METHOD FOR DETECTING A FAULT STATE OF A BATTERY CELL, DETECTION DEVICE, AND MOTOR VEHICLE

Abstract

A method for detecting a fault state of at least one battery cell of a battery having multiple battery cells. A cell voltage of a respective battery cell of the multiple battery cells is registered at a measurement time and a comparison value is determined as a function of at least one of the cell voltages and is compared to a provided first reference value. The fault state is detected as a function of a result of the comparison. A scatter value is determined, which represents a scatter of at least part of the cell voltages registered at the specific measurement time, and the fault state is determined as a function of the scatter value.

Claims

1. A method for detecting a fault state of at least one battery cell of a battery having multiple battery cells, comprising: registering a cell voltage of a respective battery cell of the multiple battery cells at a specific measurement time, determining a comparison value as a function of at least one of the cell voltages, which is compared to a provided first reference value (R1), wherein the fault state is detected as a function of a result of the comparison, and determining a scatter value is determined, which represents a scatter of at least part (Ui, Un) of the cell voltages registered at the specific measurement time, and the fault state (F) is determined as a function of the scatter value (ΔUn).

2. The method as claimed in claim 1, wherein the scatter value is determined for a subgroup of the cell voltages registered at the measurement time, wherein at least one maximum and/or minimum voltage value of the cell voltages registered at the measurement time is not comprised by the subgroup.

3. The method as claimed in claim 1, wherein the scatter value is provided as an absolute value of a difference between a maximum and minimum voltage value of the subgroup.

4. The method as claimed in claim 1, wherein at least one extreme voltage value, which represents a maximum and/or minimum voltage value of the cell voltages registered at the measurement time, is compared to a second reference value, wherein the comparison value represents the scatter value and the fault state is detected, wherein if the scatter value is at most as large as the first reference value and the at least one extreme voltage value deviates from the second reference value by at least one first limiting absolute value; and if the scatter value is greater than the first reference value and the at least one extreme voltage value deviates from the second reference value by at least one second limiting absolute value that is greater than the first limiting absolute value.

5. The method as claimed in claim 1, wherein the second reference value represents a mean value of at least the cell voltages comprised by the subgroup.

6. The method as claimed in claim 1, wherein a standard deviation or a variance for all cell voltages registered at the measurement time is determined as the scatter value.

7. The method as claimed in claim 1, wherein the cell voltages are determined for multiple successive measurement times and a respective scatter value is determined for the respective measurement times, wherein the fault state is detected as a function of a time profile of the scatter values, in particular wherein a change over time of the scatter value is determined as the comparison variable and the fault state is considered to be detected when the comparison variable exceeds the first reference value.

8. The method as claimed in claim 1, wherein the multiple consecutive measurement times are in an operating time window which is assigned to a specific operating state of the battery in which a change over time of a battery current of the battery is less than a predeterminable limiting value.

9. A detection device for detecting a fault state of at least one battery cell of a battery having multiple battery cells, wherein the detection device is designed to register a cell voltage of a respective battery cell of the multiple battery cells at a measurement time, to determine a comparison value as a function of at least one of the cell voltages, to compare the comparison value to a provided first reference value, and to detect the fault state as a function of a result of the comparison, wherein the detection device is designed to determine a scatter value, which represents a scatter of at least a part of the cell voltages registered at the specific measurement time, and to determine the fault state as a function of the scatter value.

10. A motor vehicle having a detection device for detecting a fault state of at least one battery cell of a battery having multiple battery cells, wherein the detection device is designed to register a cell voltage of a respective battery cell of the multiple battery cells at a measurement time, to determine a comparison value as a function of at least one of the cell voltages, to compare the comparison value to a provided first reference value, and to detect the fault state as a function of a result of the comparison, wherein the detection device is designed to determine a scatter value, which represents a scatter of at least a part of the cell voltages registered at the specific measurement time, and to determine the fault state as a function of the scatter value.

11. The method as claimed in claim 2, wherein the scatter value is provided as an absolute value of a difference between a maximum and minimum voltage value of the subgroup.

12. The method as claimed in claim 3, wherein at least one extreme voltage value, which represents a maximum and/or minimum voltage value of the cell voltages registered at the measurement time, is compared to a second reference value, wherein the comparison value represents the scatter value and the fault state is detected, wherein if the scatter value is at most as large as the first reference value and the at least one extreme voltage value deviates from the second reference value by at least one first limiting absolute value; and if the scatter value is greater than the first reference value and the at least one extreme voltage value deviates from the second reference value by at least one second limiting absolute value that is greater than the first limiting absolute value.

13. The method as claimed in claim 4, wherein at least one extreme voltage value, which represents a maximum and/or minimum voltage value of the cell voltages registered at the measurement time, is compared to a second reference value, wherein the comparison value represents the scatter value and the fault state is detected, wherein if the scatter value is at most as large as the first reference value and the at least one extreme voltage value deviates from the second reference value by at least one first limiting absolute value; and if the scatter value is greater than the first reference value and the at least one extreme voltage value deviates from the second reference value by at least one second limiting absolute value that is greater than the first limiting absolute value.

14. The method as claimed in claim 2, wherein the second reference value represents a mean value of at least the cell voltages comprised by the subgroup.

15. The method as claimed in claim 3, wherein the second reference value represents a mean value of at least the cell voltages comprised by the subgroup.

16. The method as claimed in claim 4, wherein the second reference value represents a mean value of at least the cell voltages comprised by the subgroup.

17. The method as claimed in claim 2, wherein a standard deviation or a variance for all cell voltages registered at the measurement time is determined as the scatter value.

18. The method as claimed in claim 3, wherein a standard deviation or a variance for all cell voltages registered at the measurement time is determined as the scatter value.

19. The method as claimed in claim 4, wherein a standard deviation or a variance for all cell voltages registered at the measurement time is determined as the scatter value.

20. The method as claimed in claim 5, wherein a standard deviation or a variance for all cell voltages registered at the measurement time is determined as the scatter value.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0031] Exemplary embodiments of the invention are described hereinafter. In the figures:

[0032] FIG. 1 shows a schematic representation of a motor vehicle having a detection device for detecting a fault state of a battery cell according to an exemplary embodiment of the invention;

[0033] FIG. 2 shows a graphical representation of the voltage curves of the cell voltages of intact battery cells with a large voltage spread in all cells;

[0034] FIG. 3 shows a schematic representation of a mechanically damaged battery cell of a battery;

[0035] FIG. 4 shows a graphical representation of the voltage curves of the mechanically damaged battery cell from FIG. 3 in comparison to the voltage curves of the other battery cells;

[0036] FIG. 5 shows a graphical representation of the voltage curves of multiple battery cells in comparison to a voltage curve of a defective battery cell;

[0037] FIG. 6 shows a schematic representation of multiple voltage curves of battery cells including a defective battery cell during a balancing process;

[0038] FIG. 7 shows a schematic representation of the end of the balancing process from FIG. 6;

[0039] FIG. 8 shows a schematic representation of the voltage curves of battery cells during a charging process with a subsequent idle phase, including the voltage curve of a defective battery cell; and

[0040] FIG. 9 shows a schematic representation of a detection device for detecting a fault state according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION

[0041] The exemplary embodiments explained hereinafter are preferred embodiments of the invention. In the exemplary embodiments, the described components of the embodiments each represent individual features of the invention to be considered independently of one another, which each also refine the invention independently of one another. Therefore, the disclosure is also intended to comprise combinations of the features of the embodiments other than those illustrated. Furthermore, the described embodiments can also be supplemented by further ones of the above-described features of the invention.

[0042] In the figures, the same reference signs designate elements that have the same function.

[0043] FIG. 1 shows a schematic representation of a motor vehicle 10 having a detection device 12 for detecting a fault state F of a battery cell 16 of a battery 18 of the motor vehicle according to an exemplary embodiment of the invention. The battery 18 can be designed as a high-voltage battery of the motor vehicle 10 or as a battery module of such a high-voltage battery. The battery 18 furthermore comprises multiple battery cells 16. One of these battery cells 16 is additionally denoted by 16a in order to be able to better illustrate and describe the method for detecting a fault state F of such a battery cell 16. Correspondingly, the procedure for how a fault state of such a battery cell 16 can be detected is explained on the basis of this first battery cell 16a, but can also be used in the same way for all other battery cells 16. If a fault state F of such a battery cell 16 is detected by the detection device 12, a signal S can be output, such as a warning signal to a driver of the motor vehicle or a signal to initiate a specific measure, such as switching off the battery, the battery cell, or the like.

[0044] A cell 16 behaves reproducibly depending on environmental conditions, for example dynamic load profiles, inhomogeneous heating, etc. However, there can be different behavior and strong spread between the cells 16 within a battery 18. As a rule, a robust diagnosis across all cells 16 is therefore sometimes very difficult. Different voltage spreads of the individual cell voltages of the battery cells 16 are to be illustrated below in different situations on the basis of several examples.

[0045] FIG. 2 shows, for example, the voltage curves of individual cell voltages UZ plotted over time tin seconds, in particular over a time period of approximately one minute. In this example, none of the cells 16 whose voltage curves are shown in FIG. 2 have a defect. As can be seen, even in the case of non-defective cells 16, there are sometimes very large voltage spreads in all cells 16, depending on the operating state. There are therefore no outliers in this example. As can also be clearly seen in FIG. 2, there are regions in which the individual cell voltage curves are very far apart, but in other regions they in turn quasi-run into one another. Especially when the individual cell voltages UZ are subject to a strong change over time, the corresponding spreads can only be resolved very poorly. This complicates the detection of possible cell defects.

[0046] FIG. 3 shows a schematic representation of a part of a battery 18 having a battery cell 16, which is again denoted as 16ain the present case and is intended to represent a defective battery cell 16a accordingly. In this example, the battery cell 16a is mechanically damaged from the outside and in particular has previous damage in the form of an intrusion of up to 3.4 millimeters. Such mechanical damage to the cell from the outside can results in a cell defect, which is accordingly noticeable in a changed cell voltage UZ. This is illustrated in FIG. 4.

[0047] FIG. 4 shows a graphical representation of the voltage curves UZ of the individual cell voltages of the battery 18 from FIG. 3, wherein the voltage curve of the damaged cell 16a is denoted by UX. The time t is plotted here in hours. The battery 18 having the damaged cell 16ais subjected to multiple charging and discharging cycles here, wherein the cell defect of the cell 16a becomes noticeable especially when changing from discharging to charging, in particular in a strongly deviating cell voltage UX. Here again it can be seen that the resulting deviation of this cell voltage UX from the other cell voltages UZ in other regions, for example during charging or discharging, is significantly less pronounced. From this it is already evident that it is difficult, for example, to define a limiting value by means of which such a fault state of a cell 16a can be detected and which is valid for all possible operating ranges of a battery 18 at the same time.

[0048] Another example is shown in FIG. 5. The curves of the cell voltages UZ of multiple intact battery cells 16 and of the cell voltage UX of a defective battery cell are also again shown here. A charging process begins in a first time phase. This loading phase is denoted by L. Moreover, this charging phase L is followed by three driving modes F1, F2, F3 having different loads. At the start of the charging process L, the voltage curves of all cells are still without any voltage deviation from one another. Only at the end of charging, i.e., at the end of the charging phase L, does a first voltage deviation of a cell 16a from the other cells 16 appear. The voltage of this defective cell 16a is again denoted by UX, and the voltage curves of the other cells are denoted by UZ. There is a small standard deviation overall in the region of this first driving mode F1. During the charging L, all cell voltages UX, UZ are therefore still close together. At the end of charging or before the driving mode F1, a cell 16a is then suddenly significantly worse. This is particularly noticeable because the other cells differ from each other by only 10 millivolts, for example. Under load, that is to say in the driving mode state F2, a greater voltage spread can be observed in all cells 16, 16a, but this is not unusual in cells 16 that are cold or heated to an unequal extent. At low load, for example in the driving state range F3, the cell voltages UZ approach each other again, wherein the cell 16a also deviates still further here with its cell voltage UX. As can be seen, the cell defect in cell 16a cannot be recognized equally well in all operating states. For example, no significant deviation of the defective cell 16a from the cell voltages UZ of the other cells 16 can be recognized in the driving range state F2. In principle, however, detection in the driving ranges F1 and F3 is possible. This knowledge is used to define suitable limiting values for detecting such a defective cell 16a, as will be explained in more detail later.

[0049] Firstly, FIGS. 6, 7, and 8 each show further examples of voltage curves of individual battery cells 16, including the voltage curve UX of a defective battery cell. FIG. 7 also shows a detail view of a detail from FIG. 6, wherein this detail is denoted by 20. FIG. 6 also shows cell balancing, wherein the balancing times run up together and initially nothing unusual occurs until the last charging process of the cells, which is again illustrated in detail in FIG. 7. It can be seen that the balancing system attempts to balance all cells 16, wherein the cell 16a deviates significantly from the last charging process with its voltage UX from the other voltages UZ and can no longer be charged.

[0050] FIG. 8 illustrates a charging process L with a subsequent idle phase R, in which the battery 18 is no longer under load. The idle phase R lasts approximately 30 hours. In this idle phase R, the cell 16a displays an increased self-discharge rate in comparison to the other cells 16 from the last charging process onwards.

[0051] In order to be able to detect these defects accurately and at an early stage, there are now multiple options. For this purpose, on the one hand, the battery variables, such as temperature and voltage UZ, UX, and variables derived therefrom, such as the standard deviation and/or an interval range or interquartile range, can be measured or calculated and stored.

[0052] For example, the battery variables can be compared to earlier values via the standard deviation under constant, comparable conditions, for example a new charging process with the same charging current, when the vehicle has started with a low discharge current, optionally also taking into consideration the state of charge or battery temperature. If a cell suddenly deviates significantly from the other cells in comparison to the time of the last diagnosis, taking into consideration the standard deviation, it can be assumed that a cell defect has occurred.

[0053] It is particularly advantageous above all to use the interquartile range or, in general, an interval range of cell voltages UZ of a specific measurement subgroup in order to determine constant conditions, for example a charging process with constant current, stationary vehicle with low discharge current, where it is possible to compare the battery variations and resulting deviations will be rather small. Under constant conditions and thus, for example, a small interquartile range or a small standard deviation, a small outlier can already indicate a cell defect. For dynamic conditions, on the other hand, the threshold can be applied significantly larger. This allows a possible cell defect to be detected in the same way for different conditions and states.

[0054] FIG. 9 shows a schematic representation of an embodiment of a detection device of a motor vehicle 10, as can be used for detecting such a cell defect F, for example. As an input variable, the detection device 12 uses the individual cell voltages Ui of all battery cells 16 of the battery 18, including the battery cell 16a initially assumed to be defective. In the present example, a distinction is also no longer made between the cell voltage assigned to a defective battery cell and the other cell voltages, but the method is described in general for all cell voltages Ui of the battery 18. In this case, a minimum cell voltage Umin of all cell voltages Ui is provided at the first input 22 and a maximum cell voltage Umax is provided at a second input 24. Furthermore, a mean value Ū is determined from all cell voltages Ui, which is also referred to as the second reference value Ū within the scope of the present invention, and is provided at a third input 26 . Furthermore, a subgroup Un is determined from the cell voltages Ui. At least the maximum and minimum cell voltage Umax, Umin are excluded to determine this subgroup. However, several of the largest and smallest voltage values can also be excluded from this subgroup Un. Furthermore, in this fourth input module 28, the absolute value between a maximum deviation of the greatest and least cell voltage of this subgroup Un is determined. In this case, this distance represents a magnitude of the scatter of this subgroup Un and is therefore referred to hereinafter as the scatter value ΔUn. In a special case, this scatter value can especially represent the interquartile range. This scatter value ΔUn is compared by means of a relational operator 30 to a predefined limiting value, which is also referred to hereinafter as the first reference value R1. In parallel thereto, the respective addition operators 32, 34 are used to determine the absolute value distance between the least voltage value Umin and the mean value Ū, on the one hand, and also the absolute value distance between the greatest voltage value Umax and the mean value Ū. The largest of these two resulting distances is selected using the maximum operator 36. As a result, this thus supplies the largest voltage difference to the mean value Ū among all input voltages Ui. If the comparison by the comparison operator 30 shows that the scatter value ΔUn is less than or equal to the first reference value R1, then the maximum deviation Δmax described above at the output of the maximum operator 36 is compared to a first threshold value S1. This comparison is in turn performed by a relational operator 38. If this limiting value S1, for example 20 millivolts, is exceeded, an error is detected and a corresponding signal is output at the output 40. Moreover, 42 denotes a logical AND operator, which ensures that both conditions have to be met for this purpose, namely that the maximum deviation Δmax exceeds the first threshold value S1 and the interquartile range or scatter value ΔUn is less than or equal to the first reference value R1. If it is not, then the exceeding of the first limit value S1 by the deviation Δmax also does not result in an error detection. In any case, however, this maximum deviation Δmax is also compared to a second threshold value S2. This second threshold value S2 thus represents a limiting value S2 by which the extreme voltage value, i.e., the least voltage value Umin or the greatest voltage value Umax, may deviate at most from the voltage mean value Ū, i.e., the second reference value Ū,or in the present example in particular can no longer deviate, so that no fault state F is detected. This comparison is again performed by a further comparison operator 43. The threshold value S2 is significantly greater than the threshold value S1 and is, for example, 400 millivolts. If the deviation Δmax is greater than this second threshold value S2, then a signal S is also output at the output 40, which indicates that an error is present. In this case, 44 represents a logical OR operator.

[0055] With a small scatter, which is represented by the scatter value ΔUn, even a small deviation from the mean value Ū can advantageously trigger a signal S, while with large scatters ΔUn, only a large deviation of a voltage value Ui from the mean value Ū results in the output of a such signal S. This makes it possible to select significantly smaller limiting values, for example at 20 millivolts, for the triggering in regions having constant conditions and thus small interquartile ranges, since in such regions even small triggers indicate a cell defect. For dynamic conditions, for example, when driving or recuperating, the threshold is applied much higher, for example at the 400 millivolts described. As a result, a detection of a cell defect can be provided which is adapted to the situation and is nonetheless always early.

[0056] Overall, the examples show how the invention can be used to identify a suddenly occurring cell defect in a particularly efficient and adapted manner.