Patent Publication Number: US-6222369-B1

Title: Method and apparatus for determining battery properties from complex impedance/admittance

Description:
This is a Continuation application of U.S. patent application Ser. No. 09/151,324, filed Sep. 11, 1998, now U.S. Pat. No. 6,037,777. 
    
    
     BACKGROUND OF THE INVENTION 
     Small-signal ac measurement techniques have proven useful in determining properties of electrochemical cells and batteries such as cranking power, percent capacity, and state-of-health. These techniques have generally utilized single-frequency measurements of a single quantity, such as conductance (e.g., U.S. Pat. Nos. 5,585,728 and 5,140,269 to Champlin), resistance (e.g., U.S. Pat. No. 3,676,770 to Sharaf et al, U.S. Pat. No. 3,753,094 to Furuishi, U.S. Pat. No. 5,047,722 to Wurst et al), or “impedance” (e.g., U.S. Pat. No. 4,697,134 to Burkum et al, U.S. Pat. No. 5,773,978 to Becker). However, considerably more information of an electrical chemical, and physical nature is contained in the continuous spectrum of complex immittance, i.e., either impedance or admittance, displayed over a range of frequencies. (See, e.g., David Robinson, “Electrochemical Impedance Spectroscopy in Battery Development and Testing”, BATTERIES INTERNATIONAL, 31, pp. 59-63, April, 1997). A big challenge for field testing batteries is to acquire such information from a relatively small number of measurements obtained at a few selected “spot” frequencies. 
     Muramatsu discloses one approach to this challenge in U.S. Pat. No. 4,678,998. He measures impedance magnitude at two frequencies. At each frequency he compares the measured magnitude with that of a predetermined experimental relationship between impedance magnitude, remaining capacity, and remaining service life. He reports that such measurements can separately determine the battery&#39;s remaining capacity and its remaining service life. Randin discloses a second approach in U.S. Pat. No. 4,743,855. He reportedly determines a battery&#39;s state-of-discharge from the argument (i-e., phase angle) of the difference between complex impedances measured at two frequencies. Bounaga discloses still another approach in U.S. Pat. No. 5,650,937. He reportedly determines state-of-charge from measurements of only the imaginary part of complex impedance obtained at a single frequency. All three of these approaches have fairly limited objectives, however. Much more information is actually contained in the complete spectrum of complex immittance than is acquired by Muramatsu, Randin, or Bounaga. 
     Equivalent circuit modeling may assist one in relating complex immittance spectra to electrical, chemical, or physical properties of a battery. A complex nonlinear least-squares curve-fitting procedure has been used by electrochemists to relate impedance spectra to nonlinear electrochemical models. (See, e.g., J. Ross Macdonald and Donald R. Franceschetti, “Precision of Impedance Spectroscopy Estimates of Bulk, Reaction Rate, and Diffusion Parameters”,  Journal of Electroanalytical Chemistry,  307, pp. 1-11, 1991; see also Bernard A. Boukamp, “A Package for Impedance/Admittance Data Analysis”,  Solid State Ionics,  18, pp.136-140, 1986). This complex procedure, however, requires measuring the complete spectral distribution of cell/battery impedance and then making initial estimates of the model&#39;s parameters to ensure ultimate convergence. 
     An equivalent circuit model is an interconnection of electrical elements introduced to represent terminal characteristics of the battery. In a linear small-signal model, these elements comprise discrete resistances capacitances and inductances. Such models have been described by a number of workers including Hampson, et al (N. A. Hampson, et al, “The Impedance of Electrical Storage Cells”,  Journal of Applied Electrochemistry,  10, pp.3-11, 1980), Willihnganz and Rohner (E. Willihnganz and Peter Rohner, “Battery Impedance”,  Electrical Engineering,  78, No. 9, pp. 922-925, September, 1959), and DeBardelaben (S. DeBardelaben, “Determining the End of Battery Life”, INTELLEC 86,  IEEE Publication CH 2328-3/86/0000-0365, pp. 365-386, 1986; and S. DeBardelaben, “A Look at the Impedance of a Cell”, INTELLEC 88,  IEEE Publication CH 2653-4/88/000-0394, pp. 394-397, 1988). However, none of these workers has disclosed means for determining component values of an equivalent circuit model from a small number of measurements obtained at a few selected “spot” frequencies. That is an important contribution of the invention disclosed herein 
     SUMMARY OF THE INVENTION 
     A device includes a microprocessor or microcontroller and measures real and imaginary parts of complex immittance of a cell or battery at n discrete frequencies, where n is an integer number equal to or greater than 2. The device determines cell/battery properties by evaluating components of an equivalent circuit model comprising 2n frequency-independent linear electrical elements. Equating measured real and imaginary parts of complex immittance to theoretical real and imaginary values appropriate to the model at each of the n discrete frequencies defines a set of 2n nonlinear equations in 2n unknowns. By introducing 2n intermediate variables, this formidable problem is made linear and is systematically solved for the values of the components of the model. Once these values are known, a table of the 2n element values contains virtually the same information as the continuous spectrum of complex immittance displayed over a range of frequencies. However, the table of values provides this information in a much more concise form that is easier to store, analyze, and manipulate. Thus, circuit element values may themselves comprise the desired result. Moreover, the circuit elements represent actual processes occurring within the battery. Accordingly, a predetermined relationship between one or more of the elements and an additional electrical, chemical, or physical property of the cell/battery may be invoked to determine the additional property. 
     The method and apparatus disclosed herein are efficient, accurate, and easily implemented with a microcontroller or microprocessor. The invention is suitable for a variety of diagnostic applications ranging from hand-held battery testers to “smart” battery chargers and battery “fuel gauges” in electric vehicles. Although a lead-acid automotive storage battery is used as an example to teach the method, the invention is equally applicable to both primary and secondary cells and batteries, and to those employed in a variety of other applications and/or employing other chemical systems. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a spectral plot of measured real part of admittance of an actual 12-volt automotive storage battery. 
     FIG. 1 b  is a spectral plot of measured imaginary part of admittance of an actual 12-volt automotive storage battery. 
     FIG. 2 depicts a general small-signal ac equivalent circuit model of a cell or battery comprising 2n frequency-independent linear elements. 
     FIG. 3 depicts the equivalent circuit model of FIG. 2 with n=2. 
     FIG. 4 a  is a plot of the experimental data of FIG. 1 a  compared with a theoretical curve calculated from the model of FIG.  3 . 
     FIG. 4 b  is a plot of the experimental data of FIG. 1 b  compared with a theoretical curve calculated from the model of FIG.  3 . 
     FIG. 5 depicts the equivalent circuit model of FIG. 2 with n=3. 
     FIG. 6 a  is a plot of the experimental data of FIG. 1 a  compared with a theoretical curve calculated from the model of FIG.  5 . 
     FIG. 6 b  is a plot of the experimental data of FIG. 1 b  compared with a theoretical curve calculated from the model of FIG.  5 . 
     FIG. 7 is a block diagram of a device for determining battery properties from spot-frequency complex immittance according to the present invention. 
     FIG. 8 is a flow chart depicting the control algorithm for the invention embodiment discloses in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The impedance of a cell or battery is a complex quantity. At a particular discrete or “spot” frequency f k , the complex impedance can be written in terms of its real and imaginary parts as 
     
       
           Z ( f   k )= R ( f   k )+ jX ( f   k )   (1)  
       
     
     where j={square root over (−1+L )}. The real quantities R(f k ) and X(f k ) are, respectively, the resistance and the reactance of the cell/battery at the frequency f k . They physically represent ratios of in-phase voltage amplitude to current amplitude, and quadrature voltage amplitude to current amplitude, respectively, at the frequency f k . 
     The admittance of a cell or battery is likewise a complex quantity. At a particular discrete or “spot” frequency f k , the complex admittance can be written 
     
       
           Y ( f   k )= G ( f   k )+ jB ( f   k ).   (2)  
       
     
     The real quantities G(f k ) and B(f k ) are, respectively, the conductance and the susceptance of the cell/battery at the frequency f k . They physically represent ratios of in-phase current amplitude to voltage amplitude, and quadrature current amplitude to voltage amplitude, respectively, at the frequency f k . 
     Complex admittance and complex impedance are related to each other by the reciprocal relationship 
     
       
           Y ( f   k )=1/ Z ( f   k ).   (3)  
       
     
     Accordingly, spectra of complex admittance and spectra of complex impedance contain exactly the same information about the cell or battery. The term “immittance” will herein denote either quantity in instances where the choice is immaterial. 
     Measured spectra of real and imaginary parts of complex admittance of a typical automotive storage battery are disclosed in FIGS. 1 a  and  1   b , respectively, over the frequency range from 5 Hz to 1000 Hz. Considerable information about a battery is expressed in such spectral plots. One sees from FIG. 1 b , for example, that the battery passes through series resonance near 250 Hz, being capacitive (B&gt;0) below this frequency and inductive B&lt;0) above. However, most of the battery information displayed in FIGS. 1 a  and  1   b  is very subtle, and not at all obvious from the plots. 
     FIG. 2 discloses a small-signal ac equivalent circuit model introduced herein to assist in reducing spectral plots of complex immittance, such as those displayed in FIGS. 1 a  and  1   b , to a small set of frequency-independent parameters. One sees that the model of FIG. 2 comprises a series interconnection of a single two-element series R-L subcircuit and n−1 two-element parallel R-C subcircuits. For n=2, this general equivalent circuit model reduces to the simple model discussed by both Willihnganz and Rohner and by DeBardelaben. 
     A complete disclosure of my method for determining circuit-model element values from measured values of spot-frequency complex immittance follows. I begin with an expression for the complex impedance of the model of FIG. 2                    Z   =                  R   +   jX     =     R1   +     j                 ω                 L1     +     1       1   /   R2     +     j                 ω                 C2         +   …              +                                1       1   /   Rn     +     j                 ω                 Cn                       (   4   )                         
     where ω=2πf is the angular frequency. The inductance and the capacitances are eliminated from this expression by writing them in terms of time constants                      τ   1     =     L1   /   R1                   τ   2     =   R2C2                          ⋮                            τ   n     =   RnCn                 (   5   )                         
     The result is                    Z   =                  R   +   jX     =       R1        (     1   +     j                 ω                   τ   1         )       +     R2     (     1   +     j                 ω                   τ   2         )       +   …              +                                  Rn     (     1   +     j                 ω                   τ   n         )       .                   (   6   )                         
     Multiplying both sides of equation (6) by the product (1+jωτ 2 ) . . . (1+jωτ n ) clears the fractions and yields 
     
       
         ( R+jX ))(1 +jωτ   2 ) . . . (1 + jωτ   n )= R 1(1 +jωτ   1 ) . . . (1 +jωτ   n )+ R 2(1 +jωτ   3 ) . . . (1+ jωτ   n )+. . . + Rn (1 +jωτ   2 ) . . . (1 +jωτ   n−1 ).   (7)  
       
     
     For n=3, equation (7) reduces to 
     
       
         ( R+jX )(1 +jωτ   2 ) . . . (1 +jωτ   3 )= R 1(1 +jωτ   1 )(1 +jωτ   2 )+ R 2(1 +jωτ   3 )+ R 3(1 +jωτ   2 )   (8)  
       
     
     Equation (7) is divided into two equations by multiplying the terms out, separating them into real and imaginary parts, and then separately equating real to real, and imaginary to imaginary. For n=3, this procedure leads to 
     Real Part: 
     
       
         (ω 2   R )(τ 2 τ 3 )+(ω X )(τ 2 +τ 3 )−ω 2   {R 1(τ 2 τ 3 +τ 3 τ 1 +τ 1 τ 2 )}+( R 1 +R 2 +R 3)= R   (9)  
       
     
     Imaginary Part: 
     
       
         (ω 2   X )(τ 2 τ 3 )−(ω R )(τ 2 +τ 3 )+ω{ R 1(τ 1 +τ 2 +τ 3 )+R2τ 3   +R 3τ 2 }−ω 3   {R 1(τ 1 τ 2 τ 3 )}= X    (10)  
       
     
     Equations (9) and (10) are nonlinear since the 2n unknown resistances and time constants appear as combinations of products. I attack this formidable problem by defining a new set of 2n intermediate variables. These new variables are the various combinations of the model&#39;s resistances and time constants that multiply functions of battery resistance, battery reactance, and frequency. For n=3, the six intermediate variables are defined by 
     
       
         ψ 1 ≡(τ 2 +τ 3 )   (11 a )  
       
     
     
       
         ψ 2 ≡(τ 2 τ 3 )   (11 b )  
       
     
     
       
         ψ 3 ≡(R1+R2+R3)   (11 c )  
       
     
     
       
         ψ 4 ≡(τ 1 +τ 2 +τ 3 )R1+τ 3 R2+τ 2 R3 (11 d )  
       
     
     
       
         ψ 5 ≡(τ 2 τ 3 +τ 3 τ 1 +τ 1 τ 2 )R1  (11 e )  
       
     
     
       
         ψ 6 ≡(τ 1 τ 2 τ 3 )R1   (11 f )  
       
     
     When expressed in terms of these new variables, equations (9) and (10) are linear. At the angular spot frequency ω k , these two equations are 
     
       
         {ω k   X (ω k ) }ψ 1 +{ω 2   k   R (ω k )}ψ 2 +{1}ψ 3 +{0}ψ 4 −{ω 2   k }ψ 5 +{0}ψ 6   =R (ψ k )   (12)  
       
     
     
       
         −{ω k   R (ω k )}ψ 1 +{ω 2   k   X(ω   k )}ψ 2 +{0}ψ 3 +{ω k }ψ 4 +{0 }ψ 5 −{ω 3   k }ψ 6   =X (ω k )   (13)  
       
     
     In general, equations such as (12) and (13) comprise a pair of linear inhomogeneous equations for the 2n intermediate variables, ψ 1 , . . . , ψ 2n . Even though linear, such equations are still not solvable since they contain an insufficient number of relationships between variables. However, by evaluating the complex impedance at n discrete frequencies, ω 1 . . . ω n , the two equations expand into a solvable set of 2n linear inhomogeneous equations in 2n unknowns. Such a system can be solved by the well-known method known as Cramer&#39;s rule. Cramer&#39;s rule expresses the 2n solutions, ψ 1 , . . . , ψ 2n , as ratios of determinants having 2n columns and 2n rows. For n=3, these six solutions are of the form 
     
       
         ψ 1 =A 1 /A D ; . . . ;ψ 6 =A 6 /A D   (14)  
       
     
     where A D  and A 1 . . . A 6  are (6×6) determinants given by                A   D     =                  ω   1          X        (     ω   1     )                 ω   1   2          R        (     ω   1     )             1       0         -     ω   1   2           0               ω   2          X        (     ω   2     )                 ω   2   2          R        (     ω   2     )             1       0         -     ω   2   2           0               ω   3          X        (     ω   3     )                 ω   3   2          R        (     ω   3     )             1       0         -     ω   3   2           0               -     ω   1            R        (     ω   1     )                 ω   1   2          X        (     ω   1     )             0         ω   1         0         -     ω   1   3                   -     ω   2            R        (     ω   2     )                 ω   2   2          X        (     ω   2     )             0         ω   2         0         -     ω   2   3                   -     ω   3            R        (     ω   3     )                 ω   3   2          X        (     ω   3     )             0         ω   3         0         -     ω   3   3                          (   15   )                 A   1     =                R        (     ω   1     )               ω   1   2          R        (     ω   1     )             1       0         -     ω   1   2           0             R        (     ω   2     )               ω   2   2          R        (     ω   2     )             1       0         -     ω   2   2           0             R        (     ω   3     )               ω   3   2          R        (     ω   3     )             1       0         -     ω   3   2           0             X        (     ω   1     )               ω   1   2          X        (     ω   1     )             0         ω   1         0         -     ω   1   3                 X        (     ω   2     )               ω   2   2          X        (     ω   2     )             0         ω   2         0         -     ω   2   3                 X        (     ω   3     )               ω   3   2          X        (     ω   3     )             0         ω   3         0         -     ω   3   3                          (   16   )                 A   2     =                  ω   1          X        (     ω   1     )               R        (     ω   1     )           1       0         -     ω   1   2           0               ω   2          X        (     ω   2     )               R        (     ω   2     )           1       0         -     ω   2   2           0               ω   3          X        (     ω   3     )               R        (     ω   3     )           1       0         -     ω   3   2           0               -     ω   1            R        (     ω   1     )               X        (     ω   1     )           0         ω   1         0         -     ω   1   3                   -     ω   2            R        (     ω   2     )               X        (     ω   2     )           0         ω   2         0         -     ω   2   3                   -     ω   3            R        (     ω   3     )               X        (     ω   3     )           0         ω   3         0         -     ω   3   3                          (   17   )                 A   3     =                  ω   1          X        (     ω   1     )                 ω   1   2          R        (     ω   1     )               R        (     ω   1     )           0         -     ω   1   2           0               ω   2          X        (     ω   2     )                 ω   2   2          R        (     ω   2     )               R        (     ω   2     )           0         -     ω   2   2           0               ω   3          X        (     ω   3     )                 ω   3   2          R        (     ω   3     )               R        (     ω   3     )           0         -     ω   3   2           0               -     ω   1            R        (     ω   1     )                 ω   1   2          X        (     ω   1     )               X        (     ω   1     )             ω   1         0         -     ω   1   3                   -     ω   2            R        (     ω   2     )                 ω   2   2          X        (     ω   2     )               X        (     ω   2     )             ω   2         0         -     ω   2   3                   -     ω   3            R        (     ω   3     )                 ω   3   2          X        (     ω   3     )               X        (     ω   3     )             ω   3         0         -     ω   3   3                          (   18   )                 A   4     =                  ω   1          X        (     ω   1     )                 ω   1   2          R        (     ω   1     )             1         R        (     ω   1     )             -     ω   1   2           0               ω   2          X        (     ω   2     )                 ω   2   2          R        (     ω   2     )             1         R        (     ω   2     )             -     ω   2   2           0               ω   3          X        (     ω   3     )                 ω   3   2          R        (     ω   3     )             1         R        (     ω   3     )             -     ω   3   2           0               -     ω   1            R        (     ω   1     )                 ω   1   2          X        (     ω   1     )             0         X        (     ω   1     )           0         -     ω   1   3                   -     ω   2            R        (     ω   2     )                 ω   2   2          X        (     ω   2     )             0         X        (     ω   2     )           0         -     ω   2   3                   -     ω   3            R        (     ω   3     )                 ω   3   2          X        (     ω   3     )             0         X        (     ω   3     )           0         -     ω   3   3                          (   19   )                   A   5     =                  ω   1          X        (     ω   1     )                 ω   1   2          R        (     ω   1     )             1       0         R        (     ω   1     )           0               ω   2          X        (     ω   2     )                 ω   2   2          R        (     ω   2     )             1       0         R        (     ω   2     )           0               ω   3          X        (     ω   3     )                 ω   3   2          R        (     ω   3     )             1       0         R        (     ω   3     )           0               -     ω   1            R        (     ω   1     )                 ω   1   2          X        (     ω   1     )             0         ω   1           X        (     ω   1     )             -     ω   1   3                   -     ω   2            R        (     ω   2     )                 ω   2   2          X        (     ω   2     )             0         ω   2           X        (     ω   2     )             -     ω   2   3                   -     ω   3            R        (     ω   3     )                 ω   3   2          X        (     ω   3     )             0         ω   3           X        (     ω   3     )             -     ω   3   3                                  and           (   20   )                 A   6     =                    ω   1          X        (     ω   1     )                 ω   1   2          R        (     ω   1     )             1       0         -     ω   1   2             R        (     ω   1     )                   ω   2          X        (     ω   2     )                 ω   2   2          R        (     ω   2     )             1       0         -     ω   2   2             R        (     ω   2     )                   ω   3          X        (     ω   3     )                 ω   3   2          R        (     ω   3     )             1       0         -     ω   3   2             R        (     ω   3     )                   -     ω   1            R        (     ω   1     )                 ω   1   2          X        (     ω   1     )             0         ω   1         0         X        (     ω   1     )                   -     ω   2            R        (     ω   2     )                 ω   2   2          X        (     ω   2     )             0         ω   2         0         X        (     ω   2     )                   -     ω   3            R        (     ω   3     )                 ω   3   2          X        (     ω   3     )             0         ω   3         0         X        (     ω   3     )                  .             (   21   )                         
     The determinants disclosed in equations (15)-(21) can be systematically evaluated from spot-frequency immittance measurements by well-known numerical expansion techniques. Once their values are known, the intermediate variables ψ 1 , . . . , ψ 6  follow from equations (14). The defining equations of the intermediate variables, equations (11a)-(11f), are then combined in particular ways to evaluate the elements of the equivalent circuit model. 
     The technique proceeds as follows. One first combines the defining equations for n−1 of the 2n intermediate variables to yield an equation for the n−1 capacitive time constants τ 2 , . . . , τ n . These n−1 equations are identified by not containing resistances. For example, for n=3, I combine equation (11a) and equation (11b) to obtain the following quadratic equation: 
     
       
         τ 2   2,3 −ψ 1 τ 2,3 +ψ 2 =0.   (22)  
       
     
     The two roots of equation (22) are given by the well-known quadratic formula                τ     2   ,   3       =         Ψ   1     2     ±             (       Ψ   1     /   2     )     2     -     Ψ   2         .               (   23   )                         
     For the general case of arbitrary n, combining the n−1 defining equations that contain no resistances leads to the following polynomial equation of order n−1: 
     
       
         τ (n−1 )−ψ 1 τ (n−2 )+. . . ±ψ n−1 =0.   (24)  
       
     
     The n−1 roots of equation (24) are the capacitive time constants τ 2 , . . . , τ n . Although general formulas similar to equation (23) do not exist to solve higher order polynomial equations, the roots of equation (24) can always be found using well-known numerical root-finding algorithms. 
     Once the capacitive time constants have been determined, the inductive time constant τ 1  follows by eliminating R 1  from the two defining equations for intermediate variables that are proportional to R 1 . For example, with n=3, I combine equations (11e) and (11f) to obtain 
     
       
         τ 1 ={(ψ 5 /ψ 6 )−1/τ 2 −1/τ 3 } −1 .   (25)  
       
     
     For the general case of arbitrary n, the expression for τ 1  is of the form 
     
       
         τ 1 ={(ψ (2n−1 )/ψ 2n )−1/τ 2 −. . . −1/τ n } −1 .   (26)  
       
     
     Thus, in principle, all n time constants are known. 
     In general, there are 2n equations defining the 2n intermediate variables. The first n−1 of these equations contain no resistances and are employed to determine the capacitive time constants. Two of the remaining n+1 equations are employed to determine the inductive time constant. By choosing either one of these two equations, along with all of the remaining n−1 unused equations, one obtains a set of n linear inhomogeneous equations in the n unknowns R1, . . . , Rn. Cramer&#39;s rule can then be invoked to solve this system for the values of these n resistances. For example, with n=3, I use equations (16c), (16d), and (16f) to obtain the following set of three linear equations in three unknowns:                      {   1   }        R1     +       {   1   }        R2     +       {   1   }        R3       =     Ψ   3                             {       τ   1     +     τ   2     +     τ   3       }        R1     +       {     τ   3     }        R2     +       {     τ   2     }        R3       =     Ψ   4                             {       τ   1          τ   2          τ   3       }        R1     +       {   0   }        R2     +       {   0   }        R3       =     Ψ   6               (   27   )                         
     Cramer&#39;s rule yields the following three solutions:              R1   =                  Ψ   3         1       1             Ψ   4           τ   3           τ   2               Ψ   6         0       0                         1       1       1               τ   1     +     τ   2     +     τ   3             τ   3           τ   2                 τ   1          τ   2          τ   3           0       0                        (   28   )                 R2   =                1         Ψ   3         1               τ   1     +     τ   2     +     τ   3             Ψ   4           τ   2                 τ   1          τ   2          τ   3             Ψ   6         0                         1       1       1               τ   1     +     τ   2     +     τ   3             τ   3           τ   2                 τ   1          τ   2          τ   3           0       0                                and           (   29   )               R3   =                  1       1         Ψ   3                 τ   1     +     τ   2     +     τ   3             τ   3           Ψ   4                 τ   1          τ   2          τ   3           0         Ψ   6                           1       1       1               τ   1     +     τ   2     +     τ   3             τ   3           τ   2                 τ   1          τ   2          τ   3           0       0                .             (   30   )                         
     Thus, in principle, all n resistance values are known. 
     Finally, I invert equations (5) to determine the single inductance value and the n−1 capacitance values from the n known time constants and the n known resistance values              L1   =       τ   1        R1             (31a)               C2   =       τ   2     /   R2             (31b)                          ⋮                           Cn   =       τ   n     /     Rn   .               (31c)                         
     This completes the disclosure of my method for determining the 2n element values. Although n is unrestricted in principle, the problem of expanding large determinants will probably limit n to the range 2≦n≦8. In summary, the steps are: 
     1. One first finds 2n intermediate variables by evaluating 2n ratios of determinants comprising 2n columns and 2n rows. The determinants comprise sums and differences of products combining the n spot frequencies with real and imaginary parts of impedance or admittance at the n spot frequencies. 
     2. The n−1 capacitive time constants are found as roots of a polynomial equation of order n−1. The polynomial&#39;s coefficients comprise n−1 of the 2n intermediate variables whose defining equations contain no resistances. 
     3. The inductive time constant is determined from an equation containing the capacitive time constants found in step 2 along with the ratio of the two intermediate variables proportional to R1. 
     4. The n resistances are determined by evaluating n ratios of (n×n) determinants. Elements of these determinants contain the n time constants found in steps 2 and 3 along with n of the remaining n+1 intermediate variables not employed in step 2. 
     5. Finally, the one inductance and the n−1 capacitances are evaluated by combining each of the n resistances with a corresponding time constant. 
     In practice, this procedure is readily implemented in software. 
     The exact procedure disclosed above can be simplified by choosing one of the n spot frequencies, say ω n , to be sufficiently high that the impedance of the series chain of R-C subcircuits is negligibly small at this frequency. A preliminary measurement of Z(ω n ) then gives approximations to R1 and L1 directly. Subtracting R1+jω k L1 from the total measured impedance Z(ω k ) at each of the n−1 remaining spot frequencies and equating this result to the theoretical impedance of the R-C subcircuit chain at each frequency leads to a system of 2n−2 equations in 2n−2 unknowns. Solving for the appropriate 2n−2 intermediate variables by Cramer&#39;s rule involves evaluating determinants that are fewer in number and smaller in size than the determinants evaluated in the exact procedure disclosed above. 
     FIG. 3 discloses an equivalent circuit model derived from FIG. 2 by letting n=2. This simple model is essentially the one discussed by Willihnganz and Rohner and by DeBardelaben in the publications cited earlier. By using the exact procedure disclosed above, I evaluated the four linear circuit elements of the model of FIG. 3 from the experimental data disclosed in FIGS. 1 a  and  1   b  at the two spot frequencies f 1 =5 Hz and f 2 =1000 Hz. The results of this evaluation procedure are displayed in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Model Element Values for n = 2 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 R1 = 4.388 mΩ 
                 R2 = 12.987 mΩ 
               
               
                   
                 L1 = 0.3885 μH 
                 C2 = 2.602 F 
               
               
                   
                   
               
            
           
         
       
     
     Theoretical curves of the real and imaginary parts of admittance as functions of frequency were calculated for the model of FIG. 3 by assuming the element values displayed in Table 1. The theoretical curves are plotted along with the measured curves for comparison in FIGS. 4 a  and  4   b . One sees that the experimental and theoretical curves agree exactly at the two spot frequencies as would be anticipated. However, away from the spot frequencies the agreement is seen to be poor. This indicates that the model of FIG. 3 does not adequately represent the battery over this frequency range. 
     Much better agreement is obtained with the n=3 model depicted in FIG.  5 . Using the exact procedures disclosed above, I evaluated the six linear circuit elements of the model of FIG. 5 from the experimental data disclosed in FIGS. 1 a  and  1   b  at the three spot frequencies f 1 =5 HZ, f 2 =70 Hz, and f 3 =1000 Hz. The results are displayed in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Model Element Values for n = 3 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 R1 = 4.381 mΩ 
                 R2 = 1.227 mΩ 
                 R3 = 13.257 mΩ 
               
               
                   
                 L1 = 0.411 μH 
                 C2 = 1.812 F 
                 C3 = 3.14 F 
               
               
                   
                   
               
            
           
         
       
     
     Theoretical curves of the real and imaginary parts of admittance as functions of frequency were calculated for the model of FIG. 5 by assuming the element values displayed in Table 2. These curves are plotted along with the measured curves for comparison in FIGS. 6 a  and  6   b . Once again one sees that the experimental and theoretical curves agree exactly at the spot frequencies. Away from these frequencies, however, the agreement is seen to also be very good. Such good agreement proves the model of FIG. 5 to be an excellent representation of the battery over the frequency range from 5 Hz to 1000 Hz. Accordingly, the n=3 model much more closely describes actual processes within the battery than does the n=2 model. 
     The excellent agreement between the experimental curves and the theoretical predictions of the model means that Table 2 contains virtually the same information about the battery as does the continuous spectrum of complex admittance displayed in FIGS. 1 a  and  1   b . However, Table 2 provides this information in a much more concise form that is far easier to store, analyze, and manipulate. Accordingly, the information displayed in Table 2 may itself comprise the desired result. 
     Moreover, since the circuit elements defined in the extended model closely describe actual processes occurring within the battery, a predetermined relationship can additionally be invoked if desired, to implement a final step of determining one or more additional battery property. For example, I have found that the battery&#39;s cold-cranking ampere (CCA) capacity is quite accurately given by 
     
       
         CCA=2662/R1  (32)  
       
     
     where R1 is expressed in milliohms. Thus, the battery whose complex admittance spectrum is disclosed in FIGS. 1 a  and  1   b  is capable of supplying 608 cold-cranking amperes. This important information is not at all obvious from the spectral plots of FIGS. 1 a  and  1   b . Other electrical properties such as state-of-charge and ampere-hour capacity; chemical properties such as ion concentration and plate composition; and physical properties such as battery temperature and effective plate area, find similar expression in the complex immittance spectra of the battery and may be determined in a comparable manner. 
     FIG. 7 discloses a block diagram of a device for determining cell/battery properties from spot-frequency complex immittance according to the present invention. Measuring circuitry  10  electrically couples to cell/battery  20  by means of current-carrying contacts A and B and voltage-sensing contacts C and D. Measuring circuitry  10  passes a periodic time-varying current i(t) through contacts A and B and senses a periodic time-varying voltage v(t) across contacts C and D. By appropriately processing and combining i(t) and v(t), measuring circuitry  10  determines real and imaginary parts of complex immittance at a measuring frequency f k ; where f k  is a discrete frequency component of waveforms i(t) and v(t). 
     Control circuitry  30  couples to measuring circuitry  10  via command path  40  and commands measuring circuitry  10  to determine the complex immittance of cell/battery  20  at each one of n discrete measuring frequencies, where n is an integer number equal to or greater than 2. This action defines 3n experimental quantities: the values of the n measuring frequencies and the values of the n imaginary and n real parts of the complex immittance at the n measuring frequencies. 
     Computation circuitry  50  couples to measuring circuitry  10  and to control circuitry  30  via data paths  60  and  70 , respectively, and accepts the 2n experimental values from measuring circuitry  10  and the values of the n measuring frequencies from control circuitry  30 . Upon a “Begin Computation” command from control circuitry  30  via command path  80 , computation circuitry  50  invokes the procedure disclosed above to combine these 3n quantities numerically to evaluate the 2n elements of the equivalent circuit model. These 2n element values may themselves comprise the desired result. However, if desired, computation circuitry  50  can also perform an additional step by relating one or more of the model element values to an additional cell/battery property to determine the additional property. 
     In practice, a single microprocessor or microcontroller running an appropriate software program can perform the functions of both control circuitry  30  and computation circuitry  50  as well as much of the function of measuring circuitry  10 . Microprocessor controlled impedance measuring apparatus is disclosed in a copending U.S. patent application. 
     FIG. 8 discloses a flow chart depicting a control algorithm for the invention embodiment of FIG.  7 . Upon entering the procedure at  100 , control circuitry  30  initializes a counter at  105  used to identify each spot frequency f k . At  110 , control circuitry  30  commands measuring circuitry  10  to excite the cell/battery with a periodic signal having a sinusoidal component at frequency f k  and to determine the real and imaginary parts of complex immittance at this frequency. At decision block  115 , control circuitry  30  determines whether this procedure has been performed at all of the desired spot frequencies. If the answer is no, the counter is incremented at  120  and the procedure repeated at a new frequency. If yes, control circuitry  30  commands computation circuitry  50  to begin the computation at  125 . Computation circuitry  50  begins at step  130  by determining the 2n intermediate variables from the values of real and imaginary parts of complex immittance at the n spot frequencies along with the n spot frequency values themselves. The 2n frequency-independent model elements are then evaluated from the 2n intermediate variable values at step  135 . Finally, at optional step  140  computation circuitry  50  can, if desired, invoke an additional predetermined relationship between one or more of the model elements and a desired cell/battery property to determine the desired property. 
     This completes the disclosure of my invention. The method and apparatus are efficient, accurate, and easily implemented with a microcontroller or microprocessor. The invention is quite general and suitable for a wide variety of diagnostic applications ranging from hand-held battery test instruments to “smart” battery chargers and battery “fuel gauges” in electric vehicles. Although a lead-acid storage battery was used as an example to teach the method, the disclosed invention is equally applicable to both primary and secondary cells and batteries, and to cells/batteries employed in a variety of other applications and/or employing other chemical systems. 
     The present invention has been described with reference to a preferred embodiment. However, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.