METHOD FOR DETERMINING THE STATE OF AT LEAST ONE CELL OF A BATTERY AND A DATA PROCESSING SYSTEM

Description

[0001] The invention relates to a method for determining the state of a cell in a battery, in particular a high-voltage battery. In particular, the method is directed towards determining the state of a cell taking into account the states of other cells in the same battery.

[0002] These high-voltage batteries are used particularly in motor vehicles to store electrical energy for powering traction drives. A battery typically consists of multiple cells, each with a terminal voltage of 1.5 to 4 volts. The cells are at least partially connected in series to provide a traction voltage of 60 to 1,500 volts DC.

[0003] Due to current and upcoming legislation, it is necessary to monitor the condition of the battery or cells during vehicle operation. In particular, determining the condition of the cells enables early detection of failures and early servicing, thus preventing cell or battery failure during operation and allowing for timely replacement of the cell or battery.

[0004] Damage to a high-voltage battery, e.g., from below, cannot be detected electronically. Therefore, a vehicle user cannot be warned of potential cell damage.

[0005] So-called thermal propagation is only detected very late. Currently, there is no way to detect a defective cell early or directly measure its temperature before it starts to burn, as the internal temperature rises. It is possible that future legislation will require users to be warned of a potential hazard early on.

[0006] The internal temperature of a cell cannot currently be measured in motor vehicles. Instead, the temperature of the cell module, i.e., the temperature of a plurality of cells, is measured indirectly. This indirect measurement of the temperature of a single cell by measuring the cell module temperature is disadvantageous because changes in the internal temperature of a cell are only detected with a significant time delay.

[0007] One method for determining the internal cell temperature is called impedance spectroscopy. In impedance spectroscopy, the phase relationship between current and voltage is evaluated to determine the real and imaginary parts of a cell's resistance. This is done by scanning the frequency response of an applied current from very low frequencies (0.1 Hz) to high frequencies (10 kHz). This method is time-consuming and requires a significant amount of energy to be drawn from the cell to apply the current. Furthermore, interference generated by the vehicle's electrical systems has a strong impact and distorts the measurement signal. The voltage and current measurements are evaluated using complex algorithms. This allows for inferences to be drawn, for example, about the absolute value of the internal cell temperature. This method is very susceptible to interference and is currently only used under laboratory conditions.

[0008] From DE 197 55 417 A1, an evaluation circuit for determining complex impedances is known. This is used for analyzing the properties of liquids.

[0009] A lithium-air battery is known from DE 10 2014 222 950 A1.

[0010] From DE 10 2014 202 927 A1, a method for operating a battery system is known. In this method, the performance status of the individual cells is determined. The weakest cells are identified and deactivated, thus slowing down their aging due to use.

[0011] The KR 102 016 339 B1 is directed to a diagnostic device and a method for determining the state of health of a plurality of battery cells.

[0012] WO 2020 / 0064932 A1 is directed to a method and a device for diagnosing battery cells.

[0013] The object of the present invention is to at least partially solve the problems cited with reference to the prior art. In particular, a method for determining the condition of a battery cell is to be proposed. Specifically, this method is intended to enable the early detection of a damaged cell during the operation of a motor vehicle and to reduce or prevent any risk to the user of the motor vehicle.

[0014] A method with the features according to claim 1 contributes to solving these problems. Advantageous further developments are the subject of the dependent claims. The features listed individually in the claims can be combined in a technologically meaningful way and can be supplemented by explanatory facts from the description and / or details from the figures, thereby showing further embodiments of the invention.

[0015] A method for determining the state of at least one cell of a battery is proposed, wherein the battery has a plurality of cells connected in series. The method comprises at least the following steps: a) Applying an alternating current to the majority of cells; b) Measuring the resulting alternating voltage at at least a first cell and a second cell; c) Analyzing the phase angle of the measured alternating voltage of each cell; wherein a difference between at least a first phase angle of a first alternating voltage measured at the first cell and a second phase angle of a second alternating voltage measured at the second cell allows for inferences about a difference in the states of at least the first cell and the second cell, whereby only the phase angle of the alternating voltages measured at the cells is evaluated relative to each other and not the phase relationship between a current and a voltage.

[0016] The above (non-exhaustive) classification of the process steps into a) to c) is primarily intended for differentiation purposes and does not impose any sequence or dependency. The frequency of the individual process steps can also vary. It is also possible that process steps may overlap, at least partially. Process steps b) and c) are most preferably carried out during step a). Step c) may be conditional and may only be executed if step b) shows that the measured alternating voltages do not exhibit a phase shift. In particular, steps a) to c) are carried out in the order listed.

[0017] In particular, the method described here, also known as "differential impedance spectroscopy," determines only the difference between the imaginary components of the cell resistances. It does not evaluate the phase relationship between a current and a voltage, but only the phase angle or phase relationship of the alternating voltages measured across the cells. Potential disturbances in the current signal affect all cells of the battery or the series-connected cells simultaneously and with the same magnitude, and are therefore not measured here as common-mode components. This method exclusively measures and evaluates changes in the imaginary components of the cell resistances relative to each other.

[0018] According to step a), in particular an alternating current, e.g. with a frequency of more than 1 Hz [Hertz], in particular between 1 Hz and 10 kHz, preferably between 10 Hz and 10 kHz, particularly preferably of 100 Hz, is impressed into the cells of the battery.

[0019] According to step b), the alternating voltage thus generated is measured at the individual cells of the battery, in particular at a selection or at all cells of the battery.

[0020] According to step c), the phase relationship of the measured AC voltages to each other is evaluated, e.g., electronically. In particular, the respective phase angle of each measured AC voltage is recorded and compared with the phase angles of the other AC voltages. The difference in phase angles can be classified using (accessible and / or predefined) limit values, thresholds, acceptance ranges, empirical values, characteristic curves, or the like, so that a conclusion can be drawn about the condition of at least one of the tested cells. This conclusion can include a probability value for the necessity of a potential or imminent failure of at least one cell, (existing or expected) damage to the cell, (additional) service requirements for at least one cell, etc. The analysis or evaluation can be performed purely computationally using data processing tools.These can then also output information about the inference.

[0021] In particular, the procedure measures only the alternating voltages applied to the cells.

[0022] In particular, the frequency can be any value between 1 Hz and 10 kHz. In particular, the alternating current can have a constant or a changing frequency. In particular, the alternating current can also have only one period.

[0023] In particular, the alternating voltages measured at the cells are each filtered by a bandpass filter, and the resulting signal is modified by an amplifier with a comparator. The signals modified by the amplifier and comparator are then combined in a phase detector. A phase shift of the measured alternating voltages can be represented by a first output signal.

[0024] The bandpass filter allows the detection of the alternating voltage that corresponds to the applied alternating current and is thus induced at the cell. The bandpass filter therefore enables the detection of an alternating voltage that has the same frequency as the alternating current. Specifically, DC components and alternating voltages of other frequencies are not detected or are filtered out.

[0025] The amplifier with comparator provides a modified signal, specifically a square wave representing the measured AC voltage filtered by the bandpass filter. If a phase shift exists relative to other AC voltages, this becomes apparent as a shift in the square wave signals along the time axis.

[0026] In particular, the signals modified by the amplifier with comparator are displayed as a second output signal, so that it is possible to show which alternating voltage(s) have a phase shift compared to other alternating voltages.

[0027] As soon as the imaginary components of the cell resistances differ and thus generate a phase shift in the alternating voltage measured at a cell, the first output signal, in particular a square wave, is produced. This allows conclusions to be drawn about a difference between the cells, e.g., a temperature increase at the beginning of thermal propagation. The duration of the square wave is proportional to, or equal to, the magnitude of the difference, i.e., the temperature difference.

[0028] If the square wave signal of the first output signal is measured or determined at the beginning of a journey in a motor vehicle where, in particular, all cell temperatures are the same, it can be assumed that at least one cell is defective. A possible cause could be a prior damaging event, e.g., the battery being pushed in by a bollard from the underside of a vehicle, or a severely aged cell.

[0029] If the absolute value of a cell temperature is measured near a cell, for example with a temperature sensor, then the phase angle of each individual AC voltage, identifiable in the second output signal, can allow conclusions to be drawn about the respective cell's internal temperature. With this additional information, the SOC (state-of-charge of the cell and, if applicable, also of the battery) or SOH (state-of-health, i.e., the cell's state of aging) can subsequently be determined more precisely.

[0030] The output signals can be evaluated by an evaluation unit.

[0031] In particular, the AC voltages between all cells can be measured, or for a cell stack of several cells (e.g., 6 or 12, etc.), a separate circuit unit is used to determine the AC voltage of the cell stack. This is particularly advantageous for constructing a high-voltage battery with cell modules or cell stacks, as the cell stacks typically do not exceed a maximum voltage of 60 volts.

[0032] The proposed method can be used to determine, in particular, temperature differences between cells, mechanical damage, or significant signs of aging in individual cells.

[0033] The circuits required to carry out the procedure can be integrated into a battery controller or implemented separately, e.g. as an application-specific integrated circuit - ASIC.

[0034] In particular, the alternating voltages measured at the cells are each filtered by a bandpass filter, and the resulting signal is modified by an amplifier. The modified signals are then fed to a lock-in amplifier, which is driven by a reference voltage signal. For each measured alternating voltage, the lock-in amplifier outputs a DC voltage, the amplitude of which is proportional to at least one phase shift between the measured alternating voltage and the reference voltage signal.

[0035] The amplifier delivers, in particular, a modified signal that represents the measured alternating voltage, filtered through the bandpass filter, as a square wave signal.

[0036] The reference voltage signal is also, in particular, a square wave signal.

[0037] The reference voltage signal has, in particular, the same frequency as the applied alternating current.

[0038] A lock-in amplifier is specifically an amplifier used to measure a weak alternating electrical signal modulated by a reference signal with known frequency and phase. The device acts as a narrowband bandpass filter, thereby improving the signal-to-noise ratio (SNR).

[0039] The two input signals of the (respective) lock-in amplifier—namely, the measured AC voltage, which has been filtered in a bandpass filter and converted into a modified signal by an amplifier, and the reference voltage signal—are multiplied together in a mixer or multiplier and then integrated in a low-pass filter. The low-pass filter allows for the attenuation of higher-frequency signals that are generated when the input signals are multiplied.

[0040] The lock-in amplifier calculates, in particular, the cross-correlation between the modified signal and the reference voltage signal for a fixed phase shift between the modified signal of a cell and the reference voltage signal. The cross-correlation for signals of different frequencies is zero. Therefore, if the frequency of the modified signal differs from that of the reference voltage signal, the lock-in amplifier provides no output signal. Only for identical frequencies does the cross-correlation yield a non-zero value and thus contribute to the output signal of the lock-in amplifier. By selecting the appropriate frequency for the reference voltage signal, the phase shift between the modified signal of each cell and the reference voltage signal used for all cells can be determined.

[0041] The lock-in amplifier provides a DC voltage signal for each cell, the value of which indicates the phase relationship of the measured AC voltage to the reference voltage signal.

[0042] A DC voltage signal from one cell that differs from the DC voltage signals of other cells indicates that there is a phase shift and the cell may be defective.

[0043] DC voltage signals that do not differ from each other indicate that there are no phase shifts and that none of the cells are defective.

[0044] In particular, the DC voltage signals are each fed to an analog-to-digital converter and converted there into a digital output signal. This digital output signal can be fed to an evaluation unit and analyzed there.

[0045] In particular, the reference voltage signal has the frequency of the applied alternating current. However, the phase relationship of the reference voltage signal is not stable relative to the phase relationship of the applied alternating current. Therefore, the alternating voltages of the cells can change over time relative to the reference voltage signal, but not relative to each other, i.e., relative to the other cells.

[0046] As soon as the imaginary part of a cell's resistance changes, e.g., due to a temperature increase at the start of thermal propagation, this cell will always have a different alternating voltage or phase angle than the other cells.

[0047] In particular, the reference voltage signal corresponds in frequency and phase to the signal of a cell of the battery filtered by the bandpass filter.

[0048] This allows the reference voltage signal to have a common phase with the measured AC voltages of the cells, especially the non-defective cells.

[0049] The reference voltage signal is generated, in particular, from the bandpass-filtered signal of the measured AC voltage of a cell. This signal is converted into a modified signal by an amplifier and used as the reference voltage signal.

[0050] In particular, the signal of a cell used for the reference voltage signal and filtered by the bandpass is shifted with respect to its phase, so that it forms the reference voltage signal of the lock-in amplifier as a phase-shifted signal of a cell.

[0051] In particular, a phase shifter is provided, which allows the phase of the reference voltage signal to be shifted. The phase shifter makes it possible to change the phase of the reference voltage signal and thus amplify only the imaginary part of the voltage (and therefore the resistance). This allows for improved adjustment of the voltage difference between the AC voltages of the cells.

[0052] In particular, the alternating voltages measured at the cells are each filtered by a bandpass filter, and the resulting signal is modified by an amplifier. The modified signals of any two cells are then fed to a differential amplifier. Each cell is connected to every other cell in a series circuit via a differential amplifier. The differential amplifier only generates a measurement signal if there is a phase shift between the modified signals of the two cells.

[0053] In particular, the cells arranged in series are each connected to the upstream and downstream cells via a differential amplifier.

[0054] This measurement signal can be fed to an analog-to-digital converter and converted there into a digital output signal. This digital output signal can then be fed to an evaluation unit and analyzed there.

[0055] Furthermore, a data processing system is proposed which is equipped, configured or programmed to carry out the described procedure, wherein the data processing system processes and compares phase positions of alternating voltages measured on a plurality of cells of a battery.

[0056] Furthermore, the process can also be carried out by a computer or with a processor of a control unit.

[0057] The procedure can be implemented in particular in a control unit or control device, the control unit being intended at least for the diagnosis, and possibly also for the operation, of the battery.

[0058] The battery can be used in a motor vehicle to store energy, whereby at least one traction drive of the motor vehicle is supplied with electrical energy via the battery.

[0059] In particular, a motor vehicle with a traction drive and the described battery as well as the data processing system is proposed.

[0060] A computer-readable storage medium may be provided, containing instructions which, when executed by a computer / processor, cause it to execute the procedure or at least part of the steps of the proposed procedure.

[0061] The explanations regarding the procedure are particularly applicable to the motor vehicle, the battery and / or the computer-implemented procedure (i.e., the computer or the processor, the data processing system, the computer-readable storage medium) and vice versa.

[0062] For the proposed method or data processing system, in particular no synchronization line is required between current excitation and voltage measurement, since the phase relationship to the current is not needed.

[0063] The circuits suitable for carrying out the procedure or

[0064] Data processing systems are particularly robust against disruptions.

[0065] In particular, no additional intelligence (µC) is required to evaluate the measured values ​​obtained using the method.

[0066] The time required to carry out the process, or the time during which the alternating current is applied, is very short, e.g., at most one second, and in particular less than two seconds. This ensures that only a small amount of energy is drawn from the at least one cell or the battery.

[0067] To generate the alternating current, an existing device in the vehicle, such as a high-voltage heater, a pulse inverter, or a heating mat control system, can be used. Similarly, the changing current of the electric drive can serve as a signal source.

[0068] The use of indefinite articles ("a", "an", "a" and "one"), particularly in the patent claims and the description reproducing them, is to be understood as such and not as a numeral. Accordingly, terms or components introduced by these articles are to be understood as occurring at least once and, in particular, may also occur multiple times.

[0069] It should be noted as a precaution that the numerical terms used here ("first", "second", etc.) primarily serve (only) to distinguish between several similar objects, quantities, or processes, and thus do not necessarily dictate any dependency and / or sequence between these objects, quantities, or processes. Should a dependency and / or sequence be required, this is explicitly stated here, or it will be obvious to a person skilled in the art upon studying the specific configuration described. Where a component can occur multiple times ("at least one"), the description of one of these components may apply equally to all or some of the multiple components, but this is not mandatory.

[0070] The invention and its technical context are explained in more detail below with reference to the accompanying figures. It should be noted that the invention is not intended to be limited by the exemplary embodiments shown. In particular, unless explicitly stated otherwise, it is also possible to extract partial aspects of the situations described in the figures and combine them with other components and findings from the present description. It should be emphasized that the figures, and especially the depicted dimensions, are only schematic. They show: Fig. 1: a diagram; Fig. 2: a first circuit; Fig. 3: a second circuit; Fig. 4: a third circuit; and Fig. 5: a fourth circuit.

[0071] The Fig. 1 Figure 1 shows a diagram. The vertical axis represents the alternating current 5 and the alternating voltage 6, 7. The horizontal axis represents time 33. The waveforms of the alternating current 5 and a first alternating voltage 6 are shown. The first alternating voltage 6 comprises an imaginary part 31 and a real part 32. The imaginary part 31 has a phase shift 20 relative to the real part 32.

[0072] One method for directly measuring the cell's internal temperature is called impedance spectroscopy. In impedance spectroscopy, the phase relationship between current 5 and voltage 6, 7 is evaluated, and the real part 32 and imaginary part 31 of a cell's resistance are determined. This is done by scanning the frequency response of an applied current 5 from very low frequencies (0.1 Hz) to high frequencies (10 kHz). This method is time-consuming, and a significant amount of energy is drawn from the cell 1, 2, 3 to apply the current 5. Furthermore, interference generated by the vehicle's electrical systems has a strong effect and distorts the measurement signal. The measured voltage 6 and current 5 are evaluated using complex algorithms. This allows, for example, conclusions to be drawn about the absolute value of the cell's internal temperature. This method is very susceptible to interference and is currently only used under laboratory conditions.

[0073] In the method described here, also known as "differential impedance spectroscopy," only the difference between the imaginary parts 31 of the cell resistances is determined. Specifically, the phase relationship between a current 5 and a voltage 6 is not evaluated, but only the phase angle 8, 9 or phase relationship of the alternating voltages 6, 7 measured at cells 1, 2, 3 to each other. Possible disturbances in the current signal affect all cells 1, 2, 3 of the battery 4, or of the cells 1, 2, 3 connected in series, simultaneously and with the same magnitude, and are not measured here as common-mode components. With this method, only changes in the imaginary parts 31 of the cell resistances to each other are measured and evaluated.

[0074] The in the Fig. 2 bis 5 The circuits 41, 42, 43, and 44 shown represent data processing systems suitable for carrying out the procedure. See the explanations regarding Fig. 1 Reference is made to each case.

[0075] Fig. 2 Figure 41 shows a first circuit. The first circuit 41 enables the execution of the method for determining the state of at least one cell 1, 2, 3 of a battery 4, wherein the battery 4 has a plurality of cells 1, 2, 3, ... to n connected in series. According to step a), an alternating current 5 is applied to the plurality of cells 1, 2, 3. According to step b), the resulting alternating voltage 6, 7 is measured at at least one first cell 1 and a second cell 2 (and at the further cells, e.g., the third cell 3 to cell n). According to step c), the phase angle 8, 9 of the measured alternating voltage 6, 7 of each cell 1, 2, 3 is analyzed.A difference at least between a first phase position 8 of a first alternating voltage 6 measured at the first cell 1 and a second phase position 9 of a second alternating voltage 7 measured at the second cell 2 allows a conclusion to be drawn about a difference in the states of at least the first cell 1 and the second cell 2.

[0076] According to step a), an alternating current 5, e.g. with a frequency 10, is impressed into the cells 1, 2, 3 of the battery 4. For this purpose, the circuit includes a switch 35 and a load 34, which is operated by the impressed alternating current 5.

[0077] The alternating voltages 6, 7 measured at cells 1, 2, 3 are each filtered by a bandpass filter 11, and the resulting signals 12, 13, 14 are modified by an amplifier 15 with a comparator. The first signal 12 is the bandpass-filtered first alternating voltage 6, which has a first phase angle 8 and was measured at the first cell 1. The second signal 13 is the bandpass-filtered second alternating voltage 7, which has a second phase angle 9 and was measured at the second cell 2. The third signal 14 is the correspondingly filtered alternating voltage of the third cell 3. The signals 16, 17, 18, each modified by the amplifier 15 with comparator, are combined in a phase detector 19. The first modified signal 16 is based on the first signal 13, the second modified signal 17 on the second signal 13, and the third modified signal 18 on the third signal 14.A phase shift 20 of the measured alternating voltages 6, 7 can be represented by a first output signal 21.

[0078] The bandpass filter 11 can detect the alternating voltage 6, 7 that corresponds to the applied alternating current 5 and is thereby induced at cells 1, 2, 3. The bandpass filter 11 thus enables the detection of an alternating voltage 6, 7 that has the frequency 10 of the alternating current 5.

[0079] The amplifier 15 with comparator provides a modified signal 16, 17, 18, which represents the measured AC voltage 6, 7, filtered by the bandpass filter 11, as a square wave signal. If a phase shift 20 exists relative to other AC voltages 7, 6, this becomes apparent through a shift of the square wave signals along the time axis (see the representation of the first output signals 21 in the diagrams).

[0080] The signals 16, 17, 18 modified by the amplifier 15 with comparator are represented as a second output signal 22, so that it is possible to show which alternating voltages 6, 7 or alternating voltages 6, 7 have a phase shift 20 compared to other alternating voltages 7, 6.

[0081] As soon as the imaginary parts 31 of the cell resistances differ and thus generate a phase shift 20 of the alternating voltage 6, 7 measured at a cell 1, 2, 3, a square wave signal is generated by the first output signal 21. This allows conclusions to be drawn about a difference between cells 1, 2, 3, e.g., a temperature increase at the beginning of thermal propagation. The duration of the square wave signal is proportional to, or equal to, the magnitude of the difference, e.g., the temperature difference (see the representation of the first output signals 21 in the diagrams; the upper diagram shows no recognizable first output signals 21, the lower diagram shows rectangular first output signals 21, which indicate the presence of a phase shift 20).

[0082] The output signals 21, 22 are evaluated by an evaluation unit 36.

[0083] Fig. 3 A second circuit, number 42, is shown. See the explanations regarding... Fig. 2 Reference is made to the first circuit 41. In contrast to the first circuit 41, the respective modified signals 16, 17, 18 are fed to a lock-in amplifier 23, which is supplied with a reference voltage signal 24, wherein the lock-in amplifier 23 outputs a DC voltage 25, 26 for each measured AC voltage 6, 7, the respective magnitude of which is proportional at least to a phase shift 20 between the measured AC voltage 6, 7 and the reference voltage signal 24.

[0084] The amplifier 15 provides a modified signal 16, 17, 18, which represents the measured alternating voltage 6, 7 filtered by the bandpass 11 as a square wave signal.

[0085] The reference voltage signal 23 is also a square wave signal. The reference voltage signal 23 has the same frequency 10 as the applied alternating current 5.

[0086] A lock-in amplifier 23 is an amplifier for measuring a weak alternating electrical signal, here the modified signal 16, 17, 18, which is modulated with a reference voltage signal 24 known in frequency 10 and phase. The device represents a narrowband bandpass filter and thereby improves the signal-to-noise ratio (SNR).

[0087] The two input signals of the respective lock-in amplifier 23, namely, on the one hand, the measured AC voltage 6, 7, which was filtered in the bandpass filter 11 and converted by an amplifier 15 into a modified signal 16, 17, 18, and on the other hand, the reference voltage signal 24, are multiplied together in a mixer 37 or multiplier and then integrated in a low-pass filter 38. The low-pass filter 38 enables the attenuation of higher frequency signals 10, which are generated when the input signals are multiplied.

[0088] The respective lock-in amplifier 23 calculates the cross-correlation between the modified signal 16, 17, 18 and the reference voltage signal 24 for a fixed phase shift 20 between the modified signal 16, 17, 18 of a cell 1, 2, 3 and the reference voltage signal 24. The lock-in amplifier 23 provides a DC voltage signal 25, 26 for each cell 1, 2, 3, here a first DC voltage signal 25 for the first cell 1 and a second DC voltage signal 26 for the second cell 2, the value of which provides information about the phase angle 8, 9 of the measured AC voltage 6, 7 relative to the reference voltage signal 24.

[0089] A DC voltage signal 25, 26 of a cell 1, 2, 3 that differs from other DC voltage signals 26, 25 of other cells 3, 2, 1 means that there is a phase shift 20 and the cell 1, 2, 3 may be defective.

[0090] The DC voltage signals 25, 26, which do not differ from each other, indicate that there are no phase shifts 20 and that none of the cells 1, 2, 3 are defective.

[0091] The DC voltage signals 25, 26 are each fed to an analog-to-digital converter 39 and converted there into a digital output signal. This digital output signal is fed to an evaluation unit 36 ​​and evaluated there.

[0092] Fig. 4 A third circuit, number 43, is shown. See the explanations regarding... Fig. 2 and 3 Reference is made to the second circuit 42. In contrast to the second circuit 42, the reference voltage signal 24 corresponds in frequency 10 and phase 8, 9 to the signal of the first cell 1 of the battery 4 filtered by the bandpass 11. Thus, the reference voltage signal 24 can have a common first phase 8 with the measured first AC voltage 6 of the first cell 1.

[0093] The reference voltage signal 24 is generated from the bandpass-filtered first signal 12 of the measured first AC voltage 6 of the first cell 1. This first signal 12 is converted into a modified signal by an amplifier 15 and used as the reference voltage signal 24. The first signal 12 of the first cell 1, used for the reference voltage signal 24 and filtered by the bandpass 11, is shifted with respect to its phase 20, so that it forms the reference voltage signal 24 of the lock-in amplifier 23 as a phase-shifted signal 27 of the first cell 1.

[0094] For this purpose, a phase shifter 40 is provided, with which the reference voltage signal 24 can be shifted with respect to the phase angle 8, 9. The phase shifter 40 allows the phase angle 8, 9 of the reference voltage signal 24 to be changed, thus amplifying only the imaginary part 31 of the voltage (and therefore the resistance).

[0095] Fig. 5 A fourth circuit, number 44, is shown. See the explanations regarding... Fig. 2 bis 4 Reference is made to the second circuit 42. In contrast to the second circuit 42, the respective modified signals 16, 17, 18 of two cells, here the first cell 1 and the second cell 2, and the second cell 2 and the third cell 3, are each fed to a differential amplifier 28. Each cell 1, 2, 3 is connected to every other cell 3, 2, 1 connected via the series circuit by a differential amplifier 28. The respective differential amplifier 28 generates a measurement signal 29 only if there is a phase shift 20 between the modified signals 16, 17, 18 of the two cells 1, 2, 3 connected via the differential amplifier 28.

[0096] This measurement signal 29 is fed to an analog-to-digital converter 39 and converted there into a digital output signal. These digital output signals are fed to an evaluation unit 36 ​​and evaluated there. Reference symbol list

[0097] 1 First cell 2 Second cell 3 Third cell 4 Battery 5 Alternating current 6 First AC voltage 7 Second AC voltage 8 First phase angle 9 Second phase angle 10 Frequency 11 Bandpass filter 12 First signal 13 Second signal 14 Third signal 15 Amplifier 16 First modified signal 17 Second modified signal 18 Third modified signal 19 Phase detector 20 Phase shift 21 First output signal 22 Second output signal 23 Lock-in amplifier 24 Reference voltage signal 25 First DC voltage 26 Second DC voltage 27 Phase-shifted signal 28 Differential amplifier 29 Measurement signal 30 Data processing system 31 Imaginary part 32 Real part 33 Time 34 Load 35 Switch 36 Evaluation unit 37 Mixer 38 Low-pass filter 39 Converter 40 Phase shifter 41 First circuit 42 Second circuit 43 Third circuit 44 Fourth circuit

Claims

1. Method for determining a state of at least one cell (1, 2, 3) of a battery (4), the battery (4) comprising a plurality of cells (1, 2, 3) connected in series; the method comprising at least the following steps: a) applying an alternating current (5) to the plurality of cells (1, 2, 3); b) measuring the alternating voltage (6, 7) thus generated at at least a first cell (1) and a second cell (2); c) analyzing a phase position (8, 9) of the measured alternating voltage (6, 7) of each cell (1, 2); with a difference at least between a first phase position (7) of a first alternating voltage (6) measured at the first cell (1) and a second phase position (9) of a second alternating voltage (7) measured at the second cell (2) generating a conclusion about a difference in the states of at least the first cell (1) and the second cell (2); characterized in that only the phase position (8, 9) of the alternating voltages (6, 7) measured at the cells (1, 2, 3) relative to one another is evaluated and not the phase relationship between a current and a voltage.

2. Method according to claim 1, wherein only the alternating voltages (6, 7) are measured.

3. Method according to either of the preceding claims, wherein the alternating current (5) has a constant frequency (10).

4. Method according to any of the preceding claims 1 to 3, wherein the alternating current (5) has a changing frequency (10).

5. Method according to any of the preceding claims, wherein the alternating current (5) has a frequency (10) with only one period length.

6. Method according to any of the preceding claims, wherein the alternating voltages (6, 7) measured at the cells (1, 2, 3) are each filtered by a bandpass (11) and the signal (12, 13, 14) thus generated is modified by an amplifier (15) having a comparator, wherein the modified signals (16, 17, 18) are combined in a phase detector (19) and a phase shift (20) of the measured alternating voltages (6, 7) can be represented by a first output signal (21).

7. Method according to claim 6, wherein the modified signals (16, 17, 18) are represented as a second output signal (22) so that it is possible to represent which alternating voltages (6, 7) have a phase shift (20) compared to other alternating voltages (7, 6).

8. Method according to any of the preceding claims 1 to 5, wherein the alternating voltages (6, 7) measured at the cells (1, 2, 3) are each filtered by a bandpass (11) and the signal (12, 13, 14) thus generated is modified by an amplifier (15), wherein the modified signals (16, 17, 18) are fed to a lock-in amplifier (23) which is supplied with a reference voltage signal (24), wherein the lock-in amplifier (23) outputs a direct voltage (25, 26) for each measured alternating voltage (6, 7), the particular magnitude of which direct voltage is proportional at least to a phase shift (20) between the measured alternating voltage (6, 7) and the reference voltage signal (24).

9. Method according to claim 8, wherein the reference voltage signal (24) corresponds in frequency (10) and phase position (8) to the signal (12, 13, 14) of a cell (1, 2, 3) filtered by the bandpass (11).

10. Method according to claim 8, wherein the signal (12, 13, 14) of a cell (1, 2, 3) used for the reference voltage signal (24) and filtered by the bandpass (11) is shifted with respect to its phase position (8) so that it forms the reference voltage signal (24) of the lock-in amplifier (23) as a phase-shifted signal (27) of a cell (1, 2, 3).

11. Method according to any of the preceding claims 1 to 5, wherein the alternating voltages (6, 7) measured at the cells (1, 2, 3) are each filtered by a bandpass (11) and the signal (12, 13, 14) thus generated is modified by an amplifier (15), wherein the modified signals (16, 17, 18) of two cells (1, 2, 3) are fed to a differential amplifier (28); wherein, via a differential amplifier (28) in each case, each cell (1, 2, 3) is connected to every other cell (3, 2, 1) connected in a series circuit; wherein the differential amplifier (28) generates a measurement signal (29) only if there is a phase shift (20) between the modified signals (16, 17, 18) of the two cells (1, 2, 3).

12. Data processing system (30) which is equipped, configured or programmed to carry out a method according to any of the preceding claims, wherein the data processing system (30) processes and compares phase positions (8, 9) of alternating voltages (6, 7) measured at a plurality of cells (1, 2, 3) of a battery (4); i.e., only evaluates the phase position (8, 9) of the alternating voltages (6, 7) measured at the cells (1, 2, 3) relative to one another and not the phase relationship between a current and a voltage.