Method and apparatus for determining the state-of-charge value of a battery

Abstract

Method for determining a state-of-charge value (SOC) of a battery (2), comprising the steps: a) Providing a battery (2) with a known temperature and known time after last charge / discharge, b) Determining an impedance value (Z) at a frequency f, c) Determining an imaginary part (Z'') of the impedance (Z) at frequency f, d) Comparison of the imaginary part (Z'') with a threshold value (S), e) if the threshold (S) is undershot, the SOC value is determined to be 100%, f) If, however, the threshold (S) is not undercut, the following steps follow: g) Determining the phase angle (φ) of the impedance (Z) as an evaluation parameter and h) Determining a SOC value of the battery (2) by comparing the phase angle (φ) with at least one reference value.

Description

[0001] The invention relates to a method and a device for determining a state-of-charge value of a battery.

[0002] Understanding the state parameters of a battery is extremely important, for example, to determine operating strategies and / or battery replacement. Therefore, some battery characteristics will first be briefly explained and defined.

[0003] The charge capacity of a battery is given in the unit ampere-hour [Ah], and the following capacities are distinguished: Nominal capacity K 20 This represents the nominal capacity specified by the manufacturer. It is the minimum capacity in [Ah] stored in a new battery, which can be achieved by discharging it for twenty hours at the defined current I. 20 at a temperature of T=(27−2+0)°C up to the shutdown criterion of U = 10.5 V can be provided.

[0004] Actual capacity K IST : Denotes the maximum capacity in [Ah] in the current state of the battery.

[0005] Residual capacity K REST The remaining capacity represents the capacity in [Ah] that is available in the current state. It is less than or equal to the actual capacity.

[0006] The state of charge (SOC) of a battery is defined as follows: SOC=KRESTKIST⋅100%

[0007] The definition of the state of health (SOH) of a battery is: SOH=KISTK20⋅100%

[0008] The SOC can be a maximum of 100%, since K IST which represents the maximum capacity, where the value of the SOH can be > 100%, since the maximum possible capacity K IST A new battery usually has the nominal capacity K specified by the manufacturer. 20 exceeds.

[0009] Aging effects in lead-acid batteries represent irreversible damage that negatively impacts component function and state of health (SOH). For example, the loss of active mass results in a lower actual battery capacity and thus a reduced amount of energy that can be stored. This loss of actual capacity is caused by cyclic charging and corrosion. Corrosion is particularly promoted by prolonged periods of low charge, especially at high temperatures. The consequences of aging due to cycling and corrosion include sludge buildup, surface sulfation, and breakage of the current-dissipating grid of the positive electrodes.

[0010] From DE 10 2009 000 337 A1 a method for determining the aging state of a battery cell is known, comprising the steps: a) Providing a battery cell, b) Recording an impedance spectrum of the battery cell, c) Determining an evaluation parameter based on the measured impedance spectrum, d) Determining the aging state of the battery cell by comparing the evaluated parameter with a reference value and transferring the result for one cell to the entire battery, disregarding possible differences in the aging state of individual cells.

[0011] Preferably, the evaluation parameter is the magnitude of the measured impedance in ohms at a specific low frequency, and the reference value is a real number with the unit ohm. The low frequency can be any frequency ≤ 10 Hz, preferably ≤ 1 Hz. A frequency in the range of 0.1 Hz to 0.3 Hz is particularly preferred. The publication describes the general application for all common accumulator technologies.

[0012] However, practical tests have shown that the SOH determination in lead-acid batteries does not yield satisfactory results using the proposed procedure steps. It should be noted that the method described in DE 10 2009 000 337 A1 does not provide a SOH as a result, since the SOH is related to KISTK20⋅100% is defined. Rather, a remaining lifetime in days is specified, which depends on the state of charge (SOC) and the temperature.

[0013] From DE 102 20 172 A1 a method for monitoring the operating state of an electrochemical device is known, in which an impedance is measured at terminals of the electrochemical device using a measuring device, wherein the operating state of the electrochemical device is monitored in an evaluation device on the basis of the magnitude of an imaginary part of the measured impedance.

[0014] From DE 10 2009 000 337 A1 a method for determining the aging state of a battery cell is known, in which an impedance spectrum is measured and evaluated.

[0015] From DE 692 29 805 T2, a method for determining the remaining capacity of a battery cell is known, wherein two impedances of the cell are measured in a frequency range of 0.001 Hz to 1 Hz. Subsequently, the slope of the imaginary part over the real part is determined, and the remaining capacity is determined by comparing the slope with a calibration curve.

[0016] From US 6 094 033 A, a method for determining the SOC value of a battery is known, wherein an impedance value is determined at a frequency, and an SOC value is determined from the parameters of an equivalent circuit and the phase angle.

[0017] The invention is therefore based on the technical problem of providing a method and a device for determining a state-of-charge value of a battery, by means of which the SOC value can be determined more reliably.

[0018] The solutions to the technical problem are provided by a method having the features of claim 1 and a device having the features of claim 3.

[0019] The method for determining the state-of-charge (SOC) value of a battery, preferably a lead-acid battery, comprises the step of providing a battery at a known temperature and at a known time since its last charge / discharge. The method further comprises the step of determining at least one impedance value at at least one frequency f.

[0020] The phase angle φ of the impedance is determined as an evaluation parameter and compared with at least one reference value, and a SOC value is assigned.

[0021] Preferably, several reference values ​​are combined into reference ranges.

[0022] As a preliminary step, the imaginary part of an impedance at a specific frequency f is determined and compared to a threshold value. If the negative imaginary part falls below the threshold value, the state of charge (SOC) is 100%. For example, at a preferred frequency of f = 10 mHz, the threshold value for lead-acid batteries is... "-0.14 mΩ". At lower frequencies, the threshold is defined as smaller. If the threshold is not undershot, the SOC is subsequently evaluated by assessing the phase angle φ and / or the slope m.

[0023] The frequency at which the imaginary part is determined is the same as the frequency at which the phase angle is determined (e.g. 10 mHz).

[0024] In one embodiment, the phase angle is determined at frequencies f below 100 mHz and above 1 mHz, whereby again the differences are more pronounced at lower frequencies, but the measurement time increases. A good compromise is a frequency of 10 mHz.

[0025] The invention is explained in more detail below with reference to a preferred embodiment. The figures show: Fig. 1. A schematic block diagram for determining the impedance of a lead-acid battery, Fig. 2 an electrical equivalent circuit diagram of a lead-acid battery, Fig. 3 an exemplary impedance curve (Nyquist diagram) of a lead-acid battery, Fig. 4. A representation of the course of the real part versus the frequency for different SOH values ​​or actual capacitances. Fig. 5 a representation of the course of the imaginary part versus the frequency for different SOH values ​​or actual capacitances and Fig. 6 A representation of the slopes in the diffusion domain as a function of the SOC.

[0026] In the Fig. Figure 1 is a simplified block diagram of a device 1 for determining the impedance of a lead-acid battery 2 with an AC voltage source 3 whose frequency f is variable. An ammeter A is connected in series with the AC voltage source 3, and a voltmeter is connected in parallel with the lead-acid battery 2. These instruments measure current and voltage in phase to determine the impedance in an evaluation unit 4. Alternatively, the source can also be configured as an AC current source. The evaluation unit 4 can then determine the state of health (SOH) and / or the state of charge (SOC) of the lead-acid battery 2, as explained below.

[0027] In the Fig. Figure 2 shows a commonly used representation of a lead-acid battery, whose elements will be briefly explained. The internal resistance R iA lead-acid battery consists of the following components: terminals, terminal bridges, cell connectors (when multiple batteries are connected in series or parallel), electrode grids, active materials, and the electrolyte, which makes up the largest proportion. An increase in internal resistance results from corrosion, loss of active material, changes in microstructures over time, as well as electrolyte concentration and temperature.

[0028] Inductive effects occur in a battery at connecting leads and their interconnection, terminal connectors and the ground structure, which is expressed by the inductance L.

[0029] Within a lead-acid battery, energy storage takes place within the electrolyte. When an overvoltage is applied, the charge carrier density distribution changes due to the potential shift in the double layer caused by the current flow. A certain amount of time is required to reach a new, stable state. This physical behavior is modeled using a capacitance C, here denoted as Cc. dl For "double layer". To understand the behavior in the double layer, the passage of charge carriers from the solid electrode to the liquid electrolyte, which also occurs, and the subsequent charge transfer within the double layer, must be modeled as a resistance that opposes the charge carriers. This can be achieved by connecting the resistor R in parallel. ct for double-shift capacity C dl This will take place. The index "ct" stands for "charge transfer".

[0030] The capacity C Dwith the parallel-connected resistor R D This model models diffusion processes. Diffusion processes are relevant when electrode reactions are inhibited by a lack of reactants. This occurs, for example, at the end of each battery charge when only a small amount of reactable PbSO4 remains on the electrode surfaces, so that the charging current is determined by the lower diffusion rate of lead ions into the double layer and hardly increases even when the charging voltage is raised.

[0031] In the Fig. Figure 3 shows an example of the impedance curve Z = Z' + jZ'' of a lead-acid battery, where it should be noted that the negative Z'' values ​​are plotted on the y-axis. The dominant parts of the equivalent circuit are shown according to... Fig. Figure 2 is shown. Here, Z' is the real part and Z'' the imaginary part of the impedance. If a straight line passes through the origin and passes through an impedance value, the angle between the line passing through the origin and the x-axis is the phase angle φ.

[0032] In the Fig. Figure 4 shows the real part Z' as a function of frequency for differently aged batteries in their resting state, where the temperature and state of charge (SOC) are the same (T = 20°C and SOC = 40%). It can be seen that the curves differ only slightly in the range above 1000 Hz. From 1000 Hz down to approximately 0.1 Hz, the only distinguishable factor is the most cycled battery, K. IST = 22.15 Ah compared to the other batteries. At frequencies < 0.1 Hz, the curves differ more significantly, with the greatest difference being observed at f = 10 mHz. However, it can be seen that the actual capacity at 10 mHz decreases non-linearly with increasing real part. In the Fig. Figure 5 shows the measurement results of the same batteries, with the negative imaginary part Z'' now shown as a function of the frequency f.

[0033] The graph shows that the imaginary part cannot differentiate the actual capacitances up to a frequency of approximately 10 Hz. Below 10 Hz, a greater difference in the curves is visible than in the graph shown in [reference]. Fig. 4. The greatest difference also occurred at a frequency of 10 mHz. It can be seen that as the imaginary part decreases, the actual capacitance also decreases, in this case in an almost linear sequence.

[0034] This is now used to determine the SOH value by assigning SOH value ranges to areas of the imaginary part Z''. By using sufficiently large ranges, even measurement outliers do not pose a problem. The comparison is preferably made between the evaluated quantity (imaginary part Z'') and reference values ​​at a fixed frequency (e.g., 10 mHz) and the same temperature when the battery is at rest.

[0035] The evaluation is preferably performed in an evaluation unit where the imaginary part Z'' is determined and compared from the current and voltage values, with the reference values ​​stored in an associated memory. The determined SOH value can then be displayed, and a recommendation for battery replacement can be issued if necessary. Furthermore, the determined SOH value can be fed to other control units, such as a battery management system.

[0036] In the Fig. Figure 6 shows the slopes for lead-acid batteries with different states of charge (SOC) in the diffusion range, where the highest frequency is, for example, 1 Hz and the lowest frequency is, for example, 10 mHz. In the graphical representation, the values ​​of the first impedance value (Z'min + jZ''min) at the highest frequency (e.g., 1 Hz) are subtracted from the subsequent values, resulting in straight lines passing through the origin. An SOC value can then be assigned to the slope m or angle α by comparison with reference values. The imaginary parts Z'' are again negative. A threshold value S is shown, which, for example, is "-0.14 mΩ" for lead-acid batteries at a frequency f = 10 mHz. As the Fig.As can be seen in section 6, only the imaginary part Z'' falls below the threshold S when SOC = 100%. Therefore, it is preferable to first evaluate whether the SOC = 100%. If the imaginary part does not fall below the threshold S, then the SOC is less than 100% and is determined by evaluating the slope m or the angle α.

[0037] Alternatively, the phase angle φ of the impedance can be determined at the lowest possible frequency, for example 10 mHz, and a SOC range can be assigned to a specific angle range. Here too, the imaginary part Z'' can first be compared with the threshold S to determine if the SOC equals 100%.

[0038] The two methods can also be combined. Preferably, the imaginary part Z'' is first compared with the threshold S. If the imaginary part falls below the threshold S, the SOC is determined to be 100% accurate. If, however, the threshold S is not exceeded, both the slope m (or the angle α) and the phase angle φ are determined at a specific frequency. The two results of the SOC determination are then combined, and weighting can be applied.

Claims

[1] Method for determining a state-of-charge value SOC of a battery (2), comprising the steps: a) providing a battery (2) with a known temperature and known time after the last charge / discharge, b) Determining an impedance value (Z) at a frequency f, c) Determining an imaginary part (Z'') of the impedance (Z) at the frequency f, d) Comparison of the imaginary part (Z'') with a threshold value (S), e) if the threshold value (S) is undershot, the SOC value is determined as 100%, f) If the threshold value (S) is not exceeded, the following steps follow: g) Determining the phase angle (φ) of the impedance (Z) as an evaluation variable and h) determining an SOC value of the battery (2) based on a comparison of the phase angle (φ) with at least one reference value. [2] Method according to claim 1, characterized bythat the phase angle (φ) is determined at a frequency f less than 100 mHz and greater than 0.1 mHz. [3] Device for determining a state-of-charge value SOC of a battery (2), comprising an alternating voltage or current source (3), means for phase-correct detection of voltage and current and an evaluation unit (4), wherein the evaluation unit (4) is designed such that an impedance value (Z) is determined and an imaginary part (Z'') of the impedance (Z) is determined therefrom, wherein the evaluation unit compares the imaginary part (Z'') with a threshold value (S), wherein if the threshold value (S) is undershot the SOC value is determined to be 100%, wherein the evaluation unit (4) is further designed such that if the threshold value (S) is not undershot a phase angle (φ) of the impedance (Z) is determined as an evaluation variable and the SOC value of the battery (2) is determined based on a comparison of the phase angle (φ) with at least one reference value.

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