Abstract:
A battery system comprises at least one battery cell and a high-voltage network connected thereto which includes a pre-charge circuit having at least one pre-charge resistor. The battery system further comprises a component including a link capacitor with a specific capacitance. A method for controlling the battery system includes measuring a first voltage at the link capacitor before charging, charging the link capacitor, and measuring a second voltage at the link capacitor after charging. The method further includes forming a voltage difference from the first and the second voltage, and determining an energy received by the pre-charge resistor based on the voltage difference at the link capacitor and based on the capacitance of the link capacitor.

Description:
[0001]    This application claims priority under 35 U.S.C. § 119 to patent application no. DE 10 2012 213 057.8, filed on Jul. 25, 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 controlling a battery system, a battery system having a battery management unit that is designed to carry out the method, and a motor vehicle having the battery system. 
         [0003]    In hybrid and electric vehicles, battery packs are connected by power contactors to the further vehicle components, such as the drive, booster generator, charging plug, etc. Often, this component is fed via a device that generates a single-phase or multi-phase a.c. voltage or pulsed d.c. voltage from the battery voltage. Due to the peak loads that occur here, such devices are provided with an electrical storage device, normally a capacitor. Such capacitors generally have a high capacitance and are also referred to as link capacitors. 
         [0004]    If a vehicle component is to be connected, a link capacitor is first charged and the vehicle component itself is then put into operation. Such a link capacitor is normally charged using a pre-charge circuit. Here, quick charging generates high currents in feed lines, structural elements of the pre-charge circuit, and the link capacitor. These currents can reduce the service life of these structural elements. Slow charging is gentle on the structural elements, but requires correspondingly more time before the vehicle components can be put into operation. 
         [0005]    DE 10 2010 038 892 A1 also describes a monitoring unit that captures operating data of a pre-charge circuit in order to estimate the current temperature of a pre-charge resistor. The monitoring unit does not use a temperature sensor arranged on the pre-charge resistor for this purpose, but measures the current flowing through the pre-charge resistor, a battery voltage, the number of starting operations per unit of time, the duration of the starting operations and the ambient temperature, and estimates from this the current temperature. If the current temperature lies above a threshold value, the pre-charge resistor may thus be overheated and cannot be operated further. 
       SUMMARY 
       [0006]    In accordance with the disclosure, a method for controlling a battery system is provided. The battery system comprises at least one battery cell and a high-voltage network connected thereto which comprises a pre-charge circuit having at least one pre-charge resistor. The battery system further comprises a component having a link capacitor with a specific capacitance. The method comprises at least the following steps: measuring a first voltage at the link capacitor before charging, charging the link capacitor, measuring a second voltage at the link capacitor after charging, forming a voltage difference from the first and second voltage, and determining an energy received by the pre-charge resistor based on the voltage difference at the link capacitor and based on the capacitance of the link capacitor. 
         [0007]    A battery system having a battery management unit that is designed to carry out the method is also proposed. 
         [0008]    In addition, a motor vehicle having the battery system is proposed, the battery system being connected to a drive system of the motor vehicle. 
         [0009]    The method according to the disclosure makes it possible to determine the energy actually received by the pre-charge resistor. In order to limit a maximum thermal loading of the pre-charge resistor, known methods usually count the number of link capacitor charging operations carried out. If a specific number is exceeded, charging via the pre-charge resistor was previously prevented. This conventional counting method does not take into consideration however the fact that the link capacitor can also be charged a number of times in succession by just a low voltage, such that the heat energy output by the pre-charge resistor over a considered period of time is lower than with practically complete charging of the link capacitor. 
         [0010]    Furthermore, the method according to the disclosure makes it possible to determine a thermal loading of the pre-charge resistor and to thus attain a better availability of a battery system. The service life of the components involved in the pre-charging operation can be increased. The reliability of battery systems can also be improved by means of the method according to the disclosure. 
         [0011]    In a further embodiment of the method, an energy received by the pre-charge resistor in a worst-case scenario can be determined. The worst-case scenario in particular comprises a situation in which the pre-charge resistor is exposed for a specific time to a high current. That is possible in the event of a fault, wherein the connected component is short-circuited and the full voltage at the pre-charge resistor drops until the battery management unit interrupts the pre-charging operation. If the full battery voltage is applied to the pre-charge resistor, the received energy can be determined by forming a quotient from battery voltage squared divided by the ohmic resistance of the pre-charge resistor, wherein the quotient is multiplied by the duration of the high current flow. The pre-charge resistor is preferably designed such that it withstands such individual current pulse loads. 
         [0012]    In a further preferred method step, the energy output by the pre-charge resistor can be determined. In order to determine whether the pre-charge resistor has received or is about to receive a critical energy, the balance of received energy and output energy is to be determined in particular. Here, the output energy is substantially dependent on the heat capacity of the pre-charge resistor, the temperature thereof, and the temperature difference from the ambient environment. It is also preferable for the pre-charge resistor to determine a maximum power over a specific period of time. This maximum power can be based on a thermal loadability of the pre-charge resistor and on the energy output by the pre-charge resistor. If the maximum power is exceeded over a specific period of time, the pre-charge resistor may be overloaded. 
         [0013]    It is preferable for the method to further comprise the following step: predicting the charging curve of the link capacitor. The charging curve of capacitors generally follows a l-e x  function, wherein x is the quotient with the time in the counter and the pre-charge resistor multiplied by the capacitor capacitance in the denominator. The l-e x  charging curve approximates the charging voltage applied to the pre-charge circuit, that is to say in particular the battery voltage, during the charging of the link capacitor. It is also preferable to measure the charging curve of the link capacitor and to compare the predicted charging curve with the measured charging curve. In the event of a deviation between the predicted charging curve and the measured charging curve, a fault can be determined. The fault may be present in the battery system or in one of its components, for example the link capacitor. 
         [0014]    In a further preferred embodiment, the method may further comprise the following step: discharging the link capacitor via a discharge circuit that comprises a discharge relay and a discharge resistor. 
         [0015]    In a further embodiment, the battery system may comprise at least one battery cell, a high-voltage network and a component. The high-voltage network is connected in particular to the at least one battery cell and basically comprises a pre-charge circuit. The pre-charge circuit may comprise an operational contactor and a series circuit formed from a pre-charge contactor and a pre-charge resistor, wherein the series circuit is connected in parallel to the operational contactor. The component preferably comprises a link capacitor, wherein the pre-charge circuit and the component can form a series circuit with the at least one battery cell. The component further comprises a discharge circuit, which preferably comprises a discharge relay and a discharge resistor connected in series to the discharge relay. The discharge circuit is in particular connected in parallel to the link capacitor. 
         [0016]    The battery system is preferably a lithium-ion battery system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Exemplary embodiments of the disclosure will be explained in greater detail with reference to the drawings and the following description. In the drawings: 
           [0018]      FIG. 1  shows a battery system in accordance with an exemplary embodiment of the disclosure, 
           [0019]      FIG. 2  shows a graph that illustrates a power consumption of a link capacitor, 
           [0020]      FIG. 3  shows a further graph that illustrates a power consumption of a link capacitor, 
           [0021]      FIG. 4  shows a method in accordance with an exemplary embodiment of the disclosure, and 
           [0022]      FIG. 5  shows a method in accordance with a further exemplary embodiment of the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Within the scope of this patent application, the terms “received energy” and “output energy” are used. Received energy is electrical energy as a result of a flow of current. An example of the received energy will be discussed within the scope of  FIG. 2 . The output energy is thermal or heat energy. 
         [0024]      FIG. 1  shows a battery system  100  in accordance with an exemplary embodiment of the disclosure that illustrates a series connection of a plurality of lithium-ion battery cells  102  and a high-voltage network  104 , wherein the high-voltage network  104  is connected to the series connection of the plurality of lithium-ion battery cells  102 . The high-voltage network  104  can be connected to a further component  106 , for example a link capacitor  108 , a discharge circuit, pulse-width-modulation inverter, or other consumers, such as electric motors, etc. 
         [0025]    The high-voltage network  104  comprises a pre-charge circuit that in turn comprises an operational contactor  110  and a series circuit formed from a pre-charge contactor  112  and a pre-charge resistor  114 , wherein this series circuit is connected in parallel to the operational contactor  110 . The pre-charge circuit and the link capacitor  108  form a series circuit with the lithium-ion battery cells  102 . The lithium-ion battery cells  102  form a battery or an accumulator. 
         [0026]    The discharge circuit comprises an end relay  116  and a discharge resistor  118  connected in series to the discharge relay  116 . The discharge circuit is connected in parallel to the component  106  or to the link capacitor  108 . 
         [0027]    The battery system  100  further comprises a battery management unit  120 , which is designed to carry out one of the methods described hereinafter with reference inter alia to  FIGS. 4 and 5 . 
         [0028]    The operation of vehicle components, such as electric motors, booster generators, and charging units as consumers at the battery system  100  causes peak loads, in particular when said vehicle components are switched on or off, said peak loads normally being buffered by an electronic storage device in the battery system  100 . The link capacitor  108  forms such an electronic storage device. If a vehicle component is connected to the battery system  100 , the link capacitor  108  is thus initially charged, then the vehicle component itself can be put into operation. The link capacitor  108  is charged from the lithium-ion battery cells  102  via the pre-charge resistor  114 , wherein the battery management unit  120  controls the pre-charge contactor  112  in such a way that it connects the pre-charge resistor  114  to the lithium-ion battery cells  102 . Once charging of the link capacitor  108  is complete, the battery management unit  120  closes the operational contactor  110  in order to put the vehicle component into operation. 
         [0029]    Quick charging of the link capacitor  108  generates high currents in feed lines and in structural elements of the high-voltage network  104 . The high currents reduce the service life of the feed lines and structural elements concerned. Slow charging would indeed be gentle on the structural elements, but requires correspondingly more time before the vehicle component can be put into operation. 
         [0030]    The current through the link capacitor  108  decreases to the extent that it is charged. In this regard,  FIG. 2  shows the curve of the power received by the pre-charge resistor  114  during charging over the period t l . The area below the power curve shown in  FIG. 2  corresponds to the energy received by the pre-charge resistor. Here, with the onset of charging of the capacitor, a high power P p  is received in the resistor and is converted into heat.  FIG. 2  shows three successive pre-charging operations, for example. 
         [0031]    Here, the loading of a plurality of charging operations carried out corresponds to the individual loads added over the period of time t v , as is shown in  FIG. 3  by the power curve P c . 
         [0032]    A method  400  in accordance with an exemplary embodiment of the disclosure is shown in  FIG. 4 . In a first step  402 , the battery management unit  120  measures a first voltage at the link capacitor  108 . In a subsequent step  404 , the battery management unit  120  controls the pre-charge circuit in such a way that the link capacitor  108  is charged. In a subsequent step  406 , the battery management unit  120  measures a second voltage at the link capacitor  108  after the charging operation and then forms, in step  408 , a voltage difference from the first and the second measured voltage. The battery management unit  120 , in a subsequent step  410 , determines an energy received by the pre-charge resistor. 
         [0033]    A method  500  in accordance with a further exemplary embodiment of the disclosure is shown in  FIG. 5 . In a first method step  502 , a start temperature of the pre-charge resistor is extracted from a non-volatile memory, or for example is determined at an ambient temperature of approximately 60° C. In a subsequent step  504 , an energy output from the pre-charge resistor in the form of heat is determined, wherein the heat output causes a cooling of the pre-charge resistor. 
         [0034]    The heat output and therefore a temperature change of the pre-charge resistor can be determined in step  504  as follows: 
         [0000]    
       
         
           
             T 
             = 
             
               
                 T 
                 - 
                 
                   ( 
                   
                     W 
                     · 
                     
                       1 
                       
                         C 
                         p 
                       
                     
                   
                   ) 
                 
               
               = 
               
                 T 
                 - 
                 
                   
                     ( 
                     
                       
                         G 
                         th 
                       
                       · 
                       
                         ( 
                         
                           T 
                           - 
                           
                             T 
                             
                               ambient 
                               , 
                               max 
                             
                           
                         
                         ) 
                       
                       · 
                       
                         t 
                         elapsed 
                       
                     
                     ) 
                   
                   · 
                   
                     1 
                     
                       C 
                       p 
                     
                   
                 
               
             
           
         
       
     
         [0035]    In this case, T is the determined current temperature of the pre-charge resistor; W is the energy output by the pre-charge resistor; C p  is the heat capacity of the pre-charge resistor in the unit of joules per kelvin; G th  is the thermal conductance in the unit of watts per kelvin at ambient temperature; T ambient,max  is the maximum ambient temperature and t elapsed  is the time elapsed during the heat output. 
         [0036]    In a subsequent step  506 , an energy received by the pre-charge resistor in the worst-case scenario is determined as follows: 
         [0000]    
       
         
           
             
               W 
               
                 w 
                 . 
                 c 
                 . 
               
             
             = 
             
               
                 
                   
                     U 
                     2 
                   
                    
                   
                       
                   
                    
                   battery 
                 
                 
                   R 
                   v 
                 
               
               · 
               t 
             
           
         
       
     
         [0037]    In this case, W w.c.  is the energy received by the pre-charge resistor  114  in the worst-case scenario in the unit of joules, given from the total voltage of the battery cells  102 , that is to say the battery voltage U battery , the ohmic resistance R v  of the pre-charge resistor, and the time t during which energy is received. 
         [0038]    In a subsequent step  508 , the temperature T w.c.  of the pre-charge resistor  114  in the worst-case scenario is determined as follows: 
         [0000]    
       
         
           
             
               T 
               
                 w 
                 . 
                 c 
                 . 
               
             
             = 
             
               T 
               + 
               
                 
                   W 
                   
                     w 
                     . 
                     c 
                     . 
                   
                 
                 · 
                 
                   1 
                   
                     C 
                     p 
                   
                 
               
             
           
         
       
     
         [0039]    In this case, T is the current temperature of the pre-charge resistor  114 , that is to say for example the ambient temperature in a first method run-through. 
         [0040]    In a subsequent step  510 , the condition as to whether the temperature in the worst-case scenario T w.c.  is less than a fixed maximum temperature for the pre-charge resistor is checked. If the condition is met, the method branches to a subsequent step  512 , in which the link capacitor  108  is charged or pre-charged. If the condition in step  510  is not met, the method  500  branches back to step  504 , in which the pre-charge resistor  114  is cooled or the cooling of the pre-charge resistor  114  is determined. 
         [0041]    In step  514 , the energy actually received by the pre-charge resistor is determined as follows: 
         [0000]    
       
         
           
             W 
             = 
             
               W 
               + 
               
                 
                   
                     
                       ( 
                       
                         
                           U 
                           battery 
                         
                         - 
                         
                           U 
                           link 
                         
                       
                       ) 
                     
                     2 
                   
                   
                     R 
                     v 
                   
                 
                 · 
                 
                   t 
                   charge 
                 
               
             
           
         
       
     
         [0042]    In this case, W is the determined energy that is received by the pre-charge resistor; U battery  is the total voltage of the battery; wherein the voltage of connection elements U link  between the battery cells and the pre-charge resistor  114  is subtracted from said total voltage; R v  is the ohmic resistance of the pre-charge resistor; and t charge  is the time elapsed during the charging of the link capacitor  108 . 
         [0043]    In a subsequent step  516 , the condition as to whether the pre-charging operation is complete is checked. If the condition is met, the method  500  branches to the next step  518 . If the condition in step  516  is not met, the method  500  branches back to step  514  and the link capacitor  108  is charged further, during which time the energy received by the pre-charge resistor continues to be determined. 
         [0044]    In step  518 , the heating or the temperature T of the pre-charge resistor  114  present once the pre-charging of the link capacitor  108  has ended is determined as follows: 
         [0000]    
       
         
           
             T 
             = 
             
               T 
               + 
               
                 W 
                 · 
                 
                   1 
                   
                     C 
                     p 
                   
                 
               
             
           
         
       
     
         [0045]    The methods described with reference to  FIGS. 4 and 5  can be used for thermal protection of the pre-charge resistor  114  in the battery system  100 , wherein the battery management unit  120  is designed to carry out such a method. The battery system  120  can in turn be used in a motor vehicle and can provide a greater reliability of the motor vehicle. 
         [0046]    In accordance with Ohm&#39;s law, instead of determining the voltage before and after the pre-charging operation, the flow of current during the pre-charging operation can also be measured. The formulas for the received energy of the pre-charge resistor apply accordingly in this regard. 
         [0047]    In a further exemplary embodiment, a further pre-charge circuit can be used instead of the pre-charge resistor in order to charge the link.