vehicle according to the invention,

4 a down-converter,

5 a schematic representation of a third embodiment of a motor vehicle according to the invention,

6 a diagram of the operating voltage as a function of the state of charge of the battery, and

7 a diagram of the torque of an electric motor as a function of the speed and the applied voltage.

1 schematically shows the electrical system of a motor vehicle according to the invention. In this particularly simple embodiment, the electrical system has a battery 1 and a consumer 2 on that between one pole of the battery 1 and the earth potential 3 is switched. The battery 1 is over the capacitor 4 also with the earth potential 3 connected. Next to it is a DC / DC converter 5 between the poles of the battery 1 connected. The DC / DC converter 5 has four connections. With one connection, the DC / DC converter is connected to a reference potential, which is the ground potential. All other voltages at the DC / DC converter are related to this potential. In addition, at the DC / DC converter an input voltage at the input 6 applied, and from the DC / DC converter, an output voltage at the output 7 output. The ratio of the output voltage to the input voltage is determined by a pulse width ratio of the DC / DC converter. In this embodiment, the DC / DC converter is directly controlled by the controller 8th triggered generated pulse wave. The pulse wave is periodic at least over several cycles and has a high and a low voltage level. The DC / DC converter includes one or more switches that are opened and closed depending on the voltage level of the pulse wave. Thus, the DC / DC converter may be a single-down-converter with, for example, a high voltage level of the pulse wave corresponding to a switch closing and a low voltage level corresponding to opening of the switch. If a synchronous converter is used, it typically has two switches which are switched in antiphase, that is, one switch is opened each time the other is closed and vice versa.

The pulse wave that controls the DC / DC converter is typically described with a pulse width ratio that describes the amount of time that the signal is at a high voltage level. If a simple buck converter is used, the output voltage is typically directly proportional to the pulse width ratio when a high signal level corresponds to closing the switch. This can be done by controlling the pulse width ratio in the control device 8th the ratio of the voltage at the output 7 to the tension in the entrance 6 of the DC / DC converter.

For simplicity, it should first be assumed that the battery 1 has no internal resistance. For example, if the battery has an output voltage of 12 volts, it is desired that the consumer 2 a voltage of 14 volts drops, so can the operating voltage by using the capacitor 4 and the DC / DC converter 5 increased by 2 volts. To achieve this, must be on the capacitor 4 drop a voltage of 2 volts. One side of the capacitor 4 is at earth potential 3 connected. Because on the capacitor 4 a voltage of 2 volts should drop off, on the other side of the capacitor 4 a voltage of 2 volts applied. This must also be the output voltage 7 of the DC / DC converter 5 opposite the earth potential 3 Be 2 volts. Because the entrance 6 of the DC / DC converter 5 with the entrance of the consumer 2 is connected in parallel, is in the state of equilibrium at the entrance 6 of the DC / DC converter 5 the same voltage as the consumer 2 , Should the consumer 2 be operated with a voltage of 14 volts, so are in this case at the entrance 6 of the DC / DC converter 5 also 14 volts.

In the DC / DC converter, therefore, a voltage of 14 volts must be present at the input 6 to a voltage at the output 7 be converted by 2 volts. The output voltage is therefore one seventh of the input voltage. To achieve this, the DC / DC converter must be used 5 be driven with a pulse wave, which is one-seventh of the period at a high voltage level. Such a pulse wave can by the control device 8th be generated for example by a digital counter. In order to achieve the most uniform possible voltage in the electrical system, it is advantageous if the pulse wave is high-frequency. Typically, DC / DC converters are operated with pulse waves in the kilohertz range. Should the DC / DC converter 5 For example, be operated at a kilohertz, in the control device, a counter can be used with a clock of, for example, 70 kilohertz. The counter can then be programmed so that the output of the control device 8th for controlling the DC / DC converter 5 after 60 cycles is set to a high voltage level and after 10 clock cycles back to a low voltage level.

In a superficial consideration of the wiring system shown is initially unclear, such as electricity, the consumer 2 has passed through the earth potential 3 back to the battery 1 can get. This is only through the connections of the reference for a reference potential of the DC / DC converter 5 to earth potential 3 and the capacitor 4 possible. First, it should be noted that a pure DC voltage analysis of the scheme of the on-board electrical system shown is completely correct, since in the DC / DC converter high-frequency switches are opened and closed and thus initially pulse-shaped current waveforms are generated. The internal structure of a DC / DC converter 5 will be detailed later with reference to 4 described. However, without a detailed consideration, it should be noted here that the capacitor 4 is connected in use of a buck converter or a synchronous converter as a DC / DC converter in parallel with a capacitor of the DC / DC converter, which acts as part of a low-pass filter in the DC / DC converter. This causes a current flow across the capacitor 4 is possible because it is high-frequency loading and unloading. In addition, current flows between the input for the reference potential of the DC / DC converter 5 and the exit 7 of the DC / DC converter 5 , As explained in detail later, for example, in the buck converter between these poles is a diode in Passage direction and a coil arranged. Thus, it can be assumed that the DC component of the current through the DC / DC converter 5 is transported.

A significant advantage of increasing the operating voltage is that voltage drops of the battery can be compensated due to high load. 2 shows a simple circuit illustrating the voltage dip in the discharge of large currents through the battery. In this simple circuit is a consumer 2 between the two poles of a battery 1 connected. In the previous embodiment, it was assumed that the battery has no internal resistance. Here, this model is to be extended so that the battery 1 from a voltage source 9 and an internal resistance 10 is constructed. When discharging not too large currents, the internal resistance 10 be modeled as an ohmic resistance, that of the voltage source 9 connected in series.

The tension, the consumer 2 is applied, or which can be measured at the poles of the battery, corresponds to the potential difference between the voltages at the point 11 and at the point 12 , The from the voltage source 9 provided voltage lies however between the points 11 and 13 at. To the consumer 2 that is, between the points 11 and 12 To determine the declining voltage, the current supplied by the battery must first be determined. The current is given by I = U: (R ₂ + R ₁₀ ), where R _{2 is} the resistance of the consumer 2 and R _{10 is} the resistance of the internal resistance 10 is. Large current outputs of the battery 1 So correspond to a low resistance of the consumer 2 , The one by the voltage source 9 discharged electricity passes through both the consumer 2 as well as the internal resistance 10 , The consumer 2 falling voltage between the points 11 and 12 The circuit thus calculates U ₂ to R ₂ × I = R ₂ : (R ₁ + R ₂ ) × U. It is easy to see that, for example, if the resistance of the consumer 2 is about the same size as the internal resistance 10 the battery 1 , between the points 11 and 12 only half the voltage of the voltage source 9 is available. So as soon as the resistance of the parallel connected consumers in the electrical system is so small that it is comparable to the internal resistance of the battery, which falls from the battery 1 emitted voltage noticeably.

This will be explained by way of example in the operation of an electric motor to support the driving performance. Initially, the battery should be considered as an ideal voltage source with an output voltage of 12 volts and no internal resistance. In addition, it should be assumed that the motor has a resistance of 50 mΩ. In this case, with an ideal voltage source, 240 amps would flow through the motor and 12 volts would still be available in the electrical system.

If the battery now has an internal resistance of 5 milliohms, using the same motor in the same speed range would reduce the voltage in the electrical system to 10.9 volts. With such a low voltage in the electrical system, the operation of all systems in the motor vehicle can no longer necessarily be ensured. As can be seen in this example, support for the battery by means of stabilization measures is required, in particular when operating low-impedance loads on the vehicle electrical system.

3 shows the electrical system of another embodiment of a motor vehicle with a circuit 17 to stabilize or increase the operating voltage of the electrical system. The circuit 17 For example, it can be designed as a compact component with three connections, in each case one connection for connection to the two poles of the battery and one earth connection. Obviously, the size of the components shown does not correspond to the actual size of the components. About the electrical system of the motor vehicle is next to the consumer 2 and possibly other consumers, which are indicated on the right by the dotted line, a starter generator 14 operated. The starter generator 14 Can also recharge the battery 1 be used. The arrows 18 indicate the current direction in the event that the starter generator 14 is operated as a motor. Thus, for example, a boost function for faster startup of a motor vehicle can be realized.

The circuit 17 for voltage stabilization or increase comprises a capacitor 4 , a DC / DC converter 5 , a control device 8th as well as two switches 15 . 16 , In the case of the starter generator 14 should be operated as a motor, should by the circuit 17 an increase in the starter generator 14 and at the consumer 2 decreasing voltage compared to that by the battery 1 provided voltage can be achieved. Thus, by the control device 8th as shown the switch 15 closed and the switch 16 open.

The resulting circuit is essentially the same as in 1 shown circuit. So here too is between the pole of the battery 1 and the earth potential 3 the capacitor 4 and it also becomes a DC / DC converter 5 between the poles of the battery 1 arranged.

The basic function of this circuit has already been related to 1 explained. Unlike in 1 shown circuit, the operating voltage is here, however, increased to a predetermined value and stabilized there. Therefore, here is a slightly more complex control device 8th used. The control device 8th here includes an analog-to-digital converter, which measures the voltage between the inputs 18 and 19 drops, converts to a digital signal. connection 18 the control device is connected to a first pole of the battery 1 connected. This corresponds to the connection 18 adjacent potential of the operating voltage of the electrical system of the motor vehicle. connection 19 is at earth potential 3 connected. By using these two terminals, the operating voltage is measured differentially against the ground potential. The control device 8th It also includes a digitally-implemented proportional-integral controller that compares the digitized input voltage to a setpoint, and calculates proportional and integral correction values. The added proportional and integral components are then converted back to an analog voltage and output 20 delivered as a reference voltage to the DC / DC converter. The DC / DC converter is designed as a voltage-controlled DC / DC converter, that is to say that the output voltage matches that at the output 20 the control device 8th is compared output voltage and is controlled by a DC / DC converter integrated control circuit, the output voltage to this voltage.

With the control device shown, it is thus possible over a wider range of values to adapt and stabilize the operating voltage over a wide voltage range by means of a digital control.

Will the starter generator 14 however, operated as a generator, it is often not desired, the circuit 17 to operate for voltage stabilization. In this case, the generator provides sufficient voltage for the electrical system and the efficiency of the motor vehicle can be increased when the DC / DC converter is not operated. In addition, it may be detrimental when charging the battery, when a capacitor is connected in series with this, since the capacitor usually has a much smaller capacity than the battery and thus a bidirectional DC / DC converter would be necessary to overcharge the capacitor to avoid. Therefore, it is far easier in this case, the switch 15 . 16 by the control device 8th switch. To charge the battery while the generator 14 is operated, so will switch 16 closed and switch 15 open.

For a better understanding of in 1 and 3 shown circuits is in 4 an example of a simple buck converter shown. A down-converter is used to switch between input 7 and earth potential 3 lying input voltage to a lower output voltage at the output 6 is spent, to convert. This will be switch 26 periodically opened and closed, whereby a rectangular-shaped voltage waveform on the other components of the DC / DC converter 5 is applied. The interruption is caused by a square wave 22 controlled, which is generated for example by an external control device or by a control loop. The simplest model for the other components of the DC / DC converter can be assumed to be the coil 24 and the capacitor 25 form a low-pass filter, which smooth the output signal and the diode 23 the correct output polarity of the voltage at the output 6 ensures. In 4 It can also be seen that there is a current flow from the earth potential 3 to the exit 6 for DC voltage of the correct polarity is easily possible. This is in the in 1 and 3 shown circuit, a current transport from the battery 1 by the consumer 2 and back to the battery 1 to enable.

5 shows a motor vehicle 27 with hybrid drive with an internal combustion engine 28 and a starter generator 14 which can also be operated during operation as a supportive electric motor. The car 27 has a 12 volt lead-acid battery as a battery 1 on. During operation of the starter generator 14 As an electric motor, large currents are required, whereby the supply of other consumers 2 is endangered with sufficient tension. It can also be in the vehicle 27 be desired when accelerating higher voltages for the operation of the starter generator 14 to provide. Therefore, in addition, the circuit 17 used to the operating voltage in the electrical system of the motor vehicle 27 keep constant or increase if necessary. The circuit 17 can be constructed, for example, as in 3 shown.

6 and 7 show on diagrams that show the operating voltage in the electrical system, or the torque of a driven electric motor, why it is advantageous to increase the vehicle electrical system voltage or stabilize.

6 shows the dependence of the operating voltage on the state of charge of the battery. There are four curves shown. The curve 29 shows the voltage dropping across an unloaded lead-acid battery, the curve 30 the voltage dropping across a loaded lead-acid battery, the curve 31 shows the operating voltage when the voltage dropping at the loaded lead-acid battery voltage is increased by an auxiliary memory, which is always assumed here by an equally loaded auxiliary memory and the curve 32 shows the operating voltage when in addition a variable DC / DC converter is used to stabilize the operating voltage in the electrical system of the motor vehicle.

The curve 29 shows that a typical lead-acid battery provides between about 11.5 and about 13 volts output voltage depending on the state of charge. Bordnetze modern motor vehicles are often designed for an optimal voltage range of 13 to 15 volts in the electrical system. This voltage range is achieved by operating a generator with the engine running. However, if the motor vehicle now has, for example, an operating mode for driving with the engine off, this additional voltage can not be made available by the generator. Thus, the voltage typically provided by a lead-acid battery is already below the desired voltage.

The voltage of a lead-acid battery could theoretically be increased by the addition of further cells, but this has the disadvantage that the internal resistance of the lead-acid battery would be increased and the weight increased. Under certain circumstances, the full capacity of the lead-acid battery could not be used to avoid too high voltages.

If the electrical system is heavily loaded, the operating voltage in the electrical system, as in curve 30 shown, further lowered. In particular, when operating electric motors for driving assistance, it may be necessary to switch off certain functions, which reduces the ride comfort.

Such a lowering of the voltage and the absence of a generator to increase the vehicle electrical system voltage can be compensated in the simplest case by switching an auxiliary storage in series with the battery. In 6 For example, a constant increase in voltage by about 2 volts is shown. Such an increase of the voltage with an auxiliary memory has two significant disadvantages. On the one hand, such a memory usually has much lower capacity than a battery, whereby an increase in the operating voltage is only possible for short time intervals, typically in the range of a few milliseconds. On the other hand, the operating voltage of the electrical system of the motor vehicle is still heavily dependent on the state of charge of the battery and the load on the battery. Therefore, all vehicle components must be designed so that they can easily operate over a wide voltage range. This may, however, be associated with considerable technical overhead.

As at curve 32 To recognize the dependence of the operating voltage from the state of charge of the battery and the output currents can be drastically reduced by the additional use of a DC / DC converter with variable output voltage. This can be achieved with relatively little technical effort a significant improvement in the stability of the operating voltage in the electrical system of a motor vehicle.

In particular, in hybrid vehicles, it is often desired to use an electric motor or starter generator to assist driving performance, particularly at low speeds and speeds.

7 shows the relationship of the speed with the torque for an exemplary electric motor. The middle curve 34 shows the torque at an operating voltage of 12 volts at the motor. This curve would be achieved with a motor on a typical lead-acid battery if it behaved like an ideal voltage source. As already explained, however, in the case of the large currents which are necessary for the operation of an electric motor for driving assistance, the battery voltage is in some cases drastically reduced.

A consequence of this lowering of the battery voltage is an example with a curve 33 showing the torque of the motor at an operating voltage of 10 volts. In particular, in the relatively low speed range of about 1000 revolutions per minute, this lowering of the operating voltage leads to a loss of torque. Thus, at 10 volts, in some cases only about 60% of the torque is available, which is available at an operating voltage of 12 volts. Even a stabilization of the operating voltage to 12 volts can thus lead to a significant increase in driving performance when using an electric motor.

In addition shows curve 35 in that with an increase in the operating voltage to 14 volts, the driving performance can be significantly improved again. It can therefore be clearly seen that a significant improvement in driving performance can be achieved, in particular when operating motor vehicles with a hybrid drive by stabilizing or increasing the operating voltage.

Claims (13)

Motor vehicle comprising a battery ( 1 ) having a first pole and a second pole, a capacitor ( 4 ), at least one consumer ( 2 ), a DC / DC converter ( 5 ) and a control device ( 8th ), whereby the at least one consumer ( 2 ) between the first pole of the battery ( 1 ) and a ground potential ( 3 ) is connected, bringing on the Consumer ( 2 ) drops an operating voltage, and wherein a reference potential of an input ( 6 ) and one output ( 7 ) of the DC / DC converter ( 5 ) the earth potential ( 3 ), wherein in at least a first operating mode of the control device ( 8th ) the capacitor ( 4 ) is connected in series with the battery, wherein the capacitor ( 4 ) with a first connection to the second pole of the battery ( 1 ) and with a second connection to the ground potential ( 3 ) and the DC / DC converter ( 5 ) with the entrance ( 6 ) with the first pole of the battery ( 1 ) and with the output ( 7 ) with the second pole of the battery ( 1 ), characterized in that the control device ( 8th ) the DC / DC converter ( 5 ) in the first operating mode such that current between the input ( 6 ) and the output ( 7 ) of the DC / DC converter ( 5 ) flows. Motor vehicle according to claim 1, characterized in that the control device ( 8th ) is designed to control a switching device, the second pole of the battery ( 1 ) in the first operating mode with the DC / DC converter ( 5 ) and the first terminal of the capacitor ( 4 ) and in at least a second operating mode with the ground potential ( 3 ) connects. Motor vehicle according to claim 2, characterized in that the control device ( 8th ) is designed for operation in the first operating mode for an operating time of at least 1 s, in particular at least 5 s, in particular at least 30 s. Motor vehicle according to claim 3, characterized in that the control device ( 8th ) for controlling the DC / DC converter ( 5 ) is formed such that on the capacitor ( 4 ) falling voltage, averaged over a switching cycle of the DC / DC converter ( 5 ), over the operating time is substantially constant. Motor vehicle according to one of claims 1 to 3, characterized in that the control device ( 8th ) for controlling the DC / DC converter ( 5 ) is configured such that in the first operating mode, the operating voltage averaged over a switching cycle of the DC / DC converter ( 5 ) essentially independent of the consumer ( 2 ) flowing load current is constant, as long as the load current does not exceed a predetermined or specifiable current limit. Motor vehicle according to claim 4 or 5, characterized in that the control device ( 8th ) a controller for controlling the capacitor ( 4 ) falling voltage or the operating voltage by driving the DC / DC converter ( 5 ), which comprises a proportional and / or an integral and / or a differential element. Motor vehicle according to one of the preceding claims, characterized in that the DC / DC converter ( 5 ) is a buck converter or a synchronous converter. Motor vehicle according to one of the preceding claims, characterized in that the battery ( 1 ) is a lead-acid battery. Motor vehicle according to one of the preceding claims, characterized in that the operating voltage in the first operating mode is higher than the battery voltage with unloaded battery ( 1 ). Motor vehicle according to one of the preceding claims, characterized in that the motor vehicle ( 27 ) is a hybrid or electric vehicle, the consumer ( 2 ) or one of the consumers ( 2 ) an electric motor or a starter generator ( 14 ), wherein the control device during acceleration or starting in the first operating mode is or changes to, wherein the operating voltage by series connection of capacitor ( 4 ) and battery ( 1 ), whereby the maximum torque of the electric motor or the starter generator ( 14 ) is increased. Motor vehicle according to one of the preceding claims, characterized in that the control device ( 8th ) or a monitoring device monitors the battery voltage and, if the voltage falls below a predetermined or predefinable minimum voltage, the control device ( 8th ) changes to the first operating mode. Motor vehicle according to one of the preceding claims, characterized in that the motor vehicle ( 27 ) a function for driving with the engine stopped ( 28 ) and the control device ( 8th ) when driving with the engine stopped ( 28 ) is in the first operating mode or changes into this. Motor vehicle according to one of the preceding claims, characterized in that the control device ( 8th ) when starting the motor vehicle ( 27 ) is in the first operating mode or changes into this.

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