Methods and apparatus for monitoring low-pass filters, as well as battery management systems, batteries, and vehicles.

Abstract

This invention relates to a method and apparatus for monitoring a low-pass filter, preferably a capacitor in the low-pass filter. Furthermore, this invention relates to a battery management system and a battery having such a battery management system. The apparatus includes a switch and a capacitor, the switch being connected in series with a resistor and the capacitor being connected in parallel with the resistor. Whenever the switch performs a switching process, the voltage changes from a first voltage value to a second voltage value in a very short time. In this case, the capacitor is used to slow down the rise / fall of the voltage. The voltage is determined by means of a voltage detection device at the time of switching and at at least one other moment (0.5 to 5 milliseconds depending on the resistor and capacitor). By determining the rate of change from the first voltage value to the second voltage value, it can be determined whether the capacitor is functioning properly and / or the resistance can be determined. Furthermore, the state of charge of the battery can be determined by determining the internal resistance of the individual battery cells.

Description

Technical Field

[0001] This invention relates to a method and apparatus for monitoring a low-pass filter. Furthermore, this invention relates to a battery management system and a battery having such a battery management system. Finally, this invention relates to a vehicle. Background Technology

[0002] Batteries, especially those based on lithium-ion technology, typically have a battery management system (BMS). Generally, a BMS is used to monitor the battery and control the charging and discharging processes of the associated battery.

[0003] Since the battery management system is functionally related in many applications, especially in vehicles, it is advantageous to monitor at least some of the functions of the battery management system itself.

[0004] Battery management systems often include low-pass filters to simplify monitoring of the battery in case of functional failures. Summary of the Invention

[0005] Therefore, the objective of this invention is to simplify the monitoring of such functional failures.

[0006] The task is solved by a method and an apparatus according to the invention. Furthermore, the task is solved by a battery management system according to the invention and a battery according to the invention. Finally, the task is solved by a vehicle having such an apparatus.

[0007] (PA1) In a method for monitoring a low-pass filter, the low-pass filter includes at least one resistor and at least one capacitor connected in parallel with the resistor, wherein a voltage across one side of the capacitor is determined, the voltage across the capacitor being switchable between a first rated voltage value and a second rated voltage value by at least one switch, wherein the method includes at least the following steps:

[0008] - Detect the first actual voltage value at the first moment.

[0009] - Perform the switching process from the first rated voltage value to the second rated voltage value.

[0010] - The actual voltage value is detected at a second moment, which is set in time after the switching process.

[0011] - Determine the first difference between the corresponding actual voltage values.

[0012] - A warning signal is provided if at least the first difference exceeds or falls below the first threshold.

[0013] Here, a low-pass filter can be understood in particular as an RC circuit.

[0014] Preferably, a variable resistor, such as a temperature-dependent resistor, is used as the resistor. The value of the resistor may correspond to the measured parameters, such as the temperature of a single battery cell, the temperature of the battery, the state of charge of a single battery cell, and / or the state of charge of a battery having multiple battery cells.

[0015] The switching process is carried out by switching, wherein the switch is advantageously integrated into an integrated switching circuit, particularly an ASIC module.

[0016] The capacitor has capacitance and is particularly constructed as a film capacitor. Here, the terms "capacitor" and "capacitance" are used synonymously. The capacitor is used, for example, to filter out interference from circuits (such as devices for monitoring individual battery cells or batteries). Advantageously, the capacitor guides rapidly changing interference voltages to ground as unimpeded as possible and thus improves the stability of the monitoring device. In a faulty state, the capacitor does not conduct.

[0017] The actual voltage value is preferably detected using a voltage detection unit. The voltage detection unit is preferably connected in series with the variable resistor. The voltage detection unit is preferably used to determine the voltage present across the capacitor.

[0018] The variable resistor can be paired with another resistor, wherein the two resistors are preferably connected in series with the voltage detection device.

[0019] Advantageously, the actual voltage value is determined at periodic time intervals. Advantageously, the first moment is set at or immediately before the switching process. For example, the first moment is set 0.01 milliseconds to 0.1 milliseconds before the switching process.

[0020] Advantageously, the switching process is performed every 10 milliseconds, thus allowing the time constant to be determined within a few milliseconds.

[0021] Advantageously, the second moment can be set such that the voltage rises / falls by a determined coefficient when the low-pass filter is functioning as specified. Particularly preferably, the second moment is set after a time constant following the switching process, wherein the time constant is proportional to the product of the capacitance of the resistor and the capacitor.

[0022] The second time point can also be set within a time range of 0.5 to 1.5 times the value of the time constant.

[0023] Alternatively, the second moment can be set such that, assuming the low-pass filter is functioning properly, the voltage rises / falls to approximately half the difference between the two rated voltage values.

[0024] Therefore, the first threshold can be selected as one-quarter and / or three-quarters of the difference in the voltage values.

[0025] The warning signal can be configured as an acoustic or optical signal, enabling the inspection of capacitors, low-pass filters, or systems with low-pass filters. Preferably, the warning signal is provided as a voltage signal, such as a TTL signal, thus activating a central monitoring system.

[0026] This invention enables the monitoring of low-pass filters in a very simple way by simply monitoring voltage values. In particular, the functionality of capacitors can be easily implemented using this invention. Furthermore, other system parameters can also be monitored simply and effectively using the methods described above.

[0027] (PA2) In an advantageous embodiment of the invention, the time constant, in particular the voltage rise or voltage fall, is determined by means of the difference, wherein the state of the resistor and / or the capacitor can be determined by means of the time constant.

[0028] In the low-pass filter considered here, the voltage typically rises / falls exponentially with a time constant. Advantageously, this time constant is the product of the resistance and the capacitance of the capacitor.

[0029] Since the exponential rise or fall of voltage is known for low-pass filters, the experimentally determined time constant can provide conclusions for functional testing of the resistor and / or the capacitor.

[0030] The time constant can advantageously correspond to the product of the resistance and capacitance. Therefore, given the capacitance, the resistance can be determined simply and with high accuracy.

[0031] The values ​​of the variable resistor and / or capacitor can be easily determined using the time constant.

[0032] (PA3) In another advantageous embodiment of the invention, the first moment is set immediately before the switching process, wherein the second moment is set at a time constant after the switching process.

[0033] Advantageously, the third moment is set in time at two time constants after the switching process.

[0034] The first moment can optionally be selected as shortly after the switching process or as the moment of the switching process itself. The switching process advantageously triggers a voltage detection unit, which detects a first voltage value during the switching process.

[0035] Preferably, the second moment is selected shortly after the switching process, advantageously choosing a value of approximately 0.01 milliseconds to 1 millisecond, given the values ​​typically chosen for the capacitor and resistor.

[0036] Preferably, the second time step is chosen such that the difference between the first voltage value and the second voltage value approximately corresponds to half of the maximum difference. The maximum difference corresponds to the voltage difference that would occur in the case of a faulty, non-conductive capacitor. In other words, the maximum difference approximately corresponds to the difference between the first rated voltage value and the second rated voltage value.

[0037] By advantageously selecting the times, a particularly reliable inspection of the low-pass filter or a particularly reliable inspection of the capacitor can be achieved.

[0038] (PA4) In another advantageous embodiment of the invention, the resistor is a variable resistor. A variable resistor is advantageously used to determine temperature, for example, the temperature of the battery or the temperature of at least one battery cell.

[0039] The resistor is preferably constructed as an NTC resistor or a PTC resistor.

[0040] Preferably, the variable resistor is constructed such that its resistance changes over time much more slowly than the switching process. Therefore, temperature changes are not filtered out by the low-pass filter.

[0041] By using variable resistors, relevant system parameters, such as temperature as a function of time, can be determined with particular precision.

[0042] (PA5) In another advantageous embodiment of the invention, a third actual voltage value is additionally determined at a third time point, said third time point being set after the second time point, wherein...

[0043] - Determine the first difference between the first actual voltage value and the second actual voltage value.

[0044] - Determine a second difference between the second actual voltage value and the third actual voltage value.

[0045] - A warning signal is provided when the first difference and the second difference are respectively below or above the threshold.

[0046] Advantageously, the third moment is set in time after twice the time constant following the switching process.

[0047] Preferably, the third voltage value is determined within a time range of 1 to 1.5 milliseconds after the switching process, based on the capacitance and resistance. Alternatively, the third time point is located between 0.5 and 1.5 times the time constant, preferably one time constant, after the second time point.

[0048] Preferably, the second and third voltage values ​​are detected using the voltage detection unit. Preferably, the voltage value detection is triggered accordingly by the switching process.

[0049] Optionally, the detection of the corresponding voltage signal can also be achieved through a trigger signal, which is used to trigger the switching process and / or the voltage detection unit.

[0050] By recording two voltage values ​​at the second and third moments after the switching process, it is possible to:

[0051] - Identify potential errors that may occur when detecting the second voltage value;

[0052] - Improved determination of the corresponding time constant, and

[0053] -Enhanced operational safety when monitoring the low-pass filter.

[0054] (PA6) In an advantageous design of the invention, the resistor is the internal resistance of a single battery cell.

[0055] Advantageously, the resistor is the internal resistance of one or more battery cells.

[0056] The design of this method is based on the observation that the state of charge (SOC) can be inferred by determining the internal resistance of a single battery cell.

[0057] Since the state of charge of the at least one battery cell changes only slowly and therefore the resistance changes only slowly, the state of charge of the at least one battery cell can be determined by means of the method described herein.

[0058] Advantageously, the corresponding difference is determined by means of the voltage detection device, wherein the at least one battery cell discharges through another resistor and a switch for a duration, for example, 1 to 10 milliseconds, preferably 2 milliseconds. Here, the other resistor is connected in parallel with the at least one battery cell. Advantageously, the switch is connected in series with the resistor. Advantageously, at least one capacitor is connected to the corresponding terminal of the at least one battery cell and to ground.

[0059] An advantageous method for checking the state of charge of at least one battery cell can be carried out using an apparatus having:

[0060] - At least one battery cell, wherein voltage is applied to the positive and negative terminals of the respective battery cell, a switch connects the positive and negative terminals via a resistor, a capacitor is connected in parallel with the resistor, and the method includes at least the following steps:

[0061] - Detect the first voltage value at the first moment;

[0062] - To perform a switching process, thereby achieving the connection between the positive and negative terminals of the corresponding battery cells;

[0063] - Detect the second voltage value at the second moment;

[0064] - Optionally, the third voltage value is detected at the third moment;

[0065] - Determine the first difference and optionally determine the second difference;

[0066] - Determine the charging status of the corresponding battery cell and / or provide a warning signal, provided that the corresponding difference is higher than a pre-defined threshold.

[0067] The design scheme described above can advantageously determine not only the function of the low-pass filter or capacitor, but also the charging state of the individual battery cells.

[0068] (PA7 old) In another advantageous embodiment of the invention, the inspection is performed during the charging process of the battery cell.

[0069] - Wherein, a first voltage value is detected before the charging process of the battery cell, and a second voltage value is detected during the charging process of the battery cell.

[0070] and / or

[0071] - wherein a first voltage value is detected during the charging process of the battery cell, and a second voltage value is detected after the charging process of the battery cell.

[0072] The above design scheme can not only check or monitor the charging status of the corresponding battery cells, but also check or monitor the function of the low-pass filter.

[0073] The status of the corresponding battery cells can also be monitored by regularly monitoring the low-pass filter, for example, by checking it once an hour.

[0074] (PA7) In another advantageous embodiment of the invention, an additional switching process is performed after the second time point, the additional switching process switching the voltage from the second rated voltage value to the first rated voltage value, the corresponding voltage value is detected after the additional switching process, and the corresponding difference is determined from the corresponding actual voltage value.

[0075] Preferably, the additional switching process is performed 5 to 15 milliseconds after the switching process.

[0076] Particularly advantageously, the above method is repeated periodically, thereby periodically switching the rated voltage value back and forth between a first rated voltage value and a second rated voltage value. Advantageously, the low-pass filter is checked after each switching process.

[0077] Advantageously, the first switching process and the other switching process are repeated periodically.

[0078] By regularly inspecting the low-pass filter, faults in the battery or battery management system can be identified early.

[0079] (PA8) In another advantageous embodiment of the invention, the voltage value is determined during the functional check of the battery management system or voltage detection unit, particularly during the “end-of-line” test.

[0080] Preferably, the method is performed during or after assembling an apparatus including a low-pass filter.

[0081] By testing the function of the resistors and / or the capacitors, the functionality of the device can be checked in a particularly simple manner. It is especially advantageous to monitor or check the low-pass filter or the capacitors after the device has been initially completed. This test, performed directly after completion, is also known as an "end-of-process" test. It is particularly advantageous that the method described herein can be performed additionally or in lieu of so-called in-circuit-test.

[0082] Particularly advantageously, the voltage value is determined by means of a control device of the apparatus (e.g., configured as an ASIC module). Advantageously, the switching process and the detection of the voltage value, as well as the triggering of at least one switching process and / or the triggering of the voltage detection device, are performed by means of the ASIC module.

[0083] By starting the method for the first time after the device has completed its operation, a portion of the routine "end-of-process" inspection can be automatically implemented. This, in particular, improves the functional safety of the device.

[0084] Description of the device

[0085] (PA9) In another advantageous embodiment of the invention, the low-pass filter is provided to at least one battery cell, wherein the device includes a low-pass filter having a capacitor and a resistor and at least one switch, wherein a voltage detection unit is configured to determine the voltage value on the capacitor, wherein the device is configured to implement the method described above.

[0086] The device is preferably integrated into or provided to a battery management system.

[0087] (PA10) In another advantageous embodiment of the invention, the device has an ASIC module, wherein the ASIC module includes the at least one switch and the voltage determination unit.

[0088] Such ASIC modules already exist, which is particularly advantageous in battery management systems. In this case, the existing ASIC module of the battery management system can be used.

[0089] Preferably, the ASIC includes a corresponding switch, wherein the corresponding switch is configured to implement the switching process.

[0090] Preferably, the ASIC is further configured to provide a reference voltage. The reference voltage may be a first rated voltage value or a second rated voltage value. The reference voltage may also be provided by a voltage divider, wherein the voltage divider is arranged between the ASIC module and a low-pass filter, particularly a capacitor. Furthermore, the voltage divider may be integrated into the low-pass filter.

[0091] Description of the battery management system

[0092] (PA11) A battery management system for at least one battery cell has the means described above.

[0093] A battery cell can be understood as a single battery cell or as multiple battery cells connected in series and / or in parallel.

[0094] Advantageously, the battery management system includes an ASIC module, wherein the above-described method is implemented by means of the ASIC module.

[0095] Alternatively or additionally, the battery management system may be configured to monitor capacitors and / or resistors according to the methods described above.

[0096] Battery instructions

[0097] (PA12) The battery, particularly the battery constructed for use in a vehicle, includes the battery management system as described above.

[0098] Preferably, the battery is configured as a dual-voltage battery, for example, a dual-voltage battery for 12 volts and 48 volts. Alternatively, the battery is configured as a high-voltage battery, preferably configured as a battery having an output voltage of 400 volts to 1000 volts.

[0099] Vehicle description

[0100] (PA13) The vehicle is preferably configured as an automobile, particularly as an automobile that is at least partially electrically driven. Alternatively, the vehicle may also be a rail vehicle.

[0101] Application of the present invention

[0102] The aforementioned device and method can be used to monitor the functional failures of resistors, particularly variable resistors and capacitors, or low-pass filters, which are components of the low-pass filter. Furthermore, the method can monitor the state of charge via the internal resistance of the battery or individual battery cells, provided the low-pass filter is functioning correctly.

[0103] In summary, the present invention relates to a method and apparatus for monitoring a low-pass filter, preferably a capacitor in the low-pass filter. Furthermore, the present invention relates to a battery management system and a battery having such a battery management system. The apparatus includes a switch and a capacitor, wherein the switch is connected in series with a resistor, and the capacitor is connected in parallel with the resistor. Whenever the switch performs a switching process, the voltage changes from a first voltage value to a second voltage value in a very short time. In this case, the capacitor is used to slow down the rise / fall of the voltage. The voltage is determined by means of a voltage detection device at the time of switching and at at least one other moment (0.5 to 5 milliseconds depending on the resistor and capacitor). By determining the rate of change from the first voltage value to the second voltage value, it can be determined whether the capacitor is functioning properly and / or the resistance can be determined. Furthermore, the state of charge of the battery can be determined by determining the internal resistance of the individual battery cells. Attached Figure Description

[0104] The present invention will now be described and illustrated in detail with reference to the accompanying drawings. The various features shown in the drawings can be combined to form other embodiments of the invention without departing from the scope of the invention. At least the described embodiments are not intended to limit the invention in any way.

[0105] In the picture:

[0106] Figure 1 A feasible device is shown;

[0107] Figure 2 This demonstrates another feasible device;

[0108] Figure 3 This demonstrates another feasible device;

[0109] Figure 4 The voltage variation curve is shown. Detailed Implementation

[0110] Figure 1 A feasible device is shown. The device includes a capacitor Cx with capacitance, wherein the capacitor Cx and a variable resistor Rx constitute a low-pass filter. The variable resistor is preferably configured as a temperature-dependent resistor, particularly an NTC (Negative Temperature Coefficient Terminator). The resistor Rx is preferably used to determine the temperature of the battery cell B1 or the battery.

[0111] The capacitance Cx and resistance Rx of capacitor Cx are subjected to a voltage U. This voltage U can be a reference voltage u-ref, which can be provided by a voltage divider, as shown by the resistor R* indicated by the dashed line. Particularly advantageously, the reference voltage u-ref can be provided by ASIC module A.

[0112] Switch S1 can be used to switch the voltage U to ground or to the second rated voltage value u2. The low-pass filter is activated by the change in voltage U. As long as capacitor Cx is functioning properly, the voltage U across capacitor Cx does not drop directly; instead, capacitor Cx discharges through the variable resistor Rx. This slow drop in voltage U can be detected using the voltage determination unit M. The voltage determination unit M is advantageously connected in parallel with capacitor Cx.

[0113] Preferably, ASIC module A is used as a voltage source. ASIC module A preferably also includes a voltage determination unit M and a switch S1.

[0114] Alternatively, another resistor R can be connected between the capacitor Cx and the variable resistor Rx.

[0115] When capacitor Cx is functioning normally, after switch S1 is closed, a portion of the charge flows through the variable resistor Rx, causing the voltage U to decrease slowly. The voltage drop can be determined, particularly based on voltage differences diff1 and diff2, by detecting the voltage as a function of time t, especially at the first time t1, the second time t2, and optionally also at the third time t3.

[0116] The voltage determination unit M can be used to check whether capacitor Cx is functioning correctly. Generally, a malfunctioning capacitor Cx has infinite resistance instead of capacitive resistance for non-constant voltages; that is, a malfunctioning capacitor behaves like an interrupted circuit. Therefore, the voltage U will drop to zero volts or the second rated voltage value within a very short time t. At the first time t1, the voltage U has already dropped to zero volts or the second rated voltage value. At the second time t2 and the third time, the voltage across capacitor Cx will be at the second rated voltage value u2 or zero volts.

[0117] Therefore, the difference between the first actual voltage value u1 and the second actual voltage value u2 is the difference between the rated voltage values. A threshold value is typically predetermined, which is lower than the difference between the rated voltage values. Therefore, an alarm signal S is output when there is a fault in capacitor Cx or resistor Rx.

[0118] Figure 2 Another feasible device is shown. The device shown here differs from the one described above in that, in addition to the variable resistor Rx, another resistor R is connected between the variable resistor Rx and the corresponding switches S1, S2. To provide the rated voltage values ​​u1, u2, a first switch S1 and a second switch S2 are provided according to the embodiment shown here. With the aid of the corresponding switches S1, S2, the voltage U can be switched to either the positive rated voltage value +U or the negative rated voltage value -U.

[0119] Furthermore, this feasible embodiment includes an additional capacitor C connected in parallel with capacitor Cx. Preferably, the additional capacitor C has a different capacitance C than capacitor Cx, thereby enabling testing, using the embodiment shown herein, to determine which of the capacitors Cx and C is faulty.

[0120] Furthermore, ASIC module A includes a first switch S1 and a second switch S2, wherein the first switch S1 and the second switch S2 are arranged and connected such that the voltage U can have three voltage levels: +U, U-ref, and -U. Additionally, ASIC module A includes two voltage determination units M, wherein each voltage determination unit M is arranged between a corresponding switch S1, S2 and a voltage tap. Alternatively or additionally, the voltage determination unit M can be inserted between the two switches S1, S2 and a resistor R.

[0121] These two voltage taps allow for setting positive and negative voltage drops, for example, between a positive voltage +U and a negative voltage -U. The structure shown here allows for testing of a low-pass filter comprising resistors R and Rx and capacitors C and Cx.

[0122] During the switching process of the second switch S2, the voltage U drops from zero volts to a negative value -U. The time constant of reducing this voltage drop is achieved through capacitors C and Cx and resistors R and Rx. This can be detected using the corresponding voltage determination unit M. The voltage drop can be described by the following function:

[0123] U(t)=±U(t1)·exp(-t / (Rx·Cx))

[0124] Here, Rx represents the total resistance, and Cx represents the total capacitance of the capacitors. Furthermore, exp represents an exponential function. The voltage U(t1) corresponds to the first actual voltage value u1 before or during the switching process. Finally, the following applies to the first and second actual voltage values ​​u2 and u3:

[0125] u2=U(t2)=±U(t1)·exp(-t2 / (Rx·Cx))

[0126] u3=U(t3)=±U(t1)·exp(-t3 / (Rx·Cx))

[0127] Figure 3 Another feasible device is shown. The device shown here is specifically used to determine the state of charge of battery cell B1. The internal resistance Rx changes as battery cell B1 charges. An ASIC module is used to provide the voltage U. Furthermore, ASIC module A includes a switch S1 and optionally includes an additional resistor R (not shown). The outputs of ASIC module A are connected to the terminals of battery cell B1. At least one of these connections is grounded through a capacitor Cx. Alternatively, both connections may be grounded through a separate capacitor Cx.

[0128] By determining the resistance Rx, both the capacitor Cx and the state of charge of the battery cell B1 or the battery containing the battery cell can be determined. Preferably, the charging or discharging process of the corresponding battery cell B1 can be briefly interrupted.

[0129] In the first case, assuming that capacitor Cx can function normally, the voltage drop can be determined before, during, and / or after the charging process of battery cell B1.

[0130] Figure 4The voltage change curve is shown. This voltage change curve illustrates the change in voltage u as a function of time t. The voltage change curve is switched to a second rated voltage value u2 at a first time t1 via a switching process. The slowed decrease of voltage U as a function of time t is achieved through the low-pass filter. At a later time, for example, approximately 10 milliseconds after the switching process, another switching process raises the second rated voltage value back to the first rated voltage value. The voltage U, detected by the voltage determination unit M, as a function of time t, is shown.

[0131] For better differentiation, the rated voltage value is shown as a dashed line, while the actual voltage change over time is shown as a solid line.

[0132] Before the switching process, at the first moment t1, the first actual voltage value u1 is detected by the voltage detection unit M.

[0133] After the switching process, at the second time t1, the second actual voltage value u2 is detected by the voltage detection unit M. Then, the first difference diff1 between the first actual voltage value u1 and the second actual voltage value u2 is determined.

[0134] Optionally, a third actual voltage value u3 is detected at a third time t3. The second difference diff2 between the second actual voltage value u2 and the third actual voltage value u3 can be determined using the third actual voltage value u3.

[0135] The time constant k can be determined by means of a first difference diff1 and optionally by means of a second difference diff2, wherein the time constant k corresponds to the product of the resistance R, Rx, in particular the variable resistance Rx and the capacitance Cx (of the capacitor Cx and optionally the additional capacitor C).

[0136] By determining at least one difference, diff1, diff2, it can be determined whether capacitor Cx or the other capacitor C, and therefore the low-pass filter, is functioning correctly. If both differences, diff1 and diff2, are equal to zero, a functional fault in the corresponding switches S1 and S2 can be identified.

Claims

1. A method for monitoring a low pass filter, wherein, The low-pass filter comprises at least one resistor (Rx) and at least one capacitor (Cx), which is connected in parallel to the resistor (Rx), wherein a voltage (u) is determined with respect to one side of the capacitor (Cx), the voltage on the capacitor (Cx) being switchable between a first nominal voltage value and a second nominal voltage value by means of at least one switch, wherein the method comprises at least the following steps: - detecting a first actual voltage value at a first point in time (tl), - carrying out a switching process from the first nominal voltage value to the second nominal voltage value, - detecting an actual voltage value at a second point in time (t2), which is set in time after the switching process, - determining a first difference value (diff1) of the first actual voltage value and the second actual voltage value, - providing a warning signal (S) if at least the first difference value (diff1) exceeds or falls below a first threshold value.

2. The method of claim 1, wherein, A time constant (k) is determined by means of the first difference value, by means of which the state of the resistor (Rx) and / or the capacitor (Cx) can be determined.

3. The method of claim 2, wherein, The time constant (k) is the time constant (k) of an exponential voltage rise or an exponential voltage fall as a function of time (t).

4. The method of any one of claims 1 to 3, wherein, The first point in time (tl) is directly before or during the switching process and the second point in time (t2) is set at one time constant after the first point in time.

5. The method of any one of claims 1 to 3, wherein, The resistor (Rx) is variable.

6. The method of any one of claims 1 to 3, wherein, A third actual voltage value is additionally determined at a third point in time (t3), which is set after the second point in time (t2), wherein - a first difference value (diff1) between the first actual voltage value and the second actual voltage value is determined, - a second difference value (diff2) between the second actual voltage value and the third actual voltage value is determined, - a warning signal (S) is provided as soon as the first difference value (diff1) and / or the second difference value (diff2) falls below or exceeds a respective threshold value.

7. The method of any one of claims 1 to 3, wherein, The resistor (Rx) is an internal resistance of a battery cell (B1).

8. The method of any one of claims 1 to 3, wherein, A further switching process is carried out after the second point in time (t2), which switches the voltage (u) from the second nominal voltage value to the first nominal voltage value, a respective actual voltage value is detected after the further switching process and a respective difference value is determined from the respective actual voltage value.

9. The method of any one of claims 1 to 3, wherein, The determination of the first actual voltage value and the second actual voltage value is carried out during production inspection of a battery management system or a voltage detection unit (M).

10. The method of claim 9, wherein, The determination of the first actual voltage value and the second actual voltage value is carried out during an "end-of-process" test.

11. An apparatus for monitoring a low pass filter, wherein, The low-pass filter is assigned to at least one battery cell (B1), the low-pass filter comprising a capacitor (Cx) and a resistor (Rx) and at least one switch (Sw), a voltage detection unit (M) being configured for determining a voltage value on the capacitor (Cx), the device being configured for carrying out the method according to any one of claims 1 to 10.

12. The apparatus of claim 11, wherein, The device further has an ASIC module comprising the at least one switch (Sw) and the voltage detection unit (M).

13. A battery management system (BMS) for at least one battery cell, the battery management system having a device according to claim 11 or 12.

14. A battery comprising a battery management system according to claim 13.

15. The battery of claim 14, wherein, The battery is configured for use in a vehicle.

16. A vehicle having a device according to claim 11 or 12.

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