Abstract:
The invention relates to a battery system ( 100 ) with a battery ( 20 ), which is designed to supply a high voltage network ( 70 ) with electric energy, and a measuring device ( 130 ) for measuring at least one insulation resistance provided between the battery ( 20 ) and a housing of the battery ( 20 ). The measuring device ( 130 ) is equipped with two measuring paths ( 140, 150 ), each of which is paired with one of two high-voltage connections ( 21, 22 ) of the battery ( 20 ), each of which comprises a series circuit that comprises a first resistor ( 142, 152 ) and a relay ( 145, 155 ), and each of which is connected between the paired high-voltage connection ( 21, 22 ) and a point ( 25 ) that has a housing potential. Furthermore, each series circuit has a semiconductor switch ( 147, 157 ). The measuring device ( 130 ) also has two functional modes in which the relays ( 145, 155 ) of the measuring paths ( 140, 150 ) are closed. When switched to a passive functional mode of the two functional modes, the measuring device ( 130 ) is designed to open each semiconductor switch ( 147, 157 ) or to keep the semiconductor switch in an open state. When switched to an active functional mode of the two functional modes, the measuring device ( 130 ) is additionally designed to alternately open and close the semiconductor switches ( 147, 157 ), to measure a first voltage which drops over the first resistor ( 142, 152 ) of each measuring path ( 140, 150 ) when the semiconductor switch ( 147, 157 ) of the corresponding measuring path ( 140, 150 ) is closed, and to determine a corresponding insulation resistance of the battery ( 20 ) using each measured first voltage.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a battery system with a battery which is designed to supply a high-voltage network with electrical energy and a measuring device for measuring at least one insulation resistance of the battery, and a corresponding method for measuring at least one insulation resistance of a battery which is designed to supply a high-voltage network with electrical energy. The invention also relates to a vehicle having such a battery system. 
         [0002]      FIG. 1  shows a battery system  10  known from the prior art, with a battery  20  configured as a high-voltage battery or traction battery, and a measuring device  30  for measuring at least one insulation resistance of the battery  20 , which is present inside or outside the battery  20 . The battery  20  comprises a plurality of series-connected battery cells. In the interests of simpler representation, said plurality of battery cells are not identified by reference numbers. Herein, a first insulation resistance  26  is the insulation resistance of the battery  20  which is present between a positive high-voltage terminal  21  of the battery  20  and a point  25  having a potential of a housing of the battery  20 , hereinafter designated as the housing potential  25 . Moreover, a second insulation resistance  27  is the insulation resistance of the battery  20  which is present between a negative high-voltage terminal  22  of the battery  20  and the housing potential  25 . The positive high-voltage terminal  21  of the battery  20  is hereinafter designated as the first high-voltage terminal  21  of the battery  20 . The negative high-voltage terminal  22  of the battery  20  is hereinafter designated as the second high-voltage terminal  21  of the battery  20 . Moreover, a third insulation resistance  28  is the insulation resistance of the battery  20  which is present between a connection point, by means of which at least two battery cells in the battery  20  are interconnected, and the housing potential  25 . 
         [0003]    The measuring device  30 , arranged in a battery control device (not represented) of the battery system  10 , comprises a first measuring path  40  and a second measuring path  50 , each of which comprises a high-resistance voltage divider  41 ,  51  and a relay  45 ,  55 , and are connectable between an assigned high-voltage terminal  21 ,  22  of the two high-voltage terminals  21 ,  22  and the housing potential  25 . Herein, the first high-voltage terminal  21  is assigned to the first measuring path  40 . The second high-voltage terminal  22  is, moreover, assigned to the second measuring path  50 . Each voltage divider  41 ,  51  is comprised of a first resistor  42 ,  52  and a second resistor  43 ,  53 . Herein, each first resistor  42 ,  52  is connected to the housing potential  25 , and each second resistor  43 ,  53  is connectable, via the corresponding relay  45 ,  55 , to its assigned high-voltage terminal  21 ,  22 . In the event of an insulation fault between the battery  20  and the housing, during the alternating closing of the relays  45 ,  55  on the measuring paths  40 ,  50 , a measurable current flux flows through at least one of the voltage dividers  41 ,  51 , such that a measurable voltage also drops across the first resistor  42 ,  52  of the at least one voltage divider  41 ,  51 . Each voltage drop across the first resistor  42 ,  52  of each voltage divider  41 ,  51  is measured via a respective measuring input  49 ,  59  of an evaluation and control unit  60  of the measuring device  30  which is assigned to the corresponding measuring path  40 ,  50 . The measuring input  49  which is assigned to the first measuring path  40  is connected, via a further resistor  46 , to that terminal of the first resistor  42  on the voltage divider  41  of the first measuring path  40  which is not connected to the housing potential  25 . For the measurement of the voltage drop across the first resistor  52  on the voltage divider  51  of the second measuring path  50 , the measuring device  30  is equipped with an operational amplifier  56  which is connected, on the input side, to the first resistor  52  on the voltage divider  51  of the second measuring path  50  and, on the output side, to the measuring input  59  of the evaluation and control unit  60  assigned to the second measuring path  50 . The evaluation and control unit  60  thus defines the respective voltage drops across the first resistors  42 ,  52  on the voltage dividers  41 ,  51 . By the alternating switching-in of the measuring paths  40 ,  50 , the voltage drop across each of the first resistors  42 ,  52  is determined and, in consideration of known system variables, the insulation resistances  26 ,  27 ,  28  of the battery  20  can be calculated in each case. The evaluation and control unit  60  is moreover designed to control the relays  45 ,  55 , i.e. to open and close the latter. 
         [0004]    The battery  20  represented in  FIG. 1  is designed to a supply a high-voltage network  70  with electrical energy. To this end, the battery  20  is connectable to the high-voltage network  70  via two further relays  71 ,  72 . Via the two further relays  71 ,  72 , the first high-voltage terminal  21  of the battery  20  is directly connectable to a first high-voltage network terminal  73  on the high-voltage network  70 , and the second high-voltage terminal  22  of the battery  20  is directly connectable to a second high-voltage network terminal  74  on the high-voltage network  70 . Between the two high-voltage network terminals  73 ,  74  on the high-voltage network  70 , a series-connected arrangement of two capacitors  75 ,  76  (Y-capacitors) is provided, connected in parallel with an intermediate circuit capacitor  77 , which forms an intermediate circuit. A connection point, by means of which the two series-connected capacitors  75 ,  76  are connected, is connected to a housing potential  25 . The high-voltage network  70  moreover comprises an inverter  78  and a motor  79 . On its input side, the inverter  78  is connected in parallel with the intermediate circuit capacitor  77 , and is designed to convert a DC voltage which can be delivered by the battery  20  into a three-phase AC voltage which is then delivered, on its output side, to the motor  79 . 
         [0005]    According to the prior art, dielectric withstand between a low-voltage network, which is galvanically separated from the high-voltage network  70 , and the battery  20 , or between the housing potential  25  and the battery  20 , must be metrologically proven, both in-factory and during the conduct of all repairs on the battery  20 . To this end, in the course of dielectric withstand testing (by a dielectric withstand test) using the battery control device (not represented) which is switched to a test mode, a test voltage is applied between the housing potential  25  and each high-voltage terminal  21 ,  22  of the battery  20  for a specified time. The test criterion to be used for this purpose is a measured current value. The test voltage here is delivered in the form of a DC voltage or an AC voltage. 
         [0006]    In order to protect the electronic measuring systems of the measuring device  30  against damage during dielectric withstand testing, and to prevent any overshoot of a predefined limiting current value by the current value of any measuring current generated in course of insulation resistance measurement, the measuring paths  40 ,  50  must be configured with a correspondingly high resistance. As the predefined limiting current value cannot be selected to be infinitely small, each measuring path  40 ,  50 , in a time during which dielectric withstand testing is in progress, and in a time during which no insulation resistance measurement is taking place, is galvanically separated from the battery  20 , and thus also from the high-voltage network  70 , wherein the relays (electromechanical switches)  44 ,  55  are opened. 
         [0007]    According to the prior art, the measuring device  30 , during each insulation resistance measurement, is connected via the relay  45  on the first measuring path  40  or via the relay  55  on the second measuring path  50  to the battery  20 . During its service life, each relay  45 ,  55  undergoes from several tens of thousands up to a hundred thousand switching operations or switching cycles. The low measuring currents which arise during insulation resistance measurements, together with high switching voltages, which comprise voltage values of several hundred volts, are conducive to low-energy corona discharge during switching operations on these relays  45 ,  55 . In combination with potential gas emissions from the relay housing material, this can lead to unwanted carbonization or film-formation on the switching contacts of the relays  45 ,  55 . A current-related switching contact transition resistance can occur on the relays  45 ,  55 , thereby invalidating the measurement. 
         [0008]    From document CN 101603986 A, a high-voltage insulation resistance measuring circuit is known, comprising a high-voltage switching unit with a plurality of relays and a voltage measuring unit wherein, in a test mode, various components of the voltage measuring unit deliver a high-voltage signal via the closed relays. 
       SUMMARY OF THE INVENTION 
       [0009]    According to the invention, a battery system is disclosed with a battery which is designed to supply a high-voltage network with electrical energy and a measuring device for measuring at least one insulation resistance which is present between the battery and a housing of the battery. In this case, the measuring device incorporates two measuring paths, each assigned to one high-voltage terminal of two high-voltage terminals of the battery, each comprising a series-connected arrangement of a first resistor and a relay, and each connected between the high-voltage terminal assigned thereto and a point at a housing potential. Each series-connected arrangement moreover comprises a semiconductor switch. The measuring device also has two functional modes, in which the relays on the measuring paths are closed. When switched to the passive functional mode of the two functional modes, the measuring device here is designed to open the respective semiconductor switch, or to maintain the latter in an open state. When switched to an active functional mode of the two functional modes, the measuring device is additionally designed to alternately open and close the semiconductor switches, to respectively measure a first voltage which drops across the first resistor of each measuring path when the semiconductor switch of the corresponding measuring path is closed, and to determine a corresponding insulation resistance of the battery using each measured first voltage. 
         [0010]    According to the invention, a method is moreover disclosed for measuring at least one insulation resistance which is present between a battery which is designed to supply a high-voltage network with electrical energy and a housing of the battery, by means of a measuring device. In this case, the measuring device incorporates two measuring paths, each assigned to one high-voltage terminal of two high-voltage terminals of the battery, each comprising a series-connected arrangement of a first resistor and a relay, and each connected between the high-voltage terminal assigned thereto and a point at a housing potential. Each series-connected arrangement moreover comprises a semiconductor switch. The measuring device also has two functional modes, in which the relays are closed. The measuring device is switched from a passive functional mode of the two functional modes, in which the semiconductor switches are respectively opened or are maintained in an open state, to an active functional mode of the two functional modes, in which the semiconductor switches are alternately closed and opened, a first voltage which drops across the first resistor of each measuring path when the semiconductor switch of the corresponding measuring path is closed is respectively measured, and the corresponding insulation resistance is determined using each measured first voltage. 
         [0011]    In the invention, the relays arranged in the measuring device according to the invention, in a state in which the measuring device is not supplied with electrical energy by the battery, i.e. a state in which no insulation measurement occurs, are closed. This means that, under normal conditions, the measuring device according to the invention is permanently galvanically connected to the battery or to a high-voltage network which is supplied by the battery. For the continued switchable execution of each measuring path or measuring channel in the measuring device, a cost-effective semiconductor switch is connected in each measuring path, in series with the corresponding relay. 
         [0012]    Preferably, the respective semiconductor switches employed in the measuring paths only fulfil such technical requirements as are necessary for the conduction or blocking of the resulting measuring signals associated with the corresponding insulation resistance measurements. It is additionally preferred that the semiconductor switches are not designed in accordance with the technical requirements necessary for the execution of a dielectric withstand test, in which a test voltage in the high-voltage range is applied or is applicable between each of the high-voltage terminals of the battery and the housing. 
         [0013]    In a preferred form of embodiment of the invention, the battery system according to the invention has a protective device for the protection of the measuring device. The protective device moreover comprises a control unit, which is designed to detect the presence of a test mode on a battery control device of the battery system, in which a test voltage is applied between at least one high-voltage terminal of the two high-voltage terminals and the housing and/or between at least one high-voltage network terminal of two high-voltage network terminals of the high-voltage network and the housing respectively, at least temporarily. Preferably, the test voltage is equal to a high voltage with a predefined characteristic. It is additionally preferred that the test voltage has a magnitude that rises continuously from a magnitude of 0 V up to a predefined magnitude, wherein the test voltage, immediately after the achievement of the predefined magnitude, is equal to a high voltage with a predefined characteristic. The control unit is moreover designed to open the relays upon the start of the test mode, and to close the relays at the end of the test mode. 
         [0014]    Preferably, each high-voltage terminal, via a further relay which is assigned thereto, is directly connectable to a high-voltage network terminal of the two high-voltage network terminals which is assigned thereto. 
         [0015]    The control unit is preferably a microcontroller arranged in the battery control device. 
         [0016]    It is additionally preferred that the protective device comprises a voltage measuring unit which is designed to measure a second voltage which is present between each high-voltage terminal and the housing and/or between each high-voltage network terminal and the housing respectively and, in the event of a magnitude of each second voltage which is equal to a predefined limiting value, or which exceeds the latter, to transmit a control signal to the control unit. In this case, the control unit is designed, in the presence of each control signal, to detect the start of the test mode and to open the relays. 
         [0017]    In the invention, the measuring device according to the invention, during each dielectric withstand test in which a critical test voltage is applied, is galvanically separated from the battery or from the high-voltage network which is to be supplied with electrical energy by the battery. To this end, galvanic separation of the measuring device according to the invention from the battery or from the aforementioned high-voltage network is executed immediately prior to the application of the critical test voltage. By this approach, the technical requirements which are to be fulfilled by the relays or electromagnetic switches which are incorporated in the measuring device in the course of their service life are significantly reduced. It is highly advantageous here that cyclical switching, to be executed under load, of the relays incorporated in the measuring device is omitted, and that the number of purely mechanical switching operations to be executed on these relays under dry load conditions is reduced to a minimum. Moreover, the risk of an occurrence of corona discharge, and of unwanted film formation on the switching contacts of said relays, is eliminated. 
         [0018]    In another preferred form of embodiment of the invention, each semiconductor switch is arranged between the point which is at the housing potential and the relay on the corresponding measuring path and/or is designed for the switching of a voltage, the magnitude of which does not exceed a further predefined limiting value. Preferably, each first resistor is directly connected to the point which is at the housing potential. Preferably, each series-connected arrangement comprises a second resistor which, specifically, is directly connected to the high-voltage terminal which is assigned to the measuring path which incorporates the corresponding series-connected arrangement. 
         [0019]    A further aspect of the invention relates to a vehicle having a battery system according to the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    Exemplary embodiments of the invention are described in detail hereinafter, with reference to the accompanying drawings. Identical components are identified by the same reference symbols. In the drawings: 
           [0021]      FIG. 1  shows a known battery system from the prior art, 
           [0022]      FIG. 2  shows a battery system according to a first form of embodiment of the invention, and 
           [0023]      FIG. 3  shows a battery system according to a second form of embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 2  shows a battery system  100  according to a first form of embodiment of the invention. The battery system  100  comprises a battery  20  for the supply of a high-voltage network  70  with electrical energy. The battery  20  delivers a battery voltage VB. Additionally, the battery  20  is connectable via two further relays  71 ,  72  to the high-voltage network  70 . 
         [0025]    The battery system  100  represented in  FIG. 2  is distinguished from the battery system  10  represented in  FIG. 1  by the design of a measuring device  130  which is incorporated in the battery system  100  for the measurement of at least one insulation resistance (not represented) which is present between the battery  20  and a housing of the battery  20 . The battery  20 , the at least one insulation resistance and the two further relays  71 ,  72  of the battery system  100  according to the first form of embodiment, together with the associated high-voltage network  70 , are each configured in an equivalent manner to the corresponding components represented in  FIG. 1 . In the interests of simpler representation, the at least one insulation resistance is not shown in  FIG. 2 , and the high-voltage network  70  is represented in a simplified manner, wherein a series-connected arrangement of two capacitors (Y-capacitors), which is present between two high-voltage network terminals  73 ,  74  of the high-voltage network  70 , is not represented. 
         [0026]    The measuring device  130  comprises a first measuring path  140  and a second measuring path  150 , which are connected respectively between a point  25  having a potential of the housing of the battery  20 , also designated here as the housing potential  25 , and a high-voltage terminal  21 ,  22  of two high-voltage terminals  21 ,  22  of the battery  20  which is assigned thereto. Herein, a first high-voltage terminal  21  of the two high-voltage terminals  21 ,  22  of the battery  20  is assigned to the first measuring path  140 . Additionally, a second high-voltage terminal  22  of the two high-voltage network terminals  21 ,  22  of the battery  20  is assigned to the second measuring path  150 . 
         [0027]    Each measuring path  140 ,  150  has a corresponding series-connected arrangement, respectively comprising a first resistor  142 ,  152 , a second resistor  143 ,  153 , a relay  145 ,  155  and a semiconductor switch  147 ,  157 . Herein, the first resistor  142  on each measuring path  140 ,  150  is directly connected to the housing potential  25  and to the corresponding semiconductor switch  147 ,  157 . Moreover, the second resistor  143 ,  153  on each measuring path  140 ,  150  is directly connected to the high-voltage terminal  21 ,  22  of the battery  20  which is assigned thereto, and to the relay  145 ,  155  on the corresponding measuring path  140 ,  150 . Consequently, the semiconductor switch  147 ,  157  on each measuring path  140 ,  150  is arranged between the first resistor  142 ,  152  and the relay  145 ,  155  of the corresponding measuring path  140 ,  150 . 
         [0028]    The measuring device  130  has an active functional mode and a passive functional mode. During the active functional mode, the relays  145 ,  155  on the measuring paths  140 ,  150  remain closed, and the measuring paths  140 ,  150  are alternately switched by means of the semiconductor switches  147 ,  157 . Each measuring path  140 ,  150  can be switched individually, wherein the semiconductor switch  147 ,  157  of the measuring path  140 ,  150  to be switched is closed, and the semiconductor switch  157 ,  147  of the other measuring path  150 ,  140  is opened, or is maintained in an open state. Each first voltage drop across the first resistor  142 ,  152  on each switched-in measuring path  140 ,  150  here is measured via a respective measuring input  149 ,  159 , which is assigned to the corresponding measuring path  140 ,  150 , of an evaluation unit  160 , which is preferably arranged in the measuring device  130 . By means of each measured first voltage, the evaluation unit  160  determines a corresponding insulation resistance (not represented) of the battery  20 . For the measurement of the first voltage drop across the first resistor  152  on the second measuring path  150 , the measuring device  130  is provided with an operational amplifier  158  which, on its input side, is connected to one terminal of the first resistor  152  on the second measuring path  150  which is not connected to the housing potential  25  and, on its output side, is connected to the measuring input  159  of the evaluation unit  160  which is assigned to the second measuring path  150 . The semiconductor switches  147 ,  157  are designed respectively for the switching of a positive or negative voltage of a maximum voltage magnitude VH, i.e. for the switching of a voltage between −VH and +VH and, accordingly, only fulfil such technical requirements as are necessary for the conduction or blocking of measuring signals arising during corresponding insulation resistance measurements. 
         [0029]    During the passive functional mode of the measuring device  130 , in which no insulation resistance measurement occurs, the relays  145 ,  155  on the measuring paths  140 ,  150  continue to remain closed, and the measuring paths  140 ,  150  are electrically isolated from the battery  20  by means of the semiconductor switches  147 ,  157 , in which the semiconductor switches  145 ,  157  are respectively opened or maintained in an open state. 
         [0030]    In the first form of embodiment of the invention, the measuring device  130  is preferably located in a battery control device  131  of the battery system  100 . 
         [0031]    The battery system  100  moreover comprises a protective device for the protection of the measuring device  130 , having a control unit  170  which, in the first form of embodiment of the invention, is a central microcontroller  170  of the battery control device  131 , which can also comprise the evaluation unit  160 . In this case, the electromechanical relays  145 ,  155  on the measuring paths  140 ,  150  are directly controlled by means of the central microcontroller  170 . 
         [0032]    If the battery control device  131  is in a test mode in which, for the execution of a dielectric withstand test, a positive or negative test voltage ±VT in the high-voltage range is applied between at least one high-voltage terminal  21 ,  22  of the two high-voltage terminals  21 ,  22  of the battery  20  and the housing potential  25 , the relays  145 ,  155  on the measuring paths  140 ,  150  are opened by means of the microcontroller  170 . The relays  145 ,  155  remain open over an entire test period. During the test mode or throughout the entire test period, the measuring device  130  to be protected is thus securely galvanically separated from the battery  20  or from the high-voltage network  70 . The open contacts of the relays  145 ,  155  block the externally applied test voltage ±VT. The microcontroller  170  is moreover connected to a vehicle interface  171  which, via a bus system interface  172 , is connected to a dielectric withstand test device  173  for the execution of dielectric withstand tests. 
         [0033]      FIG. 3  shows a battery system  200  according to a second form of embodiment of the invention. The battery system  200  comprises a battery  20  for the supply of a high-voltage network  70  with electrical energy. The battery  20  delivers a battery voltage VB. The battery  20  is connectable via two further relays  71 ,  72  to the high-voltage network  70 . Via the two further relays  71 ,  72 , a first high-voltage terminal  21  of the battery  20  is directly connectable to a first high-voltage network terminal  73  on the high-voltage network  70 , and a second high-voltage terminal  22  of the battery  20  is directly connectable to a second high-voltage network terminal  74  on the high-voltage network  70 . The battery system  200  moreover comprises a measuring device  130 , which is designed for the measurement of at least one insulation resistance (not represented) which is present between the battery  20  and a housing of the battery  20 . The battery  20 , the at least one insulation resistance and the further relays  71 ,  72  of the battery system  200  according to the second form of embodiment of the invention, together with the associated high-voltage network  70 , are each configured in an equivalent manner to the corresponding components represented in  FIG. 1 . Moreover, the measuring device  130  of the battery system  200  according to the second form of embodiment of the invention is configured in an equivalent manner to the measuring device  130  of the battery system  100  according to the first form of embodiment of the invention. In the interests of simpler representation, the at least one insulation resistance is not shown in  FIG. 3 , and the high-voltage network  70  is represented in a simplified manner, wherein a series-connected arrangement of two capacitors (Y-capacitors), which is present between the two high-voltage network terminals  73 ,  74  of the high-voltage network  70 , is not represented. For the same reason, in the measuring device  130  represented in  FIG. 3 , only two measuring paths  140 ,  150  of the measuring device  130  and two relays  145 ,  155  on the two measuring paths  140 ,  150  are provided with reference numbers. Here again, a first measuring path  140  of the two measuring paths  140 ,  150  is assigned to the first high-voltage terminal  21  of the battery  20 . Here again, moreover, a second measuring path  150  of the two measuring paths  140 ,  150  is assigned to the second high-voltage terminal  22  of the battery  20 . 
         [0034]    The battery system  200  moreover comprises a protective device for the protection of the measuring device  130  which, in the second form of embodiment of the invention, has a monitoring circuit  230  for the control of the two relays  145 ,  155  on the two measuring paths  140 ,  150  of the measuring device  130 . As soon as at least one second voltage is applicable to the measuring device  130  via the further relays  71 ,  72 , i.e. as soon as a second voltage is respectively applicable via the further relays  71 ,  72  between at least one high-voltage terminal  21 ,  22  of the two high-voltage terminals  21 ,  22  of the battery  20  and the housing potential  25 , which second voltage lies outside a provided measuring range of the measuring device  130  and, consequently, has a magnitude which exceeds a maximum magnitude, the two relays  140 ,  150  on the measuring paths  140 ,  150  are automatically opened by the monitoring circuit  230 . To this end, in the conduct of a dielectric withstand test, via the further relays  71 ,  72 , a test voltage ±VT 1 , which is applicable between at least one high-voltage terminal  21 ,  22  of the two high-voltage terminals  21 ,  22  of the battery  20  and the housing potential  25  and defined by a ramp function, is varied until a desired high voltage is achieved, such that the measuring device  130 , even in consideration of a respective time lag on the two relays  145 ,  155  of the two measuring paths  140 ,  150 , can be promptly galvanically separated from the battery  20  or from the high-voltage network  70  by means of the monitoring circuit  230 . To this end, the monitoring circuit  230  is configured for the detection of test voltages ±VT 1 , applicable in either direction of polarity, and for the comparison thereof with a predefined threshold value. 
         [0035]    The monitoring circuit  230  comprises a voltage measuring unit  231 . The voltage measuring unit  231  comprises a first voltage measurement input (not represented) and a first voltage measuring circuit  240  which is assigned thereto. Additionally, the voltage measuring unit  231  comprises a second voltage measurement input (not represented) and a second voltage measuring circuit  250  which is assigned thereto. The first measuring path  140  on the measuring device  130  and the first high-voltage network terminal  73  of the high-voltage network  70  are assigned to the first voltage measurement input and the first voltage measuring circuit  240 . Additionally, the second measuring path  150  on the measuring device  130  and the second high-voltage network terminal  74  of the high-voltage network  70  are assigned to the second voltage measurement input and the second voltage measuring circuit  250 . 
         [0036]    By means of each voltage measuring circuit  240 ,  250 , the second voltage applicable via the two further relays  71 ,  72  on the associated measuring path  140 ,  150 , i.e. the second voltage between the associated high-voltage network terminal  73 ,  74  on the high-voltage network  70  and the housing potential  25 , is detected. Each second voltage thus detected is compared with a predefined threshold value by means of a comparator  245 ,  255  on the voltage measuring unit  231  which is assigned to the corresponding voltage measuring circuit  240 ,  250 . Each voltage measuring circuit  240 ,  250  comprises a resistive voltage divider  241 ,  251  with two resistors, which is connected between the high-voltage network terminal  73 ,  74  of the high-voltage network  70  which is assigned to the corresponding voltage measuring circuit  240 ,  250  and the housing potential  25 , and an operational amplifier  242 ,  252 . In the interests of simpler representation, the resistors of the two voltage dividers  241 ,  251  on the two voltage measuring circuits  240 ,  250  are not provided with reference numbers. The operational amplifier  242 ,  252  of each voltage measuring circuit  240 ,  250  is connected, on its input side, to a connection point by means of which the two resistors of the voltage divider  241 ,  251 , arranged in the corresponding voltage measuring circuit  240 ,  250  and, on its output side, to the comparator  245 ,  255  which is assigned to the corresponding voltage measuring circuit  240 ,  250 . Both comparators  245 ,  255  of the voltage measuring unit  231  are connected respectively, on the input side, to the operational amplifier  242 ,  252  arranged in the associated voltage measuring circuit  240 ,  250  and, on the output side, via an OR function  232  on the voltage measuring unit  231 , to a control unit  233  on the monitoring circuit  230 . 
         [0037]    If one of the two comparators  245 ,  255  of the voltage measuring unit  231  detects the presence of the corresponding second voltage applicable to the measuring device  140  via the two further relays  71 ,  72  which lies outside the provided measuring range of the measuring device  130  and, consequently, has a magnitude which exceeds the maximum magnitude, the two relays  145 ,  155  on the measuring device  130  are opened by means of the control unit  233  and, in consequence, the measuring paths  140 ,  150  are securely separated from the battery  20  or from the high-voltage network  70 . By means of an appropriate latch functionality, the relays  145 ,  155  remain open until such time as notification is delivered to a central microcontroller of a battery control device on the battery system  200  of the termination of the dielectric withstand test or the termination of a corresponding test mode on the battery control device such that, in consequence, no further test voltage ±VT 1  is present between at least one high-voltage network terminal  73 ,  74  of the two high-voltage network terminals  73 ,  74  on the high-voltage network  70  and the housing potential  25 . 
         [0038]    In addition to the preceding written disclosure, for the further disclosure of the invention, reference shall hereby also be made to the representation in  FIGS. 2 and 3 .