Abstract:
A method is disclosed for adjusting a voltage of a DC-voltage intermediate circuit in a battery system having a battery and a drive system. The battery is configured to output one selectable output voltage from n+1 different output voltages. In a first step of the method, an actual value of the voltage of the DC-voltage intermediate circuit is determined, and is then compared with the various output voltages of the battery. A first selected output voltage of the battery, which is the highest voltage of those output voltages of the battery which are less than the actual value of the voltage of the DC-voltage intermediate circuit, and a second selected output voltage of the battery, which is the lowest voltage of those output voltages of the battery which are higher than the actual value of the voltage of the DC-voltage intermediate circuit, are then selected.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/063732, filed on Aug. 10, 2011, which claims the benefit of priority to Serial No. DE 10 2010 041 046.2 filed on Sept. 20, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The present disclosure relates to a method for adjusting a DC voltage intermediate circuit voltage and to a battery and a battery system having a DC voltage intermediate circuit that are embodied to perform the method. 
     BACKGROUND 
     It has become apparent that in future, both in the case of stationary applications and also in the case of vehicles such as hybrid vehicles and electric vehicles, battery systems will be used ever more frequently. In order to be able to meet particular requirements for a respective application voltage and the power that can be made available, a high number of battery cells are connected in series. Since it is necessary for the current that is provided by a battery of this type to flow through all the battery cells and a battery cell can only carry a limited amount of current, battery cells are in addition often connected in parallel in order to increase the maximum current. This can be achieved either by providing a plurality of battery cells within a battery cell housing or by connecting battery cells externally. 
       FIG. 1  illustrates the principal circuit diagram of a conventional electric drive system, such as is used for example in electric vehicles and hybrid vehicles or also in stationary applications such as when adjusting rotor blades of wind turbines. A battery  110  is connected to a DC voltage intermediate circuit and said DC voltage intermediate circuit is embodied by a capacitor  111 . A pulse-controlled inverter  112  is connected to the DC voltage intermediate circuit and sinusoidal voltages that are phase-offset with respect to each other for operating an electric drive motor  113  are supplied by said pulse-controlled inverter  112  to three outputs by way of in each case two switchable semi-conductor gates and two diodes. The capacity of the capacitor  111  that forms the DC voltage intermediate circuit must be sufficiently large in order to stabilize the voltage in the DC voltage intermediate circuit for a period of time in which one of the switchable semi-conductor gates is switched to conduct. In a practical application, such as an electric vehicle, a high capacity in the mF range is achieved. 
       FIG. 2  illustrates the battery  110  of  FIG. 1  in a detailed block diagram. A plurality of battery cells is connected in series and optionally in addition in parallel in order to achieve a battery capacity and a high output voltage required for a respective application. A charging and disconnecting device  116  is connected between the positive pole of the battery cells and a positive battery terminal  114 . Optionally, a disconnecting device  117  can in addition be connected between the negative pole of the battery cells and a negative battery terminal  115 . The disconnecting and charging device  116  and the disconnecting device  117  comprise in each case a switch  118  or  119  respectively, which switches are provided for disconnecting the battery cells from the battery terminals in order to disconnect the battery terminals from the voltage supply. Otherwise, as a result of the high DC voltage of the battery cells that are connected in series, there is a considerable potential risk for maintenance personnel or the like. A charging switch  120  having a charging resistor  121  that is connected in series to the charging switch  120  is in addition provided in the charging and disconnecting device  116 . The charging resistor  121  limits a charging current for the capacitor  111  if the battery is connected to the DC voltage intermediate circuit. For this purpose, the switch  118  is initially left open and only the charging switch  120  is closed. If the voltage at the positive battery terminal  114  achieves the voltage of the battery cells, the switch  119  can be closed and if necessary the charging switch  120  can be opened. 
     The charging switch  120  and the charging resistor  121  represent a significant amount of additional expenditure in applications in which the output is in the range of a few 10 kW and said additional expenditure is only required for the process that lasts a few hundred milliseconds for charging the DC voltage intermediate circuit. Said components are not only expensive but they are also large and heavy which is particularly troublesome when used in mobile applications such as electric motor vehicles. 
     SUMMARY 
     It is therefore proposed in accordance with the disclosure to provide a method for adjusting a voltage in a DC voltage intermediate circuit of a battery system having a battery and a drive system. The battery is connected to the drive system by way of the DC voltage intermediate circuit and comprises a number of n series-connected battery modules, each of which battery modules comprises a coupling unit and at least one battery cell that is connected between a first input and a second input of the coupling unit. The battery modules are embodied to provide either a battery module voltage or a voltage of zero in dependence upon a switching state of the coupling unit. The number n is greater than 1. The battery is embodied to provide an output voltage that can be selected from n+1 different output voltages. The method comprises at least the following steps:
         a) Determine an instantaneous value of the voltage of the DC voltage intermediate circuit;   b) Compare the instantaneous value of the voltage of the DC voltage intermediate circuit with the different output voltages of the battery;   c) Select a first selected output voltage of the battery that is the highest voltage of those output voltages of the battery that are lower than the instantaneous value of the voltage of the DC voltage intermediate circuit;   d) Select a second selected output voltage of the battery that is the lowest voltage of those output voltages of the battery that are higher than the instantaneous value of the voltage of the DC voltage intermediate circuit;   e) During a first variable period of time provide the first selected output voltage of the battery;   f) During a second variable period of time provide the second selected output voltage of the battery; and   g) Repeat the steps a) to f) until the voltage of the DC voltage intermediate circuit achieves a desired operating voltage.       

     The method of the disclosure provides the advantage that the output voltage of the battery is switched rapidly and in a controlled manner between the first and the second selected output voltage, as a consequence of which a time-averaged adjustable charging current for the DC voltage intermediate circuit is provided. Since the charging current is adjusted to a desired value by selecting suitable first and second variable periods of time and consequently said charging current can also be limited, the charging switch  120  and the charging resistor  121  of the battery systems of the prior art can be omitted as a result of which costs, volume and weight of a battery system that functions according to the method in accordance with the disclosure can be correspondingly reduced. As the battery is embodied to provide different output voltages by activating or deactivating the individual series-connected battery modules, it is possible in order to adjust the voltage of the DC voltage intermediate circuit to select the two output voltages of the battery that are the closest in value to the instantaneous value of the voltage and to switch between said two output voltages with the appropriate first and second variable periods of time in order to influence the voltage of the DC voltage intermediate circuit as desired. As a result of selecting the two closest output voltages of the battery, the ripple content of the charging current is reduced to a minimum which owing to the inevitably present inductances and capacities can only occur at a limited rate of change of the output voltage of the battery. The switching speed of the coupling units of the battery modules can also be reduced accordingly. 
     The method of the disclosure has in addition the advantage that the DC voltage intermediate circuit can be charged in a shorter period of time. In a battery system having the battery that is illustrated in  FIG. 2  and that comprises a charging and disconnecting device  116 , the DC voltage intermediate circuit is charged with a characteristic that corresponds to an exponential function with negative exponents until the switch  118  closes. This means that the maximum charging current flows at the commencement of the charging process so that the voltage of the DC voltage intermediate circuit approaches the value of the output voltage of the battery in an asymptotic manner; said maximum charging current does, however, continue to reduce as the process of charging the DC voltage intermediate circuit proceeds. However, the voltage of the DC voltage intermediate circuit can increase in a linear manner in accordance with the method of the disclosure and thus the capacity of the DC voltage intermediate circuit can be charged during the entire charging period of time to an averaged constant current that is at least a similar value to that of the initial charging current in a battery system having a charging resistor  121 . As a consequence, the first desired operating voltage is achieved correspondingly more rapidly. 
     It is preferred that the desired operating voltage is equal to a maximum of the n+1 different output voltages of the battery. The method is performed in this case until the DC voltage intermediate circuit has achieved the maximum possible voltage. The control system is subsequently deactivated so that the voltage of the DC voltage intermediate circuit is directly coupled to the output voltage of the battery. 
     The step of determining the instantaneous value of the voltage of the DC voltage intermediate circuit preferably comprises a step of measuring the voltage of the DC voltage intermediate circuit. As a consequence, it is possible to implement not only control methods but also closed loop control methods in which the closed loop control is performed in dependence upon measured values of the target measured variable, in other words the voltage of the DC voltage intermediate circuit. Alternatively, the instantaneous value of the voltage of the DC voltage intermediate circuit can be calculated by way of a state variable. 
     It is particularly preferred that the first variable period of time and the second variable period of time are determined in dependence upon a difference between the desired operating voltage and the instantaneous value of the voltage of the DC voltage intermediate circuit. The (average) current that is set during the second variable period of time is in addition to the ratio of the first variable period of time with respect to the second variable period of time also dependent upon the difference between the instantaneous value of the voltage of the DC voltage intermediate circuit and the desired operating voltage (usually equal to the maximum output voltage of the battery). In order, for example, to set a charging current for a given pair of a first and a second selected output voltage of the battery, which charging current is on average constant during the process of charging the DC voltage intermediate circuit, the first variable period of time is reduced in comparison to the second variable period of time the smaller the difference. Alternatively or additionally, the second variable period of time can also be extended in comparison to the first variable period of time. 
     The method can comprise an additional step of measuring a prevailing charging current. As a consequence thereof, a closed loop control method that is being used can also take into consideration the prevailing charging current or it can implement safety mechanisms to provide protection from inadmissibly high charging currents. 
     It is particularly preferred that the method therefore also comprises an additional step of comparing the measured prevailing charging current with a maximum permissible charging current, wherein the steps e) and/or f) is terminated if the prevailing charging current is greater than the maximum permissible charging current. 
     Following on from the two latter mentioned variants, the method can also include an additional step of determining an average charging current and of comparing the average charging current with a desired charging current, wherein the first variable period of time is extended and/or the second variable period of time is reduced if the average charging current is greater than the desired charging current, and/or wherein the first variable period of time is reduced and/or the second variable period of time is increased if the average charging current is lower than the desired charging current. 
     It is particularly preferred that a desired charging current is maintained at a constant value until the voltage of the DC voltage intermediate circuit achieves the desired operating voltage. In this manner, the voltage of the DC voltage intermediate circuit increases in a linear manner and the DC voltage circuit is charged in a shortest possible time without exceeding a maximum permissible charging current. 
     A second aspect of the disclosure provides a battery having a control unit and a number of n series-connected battery modules. Each battery module comprises a coupling unit and at least one battery cell that is connected between a first input and a second input of the coupling unit. The battery modules are embodied to provide in dependence upon a switching state of the coupling unit either a battery module voltage or a voltage of zero. The number n of the series-connected battery modules is greater than 1 so that the battery is embodied to provide a selectable output voltage of n+1 different output voltages. The control unit is embodied in accordance with the disclosure for the purpose of performing the method of the first disclosed aspect. 
     It is particularly preferred in this case that the battery cells of the battery modules are lithium-ion battery cells. Lithium-ion battery cells have the advantages of a high cell voltage and high energy content in a given volume. 
     A further disclosed aspect relates to a battery system having a battery, a DC voltage intermediate circuit that is connected to the battery, and a drive system that is connected to the DC voltage intermediate circuit. In this case, the battery is embodied in accordance with the aforementioned aspect of the disclosure. 
     It is particularly preferred that the DC voltage intermediate circuit is in this case connected directly to the battery, in other words no further components are connected between the battery and the DC voltage intermediate circuit, in particular no charging device and accordingly no charging switch and no charging resistor. In the case of embodiments of the battery system, however, it is also possible to connect further components such as current sensors between the battery and the DC voltage intermediate circuit. 
     The DC voltage intermediate circuit can comprise a capacitor or can be embodied as a capacitor. 
     The battery system can, for example, be implemented in a motor vehicle, wherein the drive system comprises an electric drive motor for driving the motor vehicle and a pulse-controlled inverter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure are explained detail with reference to the drawings and the description hereinunder, wherein like reference numbers describe like components or components that function in a like manner. In the drawings: 
         FIG. 1  shows an electric drive system in accordance with the prior art, 
         FIG. 2  shows a block diagram of a battery in accordance with the prior art, 
         FIG. 3  shows a first embodiment of a coupling unit for use in a battery, with which the method in accordance with the disclosure can be performed, 
         FIG. 4  shows a possible implementation of the first embodiment of the coupling unit with regard to the switching technology, 
         FIGS. 5 and 6  show two embodiments of a battery module having the first embodiment of the coupling unit, 
         FIG. 7  shows a second embodiment of a coupling unit for use in a battery with which the method in accordance with the disclosure can be performed. 
         FIG. 8  shows a possible implementation of the second embodiment of the coupling unit with regard to the switching technology, 
         FIG. 9  shows an embodiment of a battery module having the second embodiment of the coupling unit, 
         FIG. 10  shows a battery with which the method in accordance with the disclosure can be performed, and 
         FIG. 11  shows a block diagram of an exemplary closed loop control system in accordance with the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  illustrates a first embodiment of a coupling unit  30  for use in a battery with which the method in accordance with the disclosure can be performed. The coupling unit  30  comprises two inputs  31  and  32  and also an output  33  and is embodied to connect one of the inputs  31  or  32  to the output  33  and to disconnect the other one. 
       FIG. 4  illustrates a possible implementation of the first embodiment of the coupling unit  30  with regard to the switching technology, wherein a first and a second switch  35  and  36  respectively are provided. Each of the switches is connected between one of the inputs  31  and  32  respectively and the output  33 . This embodiment provides the advantage that it is also possible to disconnect the two inputs  31 ,  32  from the output  33 , so that the output  33  is a high impedance output, which can be useful, for example, in the case of making a repair or carrying out maintenance. In addition, the switches  35 ,  36  can be embodied simply as semi-conductor switches such as MOSFETs or IGBTs. Semi-conductor switches have the advantage of being favorably priced and providing a high switching speed, so that the coupling unit  30  can react within a comparatively short period of time to a control signal and accordingly to a change of control signal. 
       FIGS. 5 and 6  illustrate two embodiments of a battery module  40  having the first embodiment of the coupling unit  30 . A plurality of battery cells  11  is connected in series between the inputs of the coupling unit  30 . However, the disclosure is not limited to battery cells  11  being connected in series in this manner, it can also provide only one individual battery cell  11  or else a parallel connection or a combination of a series and parallel connection of battery cells  11 . In the example illustrated in  FIG. 5 , the output of the coupling unit  30  is connected to a first terminal  41  and the negative pole of the battery cells  11  is connected to a second terminal  42 . However, an almost mirror-inverted arrangement as illustrated in  FIG. 6  is possible, wherein the positive pole of the battery cells  11  is connected to the first terminal  41  and the output of the coupling unit  30  is connected to the second terminal  42 . 
       FIG. 7  illustrates a second embodiment of a coupling unit  50  for use in a battery with which the method in accordance with the disclosure can be performed. The coupling unit  50  comprises two inputs  51  and  52  and also two outputs  53  and  54 . Said coupling unit is embodied to connect either the first input  51  to the first output  53  and also to connect the second input  52  to the second output  54  (and to disconnect the first output  53  from the second output  54 ) or else to connect the first output  53  to the second output  54  (and in so doing to disconnect the inputs  51  and  52 ). In the case of particular embodiments of the coupling unit, said coupling unit can also be embodied to disconnect the two inputs  51 ,  52  from the outputs  53 ,  54  and also to disconnect the first output  53  from the second output  54 . However, it is not provided that it can also connect the first input  51  to the second input  52 . 
       FIG. 8  illustrates a possible implementation of the second embodiment of the coupling unit  50  with regard to the switching technology, wherein a first, a second and a third switch  55 ,  56  and  57  are provided. The first switch  55  is connected between the first input  51  and the first output  53 ; the second switch  56  is connected between the second input  52  and the second output  54  and the third switch  57  is connected between the first output  53  and the second output  54 . This embodiment likewise provides the advantage that the switches  55 ,  56  and  57  can be embodied simply as semi-conductor switches such as MOSFETs or IGBTs. Semi-conductor switches have the advantage of being favorably priced and providing a high switching speed, so that the coupling unit  50  can react within a comparatively short period of time to a control signal and accordingly to a change of control signal. 
       FIG. 9  illustrates an embodiment of a battery module  60  having the second embodiment of the coupling unit  50 . A plurality of battery cells  11  is connected in series between the inputs of a coupling unit  50 . Also this embodiment of the battery module  60  is not limited to battery cells  11  being connected in series in this manner, it can in turn also provide only an individual battery cell  11  or else a parallel connection or a combination of a series and parallel connection of battery cells  11 . The first output of the coupling unit  50  is connected to a first terminal  61  and the second output of the coupling unit  40  is connected to a second terminal  62 . In comparison to the battery module  40  illustrated in  FIGS. 5 and 6 , the battery module  60  provides the advantage that the battery cells  11  can be disconnected on both sides from the remaining battery by means of the coupling unit  50 , which renders it possible to replace said battery without risk during operation, since the dangerously high total voltage of the remaining battery modules of the battery is not available at any pole of the battery cells  11 . 
       FIG. 10  illustrates an embodiment of a battery with which the method in accordance with the disclosure can be performed. The battery comprises a battery module string  70  having a plurality of battery modules  40  or  60 , wherein preferably each battery module  40  or  60  comprises the same number of battery cells  11  that are connected in an identical manner. In general, the battery module string  70  can contain any number of battery modules  40  or  60  greater than 1. Also, at the poles of the battery module string  70 , charging and disconnecting devices and disconnecting devices can in addition be provided as illustrated in  FIG. 2  if this is required by safety regulations. However, disconnecting devices of this type are not required in accordance with the disclosure because the battery cells  11  can be decoupled from the battery terminals by means of the coupling units  30  or  50  that are provided in the battery modules  40  or  60 . 
       FIG. 11  illustrates a block diagram of an exemplary closed loop control system in accordance with the disclosure. The desired value of the voltage of the DC voltage intermediate circuit is compared in a superimposed closed loop control circuit with the instantaneous value of the voltage of the DC voltage intermediate circuit. If the DC voltage intermediate circuit is to be fully charged, in other words, the voltage of the DC voltage intermediate circuit is to be increased, the desired value is set directly to the value of the total voltage of all the series-connected battery modules. Naturally, the desired value can also be set to any other voltage value but in practical applications of the DC voltage intermediate circuit it is usually charged to the maximum value of the output voltage of the battery. The two-stage control of the superimposed closed loop control circuit then uses the desired value of the charging current as a desired value for an underlaying current closed loop control circuit that is implemented as a multipoint controller so that the DC voltage intermediate circuit is charged, for example, with the maximum permissible charging current or a charging current that is below the maximum permissible charging current by a safety margin. The underlaying multipoint closed loop control circuit then adjusts the desired value of the charging current using the battery that functions as a multipoint control element by transmitting corresponding control signals to the coupling units of the battery modules. 
     In detail, a desired operating voltage for the DC voltage intermediate circuit is stipulated on the input side at the point  80  and said desired operating voltage is compared at the point  81  by means of a subtraction element with an instantaneous operating voltage of the DC voltage intermediate circuit and a voltage difference is generated at the point  82 . The voltage difference is subjected to a quantizing operation in a closed loop control element  83  and said quantizing operation implements the desired two-stage closed loop control by converting the voltage difference at the point  82  into a desired charging current at the point  84 , which desired charging current can only assume two different values. Optionally, the closed loop control element  83  can also perform a hysteresis function that advantageously reduces the switching frequency of the closed loop control system. 
     An instantaneous charging current at the point  85  is subtracted in a subsequent subtraction element from the desired charging current at the point  84 . In addition, the instantaneous value of the voltage of the DC voltage intermediate circuit is converted in a closed loop control element  88  into a maximum output voltage of the battery that is, however, lower than the instantaneous value of the voltage of the DC voltage intermediate circuit and a value that is derived therefrom is added in the subtraction element, so that an adjusting variable for the current is available at the point  88  and said adjusting variable is converted in a subsequent closed loop control element  89  at the point  90  into a digitized current value for the selection of an output voltage of the battery. 
     The remaining blocks model the behavior of the DC voltage intermediate circuit. The voltage of the DC voltage intermediate circuit at the point  81  is converted by way of a proportional element  92  with a scalar factor K R  into a current value at the point  91 , which current value is subtracted in a further subtraction element from the digitized current value at the point  90  and thus delivers the instantaneous current value at the point  85 . The prevailing instantaneous current value can also be determined by means of performing a direct measurement and forming an average over an appropriate period of time and said prevailing instantaneous current value can flow into the closed loop control system at the point  85 . The closed loop control element  93  describes the integration characteristic of a capacity, such as is represented at least approximately by the DC voltage intermediate circuit, and converts the current flowing in the DC voltage intermediate circuit into the voltage of the DC voltage intermediate circuit. It also applies here that, in practice, the prevailing voltage of the DC voltage intermediate circuit is usually not calculated but rather is determined by means of performing a measurement. 
     Alternatively, the closed loop control system can also be implemented as a multipoint control using hysteresis or with minimum dwell duration in the switching states in order to limit the switching frequency of the adjusting element. The change in switching state is preferably performed in a time-discrete manner, i.e. synchronous with a pulse of, for example, 100 kHz, which would result in a maximum switching frequency of 50 kHz. 
     The disclosure relates to the idea that a battery having a coupling unit for adjusting the output voltage of the battery can be used directly as a multipoint adjusting element for the process of charging the DC voltage intermediate circuit. If, for example, the prevailing charging current is lower than a desired value of the charging current, the greater of the two selected output voltages of the battery is set. If, on the other hand, the prevailing charging current is greater than its desired value, then the lower of the two selected output voltages of the battery is set. 
     The method in accordance with the disclosure can be achieved using software functions within the scope of the control of the battery without any special additional expenditure. In this case, the different known multipoint methods with their respective advantages and disadvantages are available for integrating the battery as a multipoint adjusting element in a closed loop control circuit. Fundamentally, these methods differ with respect to the maximum occurring switching frequency and with respect to the ripple components that the charging current comprises during the charging process. The closed loop control circuit illustrated in  FIG. 11  is only an example for a possible multipoint method. 
     The disclosure renders it possible to adjust the voltage of a DC voltage intermediate circuit in a controlled manner without using a charging device. As a consequence, the charging device that is provided as standard in a practical application can be omitted, as a result of which costs are saved and the volume and weight of the entire arrangement are reduced.