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METHOD AND APPARATUS USING A
CIRCUIT MODEL TO EVALUATE CELL/
This is a Continuation-In-Part of patent application Ser. No. 09/940,684 filed Aug. 27, 2001, now U.S. Pat. No. 6,495,990, which is a Divisional of patent application Ser. No. 09/388,501 filed Sep. 1,1999, which issued as U.S. Pat. No. 6,313,607 on Nov. 6, 2001 and also claims priority to U.S. provisional application Ser. No. 60/299,876 filed Jun. 21, 2001, the contents of which are hereby incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
"Total storage capacity" (TSC), "absolute stored charge" (ASC), "state-of-charge" (SOC), "absolute cranking current" (ACC), "fully charged cranking current" (FCCC) and "state-of-health" (SOH) are important performance parameters of an electrochemical cell/battery. These six parameters are assumed herein to have the following definitions: "Total storage capacity" (TSC) denotes the total amount of charge that a fully charged battery can supply under specified discharge conditions. TSC is usually expressed in ampere-hours or in reserve capacity minutes.
"Absolute stored charge" (ASC)—also expressed in ampere-hours or reserve capacity minutes—denotes the amount of charge that a battery can supply in its current charge state. As a battery is discharged, its ASC decreases—much like the level of liquid in a fuel tank.
"State-of-charge" (SOC), or "relative stored charge", is the ratio of a battery's ASC to its TSC—generally expressed as a percentage. A battery's SOC indicates whether charging is advisable and identifies the point at which charging should be discontinued.
"Absolute cranking current" (ACC) denotes the high-rate discharge current in amperes that a battery can sustain at a specified voltage for a specified time in its present charge state. As a battery discharges, its ACC decreases.
"Fully charged cranking current" (FCCC) denotes the value that the ACC would assume if the battery were fully charged.
"State-of-health" (SOH) describes a battery's full charge capability, either its TSC or its FCCC, vis-a-vis its rated specifications. SOH identifies the point at which battery replacement is advisable.
Both ASC and TSC have traditionally been measured by performing timed-discharge tests on batteries that are partially or fully charged, respectively. Because of the time and expense involved in performing complete discharge tests, other techniques for determining ASC and TSC have been proposed. In U.S. Pat. No. 6,255,801, Chalasani claims to determine battery capacity from observations of the coup de fouet effect. O'Sullivan, in U.S. Pat. No. 6,211,654, discloses a method for predicting battery capacity from the discharge characteristics over a relatively short time period at the beginning of a full discharge.
Techniques employing time-varying signals have also been proposed. Sharaf, in U.S. Pat. No. 3,808,522, purportedly determines the ampere-hour capacity of a lead-acid battery from ac measurements of its internal resistance. Yang, in U.S. Pat. No. 5,126,675, also uses single-frequency internal resistance measurements to predict battery capacity. Muramatsu reports in U.S. Pat. No. 4,678,998 that he can determine both the remaining amp-hour capacity and the
remaining service life of a battery from measurements of the magnitude of the ac impedance at two different frequencies. Fang, in U.S. Pat. No. 5,241,275, teaches a method for determining remaining capacity from complex impedance
5 measured at two or three frequencies in the range from 0.001 to 1.0 Hz. Hampson, et al., in U.K. Patent Application GB 2,175,700A, report determining battery capacity from the frequency of the maximum value of capacitive reactance in the "impedance characteristic curve". Yoon et al., in U.S.
10 Pat. Nos. 6,208,147 and 6,160,382, claim that a battery's capacity can be found by analyzing the complete impedance spectrum over a wide frequency range. Presumably, any of these techniques, if effective, could also be used to determine SOH by comparing the TSC thus determined with a
15 rated value.
Champlin, in U.S. Pat. No. 5,140,269, shows that the percent capacity of a standby battery—and hence its SOH— can be determined from its ac conductance measured at a single frequency if the ac conductance of a reference, fully
20 charged, identically constructed, new battery is known. This method, although quite effective, requires that such ac conductance data be available, apriori.
"Absolute cranking current" (ACC) and "fully charged cranking current" (FCCC) have been traditionally measured
25 with timed, high-rate, discharge tests. Such tests have many disadvantages, however. They require heavy and cumbersome equipment, cause dangerous sparking, give imprecise results, and leave the battery in a significantly worse condition than existed before the test was performed. In
30 response to the need for a better method, Champlin pioneered a testing technique based upon single-frequency ac conductance measurements. Various aspects of this wellaccepted methodology have been disclosed in U.S. Pat. Nos. 3,873,911, 3,909,708, 4,816,768, 4,825,170, 4,881,038,
35 4,912,416, 5,572,136, 5,585,728, 5,598,098, and 5,821,756. With lead-acid batteries, SOC has been traditionally evaluated by observing the battery's open-circuit voltage or the specific gravity of its electrolyte. However, neither of these quantities provides information about the battery's
40 TSC, ASC, ACC, FCCC, or SOH. Furthermore, specific gravity measurements are messy and impossible to perform on sealed cells. Moreover, open-circuit voltage cannot be measured under load conditions and, at any rate, is imprecisely related to SOC because both "surface charge" and
45 temperature affect it.
Because of these drawbacks, several techniques for correcting voltage of lead-acid batteries to obtain SOC have been proposed. These include techniques described by Christianson et al. in U.S. Pat. No. 3,946,299, by Reni et al.
50 in U.S. Pat. No. 5,352,968, and by Hirzel in U.S. Pat. No. 5,381,096. However, such voltage correction methods are not very accurate. Furthermore, they are of little help with electrochemical systems other than lead-acid in which voltage may bear little relationship to SOC.
55 Due to these and other problems, techniques based upon ac or time-varying signals have been proposed for determining SOC. For example, Latner claims to determine SOC of NiCd batteries from ac bridge measurements of farad capacitance in U.S. Pat. No. 3,562,634. U.S. Pat. No.
60 3,984,762 to Dowgiallo purports to determine SOC from the phase angle of the complex impedance at a single frequency. In U.S. Pat. No. 4,743,855, Randin et al. assert that SOC can be determined from the argument (i.e., phase angle) of the difference between complex impedances measured at two
65 different frequencies. Bounaga, in U.S. Pat. No. 5,650,937, reportedly determines SOC from measurements of the imaginary part of the complex impedance at a single fre3
quency. Base 11 et al. purport to determine SOC from the rate of change of impedance with frequency in U.S. Pat. No. 5,717,336. Ding et al., in U.S. Pat. No. 6,094,033, broadly assert that SOC can be determined from a battery's "impedance response, which can include series and parallel equiva- 5 lent circuit parameters, i.e., resistance, capacitance, and phase angle, among others". Finally, techniques purporting to determine SOC from the transient response to an applied pulsed voltage and/or current are disclosed by Andrieu and Poignant in U.S. Pat. No. 5,530,361 and by Simon in French 10 Patent Application FR 2,749,396A. The fact that none of these methods has gained wide acceptance, however, suggests that they may not be altogether satisfactory methods for determining SOC.
SUMMARY OF THE INVENTION
Testing apparatus senses the time-varying electrical response of an electrochemical cell/battery to time-varying electrical excitation. The cell/battery may, or may not, be in service. Computation circuitry responsive to the time- 20 varying electrical response evaluates elements of a unique circuit model representation of the cell/battery. Performance parameters and physical parameters are computed from these element values. Computed performance parameters include, but are not limited to, "total storage capacity", 25 "absolute stored charge", "state-of-charge", "absolute cranking current", "fully charged cranking current", and "stateof-health". Computed physical parameters include, but are not limited to, "exchange current", "maximum exchange current", "charge transfer conductance", "maximum charge 30 transfer conductance", "double layer capacitance", and "maximum double layer capacitance". Computed parameters are either displayed to the user, employed to initiate an alarm, or used to control a process such as charging the cell/battery. 35
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of apparatus for evaluating performance parameters and physical parameters of an elec- 40 trochemical cell or battery in accordance with one embodiment of the present invention.
FIG. 2 depicts a generic 2n-element small signal circuit model representation of an electrochemical cell or battery employed in the present invention. 45
FIG. 3 depicts the six-element small signal circuit model representation of a particular fully charged 12V VRLA battery determined from complex immittance measurements at 5 Hz, 50 Hz, and 500 Hz.
FIG. 4 is a plot of the variation of the three subcircuit 50 time-constants defined in FIG. 3 as charge is removed from the battery.
FIG. 5 is a plot of values of the three conductances defined in FIG. 3 as charge is removed from the battery.
FIG. 6 is plot of values of capacitances C2 and C3 defined in FIG. 3 as charge is removed from the battery.
FIG. 7 is a plot of the "exchange current" i0 derived from "charge transfer conductance" G3 of FIG. 5 as charge is removed from the battery. 60
FIG. 8 is a plot of the "state-of-charge" SOC derived from "charge transfer conductance" G3 of FIG. 5 and "double layer capacitance" C2 of FIG. 6 as charge is removed from the battery.
FIG. 9 is a plot of the "absolute stored charge" ASC 65 determined from "double layer capacitance" C2 of FIG. 6 as charge is removed from the battery.
FIG. 10 is a plot of the "total storage capacity" TSC derived from "charge transfer conductance" G3 of FIG. 5 and "double layer capacitance" C2 of FIG. 6 as charge is removed from the battery.
FIG. 11 is a plot of the "absolute cranking current" ACC derived from series conductance G1=1/R1 of FIG. 5 as charge is removed from the battery.
FIG. 12 is a plot of the function used to correct the "absolute cranking current" ACC for "state-of-charge" SOC to obtain the "fully charged cranking current" FCCC of FIG. 13.
FIG. 13 is a plot of the corrected "fully charged cranking current" FCCC as charge is removed from the battery.
FIG. 14 is a diagram of the "commonly accepted" circuit model showing the placement of a "charge transfer resistance" and "double layer capacitance" in parallel with one another.
FIG. 15 is a diagram of the n=3 circuit model according to the present invention showing the placement of the "charge transfer conductance" and the "double layer capacitance" in two separate G-C subcircuits that are actually in series with one another.
FIG. 16 is a block diagram of apparatus for evaluating performance and physical parameters of an electrochemical cell or battery wherein an external source produces timevarying electrical excitation
DETAILED DESCRIPTION OF THE
Method and apparatus for quickly and accurately determining performance parameters and physical parameters that does not discharge the battery, does not require the battery to be fully charged, and tests batteries while "on line" would be of great value. The present invention addresses this need. It is based upon teachings disclosed by Champlin in U.S. Pat. Nos. 6,002,238, 6,037,777, 6,172,483, 6,222,369, 6,262,563, 6,313,607, and U.S. patent application Ser. Nos. 60/299,876 and 09/940,684, all of which are incorporated herein by reference.
FIG. 1 discloses a block diagram of apparatus 5 for evaluating performance parameters and/or physical parameters according to one embodiment of the present invention. Measuring circuitry 10 electrically couples to cell/battery 20 at positive terminal 15 and negative terminal 25 by means of current-carrying contacts A and B and voltage-sensing contacts C and D. Cell/battery 20 may, or may not, be in service. Under control of microcontroller circuitry 30 via control path 35, measuring circuitry 10 passes periodic time-varying excitation current i(t) through contacts A and B and senses periodic time-varying response voltage v(t) across contacts C and D. Amplification and analog to digital conversion circuitry contained within measuring circuitry 10 formulates digital representations of i(t) and v(t) samples and communicates them to microcontroller circuitry 30 via data path 40.
By appropriately processing these digital representations, microcontroller circuitry 30 computes real and imaginary parts of complex immittance—either impedance Z or admittance Y—at a measuring frequency ik; where fk is a discrete frequency contained in the periodic waveforms of i(t) and v(t). Microcontroller circuitry 30 commands measuring circuitry 10 to repeat these measurements at each one of n discrete measuring frequencies, where n is an integer number equal to or greater than 3. This action defines 2n experimental quantities: the values of the n real parts and the n imaginary parts of complex immittance at each of the n