Method for determining an internal resistance of a battery cell of an energy storage device for an electrically driveable motor vehicle, computer program and / or computer-readable medium, data processing device, electrically driveable motor vehicle, charging column

Abstract

A method for determining an internal resistance of a battery cell of an energy storage device for an electrically driveable motor vehicle, wherein the method comprises: charging the battery cell using a charging current; temporarily decreasing the charging current multiple times to a measurement charging current; measuring a cell voltage which is dependent on the charging current; and ascertaining the internal resistance based on the cell voltage and the charging current.

Description

[0001] Method for determining an internal resistance of a battery cell of an energy storage device for an electrically driven motor vehicle, computer program and / or computer-readable medium, data processing device, electrically driven motor vehicle, charging station

[0002] The present disclosure relates to a method for determining an internal resistance of a battery cell of an energy storage device for an electrically driven motor vehicle. The disclosure also relates to a computer program and / or computer-readable medium, a data processing device, an electrically driven motor vehicle comprising an energy storage device with a battery cell and a data processing device, and a charging station comprising a data processing device.

[0003] Such an energy storage device typically comprises a plurality of battery cells connected in parallel and / or series, thus forming a high-voltage storage device, also known as a traction battery. The energy storage device is configured to discharge the battery cells and provide electrical energy to operate the motor vehicle and / or to provide electrical energy externally to the vehicle, for example, via a charging station, and to be supplied with electrical energy via the charging station to charge the battery cells of the energy storage device.

[0004] Accurate modeling of the high-voltage storage system is essential for its safe, sustainable, and efficient use. For state estimation to characterize the state of the cells and / or the energy storage device, an open-circuit voltage (OCV) and / or an open-circuit potential (OCP) are typically used. This is a relationship between a cell's open-circuit voltage or an electrode potential and the cell's state of charge. This relationship is used to estimate the cell's state of charge (SOC) and / or its aging or health (SOH). The open-circuit voltage can also be used to determine a charging strategy for charging the energy storage device or cells. Measuring the open-circuit voltage is not practical during operation.The open-circuit voltage characteristic can be modeled using a model instead of measurement. However, mapping a resistance using a look-up table alone is typically not sufficiently accurate and impractical, as the open-circuit voltage characteristic is related to the internal resistance. The internal resistance, in turn, is linked to the state of charge (SOC), the aging or state of health (SOH), the temperature, and also the history, i.e., the temporal progression of charging and / or discharging. The state of charge, the state of health, the temperature, and the history form conditions that can lead to a deviation between a measured cell voltage characteristic and the open-circuit voltage characteristic and typically vary during operation.

[0005] In principle, the internal resistance can be determined, for example, via predefined pulses during a factory and / or service provider service through a predefined measurement on a test bench. However, such a pulse-based test is currently unknown and / or not used. Predefined conditions are maintained, in particular a predetermined charging rate and a predetermined temperature range, in order to determine the internal resistance reproducibly and comparably, and thus to derive the open-circuit voltage characteristic.

[0006] However, the internal resistance is also crucial for performance prediction, for example, because a higher internal resistance can lead to lower available power from the battery cell or energy storage device, and vice versa. Furthermore, the internal resistance can be used to infer a decrease in capacity, meaning that the internal resistance can directly provide initial insights into the battery's health.

[0007] It is also possible to use a dynamic discharge profile during operation in order to calculate resistance components of the internal resistance from the associated current pulses.

[0008] DE 10 2021 108 091 A1 discloses a method for characterizing and / or charging a lithium-based electrical energy storage device, wherein the device is charged in a first charging process, wherein a charging current is regulated to a predetermined current value, or the first charging process is simulated using a predetermined simulation model. A terminal voltage curve as a function of the charge introduced during the first charging process is determined and stored, and the terminal voltage curve is used to characterize the aging of the energy storage device and / or for aging-optimized charging of the energy storage device in a second charging process.

[0009] Against the background of this prior art, one object of the present disclosure is to provide a method suitable for enriching the prior art and improving at least the above-mentioned aspects of the prior art. In particular, the object of the disclosure is to enable a determination of the internal resistance during charging, and in particular under various conditions.

[0010] The problem is solved by the features of the independent claims. The subclaims contain further developments of the disclosure.

[0011] According to one aspect of the disclosure, the object is achieved by a method for determining an internal resistance of a battery cell of an energy storage device for an electrically driven motor vehicle, the method comprising: charging the battery cell with a charging current; repeatedly temporarily lowering the charging current to a measuring charging current; measuring a cell voltage dependent on the charging current; and determining the internal resistance based on the cell voltage and the charging current.

[0012] The method enables the internal resistance to be determined effectively and reliably during a charging process, particularly as a function of the state of charge. For this purpose, the charging current is applied to the battery cell, which charges the battery cell according to a charging rate. During charging, the current or power, and thus the charging rate, is throttled to generate a current pulse. The measuring charging current is thus a temporary, i.e., time-limited, applied charging current for charging the battery cell, with the measuring charging current being lower than the original charging current by a current difference. The charging current is thus reduced from the original charging current by the current difference to the measuring charging current.

[0013] The cell voltage is measured while the battery cell is being charged with the charging current and while the battery cell is being charged with the measuring charging current. The battery cell can provide a measurable cell voltage in response to the current, and thus to both the measuring charging current and the charging current, which is defined by the internal resistance. Using Ohm's law, the internal resistance can be determined for each pulse, which is defined by the temporary reduction of the charging current to the measuring charging current, as the quotient of the voltage difference between the cell voltage at the charging current and the cell voltage at the measuring charging current, and the current difference between the charging current and the measuring charging current.

[0014] The method enables the internal resistance to be calculated for various operating scenarios. This allows the internal resistance to be calculated for a variety of boundary conditions, for example, to derive an accurate representation of the open-circuit voltage characteristic. The repeated temporary reduction during charging allows the open-circuit voltage characteristic to be sampled at different charge states. The application of the method and the use of the determined internal resistance are not limited to the reconstruction of the open-circuit voltage characteristic. Based on the internal resistance, for example, a performance forecast for charging and / or providing electrical energy and / or a capacity and / or aging estimate can be made.

[0015] The temporary reduction in current results in only a brief reduction in charging power, even with multiple pulses, and thus only a marginal increase in charging time, which is barely noticeable to the user. The method can be applied to AC and / or DC charging processes, particularly for rapid charging. When determining resistance, a reduction to any measuring charging current lower than the charging current may be sufficient, and when reconstructing the open-circuit voltage characteristic, a measuring charging current comparatively lower than the charging current can be used.

[0016] It was recognized that an accurate modeling of the overpotential or overvoltage can be used to reconstruct the resting voltage characteristic. The overpotential comprises an ohmic component, a component caused by charge transfer, and a diffusive component. The internal resistance determined by the method reliably represents the ohmic component as well as the component caused by charge transfer, especially with a suitable choice of a cell- or cell chemistry-dependent time constant that defines the time points of two measurements. This allows an overpotential to be modeled, and the resting voltage to be reconstructed from the measured cell voltages.

[0017] Optionally, the charging current can be reduced periodically and / or depending on a cell parameter. The cell parameter can be the state of charge of one of the battery cells. The charging current can be reduced at regular intervals with respect to the cell parameter. By reducing the current periodically and / or depending on a cell parameter, the battery cell can be measured during charging under different conditions, especially at different states of charge. This allows the internal resistance to be comprehensively determined.

[0018] Optionally, the internal resistance is determined for each of the reductions. Each of the repeated temporary reductions can be considered a pulse that provides a characteristic voltage response to the respective reduction. By determining the internal resistance multiple times, the internal resistance can be comprehensively determined.

[0019] Optionally, the charging current can be reduced to the measurement charging current over a period of 1.5 s to 5 s. This allows the battery cell to adjust its voltage response to the current, allows the cell voltage to be measured, and simultaneously results in only a slight increase in charging time due to the reduction.

[0020] Optionally, after the temporary reduction, charging can resume at the original charging current. This allows for well-defined charging of the battery cells.

[0021] Optionally, the method includes applying a smoothing filter to smooth the determined internal resistance. Due to the discrete and temporary reduction, the internal resistance may exhibit a characteristic temporal profile, particularly comparatively strong changes when the charging current changes. Such changes can be interpreted as artifacts resulting from the reduction and reduced by smoothing. For example, the internal resistance can be smoothed using a Savitzky-Golay filter.

[0022] In other words, the above description can be summarized as follows, with reference to a specific embodiment that is described as non-limiting for the present disclosure: The disclosure relates to the determination of internal resistance during charging. It deals with the determination of the state of health of a high-voltage storage device, in particular with the estimation of degradation modes and the current capacity value from a partial charge. For this purpose, it is necessary to reconstruct the open-circuit voltage from the measured voltage. This requires accurate modeling of the overpotential, which is composed of an ohmic component, a component due to the charge transfer, and a diffusive component. In principle, the internal resistance can be determined using predefined pulses in service, i.e. a predefined measurement (e.g., 1C, 2C, 3C at a defined temperature) on a test bench.During operation, a dynamic discharge profile can be used to calculate the resistance components from current pulses. Accurate modeling of the high-voltage storage system is essential for its safe, sustainable, and efficient use. The open-circuit voltage characteristic is typically required for estimating the state of charge (SOC) and state of health (SOH). Since measuring the open-circuit voltage characteristic during operation is not practical, it must be modeled. The internal resistance depends on SOC, SOH, history, and temperature. Simply mapping the resistance using a look-up table is insufficient and impractical in most cases. The principle of the disclosure is explained below for an AC charging process. However, the principle is also applicable to a DC charging process (fast charging).During AC charging at 11 or 22 kW (at a wallbox or a public charging station), the current or power is throttled at continuous intervals to generate a current pulse (e.g., current pulse with Delta_l ~ C / 10 and Delta_t ~ 2 s). The internal resistance can be calculated for each pulse using the calculation rule (R_l = delta_U / deltaj). This allows the resistance to be calculated for various operating scenarios. Furthermore, the brief reduction in power, even with multiple pulses, results in a marginal, barely noticeable increase in charging time. The resistance modeling is in good agreement with the ohmic and charging overvoltage components. A user can automatically perform "diagnostic charging" during AC charging at home (wallbox, 11 kW), as there is hardly any increase in charging time.This application generates information about the current state of the storage device in the current temperature, SOC, and SOH ranges. Assuming a modern high-voltage storage device with a warm-charging range (10-60°C), cell-level currents range from 9A (0% SOC) to 5.5A (100% SOC). Assuming an internal resistance of 2 mOhm, a current reduction of just 3A (-C / 10) at the cell level is sufficient to change the voltage by 6mV. The measurement accuracy can be assumed to be 1mV. The maximum current rate of change is -20A / s or +40A / s. Data recording at >5Hz is possible, which allows for sufficient temporal resolution.

[0023] According to one aspect of the disclosure, a computer program and / or a computer-readable medium is provided. The computer program and / or the computer-readable medium comprise instructions which, when the program or instructions are executed by a data processing device, cause the device to perform the method according to the disclosure and / or steps thereof. Optionally, the computer program and / or the computer-readable medium comprises instructions which, when the program or instructions are executed by a data processing device, cause the device to perform the method steps described as advantageous or optional in order to achieve an associated technical effect.

[0024] According to one aspect of the disclosure, a data processing device is provided. The data processing device is configured to perform the method described above. Optionally, the data processing device is configured to perform a method step described as advantageous or optional and / or to implement a method feature in order to achieve an associated technical effect.

[0025] Optionally, the data processing device is a battery control unit.

[0026] According to one aspect of the disclosure, an electrically driven motor vehicle is provided, comprising an energy storage device with a battery cell and the data processing device described above. Optionally, the data processing device of the motor vehicle and / or the motor vehicle is configured to perform a method step described as advantageous or optional and / or to implement a method feature in order to achieve an associated technical effect.

[0027] According to one aspect of the disclosure, a charging station comprising the data processing device described above is provided. Optionally, the data processing device of the charging station and / or the charging station is configured to perform a method step described as advantageous or optional and / or to implement a method feature in order to achieve an associated technical effect.

[0028] In the following, one embodiment is described with reference to the figures.

[0029] Fig. 1 schematically shows an electrically driven motor vehicle according to one aspect of the disclosure;

[0030] Fig. 2 schematically shows a flow diagram of a method according to one aspect of the disclosure;

[0031] Fig. 3 schematically shows a section of a relationship between a charging current and time when performing a method according to one aspect of the disclosure; Fig. 4 schematically shows a relationship between a charging current and time and between a cell voltage and time, respectively, when performing a method according to one aspect of the disclosure;

[0032] Fig. 5 schematically shows a dependency between an internal resistance and time according to a method according to one aspect of the disclosure;

[0033] Fig. 6 schematically shows a dependency between an overvoltage and time according to a method according to one aspect of the disclosure; and

[0034] Fig. 7 schematically shows a rest voltage characteristic and a dependency between an absolute error and time according to a method according to one aspect of the disclosure.

[0035] Figure 1 schematically shows an electrically driven motor vehicle 50 according to one aspect of the disclosure.

[0036] The motor vehicle 50 has an energy storage device 55 and an electric drive 52. The energy storage device 55 has a plurality of battery cells 56, which and their number are shown only schematically. For example, the energy storage device 55 has a 192s5p circuit, i.e., five strings of battery cells 56 connected in parallel, each with 192 battery cells 56 per string. The capacity of a battery cell 56 is, for example, 32 Ah.

[0037] The energy storage device 55 or the battery cells 56 are configured to be supplied with electrical energy in order to charge the battery cells 56, i.e., to increase a state of charge (SOC) of the battery cells 56. The energy storage device 55 or the battery cells 56 are configured to provide electrical energy for operating the motor vehicle 50 and / or the electric drive 52, wherein the battery cells 56 are discharged, i.e., the state of charge (SOC) of the battery cells 56 decreases.

[0038] The motor vehicle 50 has a data processing device 51. The data processing device 51 is, for example, a battery control module and is configured to control and monitor the operation of the energy storage device 55. For this purpose, the data processing device 51 is configured, for example, to define a charging profile with a current I for charging the energy storage device 55 and to measure a cell voltage U of one of the battery cells 56. Figure 1 also shows a charging station 91 and a network 90 external to the vehicle. The charging station 91 and the motor vehicle 50 are configured to be connected to one another electrically and via communication technology. Thus, the energy storage device 55 can be supplied with electrical energy from the network 90 external to the vehicle via the charging station 91 to charge the battery cells 56.Optionally, electrical energy can be fed from the energy storage device 55 into the vehicle-external network 90 via the charging station 91 while discharging the battery cells 56, whereby the motor vehicle 50 and the charging station 91 are set up for so-called bidirectional charging.

[0039] The motor vehicle 50 or the data processing device 51 is configured to carry out the method 100 described with reference to Figure 2. In another embodiment, particularly in the case of bidirectional charging, the charging station 91 is configured to carry out the method 100 according to Figure 2. The charging station 91 has a data processing device 92 configured to control the charging and / or discharging of the energy storage device 55 of the motor vehicle 50.

[0040] Figure 2 schematically shows a flowchart of a method 100 according to one aspect of the disclosure. The method 100 according to Figure 2 is a method 200 for determining an internal resistance RI of a battery cell 56 of an energy storage device 55 for an electrically driven motor vehicle 50. Such a motor vehicle 50 is described with reference to Figure 1. Figure 2 is described with reference to Figure 1.

[0041] The method 100 according to Figure 2 comprises: charging 110 the battery cell 56 with a charging current IL. In this process, the battery cell 56 is subjected to the charging current IL, whereby the state of charge SOG of the battery cell increases. The charging current IL defines a charging rate or a C-rate.

[0042] The method 100 comprises: multiple temporary reductions 120 of the charging current IL to a measurement charging current IM. In this case, the charging current IL is temporarily, i.e., for a limited time, reduced to the measurement charging current IM. The measurement charging current IM defines a reduced charging rate or a reduced C-rate. The reduction 120 of the charging current IL occurs periodically and / or as a function of a cell parameter ZP, for example, regularly defined by a difference in the state of charge SOC.

[0043] The measuring charging current IM is reduced over a period DT of 1.5 s to 5 s.

[0044] Reduction 120 from the charging current IL to the measuring charging current IM is carried out with a

[0045] Reduction rate. After the temporary reduction 120, charging 110 is resumed at the original charging current IL. For this purpose, the charging current IL is increased from the measuring charging current IM to the original charging current IL. The increase in the charging current IL from the measuring charging current IM to the original charging current IL occurs at a gradient rate.

[0046] The method 100 comprises: measuring 130 a cell voltage U dependent on the charging current IL. The cell voltage U is measured for both the charging current IL and the measuring charging current IM. The determination 130 of the internal resistance RI is performed for each of the reductions 120. By repeatedly reducing the voltage 120, the internal resistance RI can be determined at different states of charge SOC.

[0047] The method 100 comprises: determining 140 the internal resistance RI based on the cell voltage U and the charging current IL. Ohm's law U = R x I is used, where the internal resistance RI is the quotient of the voltage difference between the cell voltage U at the charging current IL and the cell voltage U at the measuring charging current IM and the current difference between the charging current IL and the measuring charging current IM.

[0048] The method 100 comprises applying 150 a smoothing filter to smooth the determined internal resistance RI. By determining the internal resistance RI multiple times, the internal resistance RI is determined as a function of the state of charge SOC and / or time T. By detecting a temperature, the internal resistance RI can also be determined as a function of the temperature. The dependence of the internal resistance RI as a function of the state of charge SOC and / or time T is smoothed by the smoothing filter.

[0049] The method 100 is described in more detail with reference to Figures 3 to 7.

[0050] Figure 3 schematically shows a section of a relationship between a charging current I and time T when performing a method 100 according to one aspect of the disclosure. Figure 3 is described with reference to Figures 1 and 2.

[0051] The method 100 according to Figure 2 is described using the example of an alternating current (AC) charging process. The following parameters are exemplary and serve to illustrate the method 100. A different selection of the mentioned parameters is possible. The principle is also applicable to a direct current (DC) charging process and in particular to rapid charging. Cell currents for an AC charging process with 11 kW are approximately 4.5 A at a state of charge of 0% and approximately 2.7 A at an SOC of 100%, or approximately 9 A at a state of charge of 0% and approximately 5.5 A at an SOC of 100% for an AC charging process with 22 kW. Assuming an internal resistance RI of 2 mO and a current swing (IL-IM) of 2.5 A, Ohm's law, U = R x I, results in a required voltage swing of 5 mV. A voltage measurement resolution of 1 mV thus enables the calculation of the internal resistance RI.For example, the measurement is carried out at a rate of more than 5 Hz.

[0052] At the beginning of the section shown in Figure 2, charging 110 of one of the battery cells 56 occurs with the charging current IL. The charging current IL is, for example, 4 A.

[0053] During charging 110, the charging current IL is temporarily reduced 120 to the measured charging current IM. Figure 2 describes the temporary reduction 120 of the charging current IL.

[0054] The measuring charging current IM, for example, is 1.5 A and therefore corresponds to 37.5% of the original charging current IL. The charging current IL is reduced to the measuring charging current IM over a period DT- with a falling edge at a reduction rate of -20 A / s and over a duration DT- of 125 ms, i.e., at a reduction rate of -2.5 A / (125 ms).

[0055] The charging current IL is reduced to the measuring charging current IM over a period DT of 2 s. The current I is thus maintained at the measuring charging current IM.

[0056] The subsequent increase in current I to charging current IL after holding the measuring charging current IM occurs over a period DT+ with a rising edge at a rate of increase of +40 A / s and a duration DT+ of 62 ms, i.e., with a rate of increase of +2.5 A / (62 ms). The rate of decrease or change of current I is thus smaller in magnitude when decreasing current I from charging current IL to measuring charging current IM than the rate of increase or change of current I when increasing current I from measuring charging current IM to charging current IL. In the example shown, the period DT- with a falling edge is longer than the period DT+ with a rising edge.

[0057] Assuming a charging process to increase the SOC of the battery cells 56 from 10% to 80% and a temporary reduction 120 with a SOC difference of 2%, it can be seen that the temporary reduction 120 only results in a marginal increase in charging time. The energy not transferred due to the temporary reduction 120 is approximately: E = (80-10) / 2 x (IL-IM) x DT = 35 x

[0058] 2.5 A x 2 s = 175 As = 48.61 mAh, which corresponds approximately to 48 mAh / 32 Ah or 0.152% of the capacity of battery cell 56.

[0059] The method 100 via a charging process with a multiple temporary lowering 120 is further described with reference to Figure 4.

[0060] Figure 4 schematically shows a relationship between a charging current IL and time T in Figure 4 (A) and between a cell voltage U and time T in Figure 4 (B), each when performing a method 100 according to one aspect of the disclosure. Figure 4 is described with reference to Figures 1 to 3.

[0061] For the following analysis, in contrast to the previous analysis of Figure 3, a battery cell with a capacity of 116 Ah in new condition, an ambient temperature of 25 °C, an initial state of charge (SOC) of 20%, a temporal resolution of 5 Hz, and, for better visibility in the figures, current pulses with a current difference ID between the charging current IL and the measuring charging current IM of -10 A over a period DT of 10 °s are assumed. Thermal modeling is omitted.

[0062] Figure 4 (A) shows the relationship between a charging current IL and the time T for a charging process. During the charging process, the temporary reduction 120 of the charging current IL by a current difference ID to the measured charging current IM, as described with reference to Figure 2, occurs several times. The reduction 120 occurs periodically.

[0063] Figure 4 (B) shows the cell voltage U in response to the charging current IL according to Figure 4 (A). Figure 4 (B) also shows an enlarged section of the cell voltage U to illustrate the response to one of the reductions 120. For a given charging current IL, an increase in the cell voltage U is evident when charging the battery cell 56. When the charging current IL is reduced 120, the cell voltage U decreases, and a voltage difference UD arises, which is defined by the internal resistance RI.

[0064] The cell voltage U is measured at a frequency of 5 Hz, which depends on the battery cell 56 and its chemistry, which is why a first calculation of the internal resistance RI is possible after 1 / 5 s = 200 ms according to RI = U / IL. For one of the pulses, the internal resistance RI is calculated according to the ohmic formula: RI = UD / ID. This results in the relationship between the internal resistance RI and the

[0065] Time T

[0066] Figure 5 schematically shows a relationship between an internal resistance RI and time T according to a method 100 according to one aspect of the disclosure. The internal resistance RI according to Figure 5 results from the current I and the cell voltage U, as described with reference to Figure 4. The internal resistance RI shows a typical decrease in the internal resistance RI during charging, i.e., with increasing state of charge SOC and advancing time T. The pulses are evident as steps in the temporal progression of the internal resistance RI. The temporal progression can be smoothed by applying a filter. The overvoltage UO can be calculated based on the internal resistance RI according to Figure 5, see Figure 6.

[0067] Figure 6 schematically shows a relationship between an overvoltage UO and time T according to a method 100 according to one aspect of the disclosure. The overvoltage UO is calculated from the smoothed internal resistance RI according to Figure 5 and shows a typical drop during charging.

[0068] Figure 7 schematically shows a rest voltage characteristic curve, i.e., a relationship between voltage U and time T, in Figure 7 (A) and a dependency between an absolute error DU and time T in Figure 7 (B) depending on a method 100 according to one aspect of the disclosure. The reconstruction of the rest voltage characteristic curve is one of various possible application examples for using the internal resistance RI.

[0069] Figure 7 (A) shows the overvoltage UO together with the cell voltage U (solid line) and a resting voltage reconstructed from the internal resistance RI as the cell voltage (dashed line). Figure 7 (B) shows an absolute error DU as the difference between the curves shown in Figure 7 (A). Reference symbols (part of the description)

[0070] 50 motor vehicles

[0071] 51 Data processing device

[0072] 52 drive

[0073] 55 Energy storage device

[0074] 56 battery cells

[0075] 90 vehicle-external network

[0076] 91 charging stations

[0077] 92 Data processing device

[0078] 100 procedures

[0079] 110 stores

[0080] 120 Lowering

[0081] 130 trade fairs

[0082] 140 Investigate

[0083] 150 applications

[0084] DT period

[0085] DT+ period on rising edge

[0086] DT period on falling edge

[0087] YOU mistakes

[0088] I Current

[0089] ID current difference

[0090] IL charging current

[0091] IM measuring charging current

[0092] RI internal resistance

[0093] SOC state of charge

[0094] U cell voltage

[0095] UD voltage difference

[0096] UO overvoltage

[0097] ZP cell parameters

[0098] T Time

Claims

Claims 1. A method (100) for determining an internal resistance (RI) of a battery cell (56) of an energy storage device (55) for an electrically driven motor vehicle (50), the method (100) comprising: - charging (110) the battery cell (56) with a charging current (IL); - multiple temporary lowering (120) of the charging current (IL) to a measuring charging current (IM); - measuring (130) a cell voltage (U) dependent on the charging current (IL); and - Determine (140) the internal resistance (RI) based on the cell voltage (U) and the charging current (IL).

2. Method (100) according to claim 1, wherein the lowering (120) of the charging current (IL) takes place periodically and / or as a function of a cell parameter (ZP).

3. The method (100) according to claim 1 or 2, wherein the determination (130) of the internal resistance (RI) is carried out for each of the sinks (120).

4. Method (100) according to one of the preceding claims, wherein the charging current (IL) is reduced to the measuring charging current (IM) over a period (DT) of 1.5 s to 5 s.

5. Method (100) according to one of the preceding claims, wherein the charging (110) after the temporary lowering (120) is carried out again with the original charging current (IL).

6. Method (100) according to one of the preceding claims, comprising: - Applying (150) a smoothing filter to smooth the determined internal resistance (RI).

7. Computer program and / or computer-readable medium comprising instructions which, when the program or instructions are executed by a data processing device (51), cause the device (51) to carry out the method (100) and / or the steps of the method (100) according to one of claims 1 to 6.

8. Data processing device (51, 92), wherein the data processing device (51) is configured to carry out the method (100) according to one of claims 1 to 6.

9. Electrically driven motor vehicle (50), comprising an energy storage device (55) with a battery cell (56) and the data processing device (51) according to claim 8.

10. Charging station (91), comprising the data processing device (92) according to claim 8.

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