Abstract:
The disclosure describes a method for operating a battery system having at least a first battery module and a second battery module. The method includes activating the first battery module for a defined clock time, then activating the second battery module for the defined clock time, and at the same time deactivating the first battery module.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to patent application no. DE 10 2012 207 806.1, filed on May 10, 2012 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    The present disclosure relates to a method for operating a battery system, to a battery system which can be operated in accordance with the method, and to a motor vehicle having the battery system. 
         [0003]    Battery systems for use in motor vehicles, also called traction batteries, constitute, as is known, an interconnection of a plurality of individual battery cells or battery modules which comprise battery cells. Battery systems of this kind have a comparatively high battery capacity in comparison to consumer batteries. In order to be able to deliver a sufficient amount of power in every drive state of the motor vehicle, for example during start-up, battery systems of this kind are designed to be resistant to high currents. The rated voltage of the battery system is defined by series interconnection of battery cells or battery modules, and parallel connection of battery cells or battery modules is frequently additionally employed in order to increase the battery system capacity. 
         [0004]    Conventional batteries have an inherent internal resistance. A voltage loss which reduces the voltage which is output by the battery is dropped across the internal resistance. If current is drawn from a battery, that is to say the battery is subjected to loading, the internal resistance of said battery changes dynamically in the meantime. If, for example, a lithium-ion battery is discharged with a current, for example 1 C at a battery capacity of 60 Ah, the profile for the output voltage of the lithium-ion battery which is shown in  FIG. 1  is established. After a current jump, the voltage initially runs at a constant plateau and then drops since the internal resistance increases in the meantime. 
         [0005]    DE 10 2010 041 014 also discloses a method for operating a battery system. The battery system has a battery which, for its part, has a battery module line with a plurality of battery modules which are connected in series. Each battery module comprises a coupling unit which is designed to decouple the battery module from the battery module line, to bridge the battery module or to connect the battery module to the other battery modules in series, so that a voltage across the battery module line can be variably adjusted, that is to say can be increased or lowered. 
       SUMMARY 
       [0006]    The disclosure provides a method for operating a battery system having at least a first battery module and a second battery module. The method comprises at least the following steps:
       activating the first battery module for a defined clock time;   then activating the second battery module for the defined clock time and at the same time deactivating the first battery module.       
 
         [0009]    The disclosure further provides a battery system having at least a first battery module and a second battery module and a control unit, wherein the control unit is designed to carry out the method according to the disclosure. 
         [0010]    The disclosure also provides a motor vehicle which comprises the battery system, wherein the battery system is connected to a drive system of the motor vehicle. 
         [0011]    The method according to the disclosure can prevent the creation of high internal resistances in the battery modules. When a battery cell or a battery module which comprises a battery cell is discharged, a high internal resistance of this kind can be created in the battery module in particular by a large amount of current being drawn. In particular, a depletion effect can occur in the battery module in the event of discharge at high currents. This effect causes a drop in the voltage provided by the battery module. The effect is intensified, in particular, at a low state of charge, at a low temperature and at a high long-lasting current loading of the battery module. Furthermore, the effect can accelerate the aging of a battery module. 
         [0012]    The method according to the disclosure can reduce the creation of a high internal resistance and, in particular, the occurrence of a depletion effect by, in particular, activating the first battery module only for the defined clock time and then, after the clock time has elapsed, activating the second battery module instead of the first battery module. The first battery module is, in particular, deactivated as soon as the second battery module is activated, as a result of which the total voltage of the battery system does not substantially change but the currently unused or deactivated battery module can recover, that is to say it can reduce the state of depletion by chemical compensation processes and it can reduce its internal resistance. If the unused battery module is reactivated after a specific recovery time, its internal resistance is lower. A lower voltage drop and power loss are created in the battery module at a low internal resistance. The method according to the disclosure can further increase the efficiency of the battery system, compensate for non-uniform loading or aging of battery modules, avoid depletion effects due to permanent operation of a battery module and improve the service life of the battery system by virtue of more uniform loading of the battery modules. 
         [0013]    The method according to the disclosure regenerates, in particular, the deactivated first battery module. The first battery module is preferably deactivated for a prespecified recovery time during which chemical compensation processes reduce the internal resistance of the battery module and thus compensate for any existing state of depletion of the battery module. 
         [0014]    In a preferred refinement, the method can further comprise at least one of the following steps:
       detecting the temperature of one of the battery modules   detecting the state of charge of one of the battery modules   detecting the voltage of one of the battery modules.       
 
         [0018]    To this end, the battery modules of the battery system comprise, in particular, a temperature sensor, a voltage-measuring unit and/or a control unit, wherein the control unit is preferably designed such that it can determine the state of charge of the battery module, for example, from the detected voltage. 
         [0019]    Furthermore, the method comprises, in particular, the step of: 
         [0020]    defining the clock time as a function of at least one of the detected values from amongst temperature, state of charge and voltage. In particular, the clock time is shorter when the temperature falls. In the same way, the clock time is longer when the temperature increases. The clock time is further shortened when the state of charge is lower. In the same way, the clock time is extended when the state of charge increases. In English, the state of charge is abbreviated to SOC. 
         [0021]    When a state of depletion occurs in one of the battery modules when a large amount of current is drawn, a voltage profile which has a point of inflection is generally formed on the battery module. This point of inflection can further be determined by detecting the voltage of the battery module. If the first activated battery module reaches this point of inflection of the voltage, the clock time can be defined such that the activated battery module is deactivated and, at the same time, the second battery module is activated instead when the point of inflection of the voltage is reached. 
         [0022]    The control unit of the battery system is preferably designed such that it can activate and deactivate the battery cells. The battery modules preferably comprise a coupling unit having controllable switching elements. These switching elements can be actuated by the control unit, in particular, in such a way that the battery module provides a battery module voltage, that is to say is activated, or that the battery module provides a voltage of zero volts, wherein the battery module is bridged in this case, that is to say is deactivated. 
         [0023]    The battery system can be connected to a pulse-controlled inverter, which inverts a DC voltage which is supplied by the battery modules, via a DC voltage intermediate circuit. As an alternative, the battery system can be in the form of a battery direct inverter, wherein a plurality of battery modules, connected in series, form a battery module line and the battery system comprises a plurality of the battery module lines. In this case, the battery modules of a battery module line can preferably be actuated in such a way that they directly generate an alternating voltage and feed a load with said voltage. The plurality of battery module lines form, in particular, a polyphase system. 
         [0024]    The battery modules preferably comprise lithium-ion battery cells; therefore, the battery system is, in particular, a lithium-ion battery system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Exemplary embodiments of the disclosure will be explained in greater detail with reference to the following description and the drawings, in which: 
           [0026]      FIG. 1  shows a graph of a known voltage profile of a battery module, 
           [0027]      FIG. 2  shows a graph of a voltage profile of a battery module of a battery system according to the disclosure, 
           [0028]      FIG. 3  shows a battery system according to an exemplary embodiment of the disclosure, 
           [0029]      FIG. 4  shows a further battery system according to an exemplary embodiment of the disclosure, and 
           [0030]      FIG. 5  shows a graph for illustrating an exemplary embodiment of the method according to the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIG. 2  shows a graph of a voltage profile of a first battery module  306 . A time axis with seconds values forms the abscissa of the graph. The ordinate of the graph indicates values of the voltage profile or of the battery module voltage in volts. A high current is drawn from the battery module  306  starting from time 0 seconds. The battery module voltage drops on account of the large amount of current drawn. 
         [0032]    The battery module comprises at least one lithium-ion battery cell. A depletion of ions on the molecule surface of an electrode of the lithium-ion battery cell occurs in this at least one battery cell on account of the large amount of current drawn. This chemical process, also called the depletion effect, delays a release of ions from the electrode, and an internal resistance of the at least one lithium-ion battery cell increases. Therefore, the voltage of the battery module drops in such a way that the voltage profile exhibits a point of inflection, as can be seen in  FIG. 2  approximately at 70 seconds and 2.95 volts. 
         [0033]      FIG. 3  shows a battery system  300  according to an exemplary embodiment of the disclosure. The battery system  300  is connected to a pulse-controlled inverter  302  via a capacitor  301  which forms a DC voltage intermediate circuit. The pulse-controlled inverter  302  inverts a DC voltage which is generated by the battery system  300  into a three-phase AC voltage. The AC voltage feeds an electric motor  304  which is connected to the pulse-controlled inverter  302 . The battery system  300  comprises a plurality of battery modules, of which, by way of example, a first battery module  306  and a second battery module  308  are shown in  FIG. 3 . The battery modules  306 ,  308  form a series circuit. Charging disconnection elements  310 ,  311  are arranged at the ends of the series circuit, said charge disconnection elements being able to disconnect the series circuit of the battery module  306 ,  308  from a load or from the capacitor  301  and the pulse-controlled inverter  302  as required. The charging disconnection elements  310 ,  311  can also connect the battery modules  306 ,  308  in series with a series charging resistor in order to limit, for example, high charging currents. The battery system  300  also comprises a control unit  312 . 
         [0034]    In addition to the at least one lithium-ion battery cell, the battery modules  306 ,  308  each comprise a controllable coupling unit which is connected to the at least one lithium-ion battery cell. The coupling unit comprises controllable switching elements which are actuated by the control unit in such a way that a battery module provides its battery module voltage or provides a voltage of zero volts, wherein the battery module is bridged in this case. 
         [0035]    The control unit  312  is further designed such that it activates the first battery module  306  by means of the coupling unit for a defined clock time T t  and, after the clock time T t  has elapsed, activates the second battery module  308  by means of the coupling unit for a further clock time T t , wherein the control unit  312  deactivates the first battery module  306  by means of the coupling unit at the same time as the second battery module  308  is activated. In a similar way, a third battery module can be activated after the further clock time T t  has elapsed, wherein the second battery module  308  is deactivated at the same time, and so on. In a similar manner, a first group of battery modules can also be activated for the clock time T t  and then a second group of battery modules can be activated for the clock time T t , wherein the first group of battery modules is deactivated at the same time. 
         [0036]    While the first battery module  306  is deactivated, it can regenerate, it can counteract the depletion effect, in particular, by chemical compensation processes and, as a result, lower its internal resistance. 
         [0037]    The control unit  312  is also designed to detect various measurement values. The measurement values include: the temperature of the battery system  300 , the temperature of the individual battery modules  306 ,  308 , the voltage of the individual battery modules  306 ,  308 , that is to say the battery module voltages and the state of charge of the individual battery modules  306 ,  308 . The control unit  312  dynamically defines the clock time T t  as a function of these measurement values. The clock time T t  is shortened, for example, when the state of charge is lower. In the same way, the clock time T t  is extended when the state of charge increases. 
         [0038]    The point of inflection of the voltage profile, as shown in  FIG. 2 , is determined by the battery module voltages being detected by the control unit  312 . If the first activated battery module  306  reaches this point of inflection of the voltage, the clock time T t  can be defined such that the second battery module  308  is activated when the point of inflection of the voltage is reached and the first battery module  306  is deactivated at the same time. 
         [0039]      FIG. 4  shows a battery system  400 . In contrast to the battery system  300 , the battery system  400  can be connected directly to the electric motor  304 . Said battery system comprises three battery module lines  402  which each form a phase. The battery module lines  402  each have a plurality of battery modules, of which, by way of example, the first battery module  306  and the second battery module  308  are shown in  FIG. 4  in each case. The battery system  400  also has a control unit  404 . The control unit  404  actuates the coupling units of the battery modules  306 ,  308  in such a way that the battery module lines  402  provide AC voltages. The three battery module lines  402  form a three-phase system in this case. 
         [0040]    The control unit  404  further comprises the same functions as the control unit  312 . In particular, the control unit  404  activates the first battery module  306  of a battery module line  402  for the clock time T t  and then activates the second battery module  308  for the clock time T t  and, at the same time, deactivates the first battery modules  306 . The control unit  404  controls the battery modules  306 ,  308  in an intelligent manner such that the moment voltage provided by the battery module line  402  remains constant. The battery system  400  therefore also allows regeneration of battery modules while AC voltage is provided. 
         [0041]      FIG. 5  shows a graph  500  for illustrating an exemplary embodiment of the method according to the disclosure. A time axis forms the abscissa of the graph  500 . The ordinate of the graph  500  shows activation states  501 ,  502 ,  503 ,  504  of four battery modules and voltage values of four battery module voltages  506 . The profile of the battery module voltage  506  is formed by a large amount of current being drawn from a battery module and corresponds to the voltage profile which is shown in  FIG. 2 . The control unit  312 ;  404  can actuate the first battery module  306  in such a way that the activation states  501  are present and it can actuate the second battery voltage  308  in such a way that the activation states  502  are present. In a similar way, the control units  312 ;  404  can actuate further battery modules, so that the further activation states  503 ,  504  are present. A method, which is carried out by the control unit  312 ;  404 , for operating the battery system  300 ;  400  having at least the first battery module  306  and the second battery module  308  exhibits the following steps. 
         [0042]    In a first method step  508 , the control unit  312 ;  404  activates the first battery module  306  for the clock time T t . The clock time T t  was defined at the point of inflection of the battery module voltage  506  by the control unit  312 ;  404 . Then, that is to say, in particular, after the clock time T t  has elapsed, the control unit  312 ;  404  activates the second battery module  308  for the defined clock time T t  in a second method step  510 . In a third method step  512 , the control unit  312 ;  404  deactivates the first battery module  306  at the same time. 
         [0043]    In this case, the control unit  312 ;  404  deactivates the first battery module  306  for a recovery time T e . The recovery time T e  permits chemical compensation processes which lower the internal resistance of the first battery module  306 . 
         [0044]    The battery system  300 ;  400  having the first battery module  306 , the second battery module  308  and the control unit  312 ,  404  which carries out the method can be used, in particular, in motor vehicles. Motor vehicles of this kind can comprise the electric motor  304  which forms a drive system. In this case, the method according to the disclosure increases the range of the motor vehicle and improves the reliability since non-uniform loading and aging of the battery modules are reduced.