Abstract:
A battery module includes lithium-ion battery cells and at least one discharge circuit configured to discharge the battery cells. The discharge circuit includes a control signal input, a switch, and a resistor and is configured to close the switch in reaction to a control signal at the control signal input, in order to electrically connect terminals of the battery module. The battery module can then be reliably discharged by a corresponding control signal.

Description:
This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2012 213 926.5, filed on Aug. 7, 2012 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present disclosure relates to a battery module comprising battery cells, for example lithium-ion battery cells. Furthermore, the present disclosure relates to a battery management system and a system for supplying a drive of a machine suitable for generating torque with electrical energy. Furthermore, the present disclosure relates to a motor vehicle. 
     High-power battery systems for generating AC voltage often comprise a battery based on lithium-ion technology and an inverter having power switches arranged in series on parallel current branches. If the inverter of such a battery system comprises three or more parallel current paths, then the battery system can be embodied directly for supplying a machine suitable for generating torque for the driving of hybrid and electric vehicles. Such battery systems are also designated as traction battery systems or as traction batteries for short. 
     In order to obtain the power and energy data required in hybrid and electric vehicles and in other AC voltage-based applications, batteries have electrical voltages of up to 450 volts. In traction batteries, for this purpose, individual battery cells are connected in series and in part additionally in parallel. 
     In high-power batteries, therefore, an electrical voltage limit of 60 volts which is classified as non-critical with regard to contact by humans, is usually exceeded. 
     The basic circuit diagram of a battery module  220  according to the prior art is illustrated in  FIG. 1 . Alongside the battery cells  140  of the battery module  220 , the battery system also has a so-called charging and disconnecting device  130 , which in  FIG. 1  is arranged between the positive pole of the battery system and the battery cells  140  of the battery module  220 . By means of a disconnecting switch  120  and a disconnecting switch  125 , a positive pole of the battery cells can be electrically disconnected from the positive pole of the battery system. By means of the disconnecting switch  120 , the positive pole of the battery cells can also be electrically connected to the positive pole of the battery with low impedance, in other words with low resistance. With the disconnecting switch  120  open, the positive pole of the battery cells can also be electrically connected to the positive pole of the battery via a charging current source  110  by means of the disconnecting switch  125 . As an optional functional unit, a further disconnecting device  170  is illustrated in  FIG. 1 , by means of which the battery cells  140 , if required, can be disconnected from the negative pole of the battery system in a two-pole manner via a second disconnecting switch  150 . The battery system in  FIG. 1  additionally has a service disconnect plug  160 . This is a mechanical disconnecting plug which, in the event of accidents or other hazards, can be withdrawn by rescue services or maintenance personnel, in order to disconnect the battery in a one-pole manner. The arrangement of the service disconnect plug  160  in  FIG. 1  is by way of example: a non-symmetrical arrangement is likewise possible. 
       FIG. 2  illustrates the basic circuit diagram of an electrical drive system according to the prior art, as is used for example in electric and hybrid vehicles. The electric machine  200 , which is embodied as a polyphase machine, for example, is supplied via an inverter or pulse-controlled inverter  210 . 
     In the case of the battery systems currently known it is customary that in the event of a critical state being identified, such as an accident, for example, in which the restraint systems are triggered, the battery are disconnected from the on-board traction power supply system of the vehicle. If possible by virtue of two disconnecting devices being present, two-pole disconnection from the inverter can be effected in this case. 
     SUMMARY 
     The disclosure presents a battery module for discharging a battery which a battery system comprises. The battery module is characterized by at least one discharge circuit for discharging battery cells which the battery module comprises, wherein the discharge circuit comprises a control signal input, a switch and a resistor and is designed to close the switch in reaction to a control signal at the control signal input, in order to electrically connect terminals of the battery module. 
     The battery module can then be reliably discharged by means of a corresponding control signal. Internal short circuits that can still occur for example even a relatively long time after a mechanical force action can then no longer lead to the cells catching fire. 
     In one embodiment, the resistor is designed such that the battery cells can be discharged proceeding from a full charge state within a predefined time. 
     By means of time monitoring it is then possible to ascertain when the module is fully discharged. 
     In this case, the switch can be an electromechanical switch or a semiconductor switch. 
     This allows simple switching. 
     It is furthermore advantageous that the battery cells of the battery module are grouped in battery cell strings, wherein each battery cell string comprises a discharge circuit. 
     The discharging duration can thus be shortened. 
     An electrical connection situated between two of the battery cell strings can comprise a withdrawable service disconnect plug, wherein the at least one electrical connection is interrupted when the service disconnect plug is withdrawn. 
     This increases the safety from discharge. 
     If the battery module comprises an electronic unit for matching charge states of the battery cells by discharging the battery cells via a parallel circuit formed by ohmic resistors, the electronic unit can be used also to discharge the battery cells via the parallel circuit in reaction to the control signal. 
     The discharging duration can thus be shortened further. 
     The disclosure furthermore proposes a battery management system for a battery system, wherein the battery system comprises at least one battery module according to the disclosure. The battery management system is characterized in that the battery management system comprises a control signal output and is designed to provide a control signal at the signal output, wherein the control signal provided is suitable for causing the switch of the discharge circuit to be closed. 
     In one embodiment of the battery management system for the embodiment of the battery module which comprises an electronic unit for matching charge states of the battery cells by discharging the battery cells via a parallel circuit formed by ohmic resistors, the control signal provided is furthermore suitable for causing a discharge via the parallel circuit. 
     The disclosure finally proposes a motor vehicle comprising a machine suitable for generating torque from electrical energy and a system for supplying the machine with electrical energy. The motor vehicle is characterized in that the system comprises a battery module according to the disclosure and a battery management system according to the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the disclosure are explained in greater detail with reference to the drawings and the following description. 
         FIG. 1  shows a basic circuit diagram of a battery system according to the prior art; 
         FIG. 2  shows a basic circuit diagram of a drive system according to the prior art; 
         FIG. 3  shows a basic circuit diagram of a first exemplary embodiment of the battery module according to the disclosure, and 
         FIG. 4  shows a basic circuit diagram of a second exemplary embodiment of the battery module according to the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Development-accompanying tests on battery systems for generating AC voltage such as, for example, lithium-ion batteries having an inverter with power switches arranged in series on parallel current branches reveal that in battery systems which initially caused no problems after mechanical tests despite severe mechanical force actions on the batteries during the test, cases repeatedly occurred in which the initially inconspicuous battery systems caught fire weeks after the mechanical tests. 
     The exemplary embodiments of the present disclosure as described in greater detail below allow battery cells to be transferred to a safe state by controlled discharge after an accident or in the event of serious technical problems, such as, for example, imminent overcharging of the battery cells by a charger not functioning properly. Even in the case of the battery remaining in this safe state for a relatively long time, this cannot lead to a fire. The battery cells are discharged here to an extent such that, for example, internal short circuits can no longer lead to the cells catching fire. 
     One exemplary embodiment of the disclosure provides for carrying out, immediately after identifying an accident or a serious technical problem, by means of a battery management system, firstly at least one-pole or better, if possible, two-pole disconnection of the battery  220  by means of the opening of the disconnecting switches. 
     Subsequently, the battery management system sends a command for closing the switch  320  via the communication interface of the cycling-down circuit  300 . With the switch  320  closed, the battery cells  140  are then discharged via the resistor  310  of the cycling-down circuit  300 . 
     If a plurality of discharge circuits  301  and  302  are present, which are respectively provided for discharging battery cell strings  141  and  142  contained in the battery module, the battery management system sends the control command for closing the switches in all the battery cell strings  141  and  142 . This is illustrated by way of example for two strings in  FIG. 4 . 
     In an exemplary embodiment that is not illustrated in the drawings, the battery module additionally has an electronic unit for charge equalization. This is also designated as cell balancing. With the aid of the electronic unit it is possible to discharge the cells in a targeted manner via a parallel circuit formed by ohmic resistors and thus to match the charge states of the cells to one another. The exemplary embodiment which is not illustrated in the figures in this case comprises a driving possibility for driving the electronic unit, such that the cells are discharged simultaneously via both the cycling-down or discharge circuit and the cell balancing electronic unit. This form of additional discharge is independent of whether disconnecting devices have been disconnected the battery from a load, for example an electric motor supplied via an inverter, or whether a service disconnect plug between two battery cell strings contained in the module has been withdrawn. 
     In another exemplary embodiment which is not illustrated in the drawings, the battery module together with an inverter or pulse-controlled inverter serves for supplying an electric machine, embodied for example as a polyphase machine. The inverter comprises at least parallel current branches having power switches which are connected in series and which can be switched by external driving of the inverter. The inverter is designed for external driving which has the effect that power switches in at least one of the parallel current branches are switched on simultaneously. The other exemplary embodiment which is not illustrated in the figures in this case comprises a driving possibility for driving an electronic unit of the inverter and the disconnecting devices, such that the cells are discharged simultaneously both via the cycling-down or discharge circuit and via the charging current source and the inverter. 
     This other embodiment which is not shown is especially suitable for a system without a service disconnect plug, or for situations in which the service disconnect plug is not withdrawn. However, additional discharging via the charging current source and the inverter can always be initiated by corresponding driving by the battery management system, independently of whether or not such additional discharging is made impossible by withdrawal of the service disconnect plug or a defect on the inverter. 
     In the case of additional discharging via the charging current source  110 , the latter has to be designed for the current intensities that occur during the additional discharging. Alternatively, the battery management system can check, during discharging, whether an overloading of the current source  110  is imminent. If so, the charging current source  110  is temporarily switched off. In this case, an overloading can be identified by the battery management system in a model-based manner for example. After the checking has revealed that the charging current source  110  has recovered from the overloading on account of the switch-off and is again ready to discharge the battery cells  140 , it is switched on again by the battery management system and the battery  220  is discharged further. 
     Whether via the cycling-down circuit alone or with the aid of cell balancing electronic unit and/or inverter, the cells  140  of the battery system are in this way discharged to an extent such that an uncontrolled internal or external short circuit occurring later can no longer lead to a hazard. 
     After ascertaining that the battery  220  is sufficiently discharged, in the other exemplary embodiment which is not shown, the battery  220  can again be disconnected from the inverter  210  in a two-pole manner by the opening  11  of the disconnecting switches  120  and  150  and the simultaneous switch-off  2  of the charging current source  110 . 
     In principle, it is expedient to transfer the battery cells  140  to a discharged state in the event of technical problems. As an example, mention shall be made of a charging process for a battery  220  in an electric vehicle in which the charger, on account of a fault, does not reduce the charging current even though the battery  220  is fully charged. 
     In this case, the battery  220  would be discharged in one of the ways described.