Patent Publication Number: US-2012032681-A1

Title: Determination of the internal resistance of a battery cell of a traction battery while using resistive 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     13      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. 
     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 resistive 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, or in other words for instance when a vehicle is parked. This is not possible in the methods known at present. 
     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 especially preferably include the fact that the first time is an arbitrary time during the discharging phase or charging phase of the battery cell, and the second time is an arbitrary time during the same discharging phase or charging 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 preferably 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. Alternatively to the table, a second arithmetic unit can be provided, which reproduces the dependency of the internal resistance on the cell temperature existing during the determination of the internal resistance and on the state of charge of battery cell, existing during the determination of the internal resistance, on the basis of one or more mathematical equations. 
     The method according to the invention moreover alternatively or in addition includes the step of determining a frequency dependency of an ohmic component 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 an ohmic component 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, in particular of the ohmic component of the impedance of the battery cells, 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; and 
         FIG. 7  is a basic circuit diagram of an arrangement for the resistive 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. 7 , for the resistive cell balancing. If the battery cell  1   a  having the number n is discharged, for instance because it has a higher state of charge than other cells of the battery systems, then by switching on the transistor  10  (T Bal     —     n ), the ohmic resistor  2  (R Bal     13      n ) is connected parallel to the cell  1   a  (n). As a result, the cell  1   a  (n) is discharged. In  FIG. 7 , 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 , which processes it and forwards it to the higher-order battery state detector  17   b . 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 resistive cell balancing shown in  FIG. 7  is expanded with a first control module  3 , with which the voltage U n  applied to the battery cell  1   a  and the current (T Bal     —     n ) flowing from the battery cell  1   a  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 , via the filter comprising two resistors  11 ,  12  and a capacitor  13 , and via an A/D converter  14 . 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. 
     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. If the transistor  10  (T Bal     —     n ) is then switched on, the cell  1   a  (n) discharges via the ohmic resistor  2  (R Bal     13      n ). As a result of the switching on of the transistor  10 , 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 arrangement shown in  FIG. 1 . Naturally, the current that is flowing through the battery cell  1   a  (n) also varies in addition. This current can be determined easily via the ohmic principle for a known resistor  2  (R Bal     13      n ). Because of the considerable temperature differences that can occur in operation of the battery in a vehicle, a temperature correction of the value of the resistor  2  (R Bal     13      n ) used for determining the current of the battery cell  1   a  (n) is recommended. For that purpose, there is typically precise-enough temperature information available in the battery system, since the temperature of the battery cells  1   a  is determined and the electronics for performing the cell balancing and determining the cell voltage are logically disposed spatially directly where the battery cells  1   a  are. Thus for both the voltage and current change, which result from the switching on of the ohmic resistor  2  (R Bal     13      n ) in the cell  1   a  (n), signals that each have sufficiently great precision for the requirements are available. 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: 
     
       
         
           
             
               
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     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. Alternatively to consulting the table  6 , a second arithmetic unit can be consulted, which forms the dependency of the internal resistance on the cell temperature and on the state of charge on the basis of mathematical equations. The method presented according to the invention for determining the internal resistance can be performed for instance even with the vehicle parked. As a result, the determination of the internal resistance is not adversely affected by the superimposed “normal operation” of the battery  1 . This represents a substantial advantage over the methods known thus far. 
     The principle presented according to the invention for determining the internal resistance of the battery cells can naturally be employed 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     13      n ), it is shown by way of example how the dependency of the internal resistance 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. 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     13      n ). 
     In  FIG. 3 , a further possibility for determining the frequency-dependent 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     13      n ). The frequency-dependent internal resistance of the battery cells is again determined in accordance with equation (1). 
     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 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 “non tal 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.