Abstract:
A battery system comprises a battery including a plurality of battery cells and can be connected on an input side to a direct voltage intermediate circuit via at least one contactor, and a diagnostic device configured to diagnose a state of the at least one contactor. The battery system includes a monitoring circuit with a first branch in which the at least one contactor is arranged, and a second branch which is connected parallel thereto and in which a voltage source configured to generate a reference voltage is connected. The diagnostic device is arranged so as to evaluate a diagnostic current which is dependent on the reference voltage and flows in the monitoring circuit, and serves to determine a fault state of the at least one contactor based on a measured current value or a current profile of the diagnostic current.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2012 213 159.0, filed on Jul. 26, 2012 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    The present disclosure relates to a battery system which has a battery which comprises a plurality of battery cells and which can be connected on the input side to a direct voltage intermediate circuit via at least one contactor, and a diagnostic device for diagnosing the state of the at least one contactor. In addition, the disclosure relates to an associated method for diagnosing the state of battery contactors. 
         [0003]    It has become apparent that in future new battery systems of which very stringent demands are made in terms of reliability will be increasingly used both in stationary applications, such as, for example, in wind power plants, and in motor vehicles, such as, for example, in hybrid vehicles and electric vehicles. The background to these stringent demands is that failure of the battery of a battery system which is used can lead to a safety-related problem. The basic circuit diagram of such a battery system  10  is illustrated in  FIG. 1 . In order to achieve the required performance and energy data by means of the battery system  10 , individual battery cells  21  are connected in series and partially additionally in parallel in the battery (battery pack)  20  of the battery system  10 . In order to simplify the illustration, only a single battery cell with a reference symbol  21  has been provided in  FIG. 1 . A problem of the approach shown in  FIG. 1 , in which a large number of battery cells  21  are connected in series, is the high output voltage UB of a battery (high voltage battery)  20  which occurs with this series connection, which output voltage UB is continuously present at the on-board power system (high-voltage on-board power system)  40  of a motor vehicle unless suitable measures are used. The voltage, referred to as the “link voltage”, which is present at the on-board power system  40  has been denoted by UBN in  FIG. 1 . In the battery system  10  illustrated in  FIG. 1 , the battery  20  is connected to the on-board power system  40  of the motor vehicle by means of the capacitor  30  of a direct voltage intermediate circuit. 
         [0004]    Because of the high voltage UBN, which is continuously present at the on-board power system  40  of the motor vehicle unless suitable measures are used, as a rule contactors  50 ,  60  are used which, when necessary, disconnect the battery  20  from the on-board power system  40 . For safety reasons, a separate contactor  50 ,  60  should be respectively present both at the positive pole of the battery  20  and at the negative pole, which contactors  50 ,  60  are designed for a high battery voltage UB and are, under certain circumstances, also capable of disconnecting a short circuit current of over 1000 A. The contactor  50  is connected at its pole  51  to the battery  20 , and at its pole  52  to the on-board power system  40 . The contactor  60  is connected at its pole  61  to the battery  20 , and at its pole  62  to the on-board power system  40 . 
         [0005]    So that the disconnection of the battery  20  with its high battery voltage UB from the on-board power system  40  is ensured, one of the previously described safety requirements involves checking the function of the contactors  50 ,  60  and reliably diagnosing a malfunction. Therefore, a particularly hazardous malfunction of the contactors  50 ,  60  which has to be diagnosed is, for example, if the contactors  50 ,  60  or the contactor contacts thereof stick during correct actuation and have not opened as actuated. The contactors  50 ,  60  may also have been destroyed to such an extent that they have not closed as actuated. For the implementation of diagnostic functions it is necessary to use suitable devices and algorithms. 
         [0006]    According to the present prior art, topologies are known for implementing anti-sticking diagnostic functions which are embodied as a circuit device for implementing voltage measurements at the four contact poles of the two contactors and which are respectively arranged at the positive or the negative battery pole, between the corresponding battery pole and the on-board power system. Such circuit topologies are known, for example, from document EP 2 308 714 A2. The known circuit topologies may also include reference voltages which are connected when there are certain diagnostic configurations. In the case of open contactors, the potential differences between the four poles of the two contactors are evaluated. These potential differences are predefined, to a certain extent, by means of defined impedances or resistance dividers/voltage dividers. 
         [0007]    The methods which are known from the prior art for diagnosing a sticking state of contactors which use considerations of voltage differences as a function of the switched states have the disadvantage that they are very sensitive to voltage cross-couplings between the pole potentials. In addition, these methods are very sensitive to changes in impedance between the battery poles and the on-board power system poles. Furthermore, a complete diagnosis requires a switching frequency to be implemented with simultaneous evaluation of voltage differences. Such a sequence is run through as a rule either at the start or at the end of a driving cycle. Since switching times have to be complied with for this purpose and transient recovery processes play a role, starting delays often occur, depending on the method, specifically until the diagnosis is completed. 
         [0008]    Furthermore, document DE 10 2004 041 998 A1 discloses a method for predicting the operational capability of a relay or of a contactor in which a current which flows through the relay or the contactor is measured repeatedly. In the same method, a current temperature of the relay of the contactor is estimated by means of the measured current values and on the basis of known current temperature characteristic curves of the relay or of the contactor. A prediction about the operational capability, in particular about a sticking state, of the relay or of the contactor is then made by means of the estimated temperature. 
       SUMMARY 
       [0009]    According to the disclosure, a battery system is made available which has a battery which comprises a plurality of battery cells and which can be connected on the input side to a direct voltage intermediate circuit via at least one contactor. In addition, the battery system has a diagnostic device for diagnosing the state of the at least one contactor. In addition, a monitoring circuit, which comprises a first branch in which the at least one contactor is arranged and a second branch which is connected parallel thereto and in which a voltage source for generating a reference voltage is connected, is arranged in the battery system. The diagnostic device is arranged so as to evaluate a diagnostic current which is dependent on the reference voltage and flows in the monitoring circuit, and so as to determine the functional state, in particular a fault state, of the at least one contactor by means of a measured current value or current profile of the diagnostic current. 
         [0010]    According to the method according to the disclosure for diagnosing the state of battery contactors of a battery, a state diagnosis of at least one of the contactors is carried out by means of a monitoring circuit which has a first branch in which the at least one contactor is arranged, and a second branch which is parallel thereto and in which a voltage source for generating a reference voltage is connected. According to the method, a diagnostic current which is dependent on the reference voltage and which flows in the monitoring circuit is evaluated. 
         [0011]    In concrete terms, a battery system having a specific additional monitoring circuit and a diagnostic device is disclosed, wherein the diagnostic device is designed to diagnose a fault state of the contactor. To be more precise, the battery system and the diagnostic device are designed to respectively measure a diagnostic current, flowing through the connected, second branch of the monitoring circuit, for the contactor in a first state, assumed to be an opened switched state, and in a second state, assumed to be a closed switched state, and to be able to diagnose a malfunction of the contactor, in particular a sticking state of the contactor, by means of the measured diagnostic current. 
         [0012]    As a result, according to the disclosure, a switching topology is made available which is optimized in terms of costs and resources and which permits a particularly robust diagnosis of a contactor which is used in the open or closed state. The switching topology according to the disclosure makes it possible to implement continuous monitoring of the state of the contactors and contactor contacts used in the closed state and rapid checking in the opened state, which monitoring and checking cause only a very short starting delay since only one switching and measuring process per contactor is respectively required. 
         [0013]    In particular, the battery system according to the disclosure can comprise two contactors which are each arranged on the battery pole and are each positioned in an additional monitoring circuit, and the diagnostic device is designed to respectively measure the diagnostic currents which flow through the second connected branches of the two monitoring circuits, while the two contactors are each in the second state, assumed to be a closed switched state, and to respectively diagnose a possible malfunction of the contactors by means of the measured diagnostic currents. 
         [0014]    If both contactors are operationally capable and are in the closed switched state, a diagnostic current which is defined by the structure of the corresponding monitoring circuit must flow in each monitoring circuit. The functional state of the respective contactor can be inferred by means of the measured values of the diagnostic currents. If the expected diagnostic currents are measured in both monitoring circuits, it is possible to assume that both contactors have closed satisfactorily. If a different diagnostic current is measured in at least one monitoring circuit, a fault state of the corresponding contactor in which this contactor has not closed can be diagnosed by evaluating the measured diagnostic currents. 
         [0015]    The battery system according to the disclosure can comprise two contactors which are each arranged on a battery pole and are each positioned in an additional monitoring circuit, and the diagnostic device is designed to measure the diagnostic current which flows through the connected second branch of the monitoring circuit of at least one contactor in the first state, assumed to be an opened switched state, while the monitoring circuit of the other contactor is in the second state, assumed to be a closed switched state, and to diagnose a possible sticking state of the corresponding contactor by means of the measured diagnostic current. 
         [0016]    The diagnostic device is preferably designed to carry out the measurement of the diagnostic current through the second connected branch of the monitoring circuit of a contactor in the first state, assumed to be an opened switched state, directly after both monitoring circuits were in the second state, assumed to be the closed switched state. 
         [0017]    In other words, one of the two contactors is opened after both connectors were closed. The diagnostic current which flows through the second, connected branch of the monitoring circuit of the opened contactor is measured here and evaluated. If the corresponding contactor has opened satisfactorily, the corresponding diagnostic current is reduced, in particular is reduced to zero, that is to say is disconnected. If this contactor contact sticks, the contactor remains closed even after the opening process and the corresponding diagnostic current approximates to the expected diagnostic current in the closed state of the contactor. 
         [0018]    According to the disclosure, a sticking state which is occurring at the examined contactor can easily be diagnosed by means of the measured diagnostic current. This measurement can be carried out for each of the two contactors. 
         [0019]    In particular, the battery system according to the disclosure comprises, according to one embodiment of the disclosure, two contactors which are each arranged on a battery pole and are each placed in an additional monitoring circuit, and the diagnostic device is designed to measure the diagnostic current which flows through the connected, second branch of the monitoring circuit of at least one contactor in the first state, assumed to be an opened switched state, while the other contactor is also in the first state, assumed to be an opened switched state, and the second branch of the monitoring circuit of the other contactor is disconnected from the first branch of the monitoring circuit of the other contactor. In addition, the diagnostic device is designed to diagnose a possible sticking state of the corresponding contactor by means of the measured diagnostic current. 
         [0020]    In other words, one of the contactors is opened and the second branch of the monitoring circuit of this contactor is disconnected, while the other contactor is opened and the second branch of the monitoring circuit of this other contactor is connected. In this context, the diagnostic current which flows through the second connected branch of the corresponding monitoring circuit is measured. If no diagnostic current is detected flowing through the second connected branch of the corresponding monitoring circuit, it can be assumed that both contactors have opened satisfactorily. If the contactor contact for which the diagnostic current is measured sticks, this contactor remains closed even after the opening process and the corresponding diagnostic current approximates to the expected diagnostic current in the closed state of the contactor. The sticking state of the examined contactor can be diagnosed by means of this measured diagnostic current. This measurement can be carried out for each of the two contactors. 
         [0021]    In particular, in the second branch of at least one monitoring circuit of the battery system according to the disclosure a shunt resistor is connected in series with the additional voltage source of the monitoring circuit. In addition, in this context the diagnostic device is designed to measure the voltage which is present at the shunt resistor and to use it to determine a corresponding diagnostic current flowing through the shunt resistor. 
         [0022]    According to the disclosure, the diagnostic currents can therefore be determined by means of a simple evaluation of the voltages present at the corresponding shunt resistors. 
         [0023]    At least one monitoring circuit of the battery system according to the disclosure preferably comprises a potential-separated voltage source which can be embodied as a winding of a flyback converter. 
         [0024]    By using a potential-separated voltage source for at least one monitoring circuit, the probability of the occurrence of voltage cross-couplings between pole potentials, which would have a considerable adverse effect on the diagnosis of a possible false state of the contactors used, is drastically reduced in a very easy way with the circuit technology according to the disclosure. 
         [0025]    According to one preferred embodiment of the disclosure, the voltage source is connected in an orientation which is opposed to the orientation of the battery voltage. In another embodiment, the voltage source is connected with the same orientation as the battery. 
         [0026]    In addition, the second branch of the monitoring circuit can have a plurality of reference resistors and/or a diode, which are arranged in a series circuit with the voltage source. In this context, the diode is preferably poled in series with the battery cells with respect to its direction of flow. 
         [0027]    A second inventive branch of a monitoring circuit is preferably designed such that it can be deactivated, in particular by means of a switch arranged in the second branch. 
         [0028]    In one very preferred embodiment of the battery system according to the disclosure, the diagnostic device comprises at least one analog/digital converter and an evaluation unit for evaluating digital signals which can be embodied, in particular, as microcontrollers, wherein the analog/digital converter is designed to convert a detected diagnostic current or a voltage which is present and detected at the shunt resistor into a digital signal and to transfer the digital signal, in particular to transfer it with separated potentials, to the evaluation unit. In this context, the microcontroller can be embodied within the electronics of the battery management system. 
         [0029]    Since the analog/digital converter is designed to transfer the digital signal of the evaluation unit with separated potentials, with the circuit topology according to the disclosure the probability of the occurrence of destructive voltage cross-couplings between pole potentials is easily minimized. 
         [0030]    According to one preferred development of the method according to the disclosure, a closed state of a contactor is diagnosed if the measured diagnostic current corresponds to a predefined reference resistor arranged of the reference voltage and in the monitoring circuit, in particular in the second branch of the monitoring circuit, wherein opening of the contactor is diagnosed if a reduction occurs in the measured diagnostic current, in particular a reduction to the value zero. 
         [0031]    This development according to the method can be carried out with the battery system according to the disclosure, in which the reference voltage source has opposed poling to that of the battery. On the other hand, if the reference voltage source is poled with the battery, the method can also be carried out in such a way that an initially increasing current indicates opening of the contactor. 
         [0032]    Furthermore, according to the method a satisfactory open state of a contactor can be diagnosed if, in response to a connection of the second branch, which is performed, for example, by a switch arranged in the second branch, it is not possible to measure any diagnostic current which differs from zero. In this context, a malfunction, in particular sticking of the contactor, can be diagnosed if a diagnostic current which differs from the current value zero is measured in response to the connection of the second branch. 
         [0033]    This embodiment of the method is preferably carried out in such a way that when the second branch is connected the other second branch, which is associated with another contactor, remains open, that is to say disconnected. 
         [0034]    The battery which forms part of the battery system according to the disclosure is preferably a lithium ion battery. 
         [0035]    Yet a further aspect of the disclosure relates to a motor vehicle having the battery system according to the disclosure. The use of the battery system according to the disclosure in a motor vehicle, in particular for supplying the on-board power system of the motor vehicle, considerably improves the driving safety of such a motor vehicle. 
         [0036]    Advantageous developments of the disclosure are specified in the dependent claims and described in the description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Exemplary embodiments of the disclosure are explained in more detail by means of the drawings and the following description. In the drawings: 
           [0038]      FIG. 1  shows a battery system which is known from the prior art and has a battery with a plurality of battery cells, wherein a contactor is arranged at each of the battery poles, 
           [0039]      FIG. 2  shows a battery system according to a first embodiment of the disclosure with a battery with a plurality of battery cells, wherein a contactor which is positioned in a separate monitoring circuit is arranged at each of the battery poles, 
           [0040]      FIG. 3  shows a battery system according to the first embodiment of the disclosure together with the current flows at the monitoring circuits with the contactors at the battery poles closed, 
           [0041]      FIG. 4  shows the battery system according to the first embodiment of the disclosure together with the current flows at the monitoring circuits with the contactor at the positive battery line pole opened and the contactor at the negative battery pole closed, 
           [0042]      FIG. 5  shows the battery system according to the first embodiment of the disclosure, in which the monitoring circuits with opened contactors on the two battery poles for checking non-sticking of the contactor at the negative battery pole are illustrated, and 
           [0043]      FIG. 6  shows the battery system according to the first embodiment of the disclosure together with the current flows at the monitoring circuits from  FIG. 5  for a case in which the contactor at the negative battery pole is sticking. 
       
    
    
     DETAILED DESCRIPTION 
       [0044]      FIG. 2  illustrates the basic circuit diagram of a battery system  10  according to a first embodiment of the disclosure. The battery system  10  comprises a battery  20  with a plurality of battery cells  21  which are connected in series and partially additionally in parallel. In order to simplify the illustration, just one of the illustrated battery cells is provided with a reference symbol  21  in  FIG. 1 . The output voltage of the battery  20  is also denoted by UB here. In the battery system  10  illustrated in  FIG. 1 , the battery  20  can be connected to the on-board power system  40  of a motor vehicle via two contactors  50 ,  60  by means of the capacitor  30  of a direct voltage intermediate circuit. If both contactors  50 ,  60  are functionally capable and are each in a closed state, the battery voltage UB is present at the on-board power system (high-voltage on-board power system)  40  of the motor vehicle. The voltage (link voltage) which is present at the on-board power system  40  is denoted by UBN in  FIG. 2 . 
         [0045]    The contactors  50 ,  60  can, if they are functionally capable and in the opened state, disconnect the battery  20  from the on-board power system  40 . The contactor  50  is connected at its pole  51  to the positive battery pole (not characterized) and at its pole  52  to the on-board power system  40 . The contactor  60  is connected at its pole  61  to the negative battery pole (not characterized) and at its pole  62  to the on-board power system  40 . The diagnostic voltages which are present at the poles  51 ,  52  of the contactor  50  and at the poles  61 ,  62  of the contactor  60  are denoted by UD 1  and UD 2 , respectively. 
         [0046]    In the battery system  10  according to the first embodiment of the disclosure, each of the two contactors  50 ,  60  is positioned in an additional monitoring circuit  150 ,  160  which can be disconnected. Each of these monitoring circuits  150 ,  160  comprises a first branch  151 ,  161  in which the corresponding contactor  50 ,  60  is arranged, and a second branch  152 ,  162  which can be connected in parallel with the corresponding first branch  151 ,  161  in each case by means of a switch  153 ,  163  and comprises a separate potential-separated voltage source  154 ,  164  which is connected in series with the corresponding switch  153 ,  163  and which respectively supplies the corresponding monitoring circuit  150 ,  160 . The potential-separated voltage sources  154 ,  164  each supply a known reference voltage UR 1  and UR 2 , respectively. The potential-separated voltage sources  154 ,  164  can be implemented cost-effectively by means of, for example, a separate winding of a flyback converter. 
         [0047]    If a contactor  50 ,  60  is closed, a defined diagnostic current flows in the corresponding monitoring circuit  150 ,  160 , which diagnostic current can be detected by means of a diagnostic device  70  arranged in the battery system  10 . This can be implemented, for example, by virtue of the fact that the two branches  152 ,  162  of the monitoring circuits  150 ,  160  each comprise a shunt resistor  155 ,  165  which is connected in series with the corresponding voltage source  154 ,  164 , and the voltage which is present at a shunt resistor  155 ,  165  is detected and evaluated by means of the evaluation unit  90  for the purpose of determining the diagnostic current flowing through the corresponding shunt resistor. The resistance values of the shunt resistors  155 ,  165  are denoted by RS 1  and RS 2 . In addition, each of the second branches  152 ,  162  of the monitoring circuits  150 ,  160  can respectively comprise two further resistors  156 ,  157  and  166 ,  167 , respectively, which are connected in series with the respective second branch  152 ,  162  and whose resistance values are denoted by R 1 , R 2  and R 3 , R 4 , respectively, in the drawing. 
         [0048]    The voltage which is present at a shunt resistor  155 ,  165  can be respectively made available by an analog/digital converter  81 ,  82  which is arranged in the evaluation unit  90 , with potential separation, which analog/digital converter  81 ,  82  converts this voltage which is made available into a digitized signal and transfers the digitized signal with potential separation to a microcontroller, arranged in the evaluation unit  90 , for the purpose of evaluation. The state of the contactor  50 ,  60  monitored in this way can be inferred by means of the evaluation of the digitized signal. 
         [0049]      FIG. 3  shows the battery system  10  (illustrated in  FIG. 2 ) according to the first embodiment of the disclosure together with the current flows at the monitoring circuits  150 ,  160  for a case in which both contactors  50 ,  60  which are arranged at the battery poles and the associated monitoring circuits  150 ,  160  are closed. For the purpose of simplifying the illustration, in  FIG. 3  all voltages and currents which occur and which are relevant for the description of the current flows at the monitoring circuits  150 ,  160  were provided with reference symbols only for the other components which are mainly discussed here. 
         [0050]    The monitoring of the contactors  50 ,  60  in the “closed” setpoint state is described in more detail with reference to  FIGS. 3 and 4 . 
         [0051]    If the contactors  50 ,  60  are closed, a defined diagnostic current flows in each monitoring circuit  150 ,  160 . The values of the diagnostic currents ID 1 , ID 2  flowing through the second branches  152 ,  162  are specified in the relations (1) and (2). The current flows which are respectively present in the monitoring circuits  150 ,  160  are denoted by  250  and  260  in  FIG. 3 . In addition, in  FIG. 3  the current flow which passes via the contactor  60 , via the battery  20 , via the contactor  50  and via the input of the on-board power system  40  was denoted by 100. The current which is associated with the current flow  100  is denoted by I. 
         [0052]    The following relationship (1) relates to the monitoring circuit  150  of the contactor  50  which is arranged at the positive battery pole: 
         [0000]        ID 1=( UR 1 −UD 1)/( R 1 +R 2 +RS 1)   (1)
 
         [0053]    The relationship (2) relates to the monitoring circuit  160  of the contactor  60  which is arranged at the negative battery pole: 
         [0000]        ID 2=( UR 2 −UD 2)/( R 3 +R 4 +RS 2)   (2)
 
         [0054]      FIG. 4  shows the battery system  10  (illustrated in  FIG. 2 ) according to the first embodiment of the disclosure together with the current flows at the monitoring circuits  150 ,  160  for the case in which the contactor  50  which is arranged at the arranged positive battery pole is opened, the second branch  152  of the associated monitoring circuit  150  is connected and the contactor  60  which is arranged at the negative battery pole and the associated monitoring circuit  160  are closed. In order to simplify the illustration, all the voltages and currents which occur and which are relevant for the description of the current flows at the monitoring circuits  150 ,  160  are again provided with reference signs only for various selected components. The values of the diagnostic currents which flow through the second branches  152 ,  162  of the monitoring circuits  150 ,  160  are also denoted by ID 1  and ID 2  here. In  FIG. 4 , the current flow which, because the contactor  50  is opened, passes via the second branch  152  of the monitoring circuit  150  and additionally also via the battery  20 , via the contactor  60  and via the input of the on-board power system  40  is denoted by  250  in  FIG. 4 . 
         [0055]    In  FIG. 4 , the current flows  250 ,  260  are illustrated for the case in which, directly after the two contactors  50 ,  60  and the associated monitoring circuits  150  and  160  were closed, one of the two contactors  50 ,  60 , here the contactor  50  arranged at the positive battery pole, has been opened. 
         [0056]    If a contactor opens, the associated diagnostic current, here ID 1 , will be reduced and, under certain circumstances, disconnected as soon as the on-board power system voltage UBN, that is to say the high voltage present at the on-board power system, becomes lower than the battery voltage UB. 
         [0057]    In the following relationship (3), the value of the diagnostic current ID 1  which is associated with the current flow  250  is specified: 
         [0000]        ID 1=(( UR 1 −UD 2−( UB−UBN ))/( R 1 +R 2 *RS 1)   (3)
 
         [0058]    As soon as the on-board power system voltage UBN becomes lower than the difference (UB−UR 1 ) between the battery voltage UB and the reference voltage UR 1 , the current flow  250  is completely disconnected. 
         [0059]    The evaluation of the digitized voltage drop at the measuring shunt resistor  155  whose resistance value is denoted by RS 1  here makes it possible for the diagnostic current ID 1  to be monitored and for opening of the contactor  50  to be diagnosed immediately as soon as the diagnostic current changes or fails entirely. When the contactor  50  is opened, the diagnostic current will change immediately as soon as the on-board voltage UBN changes in comparison with the on-board voltage value in the closed state of the contactor  50 . 
         [0060]    The same measurement principle can be applied for the contactor  60  if the contactor  60  is opened directly after the contactors  50 ,  60  and the associated monitoring circuits  150 ,  160  were closed. 
         [0061]    The monitoring of the contactors  50 ,  60  in the “open” setpoint state is described in more detail below with reference to  FIGS. 5 and 6 . In the case of the “contactors open” setpoint state the objective is to diagnose whether a contactor  50 ,  60  is not open as it should be but instead sticking. For this purpose, one measurement should be carried out per contactor  50 ,  60 . 
         [0062]      FIG. 5  shows the battery system  10  (illustrated in  FIG. 2 ) according to the first embodiment of the disclosure for a case in which the monitoring circuits  150 ,  160  are used with contactors opened at both battery poles for checking for nonsticking of the contactor  60  at the negative battery pole. In order to simplify the illustration, in  FIG. 5  all the voltages and currents which occur and are relevant for the description of the states used in the specified case were provided with reference symbols, but only the essential miscellaneous components were provided with reference symbols. 
         [0063]    During the measurement for diagnosing the contact state of the contactor  60  at the negative battery pole, the switch  163  of the monitoring circuit  160  of this contactor  60  is closed and the switch  153  of the monitoring circuit  150  of the contactor  50  at the positive battery pole is used in an open switched state. If both contactors  50 ,  60  are open, a diagnostic current ID 1 , ID 2  cannot flow in any of the monitoring circuits  150 ,  160 . In this case, the relationship (4): 
         [0000]      ID1=ID2=0   (4)
 
         [0000]    then applies for the diagnostic currents ID 1 , ID 2 . 
         [0064]      FIG. 6  shows the battery system  10  according to the first embodiment of the disclosure together with the current flows at the monitoring circuits  150 ,  160  (illustrated in  FIG. 5 ) for the case in which the contactor  60  at the negative battery pole is sticking. The values of the diagnostic currents flowing through the second branches  152 ,  162  of the monitoring circuits  150 ,  160  are also denoted by ID 1  and ID 2  in  FIG. 6 . The current flow which, because the contactor  60  is sticking, is present in the monitoring circuit  160  or does not disappear has been denoted by  260  in  FIG. 6 . 
         [0065]    If the contactor  60  which is arranged at the negative battery pole were to stick, a diagnostic current ID 2 , which has been specified in the relationship (5), would flow through the second branch  162  of a corresponding monitoring circuit  160 . The detection of the diagnostic current ID 2  makes it possible to diagnose a sticking contactor  60  at the negative battery pole. In this case, no diagnostic current ID 1  would flow through the second branch  152  of the monitoring circuit  150  of the contactor  50  arranged at the positive battery pole and the value of this diagnostic current ID 1  would therefore be zero. 
         [0000]        ID 2=( UR 2 −UD 2)/( R 3 +R 4 +RS 2)   (5)
 
         [0066]    The measuring procedure for diagnosing a sticking state of the contactor  50  which is arranged at the positive battery pole is analogous to the measuring procedure which has just been presented above, for diagnosing a sticking state of the contactor  60  which is arranged at the negative battery pole.