DE102019110349A1 - Method for determining mechanical defects in a battery system and battery system - Google Patents

Abstract

A method for determining mechanical defects in a battery system (10), in particular a high-voltage storage device in a motor vehicle, wherein the battery system (10) comprises at least one module (14) with a plurality of battery cells (18), and wherein the battery system (10) is a sensor system (12) comprises the following steps: - providing the battery system (10); - measuring the impedance of at least one battery cell (18) of the at least one module (14) of the battery system (10) by means of the sensor system (12); - transmitting the at least one measured value to an evaluation unit (16); and comparing the measured value with at least one reference value in order to infer a change in impedance characteristic of a mechanical defect on the basis of the comparison. A battery system is also specified.

Description

The invention relates to a method for determining mechanical defects in a battery system, in particular a high-voltage storage device in a motor vehicle, and to such a battery system.

In electric vehicles and hybrid vehicles, suitably dimensioned battery systems are used as high-voltage storage devices for the sole or auxiliary electric drive of the motor vehicle. The high-voltage storage system contains several modules, also called battery modules, in which several battery cells are combined. The individual battery cells can be connected in series and / or in parallel. Typically, lithium-ion accumulators or batteries are used as battery cells.

When the motor vehicle is in operation, the state of the individual battery cells, namely the so-called “state of health” (SoH) or “state of charge” (SoC), must be constantly monitored. One possibility of providing appropriate monitoring is to determine the impedance of the battery cells. It is known that the impedance values of a battery cell change with advancing age, whereby corresponding conclusions can be drawn about the state of the battery cell.

From the DE 10 2016 216 777 A1 For example, a measuring method of a measuring device is known with which the impedance of several modules of a battery storage device connected together to form a string can be detected.

The battery cells in the individual modules of the battery system are usually mechanically braced against one another, for example by arranging a frame around the battery cells. However, the battery cells expand in the course of their lifetime due to aging effects, which can lead to mechanical stresses in the module.

In extreme cases, these mechanical stresses can lead to the failure of the frame, as a result of which the mechanical stressing of the individual battery cells is no longer guaranteed. This can lead to failure of individual modules or the entire battery system.

The object of the invention is to provide a possibility of reliably detecting mechanical defects in a battery system, in particular a high-voltage storage device in a motor vehicle.

• - Bereitstellen des Batteriesystems;
• - Messen der Impedanz von wenigstens einer Batteriezelle des zumindest einen Moduls des Batteriesystems mittels des Sensorsystems;
• - Übermitteln des zumindest einen Messwerts an eine Auswerteeinheit; und
• - Vergleichen des zumindest einen Messwerts mit zumindest einem Referenzwert, um aufgrund des Vergleichs auf eine für einen mechanischen Defekt charakteristische Veränderung der Impedanz zu schließen.

The object is achieved according to the invention by a method for determining mechanical defects in a battery system, in particular a high-voltage storage device in a motor vehicle, the battery system comprising at least one module with a plurality of battery cells, and the battery system having a sensor system comprising the following steps:

• - Provision of the battery system;
• Measuring the impedance of at least one battery cell of the at least one module of the battery system by means of the sensor system;
• - Transmission of the at least one measured value to an evaluation unit; and
• Comparing the at least one measured value with at least one reference value in order to infer a change in impedance characteristic of a mechanical defect on the basis of the comparison.

The basic idea of the present invention is that mechanical defects also lead to a characteristic change in the impedance of the battery cells and this characteristic change in the impedance can be determined by means of the sensor system and the evaluation unit. In other words, the impedance of the at least one battery cell is detected via the sensor system, the detected impedance being evaluated by the evaluation unit in order to detect the characteristic change.

For this purpose, the evaluation unit can access a reference value stored in a memory in order to compare this with the current measured value that has been measured by the sensor system.

The battery system is in particular a high-voltage storage device, for example a traction battery of a motor vehicle such as an electric car or a plug-in hybrid.

The at least one measured value that has been transmitted to the evaluation unit can also be at least temporarily stored, in particular stored, in the evaluation unit.

The reference value with which the determined measured value is compared can be both a value stored in the evaluation unit, for example from the manufacturer, and an earlier measured value.

Furthermore, the reference value can have been determined from several previous measured values, for example the average of a specific number of previous measured values or a moving average of a specific number of previous ones Readings. The reference value can also have been determined based on a predetermined value which is adapted accordingly, for example due to aging.

Likewise, not only a singular measured value can be used, but several measured values, for example, to determine the change in impedance over time. The change over time can then be compared with a reference value which, for example, represents the age-related change. If the measured change in impedance over time deviates from the age-related change by a certain value, the characteristic change may be present.

The impedance can be described as a complex quantity with a real part and an imaginary part. It is thus possible to evaluate the real part and the imaginary part of the impedance separately from one another. The reference value can accordingly also be described as a complex variable with a real part and an imaginary part. In this way, characteristic changes can also be recognized separately from one another, only in the real part or the imaginary part. In particular, some changes can be identified more in the real part than in the imaginary part or vice versa.

The characteristic change in the impedance in the presence of a mechanical defect is, in particular, a sudden change in the measured value, in particular compared to the previous measured value. Thus, even comparatively small changes in the impedance that occur in the event of mechanical defects can be reliably distinguished from other influences, for example due to changes due to temperature changes.

The differentiation can also be made easier by taking known changes to the real part and / or imaginary part into account. A temperature change has a comparable effect on both the real part and the imaginary part of the impedance, while in the case of a mechanical defect the influence on the real part is greater than that on the imaginary part of the impedance.

Alternatively and / or in addition, only the magnitude of the impedance, in particular the change in impedance, can also be the characteristic change.

In particular, low voltages are used for the impedance measurement, for example a voltage of 1 to 2 mV.

The impedance can be measured using an alternating voltage signal with a frequency in the range from 100 Hz to 10 kHz, in particular with a frequency greater than 500 Hz.

High frequencies enable the impedance to be measured with as little interference as possible. If the impedance is measured at lower frequencies, in particular near the AC mains frequency of 50 Hz in the motor vehicle, the probability increases that sources of interference falsify the measurement. In general, sources of interference occur primarily in the low frequency range due to various consumers in the motor vehicle, so that the impedance can be measured especially at higher frequencies.

Since temperature fluctuations can have a much stronger effect on the measured impedance than mechanical changes, the temperature of the battery cell must be taken into account when comparing the measured value with the reference value. As already explained, the real part and the imaginary part can be analyzed independently of one another. This is helpful because changes in temperature affect both the real part and the imaginary part, whereas a mechanical defect has a greater influence on the real part. The dependence of the measured impedance on the temperature decreases with higher frequencies. An impedance measurement at high frequencies is therefore also advantageous because incorrectly determined temperatures cause less measurement errors.

In addition, with higher frequencies it is possible to select the measurement periods for the impedance comparatively short. If possible, the impedance measurement should be as short as possible, since no interference should occur during the measurement. Accordingly, there should be no load changes in the battery system during the measurement, for example (strong) acceleration. A constant current, on the other hand, does not lead to falsification of the measured impedance, so that the impedance measurement is also possible while driving.

In a variant, the impedance can be measured with at least two different frequencies. The impedance of a battery cell changes to different degrees at different frequencies, since different influencing factors have a stronger effect depending on the frequency range. By measuring the impedance with different frequencies, it can thus be ensured in a simple manner that a single source of interference does not accidentally cause a disproportionately large deviation in the impedance at a single measurement frequency. A higher number of frequencies for the impedance measurement therefore increases the accuracy of the method, but a longer measurement time is required. Accordingly, a compromise must be chosen between the desired accuracy and measurement duration.

The excitation signal can also have several frequencies, in particular two frequencies, so that the measurement time does not have to be extended. However, the outlay on equipment is higher in this case, so that a compromise must be chosen between the desired accuracy and the costs.

The impedance can be recorded with a resolution of 10 ^-3 mOhm, i.e. 0.1 µOhm. The internal resistance of a cell in a typical high-voltage storage system is often less than 1 mOhm. The internal resistance of the high-voltage battery can also be significantly greater through further components of the high-voltage storage unit, for example cable connections. Correspondingly, the changes in the measured impedance in the case of mechanical defects are usually very small, so that a sufficiently accurate resolution is necessary during the impedance measurement in order to be able to reliably detect it. The measurement accuracy can accordingly be in a range from 0.005 to 0.01 mOhm (5 to 10 µOhm).

Furthermore, if the impedance deviates by more than 0.2 mOhm (1 µOhm), a warning signal can be issued and / or a notification can be sent to a workshop. The measured value can thus have a deviation from the reference value, the warning signal or the notification only occurring if there is a deviation of more than 0.2 mOhm. Basically, the deviation in the impedance from which the warning signal or the notification is to be output must be adapted to the expected characteristic change in impedance in order to be able to reliably detect a mechanical defect without false reports.

The values used, in particular the value for the output of the warning signal, basically depend on the specific number of batteries connected in parallel.

In this way, the safety of the battery system can be increased even further, since mechanical defects are detected immediately and, for example, communicated to the driver of the motor vehicle.

The sensor system can comprise a voltage and / or a current measuring device.

In a further variant, each of the battery cells of the at least one module of the battery system can be assigned a sensor of the sensor system. For example, a chip that monitors the respective battery cell is applied to each of the battery cells. The chip can have a voltage and / or a current measuring device as well as further measuring devices, for example a temperature sensor, in order to monitor the temperature of the assigned battery cell.

In this variant, the changes in the impedances of the individual battery cells of the at least one module can also be compared with one another. In the event of a break in the frame surrounding the battery cells, it is to be expected that the mechanical stresses on all battery cells will decrease, whereby each of the sensors should detect a change in the impedance characteristic of a mechanical defect for each of the battery cells of the module. However, there may be gradual differences, as the mechanical tension decreases more, especially in the immediate vicinity of the broken frame. In any case, however, the impedance changes essentially at the same time for each battery cell, so that the mechanical defect can be detected with a sufficiently high resolution.

Furthermore, a time profile of the impedance can be recorded, in particular with the time profile being at least temporarily stored. In this respect, several measured values are used that are used to determine a mechanical defect. The storage or intermediate storage can take place in the evaluation unit. In this way, the change in impedance over time can also be taken into account. It is to be expected that in the event of a mechanical defect, for example a break in the frame surrounding the battery cells, a rapid or sudden change in the impedance occurs. At least the change in impedance in a short period of time is greater than would be expected solely from the aging effects of the battery cells.

The reference value used for the comparison can also be adapted over time. This ensures that a change in impedance that would suggest a mechanical defect is not determined from the originally stored reference value, for example the reference value stored in the delivery state, due to a change in impedance caused by aging effects. It is thus possible to reliably differentiate between a mechanical defect and other influences on the impedance over the life of the battery system. In other words, the reference value is subject to an aging model, so that the change in impedance due to the aging of the battery system is taken into account accordingly in the reference value.

The object is also achieved by a battery system, in particular a high-voltage storage device in a motor vehicle, wherein the battery system comprises at least one module with a plurality of battery cells, and wherein the battery system has a sensor system, the battery system being set up to carry out the method of the type described above. In this respect, reference is made to the advantages mentioned above.

• - 1 einen schematischen Aufbau eines erfindungsgemäßen Batteriesystems;
• - 2 einen schematischen Ablauf des erfindungsgemäßen Verfahrens;
• - 3 ein Nyquist-Diagramm beispielhafter Messwerte der Impedanz vor und nach einem mechanischen Defekt; und
• - 4 ein Bode-Diagramm beispielhafter Messwerte der Impedanz vor und nach einem mechanischen Defekt.

Further advantages and properties of the invention emerge from the following drawings. In these show:

• - 1 a schematic structure of a battery system according to the invention;
• - 2 a schematic sequence of the method according to the invention;
• - 3 a Nyquist plot of exemplary measured values of impedance before and after a mechanical failure; and
• - 4th a Bode diagram of exemplary measured values of the impedance before and after a mechanical defect.

In 1 is a schematic structure of a battery system according to the invention 10 shown for an at least partially electrically powered motor vehicle.

The battery system 10 includes a sensor system 12 , multiple modules 14th and an evaluation unit 16 that come with the sensor system 12 cooperates, as will be explained below.

Each of the modules 14th has multiple battery cells 18th on that by means of a frame 20th are mechanically braced together. The individual battery cells 18th of a module 14th are connected in series in the embodiment shown. The modules 14th are in turn also connected in series.

Each of the battery cells 18th is a sensor 22nd assigned, the sensors 22nd Part of the sensor system 12 are. The sensors 22nd are in the embodiment shown in sensor units 23 connected together, each sensor unit 23 one of the modules 14th assigned. The sensor 22nd can in particular be a Smart Cell Chip (SCC).

Any of the sensors 22nd has a tension measuring device 24 , for example by means of an integrated shunt resistor and / or a current measuring device 26th on. Basically the sensor could 22nd also have further measuring devices, for example a temperature sensor, via which the temperature of the assigned battery cell 18th can be captured.

The modules 14th are constructed identically in the embodiment shown, the individual battery cells 18th within each module 14th as well as the battery system 10 can also distinguish from one another, in particular due to aging effects of the battery cells that are differently pronounced 18th .

Basically, the modules 14th also differ from each other. For example, it is conceivable that not all battery cells 18th in all modules 14th Sensors 22nd have, but for example only one battery cell 18th per module 14th .

Due to the aging of the battery cells 18th these expand over the life of the battery system 10 out, creating a mechanical tension in the frame 20th of the modules 14th increases. These mechanical stresses can lead to the frame 20th breaks. In order to be able to reliably detect such a mechanical defect, the method according to the invention for determining mechanical defects, which is explained below, can be used.

It gets the battery system first 10 provided (step S1 in 2 ), especially in a motor vehicle (not shown here).

Then the impedance of at least one battery cell 18th at least one module 14th by means of the sensor system 12 measured (step S2 2 ). The measurement can be carried out via a control unit 28 to be controlled.

The sensor system is used to measure the impedance 12 For example, an alternating voltage signal is generated, the frequency of the alternating voltage signal being in a range from 100 Hz to 10 kHz.

In principle, it is sufficient to determine the impedance at a single frequency, but several, in particular two, frequencies can also be used in order to increase the accuracy or the measurement reliability.

The impedance is recorded with a resolution of 10 ^-4 Ohm.

The at least one recorded measured value of the impedance is then sent to the evaluation unit 16 transmitted (step S3 in 2 ). The at least one measured value can be (temporarily) stored there.

In the embodiment shown, the evaluation unit 16 Part of the sensor system 12 .

However, it is also conceivable that the evaluation unit 16 not part of the sensor system 12 and only for data exchange with the sensor system 12 is set up.

In this respect, the evaluation unit would be 16 separate from the sensor system 12 educated.

In the evaluation unit 16 , in particular a memory of the evaluation unit 16 , a reference value is stored with which the transmitted measured value is compared (step S4 in 2 ).

If the measured value of the impedance deviates from the reference value by more than 0.2 mOhm, the battery system 10 a warning signal can be issued and / or a notification is sent to a workshop.

In addition, the measured value can also be used in the evaluation unit 16 adjust the stored reference value.

For example, the last determined measured value of the impedance replaces the previously stored reference value at regular intervals if no mechanical defect was found. In this way, small changes in the impedance due to aging effects of the battery cells 18th are included in the reference value.

A time profile of the determined impedances can also be used in the evaluation unit 16 recorded and / or at least temporarily stored. This allows the evaluation unit 16 the temporal course of the impedance and the change in impedance are determined and taken into account when comparing with the reference value.

A (moving) average from a specified number of previous measured values can also be used to compare this with the reference value.

In the 3 and 4th exemplary curves of the impedances over a frequency spectrum from 1 to 1000 Hz are shown, with in 3 a representation as a Nyquist diagram, i.e. the imaginary part versus the real part of the impedance, and in 4th a representation as a Bode diagram is used, that is, the course of the impedance, in particular the absolute value and the phase, versus the frequency.

The dashed lines in the 3 and 4th show the course of the impedance of a battery cell 18th in the initial state, i.e. with the frame intact 20th .

The solid lines in the 3 and 4th show the course of the impedance over the same frequency spectrum for the same battery cell 18th after the frame 20th has been damaged.

After damaging the frame 20th a clear change in impedance that is greater than 0.2 mOhm can be seen over the entire frequency range.

The changes in impedance from the evaluation unit can correspond accordingly 16 identified as a characteristic change after a mechanical defect.

In particular, such a change occurs suddenly, that is to say briefly in time, whereas aging effects or other effects influencing the impedance occur gradually, even if, viewed in absolute terms, they have a greater influence on the impedance.

In this respect, the impedance, in particular its change, is continuously monitored in order to determine the change in impedance characteristic of the mechanical defect.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• DE 102016216777 A1 [0004]

Claims (10)

Method for determining mechanical defects in a battery system (10), in particular a high-voltage storage device in a motor vehicle, wherein the battery system (10) comprises at least one module (14) with a plurality of battery cells (18), and wherein the battery system (10) comprises a sensor system ( 12), comprising the following steps: - Provision of the battery system (10); - Measuring the impedance of at least one battery cell (18) of the at least one module (14) of the battery system (10) by means of the sensor system (12); - Transmission of the at least one measured value to an evaluation unit (16); and Comparing the at least one measured value with at least one reference value in order to infer a change in impedance characteristic of a mechanical defect on the basis of the comparison. Procedure according to Claim 1 , characterized in that the impedance is measured via an alternating voltage signal with a frequency in the range from 100 Hz to 10 kHz. Procedure according to Claim 1 or 2 , characterized in that the impedance is measured with at least two different frequencies. Method according to one of the preceding claims, characterized in that the impedance is recorded with a resolution of 10 ^-3 mOhm. Method according to one of the preceding claims, characterized in that if the impedance deviates by more than 0.2 mOhm, a warning signal is output and / or a notification is sent to a workshop. Method according to one of the preceding claims, characterized in that the sensor system (12) comprises a voltage (24) and / or a current measuring device (26). Method according to one of the preceding claims, characterized in that each of the battery cells (18) of the at least one module (14) of the battery system (10) is assigned a sensor (22) of the sensor system (12). Method according to one of the preceding claims, characterized in that a time profile of the impedance is recorded, in particular wherein the time profile is at least temporarily stored. Method according to one of the preceding claims, characterized in that the reference value used for the comparison is adapted over time. Battery system, in particular a high-voltage storage device in a motor vehicle, the battery system (10) comprising at least one module (14) with several battery cells (18), and the battery system (10) having a sensor system (12), the battery system (10) for this purpose is set up, the method according to one of the Claims 1 to 9 perform.

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