Patent Publication Number: US-2012046892-A1

Title: Determination of the internal resistance of a battery cell of a traction battery while using inductive cell balancing

Description:
PRIOR ART 
     The present invention relates to a method and an apparatus for determining the internal resistance of a battery cell of a battery, in particular a traction battery, as generically defined by the preambles to claims  1  and  6 . 
     It is remarkable that in future, both in stationary applications such as wind farms and in vehicles such as hybrid and electric vehicles, new battery systems will increasingly come into use. In the present specification, the terms battery and battery system are used, in accordance with conventional linguistic usage, for the terms accumulator and accumulator system, respectively. 
     The basic functional construction of a battery system in the prior art is shown in  FIG. 4 . To achieve the requisite power and energy data with the battery system, in a battery cell  1   m  individual battery cells  1   a  are connected in series and sometimes in parallel as well. For a series circuit of battery cells, the basic circuit diagram of a so-called traction battery for hybrid or electric vehicles is shown in  FIG. 5 . Between the battery cells  1   a  and the poles of the battery system is a so-called safety and fuse unit  16 , which for instance takes on the task of connecting and disconnecting the battery  1  to and from external systems and protecting the battery system against impermissibly high currents and voltages and also provides safety functions, such as the unipolar disconnection of the battery cells  1   a  from the battery system poles when the battery housing is opened. A further function unit is formed by the battery management  17 , which in addition to the battery state detection  17   a  also performs communication with other systems as well as the thermal management of the battery  1 . 
     The function unit called battery state detection  17   a  shown in  FIG. 4  has the task of determining the actual state of the battery  1  as well as predicting the future behavior of the battery  1 , such as predicting its service life and/or predicting its range. Predicting future behavior is also called prediction. The basic structure of a model-based battery state detection is shown in  FIG. 6 . The model-based battery state detection and battery state prediction shown is based on an evaluation of the electrical variables of battery current and voltage as well as the temperature of the battery  1  by means of an observer  17   b  and a battery model  17   c  in a known manner. The battery state detection can be done for individual cells  1   a  of a battery  1 ; in that case, this is done on the basis of the corresponding cell voltage, cell current, and cell temperature. The battery state detection can also be done for the entire battery  1 . This is then done—depending on the requisite precision—either by evaluating the states of the individual cells  1   a  of the battery and an aggregation, based on that, for the entire battery  1 , or directly by evaluating the total battery voltage, the battery current, and the battery temperature. A common feature of all the methods in the prior art is that the courses of current, voltage and temperature that occur in normal operation of the battery  1  are used both for determining the battery state and for predicting the future behavior. 
     In  FIG. 7 , the functional principle of an arrangement for so-called resistive cell balancing of battery cells  1   a  is shown. The task of cell balancing is, in a series circuit of a plurality of individual cells  1   a , to ensure that the cells  1   a  all have the same state of charge and the same cell voltage. Because of the intrinsic asymmetries among the battery cells  1   a , such as slightly different capacitance and slightly different self-discharging, this could not be done without additional provisions while the battery is in operation. In resistive cell balancing, the battery cells  1   a  can be discharged by switching on an ohmic resistor  2  disposed parallel to the cell. In  FIG. 6 , the resistor  2  is switched on with the value R Bal     —     n  via the transistor  10  (T Bal     —     n ) parallel to the cell  1   a  having the number n. By discharging those cells  1   a  that have a higher state of charge and a higher voltage than the cells  1   a  with numbers n with the least state of charge and the least voltage, the states of charge or voltages can be made symmetrical over all the cells  1   a  of the battery  1 . The voltage applied to a cell  1   a  is supplied, for evaluation, via a filter comprising two resistors  11 ,  12  and a capacitor  13  and via an A/D converter  14 , to a control and evaluation unit  15 , of which there is one for each cell  1   a , and which communicates with a higher-order central control unit, such as the battery status detector  17   a . In lithium-ion batteries, which comprise a series circuit of a plurality of individual cells  1   a , the use of resistive cell balancing is state of the art. Still other methods for cell balancing exist that can in principle function without loss, such as so-called inductive cell balancing. 
     In  FIG. 8 , the fundamental principle of so-called inductive cell balancing is shown. Here all the cells  1   a  are connected to a circuit for cell voltage detection and for control of the inductive cell balancing, which circuit has an inductive resistor  2  as an energy storing means. The term inductive cell balancing is used when the circuit concept for adapting the cell voltages or the state of charge of the cells is based on an inductive buffer storage of the electrical energy transported in the process. The buffer storage can—depending on the circuit concept—be done in chokes or repeaters. 
     In inductive cell balancing, in a first step, energy is drawn from one or more cells and buffer-stored in the inductive resistor  2 . In a second step, the buffer-stored energy is re-stored into one or more battery cells  1   a . As examples, the following can be named:
         drawing energy from one cell and re-storing it in one or more cells, in which re-storing is not done into the cell from which the energy was drawn;   drawing energy from one cell and re-storing it in one or more cells, with some of the energy being re-stored in the cell from which energy from drawn;   drawing energy from a plurality of cells and re-storing it in one or more cells, without re-storing it into those cells from which energy was drawn;   drawing energy from a plurality of cells and re-storing it in one or more cells, with some of the energy being re-stored into those cells from which energy was drawn.       

     In  FIG. 9 , as an example a circuit principle for inductive cell balancing is shown, in which chokes are used for buffer-storing energy. If the battery cell  1   a  (n) is discharged, for instance because it has a higher state of charge than other cells of the battery system, then by the switching on of the transistor  10  (T Bal     —     n ), a current is built up via the chokes  2   a  (L n     —     upper ) and  2   b  (L n     —     lower ), by which current the cell  1   a  (n) is discharged. After the shutoff of the transistor  10  (T Bal     —     n ), the current is switched through the choke  2   a  (L n     —     upper ) into a current path via the diode  2   c  (D n+1     —     1 ) and charges the battery cell  1   a  (n+1), and the current through the choke (L n     —     upper ) is switched into a current path via the diode  2   d  (D n−1     —     1 ) and charges the battery cell  1   a  (n−1). In this way, with the arrangement shown in  FIG. 9 , energy can be drawn from one cell  1   a  (n) and transported into the two adjacent cells  1   a  (n+1, n−1), in order to perform the cell balancing. 
     In  FIG. 9 , for the sake of clarity, the detection of the cell voltages is not shown. Such detection is necessary in lithium-ion batteries both for determining the state of charge and for performing the cell balancing as well as for monitoring the adherence to the upper and lower voltage limit values of the cells. Thus the information on the individual cell voltages is available to the system. 
     It is the object of the present invention to present a novel concept for determining the internal resistance of the individual cells of a battery system, with which the battery state detection and prediction, compared to the present state of the art, can be achieved more robustly and precisely, and independently of the operating state of the battery. 
     DISCLOSURE OF THE INVENTION 
     The method of the invention having the characteristics of claim  1  and the apparatus of the invention having the characteristics of claim  6  have the advantage over the prior art that they can be used for determining the internal resistance of battery cells in battery systems with inductive cell balancing, with no or only slight additional electronic circuitry expense. This method and apparatus have the advantage over the present prior art that for determining the internal resistance, again and again the same course of operation can be brought about, and as a result, especially robust, precise determination becomes possible. Moreover, the novel method and the novel apparatus have the advantage that they can be used even in phases of operation in which the battery is not outputting or drawing any power at its poles, and/or in which the battery, including the battery cell, is being charged, or in other words for instance when a vehicle is parked. This is not possible in the methods known at present, for instance with the vehicle parked. In the last instance, in which use takes place during charging of the battery, a superposition of the balancing current on the charging current takes place, which according to the invention is preferably taken into account. Determining the internal resistance in the phases of operation mentioned above is impossible in the methods known so far. 
     The dependent claims show preferred refinements of the invention. 
     Especially preferably, the method and the apparatus of the invention include the feature that the first time is selected such that the first current is equal to zero, and the second time is an arbitrary time during the ensuing discharging phase or charging phase of the battery cell. 
     Alternatively, the method and the apparatus of the invention espe4cially preferably include the fact that the first time is an arbitrary time during the discharging phase of the battery cell, and the second time is an arbitrary time during the same discharging phase of the battery cell. 
     Alternatively or in addition, the method of the invention includes the step of determining an aging-dependent increase in the internal resistance of the battery cell on the basis of a known dependency of the internal resistance on a cell temperature existing during the determination of the internal resistance and a state of charge of the battery cell existing during the determination of the internal resistance. The corresponding preferred refinement of the apparatus of the invention for this purpose includes a table, which stores in memory a dependency of the internal resistance on a cell temperature existing during the determination of the internal resistance and on a state of charge of the battery cell existing during the determination of the internal resistance, and a first evaluation unit, which determines an aging-dependent increase in the internal resistance of the battery cell on the basis of the determined internal resistance and of consulting the table. 
     The method according to the invention moreover alternatively or in addition includes the step of determining a frequency dependency of the internal resistance of the battery cell by means of a variation of a frequency of an excitation of the resistive cell balancing during a plurality of successive determinations of the internal resistance and/or by means of a variation of a pulse-duty factor of an excitation of the resistive cell balancing during a plurality of successive determinations of the internal resistance. The corresponding preferred refinement of the apparatus of the invention for this purpose includes a second control module for varying a frequency of an excitation of the resistive cell balancing during a plurality of successive determinations of the internal resistance and/or for varying a pulse-duty factor of an excitation of the resistive cell balancing during a plurality of successive determinations of the internal resistance, and a second evaluation unit for determining a frequency dependency of the internal resistance of the battery cell by means of evaluating the plurality of successive determinations of the internal resistance. In this preferred embodiment, the internal resistance is also determined by the novel method, as a function of the frequency of the excitation. 
    
    
     
       DRAWINGS 
       One exemplary embodiment of the invention will be described in detail below in conjunction with the accompanying drawings. In the drawings: 
         FIG. 1  is a basic circuit diagram of a first preferred embodiment of an apparatus according to the invention for determining the internal resistance of a battery cell; 
         FIG. 2  shows a first example for the excitation of the battery cells, for determining the frequency dependency of the internal resistance by way of varying the excitation frequency; 
         FIG. 3  shows a second example for the excitation of the battery cells, for determining the frequency dependency of the internal resistance by way of varying the pulse-duty factor; 
         FIG. 4  shows a functional construction of a battery system in accordance with the prior art; 
         FIG. 5  is a further basic circuit diagram of a battery system in accordance with the present prior art; 
         FIG. 6  is a basic circuit diagram of model-based battery state detection and prediction in accordance with the prior art; 
         FIG. 7  shows a basic circuit diagram of an arrangement for resistive cell balancing in accordance with the prior art; 
         FIG. 8  shows a basic circuit diagram of an arrangement for inductive cell balancing of the battery cells in accordance with the prior art; and 
         FIG. 9  shows an example of a circuit for implementing inductive cell balancing of the battery cells in accordance with the prior art. 
     
    
    
     PREFERRED EMBODIMENTS OF THE INVENTION 
     Preferred embodiments of the invention will be described below in detail, in conjunction with the drawings. 
     In  FIG. 1 , a preferred embodiment of the apparatus of the invention is shown; this is an expansion of the circuitry principle, shown in  FIG. 9 , for inductive cell balancing. In  FIG. 1 , a filter circuit  11 ,  12 ,  13  for preparing the differential voltage signal of the cell  1   a  (n) for an analog/digital converter  14  is also shown. By way of this converter, the cell voltage is furnished while adhering to the sampling theorem of a control and evaluation unit  15  of the cell  1   a  (n), which unit processes it and forwards it to the higher-order battery state detector  17   b . The control and evaluation units  15  of all the cells  1   a  communicate with one another, in order to perform the cell balancing. Optionally along with the additional circuit elements shown, though they can preferably also be integrated with the control and evaluation unit  15 , the circuit used for the cell balancing is also used for determining the internal resistance of the cell in accordance with the invention. In the invention, the circuit for the inductive cell balancing shown in  FIG. 9  is expanded with a first control module  3 , with which the voltage U n  applied to the battery cell  1   a  (n), or the voltage U L  applied to the chokes  2   a ,  2   b , and the current (T Bal     —     n ) flowing from the battery cell  1   a  (n) are detected at various times during the charge withdrawal. This can be done either via a direct current and voltage measurement or via the control and evaluation unit  15  of the cell  1   a  (n), which detects at least the battery voltage U n , or the voltage U L  applied to the chokes  2   a ,  2   b , via the filter comprising two resistors  11 ,  12  and a capacitor  13 , and via an A/D converter  14 , from which, as described below, the current (T Bal     —     n ) flowing from the battery cell  1   a  (n) can be calculated. The first control module  3  is connected to an arithmetic unit  4 , which as described below calculates the internal resistance of the battery cell as the quotient of the difference between two detected voltage values and the difference of two detected current values. 
     The charge that in the first step was drawn from the cell or cells  1   a  can be calculated as follows by way of the voltage/time area, for a known inductance of the reservoir  2  used for buffer-storage of the energy:
         The course over time of the current in the inductive component  2  is       

     
       
         
           
             
               
                 
                   
                     I 
                     L 
                   
                   = 
                   
                     
                       1 
                       L 
                     
                      
                     
                       ∫ 
                       
                         
                           U 
                           L 
                         
                          
                         
                            
                           t 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     The maximum current at the end of the first step will be called I Lmax .
         The charge drawn in the first step can be calculated as follows:       

         Q   ent   =∫I   L   dt   (2)
 
     The voltage U L  at the inductive component can then—on the assumption of ideal electronic switches  10  with an on-state resistance toward 0 as well as an ideal inductive component  2  which has no internal ohmic resistance—be ascertained simply from the voltage or voltages U n , of those cells from which the energy was drawn. Via equations (1) and (2), the discharging current as well as the charge drawn from the cells  1   a  can thus be determined. The non-ideal properties of the electronic switches  10  and of the inductive components  2 , given suitable dimensioning of the components, result in only slight errors in ascertaining the charge that is drawn from the cell or cells  1   a.    
     In an equivalent form, in the second step the re-storing of the buffer-stored energy into the cell or cells  1   a  can be calculated:
         The course over time of the current in the inductive component  2  is       

     
       
         
           
             
               
                 
                   
                     
                       I 
                       L 
                     
                     = 
                     
                       
                         I 
                         Lmax 
                       
                       + 
                       
                         
                           1 
                           L 
                         
                          
                         
                           ∫ 
                           
                             
                               U 
                               L 
                             
                              
                             
                                
                               t 
                             
                           
                         
                       
                     
                   
                   , 
                   
                     
                       in 
                        
                       
                           
                       
                        
                       which 
                        
                       
                           
                       
                        
                       
                         U 
                         L 
                       
                     
                     &lt; 
                     0 
                   
                 
               
               
                 
                   ( 
                   
                     3 
                      
                     a 
                   
                   ) 
                 
               
             
           
         
       
     
     Once the current I L  has assumed the value 0, the following applies: 
         I   L =0  (3b)
         The charge fed back in the second step can be calculated as follows:       

         Q   zu   =I   L   dt   (4)
 
     Thus the course of current during transporting of the charge and the drawn or fed-back charge can be determined during the cell balancing. 
     On the basis of this information, the internal resistance of the cells can be ascertained as follows: 
     Let the starting point for explaining the mode of operation be an operating state in which the battery is not outputting or drawing any power at its terminals. In this state, no current flows through the battery cells  1   a . If the transistor  10  (T Bal     —     n ) is then switched on, the cell  1   a  (n) discharges via the chokes  2   a ,  2   b  (L n     —     upper  and L n     —     lower ). As a result of the switching on of the transistor  10  (T Bal     —     n ), the cell voltage varies in comparison to the outset state (when no power is being output or drawn), and this voltage is detected by means of the aforementioned arrangement for cell voltage measurement. Naturally, the current that is flowing through the battery cell  1   a  (n) also varies in addition. This current can be determined in the manner described above. 
     The temperature-dependent, state-of-charge-dependent and aging-dependent internal resistance R i     —     n  of the battery cell  1   a  (n) can thus be determined for instance as follows: 
     
       
         
           
             
               
                 
                   
                     
                       R 
                       i_n 
                     
                      
                     
                       ( 
                       
                         Temp 
                         , 
                         
                           S 
                            
                           
                               
                           
                            
                           O 
                            
                           
                               
                           
                            
                           C 
                         
                         , 
                         Aging 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         U 
                         n 
                       
                        
                       
                          
                         
                           
                             T 
                             
                               Bal_n 
                                
                               
                                   
                               
                                
                               ON 
                             
                           
                           - 
                           
                             U 
                             N 
                           
                         
                          
                       
                        
                       
                         T 
                         
                           Bal_n 
                            
                           
                               
                           
                            
                           OFF 
                         
                       
                     
                     
                       
                         I 
                         Bal_n 
                       
                       | 
                       
                         T 
                         
                           Bal_n 
                            
                           
                               
                           
                            
                           ON 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     For ascertaining the inner resistance, besides the output state (cell not loaded), an arbitrary time during the discharge phase of the cell  1   a  (n) can be used, for which the cell voltage and the cell current are ascertained in the manner described. 
     For a known dependency of the internal resistance on the cell temperature and on the state of charge of the cell, the aging-dependent increase in the internal resistance of the battery cell can be determined. For that purpose, the arithmetic unit  4  is connected to a first evaluation unit  7 , which determines the aging-dependent increase in the internal resistance of the battery cell  1   a  (n) on the basis of the determined internal resistance and by consulting a table  6 , which stores in memory the dependency of the internal resistance on the cell temperature, existing during the determination of the internal resistance, and on a state of charge of the battery cell  1   a , existing during the determination of the internal resistance. 
     In a modification of the method, for determining the inner resistance of the cell  1   a  (n), it is also possible to use two or more times during the discharge phase of the cell  1   a  (n). Then the determination of the inner resistance is done via the differential voltage and the change in the balancing current, which occur between the observed times: 
     
       
         
           
             
               
                 
                   
                     
                       R 
                       i_n 
                     
                      
                     
                       ( 
                       
                         Temp 
                         , 
                         
                           S 
                            
                           
                               
                           
                            
                           O 
                            
                           
                               
                           
                            
                           C 
                         
                         , 
                         Aging 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           U 
                           n 
                         
                       
                       | 
                       
                         T 
                         
                           Bal_n 
                            
                           
                               
                           
                            
                           ON 
                         
                       
                     
                     
                       
                         Δ 
                          
                         
                             
                         
                          
                         
                           I 
                           Bal_n 
                         
                       
                       | 
                       
                         T 
                         
                           Bal_n 
                            
                           
                               
                           
                            
                           ON 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     The method presented according to the invention for determining the inner resistance can for instance also be performed with the vehicle parked. As a result, the determination of the inner resistance is not adversely affected by the superimposed “normal operation” of the battery  1 . This represents a substantial advantage over the methods known until now. 
     The procedure according to the invention described for ascertaining the inner resistance of the battery cells  1   a  (including in the modification described) can also be employed analogously during the phases in which the energy buffer-stored in the inductive components  2 ;  2   a ,  2   b  is fed back into the cells  1   a . During these phases as well, the information about the actual values of the cell voltages U n  and the information about the courses over time of the balancing currents I L  are available to the system. Thus the method can be employed for ascertaining the internal resistance. 
     The principle presented according to the invention for determining the internal resistance of the battery cells can naturally be employed (including the phases of operation in which the battery is being charged) during the “normal operation” of the battery  1  as well. Then, to determine the internal resistance, the influence of the battery current flowing in the cell  1   a , which at that time is superimposed on the balancing current, must be taken into account. However, this procedure is worthwhile only in operating states in which the battery  1  is being charged or discharged with low currents. For that purpose, the internal resistance R i     —     n  of the battery cell  1   a  (n) is determined again from the quotient of the differences in the cell voltage and in the cell current at two times. 
     In phases of operation in which the battery  1  is being charged or discharged with high currents, it makes little sense to bring about an additional “excitation” of the cell by loading via the balancing current. During such phases of operation, according to the invention the use of the method, employed in the prior art, is preferably employed for determining the internal resistance from the cell voltage and the cell current that result from the “normal operation” of the battery  1 . 
     With the method presented for determining the internal resistance of the battery in accordance with the invention, one of the essential pieces of information that are required for battery state detection and prediction—the temperature-dependent, state of charge-dependent and aging-dependent change in the internal resistance of the battery cells—can be determined in all operating states of the battery. In the methods known until now, the internal resistance can be determined only in phases of operation in which the battery current changes significantly during the “normal operation”. In this way, it is possible to perform the determination of the internal resistance of the battery cells substantially more robustly and precisely than in the prior art. 
     According to the invention, the dependency on the frequency of the excitation is preferably determined. To that end, the following procedures are preferably employed:
         variation of the frequency of the excitation at a constant pulse-duty factor   variation of the pulse-duty factor of the excitation at a constant frequency   combination of the first two.       

     In  FIG. 2 , in the two courses over time for the triggering of the transistor  10  (T Bal     —     n ), it is shown by way of example how the dependency of the impedance of the battery cells on the frequency of the excitation can be determined. The pulse-duty factor of the excitation is shown symmetrically in  FIG. 2 ; that is, the ON time and the OFF time of the transistor are equal. In principle, the method can also be attained with asymmetrical pulse-duty factors; all that has to be taken into account is a maximum pulse-duty factor, which depends on the type and size of the energy reservoir  2  and cells  1   a , since the buffer-stored energy has to be re-stored into the cells  1   a . The frequency of the excitation is varied to determine the frequency dependency of the internal resistance. In  FIG. 2 , the courses are shown for two frequencies. In addition in  FIG. 2 , the measurement times are shown in the form of upward-pointing arrows, in which the internal resistance can be determined in accordance with equation (1). The measuring times are each selected here before and after a change in the switching state of the transistor  10  (T Bal     —     n ). 
     In  FIG. 3 , a further possibility for determining the frequency dependency of the internal resistance of the battery cells is shown. Here, the pulse-duty factor of the excitation is varied, while the frequency is kept constant. In this method as well, the measuring times, shown as upward-pointing arrows, are each selected before and after a change in the switching state of the transistor  10  (T Bal     —     n ). The frequency-dependent internal resistance of the battery cells is again determined in accordance with equation (5), or with suitable modification of the procedure, in accordance with equation (6). 
     In principle, combinations of the two methods described are naturally also possible for describing the internal resistance as a function of the excitation. The methods according to the invention make it possible, similarly to the procedure in what is known as impedance spectroscopy, to determine the frequency dependency of the internal resistance. In contrast to impedance spectroscopy, the methods according to the invention can be implemented without complicated additional measurement electronics. Only with regard to detecting the cell voltages are more-stringent demands in terms of dynamics and sampling frequency required, compared to the circuits conventionally used in battery systems. 
     To change the frequency and/or the pulse-duty factor of the excitation, a second control module  8 , which is coupled to the first control module  3  and to the control and evaluation unit  15 , is provided according to the invention. The second control module  8  is also connected to a second evaluation unit  9 , which is likewise connected to the arithmetic unit  4 . The second evaluation unit  9  determines the frequency dependency of the ohmic component of the internal resistance of the battery cell by evaluating the plurality of successive determinations of the internal resistance, taking into account the change in the frequency and/or the pulse-duty factor of the excitation. 
     With the preferred method presented for determining the frequency dependency of the internal resistance of the battery cells, it is equally possible for one piece of the essential information required for battery state detection and prediction—that is, the temperature-dependent, state-of-charge-dependent and aging-dependent change in the internal resistance of the battery cells—to be determined. In contrast to the methods known until now, the internal resistance can be determined only in phases of operation in which the battery current changes significantly during the “normal operation”. In this way it is possible to perform the successful determination of the internal resistance of the battery cells substantially more robustly and precisely, compared to the prior art. 
     In addition to the above written disclosure, the disclosure in the drawings is also expressly noted here.