Abstract:
The disclosure presents a method for starting up a battery system having a battery, a DC voltage intermediate circuit which is connected to the battery, and a drive system which is connected to the DC voltage intermediate circuit. The battery has a large number of battery modules which are connected in series and which each comprise a coupling unit and at least one battery cell which is connected between a first input and a second input of the coupling unit. The method comprises a step for decoupling the battery cells of all of the battery modules which are connected in series by outputting a corresponding control signal to the coupling units of the battery modules which are connected in series. All of the battery modules which are connected in series are then bridged at the output end, and therefore an output voltage of the battery is zero.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/063967, filed on Aug. 12, 2011, which claims the benefit of priority to Serial No. DE 10 2010 041 009.8, filed on Sep. 20, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     The present disclosure relates to a method for starting up a battery system having a DC voltage intermediate circuit and a battery and a battery system having a DC voltage intermediate circuit that are embodied to perform the method for starting up said battery system. 
     BACKGROUND 
     It has become apparent that in future, both in the case of stationary applications and 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 voltage for a respective application and to be able to provide 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 more 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 starting up 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. The battery comprises a plurality of series-connected battery modules that are connected in series and in each case comprise 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 method comprises at least the following steps:
         a) Disconnect the battery cells of all the series-connected battery modules by transmitting a corresponding control signal to the coupling units of the series-connected battery modules;   b) Provide a bridge on the output side of all the series-connected battery modules so that an output voltage of the battery is zero;   c) Connect the battery cells of one of the series-connected battery modules and terminate the bridge on the output side of one of the series-connected battery modules by terminating the transmission of the corresponding control signal to the coupling unit of one of the series-connected battery modules;   d) Repeat the step c) for in each case a further one of the series-connected battery modules at least until a voltage of the DC voltage intermediate circuit achieves a first desired operating voltage.       

     The method of the disclosure provides the advantage that the output voltage of the battery and consequently also the voltage of the DC voltage intermediate circuit is increased incrementally, so that, owing to the relatively small voltage difference in each increasing step between the output voltage of the battery and the voltage of the DC voltage intermediate circuit, the charging currents that flow in the capacitor of the DC voltage intermediate circuit in order to adjust the voltage of the DC voltage intermediate circuit to the output voltage of the battery are also comparatively small. In this manner, the charging switch  120  and the charging resistor  121  of the battery systems of the prior art are redundant and the costs, volume and weight of a battery system that functions according to the method in accordance with the disclosure are correspondingly reduced. 
     The method of the disclosure has in addition the advantage that the DC voltage intermediate circuit is 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 magnitude 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 is continuously increased incrementally in accordance with the method of the disclosure, so that said voltage demonstrates a stepped progression that is approximately linear in the middle. The increase of the averaged voltage of the DC voltage intermediate circuit corresponds to the average charging current that is at least approximately constant over the entire charging process, as a consequence of which the first desired operating voltage is achieved correspondingly more rapidly. 
     It is preferred that the method comprises an additional step e) of starting up the drive system that is connected to the DC voltage intermediate circuit if the voltage of the DC voltage intermediate circuit achieves a second desired operating voltage. The second desired operating voltage can be equal to the first desired operating voltage; the method for starting up the battery system then terminates upon the first and accordingly second desired operating voltage being achieved and the drive system being started up. Alternatively, the second desired operating voltage can be lower than the first desired operating voltage. In this case, the drive system is started up before the voltage of the DC voltage intermediate circuit achieves the first desired operating voltage, and is operated at a reduced output until the first desired operating voltage is achieved. 
     It is preferred that the step c) is repeated until the battery cells of all the series-connected battery modules are connected, in other words are connected in series. In this case, the first desired operating voltage is equal to the maximum output voltage of the battery that arises by connecting the battery cells of all battery modules in series. The maximum output voltage of the battery corresponds to the maximum possible drive output of the drive system. 
     A second aspect of the disclosure provides a battery having a control unit and a plurality of series-connected battery modules. Each battery module comprises in so doing 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 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 so doing that the battery cells of the battery module 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 so doing, 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 so doing connected directly to the battery, in other words no further components are provided between the battery and the DC voltage intermediate circuit, in particular 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. 
     A fourth aspect of the disclosure provides a motor vehicle having a battery system in accordance with the aforementioned aspect of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure are explained in 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 
         FIGS. 11 and 12  show graphs of the voltage of the DC voltage intermediate circuit for a battery system in accordance with the prior art and for a battery 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  35 ,  36  is connected between one of the inputs 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, for example. 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 time period 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  50 , 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, for example. 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 time period 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 is in turn also possible to 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 . 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 the running operation, since the dangerous 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  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, charging and disconnecting devices and disconnecting devices can in addition be provided at the poles of the battery module string  70  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 disconnected from the battery terminals by means of the coupling units  30  or  50  that are provided in the battery modules  40  or  60 . 
       FIGS. 11 and 12  illustrate graphs of the voltage of the DC voltage intermediate circuit for a battery system in accordance with the prior art and a battery system in accordance with the disclosure. 
       FIG. 11  illustrates the graph for a battery system in accordance with the prior art. At the point in time t 0 , the battery is connected by way of the charging switch  120  and the charging resistor  121  to the capacitor  111  of the DC voltage intermediate circuit, wherein the latter is fully discharged at this point in time. The voltage of the DC voltage intermediate circuit rises initially rapidly but the rate of increase then reduces continuously. Only at the point in time t 11  is the voltage of the DC voltage intermediate circuit of such a magnitude that the difference ΔV between the output voltage of the battery and the voltage of the DC voltage intermediate circuit is sufficiently small in order to close the switch  118  and to charge the DC voltage intermediate circuit rapidly up to the output voltage of the battery without limiting the current by means of the charging resistor  121 . 
       FIG. 12  illustrates the corresponding graph for a battery system in accordance with the disclosure. At the commencement of the charging process the voltage of the DC voltage intermediate circuit is in turn zero, in other words, the capacitor of the DC voltage intermediate circuit is fully discharged. The first battery module is activated at the point in time t 0 , so that the output voltage of the battery corresponds to the voltage of a battery module. The charging current is not limited by a charging resistor, so that the voltage of the DC voltage intermediate circuit rapidly rises, however, the charging current does not become inadmissibly high because the voltage difference between the output voltage of the battery and the voltage of the DC voltage intermediate circuit is comparatively small. As soon as the voltage of the DC voltage intermediate circuit approaches the magnitude of the output voltage of the battery (point in time t 21 ), the next battery module is activated, as a consequence of which the output voltage of the battery increases by the voltage of a battery module and the voltage of the DC voltage intermediate circuit in turn follows rapidly the output voltage of the battery. The process of switching in a further battery module is then repeated in each case until the voltage of the DC voltage intermediate circuit achieves the first desired operating voltage and accordingly all battery modules are activated (points in time t 22 , t 23 , t 24 , t 25 ). In the illustrated example, the battery comprises five battery modules; it is, however, naturally possible to provide any number of battery modules greater than 1. The higher the number of battery modules, the smaller the steps in the voltage progression of the voltage of the DC voltage intermediate circuit and consequently also the maximum charging current. 
     The comparison of the two graphs of the voltage of the DC voltage intermediate circuit demonstrates that the DC voltage intermediate circuit in accordance with the disclosure is charged considerably more rapidly than is usual in the prior art. As a consequence, a drive system that is connected to the DC voltage intermediate circuit can start up more rapidly, which is of particular interest for applications where safety is concerned.