JP2003317810A - Battery characteristics estimation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a battery characteristics estimation method capable of quickly and easily estimating self-discharge amount. <P>SOLUTION: The battery characteristics estimation method comprises an internal impedance measuring process measuring the internal impedance for a plurality of frequencies under the condition that the migration velocity of electric charge at the electrode reaction of the battery is sufficiently higher than that of matter, a reaction resistance measuring process calculating the value of reaction resistance of which, the real part is plotted on the X-axis and the imaginary part is plotted on the Y-axis of an impedance circle on a two-dimensional coordinates, and a minute short circuit estimation process estimating the presence of minute short circuits from the value of the reaction resistance. The minute short circuit can be detected by measuring the valve of the reaction resistance because the value of reaction resistance estimated by measuring the internal impedance sharply varies mainly depending on the size of the minute short circuit. Since the minute short circuit can be quickly measured, the internal impedance can also be quickly detected. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery characteristic evaluation method capable of highly accurately detecting a minute internal short circuit occurring in a battery.

[0002]

2. Description of the Related Art In the fields of information-related equipment and communication equipment,
Personal computers, video cameras, mobile phones, etc. are becoming smaller. As a power source used for these devices, a secondary battery having high energy density and high output density has been put into practical use and has come into widespread use. On the other hand, also in the field of automobiles, development of electric vehicles / hybrid vehicles is urgently required due to environmental problems and resource problems, and secondary batteries are also being considered as a power source used for the electric vehicles and the like.

By the way, the electromotive force of a battery decreases as it is left (self-discharge). From the viewpoint of battery efficiency, the self-discharge amount is preferably small. Even if batteries are created by the same method, the value of the self-discharge amount varies slightly for each battery. A major cause of self-discharge is the presence of minute short circuits due to conductive paths formed inside the battery.

Therefore, the batteries are evaluated for all the manufactured batteries by measuring the self-discharge amount before being applied to the products and whether they have proper characteristics when used in the products. It is preferable. This is especially true when applied to products with high requirements for batteries such as electric vehicles.

Conventionally, as a method of measuring the self-discharge amount, a method of measuring the terminal voltage after fully charging the battery and leaving it in a room temperature atmosphere for a certain period of time, and an acceleration test in a high temperature atmosphere of the fully charged battery are performed. Method, JP 2000
There is a method of estimating from the value of the voltage drop after the charged battery disclosed in Japanese Patent Laid-Open No. 133319 is stored at a low temperature.

[0006]

However, the conventional method for measuring the self-discharge amount has the following inconveniences. That is, for a battery having a small self-discharge amount such as a lithium-ion secondary battery, it is necessary to leave the battery for at least 2 weeks to 1 month in order to measure the self-discharge amount at room temperature. It took a very long time to complete, and it was not suitable for 100% inspection of the manufactured batteries. In addition, although the self-discharge amount measurement time can be shortened in the accelerated test, if the battery is left in a high temperature atmosphere, deterioration of the battery will progress. Further, it is impossible to distinguish between a micro short circuit and self-discharge due to other causes. In other words, in order to detect a micro short circuit, it is necessary to take a long time to detect a micro short circuit or sufficient detection accuracy cannot be obtained if the voltage with a small change width and the storage characteristics that are caused by multiple factors are investigated. The problem arises.

Therefore, it is an object of the present invention to provide a battery characteristic evaluation method capable of measuring the self-discharge amount of a battery quickly and simply.

[0008]

MEANS FOR SOLVING THE PROBLEMS AND EFFECTS OF THE INVENTION The inventors of the present invention have earnestly studied for the purpose of solving the above problems, and have obtained the following findings. That is, under the condition that the mass transfer rate in the electrode reaction of the battery is sufficiently higher than the charge transfer rate, the relationship between the battery overvoltage η and the current I is | η | = a
+ B · logI: (a and b are constants) is satisfied (Tafel equation). Differentiating this equation by the current value (I), dη / dI = R = C / I; (R is reaction resistance, C
Is a constant). Therefore, under the condition that the charge transfer rate is sufficiently slower than the mass transfer rate, the reaction resistance is greatly reduced when a current flows even though the flow rate is minute.

That is, the presence or absence of a micro short circuit can be inferred by measuring the reaction resistance under conditions in which no current is externally input or output to or from the battery. That is, when the measured reaction resistance value is smaller than the internal resistance value of the battery having no internal micro short circuit, it can be estimated that the leak current resulting from the micro short circuit inside the battery is flowing. It can be inferred that there is an amount of minute short circuit proportional to the decrease amount of the reaction resistance value inside the battery. Therefore, by obtaining the value of the reaction resistance of the battery, it is possible to quickly infer the existence of a micro short circuit inside the battery,
The inventors have made the following inventions with the idea that the amount of self-discharge in the future can be predicted.

That is, the method for evaluating the characteristics of the battery of the present invention is to measure the internal impedance of the battery at a plurality of frequencies under the condition that the mass transfer rate in the electrode reaction of the battery is sufficiently higher than the charge transfer rate. A measurement step, a reaction resistance calculation step of calculating a value of reaction resistance from an impedance circle in a two-dimensional coordinate defined by plotting the real part of each internal impedance on the X axis and the imaginary part on the Y axis, and the reaction A step of estimating the presence of a micro short circuit in the battery from the value of the resistance, and a micro short circuit estimating step (claim 1).

Since the magnitude of the value of the reaction resistance based on the measurement of the internal impedance largely changes mainly depending on the size (quantity) of the minute short circuit, the minute short circuit can be detected with high accuracy by measuring the value of the reaction resistance. It becomes possible to do. Also,
Since the internal impedance can be measured quickly, a minute short circuit can be detected quickly.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The battery characteristic evaluation method of the present invention comprises:
It has an internal impedance measuring step, a reaction resistance calculating step, and a micro short circuit estimating step. The battery to which the battery characteristic evaluation method of the present invention can be applied is not particularly limited. Further, the method of the present invention is preferably performed after general conditioning of the target battery in order to reduce variation in results, but the conditioning is not particularly required. Further, the method of the present invention can be adopted not only as a means for quality control immediately after the production of a battery but also as a means for finding a micro short circuit newly generated in a battery in use. That is, by periodically applying the method of the present invention to a battery in use, it is possible to quickly obtain information such as the life of the battery.

(Internal Impedance Measuring Step) The internal impedance measuring step is a step of measuring the internal impedance of the battery at a plurality of frequencies under the condition that the mass transfer rate in the electrode reaction of the battery is sufficiently higher than the charge transfer rate. . The charge transfer rate can be made sufficiently higher than the mass transfer rate by controlling the temperature of the battery within a predetermined temperature range. Whether or not the mass transfer rate is sufficiently higher than the charge transfer rate will be described in the section of the reaction resistance calculation step described later.

As a preferable predetermined temperature range, an appropriate value varies depending on the type of battery to be evaluated. If the battery to be measured is a nickel-hydrogen secondary battery, -30
When the temperature is -15 ° C, the condition is such that the charge transfer rate is sufficiently higher than the mass transfer rate. Similarly, when the battery is a lithium battery, the charge transfer rate is measured within a range of −35 to 25 ° C., and when the battery is a fuel cell, within a range of 25 to 1000 ° C. The condition is sufficiently slower than the moving speed.

The method for measuring the internal impedance of the battery is not particularly limited, but the method generally called the AC impedance method is preferable. Examples of the measuring method in the AC impedance method include analog methods such as Lissajous method, AC bridge method, and phase discrimination method, and digital methods such as digital Fourier integration method and fast Fourier transform method by noise application.

The Lissajous method is a method in which the voltage and current of the input signal to the battery and the voltage and current of the response signal are directly recorded in two channels and the amplitude and phase difference are directly obtained. The AC bridge method is a method in which an equilibrium point of a Wheatstone bridge having an RC element including a battery on each side is obtained and the impedance is calculated from the equilibrium point. The phase discrimination method is a method of detecting the amplitude and phase difference of a response signal with respect to an input signal by using a lock-in amplifier.

The digital Fourier integration method is a method in which an input signal consisting of a sine wave of each frequency is stepwise changed and applied, and a complex quantity is obtained by digital processing of the potential and current of the response signal by Fourier integration to calculate the impedance. Is. In other words, it can be said that the analog phase discrimination method is digitized. The fast Fourier transform method using noise application applies noise (input signal) containing multiple frequencies to a battery and Fourier transforms time domain data consisting of voltage and current of response signal measured to frequency domain data to transfer impedance. It is a method to obtain as a function.

In the internal impedance measuring step, 1 MHz is used as a plurality of frequencies for measuring the internal impedance.
It is preferable to determine in the range of z to 100 mHz. When the measurement frequency is within this range, the internal impedance can be easily measured with high accuracy. It is necessary to measure at least two frequencies for measuring the internal impedance, and it is preferable to select and measure about 5 to 10 types of frequencies. 1 kHz when selecting multiple frequencies
at least one in the range of z to 100 Hz and 10 Hz to 1
By selecting at least one in the range of 00 mHz, the reaction resistance can be calculated with higher accuracy in the reaction resistance calculation step described below. The amplitude of the alternating current applied to the battery is not particularly limited, but is preferably about 1 mV to 10 mV.

(Reaction Resistance Calculation Step) The reaction resistance calculation step is a two-dimensional process in which the real part of the internal impedance at each frequency measured in the internal impedance measurement step is plotted on the X axis and the imaginary part is plotted on the Y axis. It is a step of calculating the value of the reaction resistance from the impedance circle in the coordinates. The method of plotting the real part on the X axis and the imaginary part on the Y axis is generally called Cole-Cole plot. As a method of calculating the value of the reaction resistance from the impedance circle, a method of calculating from the distance between the intersection points of the obtained impedance circle and the X axis (imaginary part is 0) is adopted. That is, the impedance circle is estimated from the combination of the internal resistance values measured in the internal impedance measuring step, and the reaction resistance is calculated.

Whether or not the charge transfer rate is sufficiently faster than the mass transfer rate in the electrode reaction of the battery can be judged by the shape in the two-dimensional coordinates of the Cole-Cole plot. Specifically, when the Cole-Cole plot draws an almost perfect semicircle as shown in FIG. 1, it can be determined that the charge transfer rate is sufficiently higher than the mass transfer rate. That is, it means that the battery can be approximated by an equivalent circuit as shown in FIG. The arrow in the graph indicates the high frequency side, Rsoln in the graph and the equivalent circuit are solution resistances, Cdl is the double layer capacitance, and r.
Means reaction resistance. Here, under the condition that the charge transfer speed is not sufficiently higher than the mass transfer speed, the battery shows a straight line rising to the low frequency side as shown in FIG. 2, and the battery is represented by the equivalent circuit shown in FIG. . The straight line rising upward is derived from Zw (Warburg impedance: diffusion impedance for a mass transfer system assuming linear semi-infinite diffusion) in the equivalent circuit of the battery. If the charge transfer speed is not sufficiently higher than the mass transfer speed, the Zw component cannot be ignored.

(Micro-short circuit estimating step) The micro-short circuit estimating step is a step of estimating the existence of a micro short circuit from the value of the reaction resistance calculated in the reaction resistance calculating step. It is considered that the smaller the value of the reaction resistance is, the larger (the number of) micro short circuits are. Here, when the value of the reaction resistance is less than or equal to a predetermined value, it is possible to assume that a problematic micro short circuit exists, and perform the subsequent steps (for example, determine that the battery is defective). The predetermined value can be determined in relation to the actual battery. Specifically, the amount of self-discharge of actual batteries is measured over a long period of time, and batteries that are supposed to be usable under actual usage conditions and batteries that are not expected to be used under actual usage conditions are selected. By applying the evaluation method of the invention, the boundary value of the reaction resistance between a usable battery and a battery that cannot withstand use can be set to a predetermined value. Once the predetermined value for selecting the battery is determined, it is not necessary to re-examine by re-testing or the like unless there is a significant change in the battery configuration, so that the method of the present invention can be performed easily.

[0022]

EXAMPLE Nine commercially available Ni-MH batteries (6.5 Ah,
The test was carried out using a single size). The 9 batteries were divided into 3 types of 3 each, the first group was subjected to the test as it was, the second group was left to stand in the atmosphere of 65 ° C for 45 days and then subjected to the test, and the third group was subjected to the 65 ° C. The sample was left for 90 days in the above atmosphere and then subjected to the test. In the test, the characteristic evaluation by the method of the present invention and the storage characteristic (capacity change) by the conventional method were performed.

(Characteristic Evaluation Method of the Present Invention) After adjusting the state of charge of the test battery to SOC 60%, the temperature of the test battery was kept constant by holding it in an atmosphere of −15 ° C. for 8 hours. After that, frequency analyzer (FRA5080 manufactured by Hokuto Denko)
And potentio / galvanostat (Hokuto Denko, HZ3
000 (-10 to 10 A)) was used to measure the internal impedance of each battery independently (internal impedance measurement step). The measurement conditions were an amplitude of ± 5 mV and a frequency of 10 kHz to 100 mHz.

The reaction resistance was calculated from the distance of the intersection of the impedance circle and the X axis when the real part of the measured internal impedance value was plotted on the X axis and the imaginary part was plotted on the Y axis (Cole-Cole plot). Resistance calculation process). Here, the shape of the Cole-Cole plot is as shown in the graph of FIG. 1, and it was revealed that the charge transfer rate of the test battery was sufficiently higher than the mass transfer rate under the measurement conditions.

(Measurement of conventional storage characteristics) A test battery is SO
The discharge amount was calculated from the remaining capacity after being left at 25 ° C. for 4 weeks after adjusting to C60%.

(Results) FIG. 3 shows the results of plotting the discharge amount measured in the storage characteristic measurement by the conventional method on the X axis and the reaction resistance value measured by the method of the present invention on the Y axis.
As is clear from FIG. 3, a good correlation was observed between the discharge amount measured by the conventional method and the value of the reaction resistance measured by the method of the present invention. That is, it became clear that the value of the reaction resistance decreases as the discharge capacity increases, and the deterioration of the storage characteristics can be estimated by a simple method of measuring the value of the reaction resistance.

[Brief description of drawings]

FIG. 1 is a diagram showing a Cole-Cole plot and an equivalent circuit of a battery in which a charge transfer rate is sufficiently higher than a mass transfer rate.

FIG. 2 shows a Cole-Cole plot and an equivalent circuit of a battery in which the charge transfer rate is not significantly faster than the mass transfer rate.

FIG. 3 is a graph showing the results of the examples.

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Claims (7)

[Claims] 1. An internal impedance measuring step of measuring an internal impedance of the battery at a plurality of frequencies under a condition that a mass transfer rate in an electrode reaction of the battery is sufficiently higher than a charge transfer rate, and The reaction resistance calculation step of calculating the value of the reaction resistance from the impedance circle in the two-dimensional coordinates defined by plotting the real part on the X-axis and the imaginary part on the Y-axis, and a micro short circuit of the battery from the value of the reaction resistance. A method for evaluating characteristics of a battery, comprising: 2. The battery characteristic evaluation method according to claim 1, wherein the step of estimating the micro short circuit is a step of estimating that the micro short exists when the value of the reaction resistance is equal to or less than a predetermined value. 3. The battery characteristic evaluation method according to claim 1, wherein the plurality of frequencies in the internal impedance measuring step are in the range of 1 MHz to 10 mHz. 4. The method for evaluating the characteristics of a battery according to claim 1, wherein the internal impedance measuring step is performed under the condition that the Tafel equation is satisfied. 5. The battery characteristic evaluation method according to claim 1, wherein the battery is a nickel-hydrogen secondary battery, and the internal impedance is measured at −30 to 15 ° C. in the internal impedance measuring step. . 6. The battery characteristic evaluation method according to claim 1, wherein the battery is a lithium battery, and the internal impedance is measured at −35 to 25 ° C. in the internal impedance measuring step. 7. The battery characteristic evaluation method according to claim 1, wherein the battery is a fuel cell, and the internal impedance is measured at 25 to 1000 ° C. in the internal impedance measuring step.