Method and device for determining the state of an accumulator, as well as a computer program

Abstract

Method for determining the state of an accumulator (1) by applying an alternating current signal to terminal contacts (2) of the accumulator (1) and measuring at least one electrical response signal of the accumulator (1) that arises in reaction to the applied alternating current signal and determining the state at least taking the response signal into account, wherein, in determining the state, a distinction is made between at least two different state effects of the accumulator (1) that influence the state and at least one of these state effects is quantified independently of other state effects, wherein the accumulator (1) is a lithium-ion accumulator and, with respect to the state, a distinction is made at least between the two state effects Solid Electrolyte Interphase (SEI) and Lithium Plating, wherein at least one state effect, namely that of the change in the Solid Electrolyte Interphase,by evaluating nonlinear response signals of one or more frequencies, which are harmonics of the applied alternating current signal and which can lie in the entire frequency range, and at least one state effect, namely lithium plating, is detected by evaluating nonlinear response signals in a predetermined, limited frequency range.

Description

[0001] The invention relates to a method for determining the state of a battery by applying an alternating current signal to the battery's terminals and measuring at least one electrical response signal from the battery, generated in reaction to the applied alternating current signal, and determining the state at least by considering the response signal. The invention further relates to a device for determining the state of a battery with which such a method can be carried out. The invention also relates to a computer program for carrying out such a method.

[0002] An accumulator is an arrangement of one or more electrochemical cells that can supply electrical energy or absorb electrical energy when being charged.

[0003] The condition of a battery can be, for example, its aging state, also referred to as its state of health (SOH). The condition of a battery can be determined, for instance, by measuring or estimating its maximum remaining usable capacity. However, determining the condition in this way has the disadvantage that the remaining battery life is often misjudged, leading to premature replacement. This is particularly costly for large batteries, such as those required for electric vehicles.

[0004] A battery and a method for determining the aging state of a battery are known from DE 10 2014 001 260 A1. Further methods for investigating battery states are known from: Harting, Nina et al.: Nonlinear frequency response analysis (NFRA) of lithium-ion batteries. In: Electrochimica acta, Vol. 248, 2017, pp. 133-139. Wolff, Nicolas et al.: Nonlinear frequency response analysis of lithium-ion batteries: a model-based assessment. In: Electrochimica acta, Vol. 260, 2018, pp. 614-622.

[0005] The invention is based on the objective of determining the state of an accumulator more precisely and realistically.

[0006] This problem is solved by a method according to claim 1. This involves differentiating between at least two different state effects of the accumulator that influence its state when determining its condition, and quantifying at least one of these state effects independently of other state effects. In this way, the "true" state of the accumulator can be assessed much more realistically. Depending on the type of accumulator, e.g., lithium-ion, lithium-polymer, nickel-metal hydride, nickel-cadmium, or lead-acid, specific state effects can be considered and differentiated between them. The state effects of the accumulator can, in particular, be state-change effects, i.e., effects by which at least one state variable of the accumulator changes reversibly, irreversibly, or partially reversibly.

[0007] The method according to the invention is suitable for determining various types of states of a battery, e.g., for determining its aging state. The method according to the invention is also suitable, for example, for the incoming inspection of battery cells (purchase), for production verification, e.g., to detect errors and / or deviations in production, or for further state determinations, such as the temperature variation between individual cells, detection of coolant failure or inhomogeneity in cooling, or a specific mechanical load on the battery. Finally, the current state of charge of the battery can also be determined using the method according to the invention, for example, in combination with the phase shift evaluation explained below.

[0008] The various state effects of the battery mentioned above, which are differentiated, can be any chemical and / or physical effects, generally specific to the cell type. These state effects can include, in particular, aging effects of the battery or other effects that influence or impair its performance. For example, the state effects may be defined in such a way that changes in the state of charge that normally occur during battery operation are not included.

[0009] As a rule, such state effects result not only in losses of capacity but also in losses of power, including through an increase in internal resistance and a decrease in the active electrode surfaces.

[0010] Lithium-ion batteries can exhibit various conditions, such as solid electrolyte interphase (SEI) or lithium plating, which is particularly prevalent at low temperatures. In addition to these effects, local heat accumulations, known as hotspots, can also develop and influence the battery's performance.

[0011] As mentioned, the state is determined at least by considering the response signal. Therefore, other influencing factors can also be taken into account when determining the state, such as the total operating time or specific load conditions of the battery.

[0012] The determination of the state, and in particular the differentiation between various state effects, can be carried out with respect to the entire (multi-cell) accumulator, with respect to one, several, or all individual cells, or with respect to groups of individual cells of the accumulator. Accordingly, for example, an overall state of the entire accumulator or a state of individual cells or groups of cells can be determined.

[0013] Quantifying a state effect involves determining a relative or absolute numerical value that represents the state effect. This value can be processed, for example, in a battery management system to manage the battery and ensure appropriate battery management, thereby reducing or minimizing the progression of this or other state effects. The value can also be used to control a signal transmitter, such as a warning light, or displayed visually on a vehicle screen or diagnostic device, for example, when the vehicle is in a workshop.

[0014] Lithium plating can lead to safety-critical conditions during battery operation. Lithium plating is often caused by improper charging, so a differentiated assessment of this condition can significantly extend the battery's lifespan.

[0015] For example, depending on at least one quantification of a state effect, at least one operating parameter during charging and / or discharging of the battery can be influenced. For instance, if lithium plating occurs more frequently, the battery can be charged using a different charging algorithm in the future, such as with a reduced charging current and / or altered charging timing. This function can be performed automatically by the battery management system, depending on at least one quantification of a state effect.

[0016] According to an advantageous embodiment of the invention, the response signal is analyzed by one or more analytical methods to differentiate between at least two different state effects and / or to quantify at least one of these state effects independently of other state effects. This allows for a very efficient differentiation between different state effects. Advantageously, analytical methods with high differentiation capability are used.

[0017] According to an advantageous embodiment of the invention, one analysis method is nonlinear frequency response analysis. Nonlinear frequency response analysis (NFRA) advantageously allows differentiation between state effects based on their characteristic impact on the nonlinear frequency spectrum. For example, this allows for a clearer distinction between lithium plating and solid electrolyte interphase. In nonlinear frequency response analysis, both the sum of harmonics and the amplitudes of individual harmonics, starting with the second harmonic, are analyzed, for example, after applying an alternating current with an amplitude that covers the exponential current-voltage range. Phase shifts of the harmonics can also be utilized.

[0018] The solid electrolyte interphase, or its growth, increases the nonlinear response signals over a wide frequency range, whereas lithium plating specifically or more strongly affects a frequency range or harmonic and influences the nonlinear signals in this range.

[0019] According to an advantageous embodiment of the invention, an analytical method is provided that electrochemical impedance spectroscopy or a related analytical method. This allows for an additional analysis of the battery's state as well as a more refined differentiation between state effects. While electrochemical impedance spectroscopy can quantify a general battery state, it cannot distinguish between the previously mentioned state effects of solid electrolyte interphase and lithium plating. An analytical method related to electrochemical impedance spectroscopy is, for example, one that can be converted into electrochemical impedance spectroscopy results through linear system analysis. Thus, instead of impedance, another signal can be analyzed, such as switch-off measurements, step signals, or ramps as the input signal.Such input signals can be evaluated in a similar way to impedance, for example, using a Fast Fourier Transform (FFT).

[0020] According to an advantageous embodiment of the invention, the applied alternating current signal is applied successively multiple times during the process, with the frequency of the alternating current signal being changed between successive application processes. After each application process, the electrical response signal of the accumulator, generated in reaction to the applied alternating current signal, is measured, and the state is determined, at least with consideration of the response signal. In this way, even more evaluable data can be obtained for a particularly fine differentiation of various effects influencing the state of the accumulator. For example, the input frequency of the applied alternating current signal can be varied within a predetermined frequency range, such as from a few millihertz to several kilohertz.For example, input frequencies can be selected in the range of 1 kHz to 1 Hz, or from 1 kHz to 0.1 Hz.

[0021] When performing a nonlinear frequency response analysis, if several AC signals with different input frequencies are applied sequentially, this can be done, for example, with two, three, or four different frequencies. For electrochemical impedance spectroscopy or a related analysis method, the same frequency range as previously described can be used. Here, it is advantageous to provide a finer subdivision in the input frequencies, for example, by using five different frequencies per decade of the input frequency. In this case, the input frequency can be varied in the intended frequency range with at least 20 or at least 50 steps, i.e., at least 20 or at least 50 different input frequencies.

[0022] According to the invention, the accumulator is a lithium-ion accumulator and its state is differentiated at least between two state effects: solid electrolyte interphase and lithium plating. In this way, the invention is particularly suitable for diagnosing lithium-ion technology, which is important for electric vehicles.

[0023] According to the invention, at least one state effect, namely the solid electrolyte interphase, is detected by evaluating nonlinear response signals of one or more frequencies, which are harmonics of the applied alternating current signal and which can lie across the entire frequency range. The term "entire frequency range" here refers to the frequency range that can be measured with conventional methods, which naturally ends at higher harmonics, for example, the tenth or fifteenth harmonic. The evaluation of nonlinear response signals can also be performed with respect to only a portion of the frequency range, for example, by evaluating the first harmonic of the response signal. In this context, the first harmonic is considered the fundamental frequency; accordingly, the first overtone is considered the second harmonic, the second overtone the third harmonic, and so on.

[0024] Similarly, at least one state effect, in particular the solid electrolyte interphase, can be quantified by evaluating nonlinear response signals of one or more frequencies, which are harmonics of the applied AC signal and can lie across the entire frequency range. For example, a numerical value can be obtained from the amplitude of the evaluated nonlinear response signals, which quantifies the state effect.

[0025] According to the invention, at least one state effect, namely lithium plating, is detected by evaluating nonlinear response signals within a predetermined, limited frequency range. For example, the state effect can be detected by evaluating nonlinear response signals of one or more frequencies that are harmonics of the applied AC signal and lie within the predetermined, limited frequency range. The predetermined, limited frequency range can be specified by the manufacturer for a particular battery brand. The manufacturer specifies, for example, a characteristic frequency or a frequency range suitable for detecting lithium plating by nonlinear frequency response analysis.

[0026] To detect the state effect, a comparison of the response signals for different harmonics can be performed, for example, for the nth harmonic and the mth harmonic, where n ≠ m. As a concrete example, a comparison of the amplitudes of the second harmonic with the third harmonic can be made. If the amplitude of the third harmonic is greater than the amplitude of the second harmonic, the state effect of lithium plating can be diagnosed in a lithium-ion battery. This state effect can be quantified in the same way. For example, a numerical value that quantifies the state effect can be obtained from the ratio of the amplitudes of the evaluated nth harmonic and mth harmonic.

[0027] According to an advantageous embodiment of the invention, hotspots are determined and / or identified by evaluating the response signal, in particular by detecting that nonlinear response signals are reduced in a predetermined frequency range, e.g., compared to a value typical for the accumulator. This has the advantage that local heat accumulations are also diagnosed without requiring additional complex hardware, such as a temperature sensor on each individual accumulator cell. If hotspots are detected, a warning signal can be generated, or the accumulator can be completely or partially shut down.

[0028] According to an advantageous embodiment of the invention, at least one phase shift between at least a part of the battery's response signal and the applied AC signal is evaluated to determine the battery's state. This allows for further differentiation of the battery's state, enabling even finer state differentiation. For example, the phase shift between one or more harmonics of the response signal relative to the applied AC signal can be analyzed. The phase shift between the fundamental frequency of the response signal relative to the applied AC signal can also be analyzed.

[0029] The aforementioned task is also solved by a device for determining the state of an accumulator, comprising an AC imprinting device for imprinting an AC signal onto the accumulator's terminals, a measuring device for measuring the accumulator's electrical response signal, and an evaluation unit configured to execute a method of the type described above. The advantages described above can also be realized in this way. The evaluation unit can, for example, include a computer, such as a microprocessor or microcontroller, which executes a computer program, thereby carrying out a method of the type described above. The aforementioned device, or at least its evaluation unit, can, for example, be a vehicle battery management system or a part of such a system.

[0030] The aforementioned task can also be solved by a computer program using program code tools, designed to carry out a procedure of the type described above, when the procedure is executed on a computer. This also allows the previously described advantages to be realized.

[0031] The invention is explained in more detail below with reference to an exemplary embodiment and the accompanying drawings. The drawings show in Fig. 1. a device for determining the state of an accumulator and in Fig. 2 measurement results.

[0032] The Fig. Figure 1 shows an accumulator 1 with two terminals 2. An AC imprinting device 3 and a measuring device 4 are connected to the terminals 2. An AC signal can be imprinted onto the terminals 2 by means of the AC imprinting device 3. The electrical response signal of the accumulator 1, generated in reaction to the imprinted AC signal, can be measured by the measuring device 4. The signal measured and processed by the measuring device 4 is output to a number of analysis units, in this case two analysis units 5 and 6.

[0033] In analysis block 5, a first analysis method is performed, and in analysis block 6, a second analysis method is performed, each analyzing the signal output by the measuring device 4. For example, the first analysis method could be nonlinear frequency response analysis, and the second analysis method electrochemical impedance spectroscopy. Analysis results from analysis blocks 5 and 6 are then transmitted to an evaluation unit 7. In the evaluation unit 7, the analysis results are evaluated to determine the state of the accumulator, differentiating between at least two different state effects. One, several, or all of these state effects can be quantified, i.e., characterized by a numerical value.For example, the detection of specific characteristics of nonlinear response signals across the entire frequency range is used to quantify the state effect of Solid Electrolyte Interphase. Characteristic data from nonlinear response signals within a predetermined, limited frequency range are used to quantify the state effect of Lithium Plating.

[0034] The results of the condition assessment, e.g., the quantified data, can be visually displayed, for example, on a display device 9. When certain limit values ​​for the condition effects are reached, a warning signal can be generated in a vehicle, e.g., a visual or audible signal. A direct numerical display of the condition effects is also possible. Furthermore, the condition effects can be displayed on a diagnostic device, for example, during a workshop inspection by recording the numerical data in a workshop log.

[0035] The evaluation unit 7 also controls the AC imprinting device 3 such that the AC signal is imprinted at the connection contacts 2 at desired times, for example at regular intervals, and the described measurement process is started. The entire process can be controlled, for example, by a computer 8, which is part of the evaluation unit 7.

[0036] The Fig. Figure 2 shows measurement results determined using the inventive method on a lithium-ion battery. The number of charge and / or discharge cycles of the battery is plotted on the abscissa axis in each diagram, and the amplitude of the respective i-th harmonic Y is plotted on the ordinate axis. iThe amplitudes of the first, second, and third harmonics Y1, Y2, and Y3 were evaluated as examples. Diagrams A and B show results from a battery in which SEI growth is the predominant state effect. Diagram A shows the measurement results for the EIS analysis method, and Diagram B for the NFRA analysis method. Diagrams C and D show results from a battery in which lithium plating is the predominant state effect. Diagram C shows the results for the EIS analysis method, and Diagram D shows the results for the NFRA analysis method. The frequency of the applied AC signal was set to 50 Hz, which is a characteristic frequency for electrochemical reactions in such batteries.

[0037] It is evident that Y1 changes to the same extent at 50 Hz for the state effects SEI growth and lithium plating, and that the measurement results do not allow for a differentiation of the state effects. A comparison of the measurement results in diagrams A and B reveals that SEI growth is the predominant state effect in this case, because in the NFRA analysis, Y3 is smaller than Y2 in all cases. In diagrams C and D, a comparison shows that lithium plating is the predominant state effect, because in diagram D, after a certain number of cycles, Y3 is larger than Y2.

[0038] It should be noted that the EIS and NFRA analysis methods must be applied in a cell-type-specific manner. Therefore, for example, the manufacturer of the accumulator cells must provide relevant information on how condition effects of the respective cell type affect the EIS and NFRA analysis methods.

Claims

[1] A method for determining the state of an accumulator (1) by applying an alternating current signal to terminal contacts (2) of the accumulator (1) and measuring at least one electrical response signal of the accumulator (1) that arises in response to the applied alternating current signal and determining the state at least taking into account the response signal, wherein, in determining the state, a distinction is made between at least two different state effects of the accumulator (1) that influence the state and at least one of these state effects is quantified independently of other state effects, wherein the accumulator (1) is a lithium-ion accumulator and, with respect to the state, a distinction is made between at least two state effects, solid electrolyte interphase (SEI) and lithium plating, wherein at least one state effect, namely that of the change in the solid electrolyte interphase,by evaluating nonlinear response signals of one or more frequencies, which are harmonics of the applied alternating current signal and which can lie in the entire frequency range, and at least one state effect, namely lithium plating, is detected by evaluating nonlinear response signals in a predetermined, limited frequency range. [2] Method according to claim 1, characterized by , that for the differentiation between at least two different state effects and / or the quantification of at least one of these state effects independently of other state effects, the response signal is analyzed by one or more analysis methods. [3] Method according to claim 2, characterized by , that one analysis method is nonlinear frequency response analysis. [4] Method according to any one of claims 2 to 3, characterized bythat an analytical method is electrochemical impedance spectroscopy or a related analytical method. [5] Method according to any one of the preceding claims, characterized by , that the applied alternating current signal is applied several times successively during the process, whereby the frequency of the alternating current signal is changed between successive imprinting processes, and after each imprinting process the electrical response signal of the accumulator (1), which arises in reaction to the applied alternating current signal, is measured and the state is determined at least taking into account the response signal. [6] Method according to any one of the preceding claims, characterized by , that hotspots can be determined and / or identified by evaluating the response signal, in particular by detecting that nonlinear response signals are reduced in a predetermined frequency range. [7] Method according to any one of the preceding claims, characterized by , that depending on at least one quantification of a state effect, at least one operating parameter is influenced during the charging and / or discharging of the accumulator (1). [8] Method according to any one of the preceding claims, characterized by , that for the determination of the state of the accumulator (1) at least a phase shift between at least a part of the response signal of the accumulator (1) and the applied alternating current signal is evaluated. [9] Device for determining the state of an accumulator (1), comprising an AC imprinting device (3) for imprinting an AC signal on terminal contacts (2) of the accumulator (1), comprising a measuring device (4) for measuring the electrical response signal of the accumulator (1), and comprising an evaluation device (5, 6, 7) configured to perform a method according to one of the preceding claims. [10] Computer program with program code means, configured to carry out a method according to any one of claims 1 to 8, when the method is executed on a computer (8).

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