DE102013226925A1 - Hybrid determination of an equivalent circuit electrochemical energy storage - Google Patents

Abstract

An electronic control unit, a means of locomotion with an electronic control unit and a method for determining the components of an equivalent circuit (1) of an electrochemical energy store are proposed. The method comprises the steps of determining a first parameter of a first component (R0, R1, C1) of the equivalent circuit (1) via a periodic excitation of the energy store with variable frequency, determining a second parameter of a second component (R2, C2) of the equivalent circuit (1 ) via a step-like excitation, wherein the periodic excitation is made up to a lower limit frequency, and wherein the cut-off frequency is above a frequency range which would be required to determine the second parameter by means of the periodic excitation.

Description

State of the art

The present invention relates to the determination of the components of an equivalent circuit of an electrochemical energy storage device and to an electronic control device configured for carrying out the method and a means of locomotion with such an electronic control device. In particular, the present invention relates to a possibility for improved / more accurate determination of the parameters and ways of saving time.

In order to characterize a battery cell, equivalent circuits and various methods for determining their values are used in the prior art. An example is the electrochemical impedance spectroscopy (EIS), another example is the current pulse method. Both methods are basically known in the prior art. Each of them makes it possible to determine the values ("parameters") of the discrete components of the equivalent circuit diagrams and both have their strengths and weaknesses.

DE 10 2007 061 122 A1 shows a method and a system for monitoring an electrochemical energy storage, in which a cyclic excitation of the electrochemical energy storage and monitoring its reaction take place during transient sections. By a variable timing in the control of a load device and different frequencies are possible.

US 6,687,631 B2 shows an impedance spectrometer and an associated method in which a Laplace transform is performed on the basis of measured variables, which are determined in different ways. As a disadvantage of the use of harmonic frequencies, the document calls the hardware and the time required.

EP 1 590 679 B1 shows a method for estimating state variables and parameters for an electrochemical energy storage, wherein the electrochemical energy storage is represented by a plurality of submodels, which apply differently well at different frequency ranges. Therefore, the submodels are preceded by a matched filter to improve the imaging accuracy of the overall model.

If an impedance measurement (IM) is carried out, for example, on a lithium-ion battery cell (LIB), the decomposition of the impedance can be used to determine a real and an imaginary part for each measured frequency. In general, the collected data is displayed as a Nyquist plot (NYP). In this the imaginary part of the resistance is applied against the real part. Each individual value represents a specific measurement frequency. Different ESBs can be derived from the Nyquist representation. In 1 is an exemplary arrangement with an energy storage 2 shown, between its terminals an equivalent circuit diagram (ESB) 1 is shown in the form of a series connection of two parallel circuits and a single ohmic resistance R0. The two parallel circuits each consist of a resistor R1 and a first capacitor C1 or an ohmic resistor R2 and a second capacitor C2. Basically, the number of RC elements or any existing inductances varies with the accuracy of the model. The different values for the capacitances, resistances and optionally inductances can be transferred from the NYP to the in 1 transfer illustrated ESB.

Another method to derive an ESB is to perform a pulse measurement. The battery cell is charged with a defined current pulse. After reaching a defined state of charge, the current pulse is switched off and recorded the resulting voltage waveform at the terminals of the battery cell. From this method can also determine the above sizes. Both the pulse measurement and the IM have advantages and disadvantages. The ohmic resistance R0 and the first RC element can be derived very easily and quickly from the impedance data, while the second RC element requires a long measurement time. The latter is because very low frequencies are required and the result is delayed by the correspondingly long measurement available. By the pulse measurement, in turn, the second RC element can be determined quickly and accurately, while the ohmic resistance R0 and the first RC element is determined inaccurately.

It is therefore an object of the present invention to provide a technical realization for the rapid and accurate determination of the parameters of an equivalent circuit diagram for an electrochemical storage.

Disclosure of the invention

The above object is achieved by a method with the features of claim 1 and by an electronic control unit with the features of claim 9 and a means of locomotion with the features of claim 10. To this end, the method according to the invention comprises the following steps: First, a first parameter of a first component of the equivalent circuit is determined by means of a periodic excitation of the energy store with a variable frequency. Such a method is basically known in the art as Impedance spectroscopy (EIS) known. As the first component, for example, an ohmic resistance, an inductance and / or an ohmic resistance in parallel with a capacitance in question. The first parameter of this component may accordingly be an ohmic resistance value, a capacitance value, an associated impedance, or the like. In general, the first parameter is used for the characterization and technical determination of the dimensioning of the first component. In a second step, a second parameter of a second component of the equivalent circuit is determined via a step-shaped electrical excitation. For the second parameter and the second component, the statements made in connection with the first parameter and the first component apply accordingly. The step-shaped electrical excitation establishes a temporary load state of the electrochemical energy store with a predefined size. Also, the step-shaped excitation between different predefined state of charge ("state of charge", SOC) take place with, for example, substantially constant load. Since the step-shaped electrical excitation is subsequently withdrawn, it is also possible to speak of a substantially rectangular excitation or load. According to the invention, the periodic excitation is made up to a lower limit frequency of the variable frequency. In this way, frequency ranges are omitted, which would justify a particularly time-consuming measurement. The cutoff frequency is above such a frequency range which would be required for determining the second parameter (determined according to the invention by the stepped electrical excitation) by means of the periodic excitation. In other words, according to the invention, the second parameter is not determined via the periodic excitation but via the stepped excitation. In this way, time is saved, which can be invested to increase the accuracy of measurement. The result is a particularly accurate and fast method to determine the parameters for equivalent circuits electrochemical energy storage with automated technical processes.

The dependent claims show preferred developments of the invention.

While the methods used according to the invention can in principle also be used overlapping, the idea underlying the time saving lies in the fact that the first parameter is determined exclusively on the basis of the periodic excitation up to the lower limit frequency. Preferably, the second parameter can be determined according exclusively by the step-shaped electrical excitation. The excitation variable is preferably a current which can be designed as a charging and / or discharging current. It is also possible to use an electrical voltage as an excitation for the method according to the invention.

More preferably, the first component may comprise a first parallel connection of a first ohmic resistor and a first capacitor. In particular, a further ohmic resistor arranged in series with the first parallel circuit can also be provided. The complex resistance of the first component or the concrete impedances of its electrical components can be inventively determined by the periodic excitation with variable frequency quickly and accurately.

Accordingly, the second component may comprise a second parallel connection of a second ohmic resistor and a second capacitor. The second component may also comprise a third parallel circuit of a third ohmic resistor and a third capacitor, the third parallel circuit being arranged in series with the second parallel circuit. For the second component and the discrete electrical components contained in it, the same applies in connection with the first component above, with the difference that the step-shaped electrical excitation according to the invention is used in order to enable an exact and time-saving determination.

The step-shaped electrical excitation may comprise, for example, a temporary application of a positive current ("charging current") or a temporary application of a negative current ("discharging current"). The loading may last several seconds, a few minutes, or an even longer time (e.g., 10 minutes). For a person skilled in the art, the choice of the duration of the step-shaped electrical excitation as a function of a specific electrochemical energy store or concrete additional circumstances is merely a technical exercise.

By contrast, the periodic electrical excitation can be carried out, for example, with a sinusoidal signal, a plurality of sinusoidal signals or signals containing sinusoidal signals. In particular, a so-called sinusoidal sweep, which has a substantially continuous frequency change of the electrical signal over time, can be used. In this way, the course in the NYP can be particularly well prepared metrologically.

The limit frequency used according to the invention for the periodic excitation is based on the capacitance as well as on the used storage technology of the electrochemical Energy storage dependent and, for example, in a range between 0.01 Hz and 5 Hz, in particular between 0.1 Hz and 2 Hz, preferably between 0.5 Hz and 1 Hz. This results in a suitable synthesis of the results of the excitation results used according to the invention. In other words, the quality of the result is maximized with the least amount of time possible.

According to a second aspect of the present invention, an electronic control unit is proposed, which may be configured as known in the art "Battery Management System" (BMS). These electronic control units are especially, but not exclusively, used in automotive applications. The person skilled in the art is familiar with further fields of application to which the scope of application and protection of the present invention likewise extends. The electronic control unit has electrical connections for connecting an electrochemical energy store or such a comprehensive on-board network. Furthermore, the electronic control unit has an evaluation unit which is set up to carry out a method as has been described in detail above in connection with the first aspect of the invention. The evaluation unit may comprise, for example, a processor (microcontroller, nanocontroller or the like). It can be connected to memory means which contain instructions on the basis of which the processor can carry out the method steps according to the invention. The features, combinations of features as well as the advantages resulting therefrom correspond to those described in connection with the first-mentioned aspect of the invention such that, in order to avoid repetition, reference is made to the above statements.

According to a third aspect of the present invention, a means of transportation is proposed, which may be, for example, an electrically drivable vehicle. The means of locomotion has an energy store, which can be configured, for example, as an electrochemical energy store. In addition, the means of locomotion comprises an electronic control unit, as described in connection with the second aspect of the invention above. Reference is also made to the second aspect of the invention to the features, feature combinations and the advantages resulting therefrom in order to avoid repetition.

Brief description of the drawings

Hereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings. In the drawings:

1 a schematic view of a means of transport according to an embodiment of the present invention;

2 a timing diagram illustrating a stepwise excitation of an electrochemical energy store;

3 a schematic representation of the synthesis of the method used in the invention for the determination of parameters of the components of an equivalent circuit diagram of an electrochemical energy storage;

4 an illustration of an alternative approach for determining the parameters by means of a step-shaped excitation; and

5 a flowchart illustrating steps of an embodiment of a method according to the invention.

Embodiments of the invention

1 shows a car 20 , in which an electrochemical energy store 2 with a schematically arranged in this equivalent circuit 1 located. Between the electrodes of the electrochemical energy store 2 is an electronic control unit 3 with an evaluation unit contained therein 4 arranged to determine the inventive components of the equivalent circuit 1 perform. In the equivalent circuit 1 is the ohmic resistance R0 for the resistance of the electrolyte and ohmic conduction and contact resistance. The resistor R1 and the ohmic resistor R2 stand for resistances. The first capacitance C1 represents the SEI layer ("solid electrolyte interface") and the second capacitance C2 models the double layer (English "double-layer",). The illustrated equivalent circuit 1 can be derived for example by an impedance measurement of lithium-ion battery cells. The determination of the parameters of their components can be made according to the invention faster and more accurate.
R0 = pure ohmic component
R1C1 = pure ohmic component parallel to capacitive component.
R2C2 = pure ohmic component parallel to capacitive component.
Capacity C1 <C2 follows from this
Time constant RC1 <RC2 follows from this
RC1 corresponds to a fast-reacting circuit (relatively speaking)
RC2 corresponds to a slow-reacting circuit (relatively speaking)

2 shows a voltage or current-time diagram in which an applied to an electrochemical energy storage Storm 5 to step-shaped excitation to a course of the terminal voltage also shown 6 of the electrochemical energy storage leads. In a first period of time (0 to 0.3 days), several complete charge-discharge cycles are sequentially performed, through which the terminal voltage varies between 2.8 and 4.1 volts. In a second period (0.3 to 1.2 days), the state of charge of the electrochemical energy storage is stepped by a pulse-shaped current drain 8a of 25A (equivalent to -1C), whereby the charge state is lowered in sections from 100% to 0%. The descending stages of the electrical terminal voltage 6 each show in the current drain burglary 7 in the wake of which the relaxation to a substantially constant terminal voltage 6 leads. In a third time range (1.1 to 2.1 days), the reverse process, namely a rise from 0% to 100% in sections of the state of charge takes place. For this purpose charging current pulses 8b used in the amount of 1C to step-electrically energize the electrochemical energy store. Between the current pulses 8a . 8b a substantially constant terminal voltage can be seen.

3 shows an overview of the inventively used suggestions in the context of determining parameters of the components of an equivalent circuit diagram 1 an electrochemical energy storage. The equivalent circuit diagram 1 corresponds to the in 1 shown equivalent circuit diagram 1 , In the left part of the 3 there is a time diagram for terminal voltage 6 and terminal current 5 according to the in 2 shown illustration. Only a single burglary 7 is shown enlarged to illustrate the apparent sizes. The first falling edge of the discharge current 5 leads to a corresponding falling edge of the terminal voltage 6 from which the ohmic resistance R0 can be determined. The relationship of this phenomenon is indicated by the connection line A with the electrical component R0. At the base of the falling edge of the terminal voltage 6 there is a time course of the terminal voltage 6 which depends on the parameters of the ohmic resistor R1 and the first capacitor C1 can close. This relationship is indicated by the connecting line B. The bottom of the burglary 7 in the terminal voltage 6 is formed by the parameters of the parallel connection of an ohmic resistor R2 and a second capacitor C2 as a second component. This relationship is indicated by the connecting line C. Above the equivalent circuit diagram 1 is a NYP 10 for the equivalent circuit 1 represented underlying electrochemical energy storage. While the abscissa indicates the real part of the impedance as a function of a frequency of the periodic excitation, the ordinate is assigned to the corresponding imaginary part. Also in the NYP 10 find the components of the equivalent circuit 1 again. The corresponding areas are also identified by the connecting lines A, B, C. While the ohmic resistor R0 through a section of the NYP arranged substantially on the abscissa 10 is represented, the first RC1 member (R1, C1) is responsible for a first arcuate portion. This range results from periodic excitation above the limit frequency defined according to the invention. Around the sections of the NYP 10 To determine, very low frequencies are required, which require a time-consuming measurement. According to the invention, the second RC1 element (R2, C2) is replaced by the links in FIG 3 shown step-shaped electrical excitation determined and above the cutoff frequency exactly and time-saving determinable components of the first component (R0, R1, C1) are from the NYP 10 determined. In this way, a time-consuming determination in low frequency ranges of the impedance spectroscopy as well as a complicated and subject to considerable tolerances determining the parameters of the first component from the result of the stepped electrical excitation can be avoided.

4 shows a consideration of an alternative section in the diagram due to a stepped electrical excitation. In contrast to the first falling edge, the first rising edge of the terminal voltage 6 after passing through the burglary 7 used to determine the second (and any third) RC element. Here, the voltage value U _{63 is} considered, which marks the 63% mark within the area marked ΔR. As shown, the time constant τ can be derived from the degressive rising curve of the terminal voltage 6 using the voltage value U ₆₃ . Where RE is composed of R0 + R1. RE is strongly dependent on the different contacting methods and installed or used materials. R0 will increase when the temperature drops. The electrolyte becomes more viscous (viscous) the internal resistance of the cell will increase and RE will grow.

A faulty contact will therefore affect RE accordingly. The ohmic part becomes larger so that R0 and finally also RE.

5 1 is a flowchart illustrating steps of an embodiment of a method according to the invention. In step 100 a first parameter of a first component of the equivalent circuit of an electrochemical energy store is determined via a periodic excitation of the energy store with variable frequency. The periodic excitation is made only up to a lower limit frequency, the Limit frequency is chosen such that a frequency range which would be required to determine the second parameter by means of the periodic excitation, below it. The second parameter of the second component of the equivalent circuit is different from the first parameter in step 200 determined by a step-shaped electrical excitation. If the transfer criterion (eg the cutoff frequency) and / or the execution of the two aforementioned (sub-) methods are performed automatically in a measuring device, then their results can be automatically assembled and the parameters of the first and second components faster and more accurate than by the in stand The technique known to be determined.

Although the aspects and advantageous embodiments of the invention have been described in detail with reference to the embodiments explained in connection with the accompanying drawings, modifications and combinations of features of the illustrated embodiments are possible for the skilled person, without departing from the scope of the present invention, the scope of protection the appended claims are defined.

LIST OF REFERENCE NUMBERS

1

equivalent circuit

2

energy storage

3

Electronic control unit

4

evaluation

5

electricity

6

terminal voltage

7

Burglary of the terminal voltage 6

8a

Current pulse of the negative current 5

8b

Current pulse of the positive current 5

10

Nyquist-Plot, NYP

20

car

100, 200

 steps

A, B, C

 Relationships between subgraphs and electrical components

C1, C2

First / second capacity

I

electricity

p

Time

R0, R1, R2, RE

 Ohmic resistance

τ

time constant

U

tension

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102007061122 A1 [0003]
• US 6687631 B2 [0004]
• EP 1590679 B1 [0005]

Claims (10)

Method for determining the components of an equivalent circuit ( 1 ) of an electrochemical energy store ( 2 ) comprising the steps - determining ( 100 ) of a first parameter of a first component (R0, R1, C1) of the equivalent circuit ( 1 ) via a periodic excitation of the energy store ( 2 ) with variable frequency, - Determine ( 200 ) of a second parameter of a second component (RC, C2) of the equivalent circuit ( 1 ) via a stepped excitation, - wherein the periodic excitation is made up to a lower limit frequency, and - wherein the cutoff frequency is above a frequency range which would be required to determine the second parameter by means of the periodic excitation.  The method of claim 1, wherein the first parameter is determined solely on the basis of the periodic excitation up to the lower limit frequency.  Method according to Claim 1 or 2, wherein the first component (R0, R1, C1) comprises a first parallel circuit comprising a first ohmic resistor (R1) and a first capacitor (C1), in particular also a further ohm arranged in series with the first parallel circuit 'resistance (R0) includes.  Method according to one of the preceding claims, wherein the second component (R2, C2) comprises a second parallel connection of a second ohmic resistor (R2) and a second capacitor (C2).  The method of claim 4, wherein the second component (R2, C2) comprises a third parallel circuit of a third ohmic resistor and a third capacitor arranged in series with the second parallel circuit. Method according to one of the preceding claims, wherein the stepwise excitation comprises a temporary application of a positive current ( 8b ) or temporary exposure to a negative current ( 8a ).  Method according to one of the preceding claims, wherein the periodic excitation comprises an excitation with a sinusoidal signal or a plurality of sinusoidal signals and / or with a sine wave.  Method according to one of the preceding claims, wherein the cut-off frequency is in a range between 0.01 Hz and 5 Hz, in particular between 0.1 Hz and 2 Hz. Electronic control unit ( 3 ), especially for automotive use, which - electrical connections for connecting an electrochemical energy store ( 2 ), and - an evaluation unit ( 4 ), wherein the evaluation unit ( 4 ) is arranged to carry out a method according to one of the preceding claims. Means of transport, in particular electrically driven vehicle ( 20 ), which - an electrical energy storage ( 2 ), and - an electronic control unit ( 3 ) according to claim 9.

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