DE102020202857B4 - Device and method for thermal monitoring of a battery - Google Patents

Abstract

Device for monitoring a battery (30), in particular a lithium-ion battery to prevent thermal runaway, with the battery (30) having a plurality of cells (6) arranged next to one another within a housing (16) under a prestress in a transverse direction (8), wherein a measuring arrangement (36) is provided for detecting an expansion of at least one of the cells in the transverse direction (8), and the measuring arrangement (36) has a plurality of sensors (12) and an evaluation unit (18, 34), characterized in that the sensors (12) are each arranged between two cells (6) adjacent in the transverse direction (8) and are connected to the evaluation unit (18, 34), so that an expansion of one of the cells (6) in the transverse direction (8) is detected by the measuring arrangement (36) and monitored with regard to exceeding a critical value for the expansion, wherein the critical value indicates a potential occurrence of a thermal runaway, characterized in that a detection unit (40) for Detection of a current operating temperature of the battery (30) is provided and the evaluation unit (18,34) is set up such that the critical value is set as a function of the current operating temperature.

Description

The invention relates to a device and a method for thermal monitoring of a battery, in particular a lithium-ion battery, to avoid a so-called “thermal runaway”.

The problem of so-called "thermal runaway" is well known with rechargeable batteries, especially lithium batteries and especially with high-voltage vehicle batteries used for electric traction motors. This is an exothermic chemical reaction within a battery cell in which a self-reinforcing and heat-producing chemical process leads to overheating, possibly resulting in an explosion of the cell or a fire. Such thermal runaway starts within a cell of a battery, for example due to overload or a short circuit. If the resulting increased temperature exceeds a threshold temperature, the self-reinforcing chemical process begins, which further heats up the cell temperature until the cell explodes or catches fire. This also affects neighboring cells, which can also begin to burn due to the higher heat. The spreading fire can then spread very quickly to the entire battery and, in the case of a vehicle battery, to the vehicle itself.

From the DE 10 2009 034 854 A1 and the WO 2019 146 960 A1 Methods and devices for monitoring the charge state of a battery can be found there.

The DE 10 2009 034 854 A1 To this end, it provides for the detection of a change in at least one non-electrical physical quantity of the battery as a measure of the state of charge. In particular, a change in volume or a changing pressure is detected. In this case, lithium batteries with graphite as the active material are monitored in particular, where a change in the state of charge leads to a change in volume. To detect the change in volume, for example, a housing delimiting a cell stack is provided with a plate that can be moved on one side and forms an electrode. When there is a change in volume, the distance to a second electrode changes as a result of moving the movable plate. A change in capacity caused by this is detected.

The WO 2019 146 960 A1 describes a pressure sensor in different versions, which is applied to the outside of a battery and detects a pressure change due to a volume change.

Out of DE 10 2015 211 598 A1 The arrangement of force sensors between two adjacent cells of a battery for battery monitoring can be seen.

According to US 2006 / 0 246 345 A1 A piezoelectric sensor is arranged to detect a change in pressure between the cells of a battery.

US 2017/ 0 324 110 A1 describes the arrangement of capacitive elements between the cells to detect expansions.

Based on this, the invention is based on the object of specifying a device and a method for monitoring a battery, especially a rechargeable battery (accumulator) and in particular a lithium-ion battery, in order to avoid a thermal runaway.

The object is achieved according to the invention by a device having the features of claim 1 and by a method having the features of claim 13. The advantages and preferred embodiments mentioned with regard to the device are also to be transferred to the method.

The device comprises a rechargeable battery (accumulator), in particular a lithium-ion battery, which has a plurality of, in particular, cuboid-shaped cells that are arranged next to one another within a housing under a bias voltage in a transverse direction. The device further comprises a measuring arrangement for detecting an expansion of at least one of the cells in the transverse direction. For this purpose, the measuring arrangement has a plurality of, in particular, capacitive sensors and an evaluation unit. The sensors are each arranged between two cells adjacent in the transverse direction and are connected to the evaluation unit. If one of the cells expands in the transverse direction, this measuring arrangement detects a change in distance between the sensors and the evaluation unit. When capacitive sensors are used, the change in distance causes a change in capacitance, which is evaluated by the evaluation unit.

This measurement setup monitors the battery for the occurrence of thermal runaway. As soon as a critical expansion or change in distance between two neighboring cells is detected that exceeds a critical value, which is characteristic of the potential occurrence of thermal runaway, appropriate countermeasures are initiated. One such countermeasure could be reducing the load on the battery. or to disconnect the battery from the rest of the (on-board) electrical system, especially of a motor vehicle. The device, especially the evaluation unit, is designed and configured to carry out these steps.

The critical value for the change in distance is generally greater than a normal operating change due to normal, operational temperature fluctuations within a permissible operating temperature range.

This measure therefore ensures predictive detection of a potential thermal runaway and reliably prevents its occurrence. A particular advantage of the measurement setup is that the capacitive sensor, in particular, performs an individual measurement between two adjacent cells in the transverse direction, allowing each cell to be monitored individually. This is of particular importance.

The batteries considered here, especially vehicle batteries, are constructed in such a way that several cells are combined to form a cell module. Within a cell module, the individual cells are typically connected in series, so that the voltage of the cell module is the sum of the voltages of the individual cells. The cell module itself typically has a module housing. The individual cells combined in a cell module are typically held against one another within the module housing. This means that a certain amount of pressure is exerted on the individual cells even in their initial state. This achieves the highest possible packing density of the individual cells and an overall compact battery structure. Several such cell modules are in turn interconnected to form the entire battery and are typically combined within a battery housing. The individual cell modules are also typically connected in series to achieve the desired battery voltage. Depending on the application, especially in the case of a battery for supplying an electric traction motor of a motor vehicle, the battery voltage and the corresponding supply voltage for the electric traction motor is typically several hundred volts, typically greater than 300 volts and, for example, in the range between 300 and 1,000 volts.

Whenever the term "cell" is used here, this refers to a single (galvanic) individual cell having a (single) cathode and a (single) anode, with a separator arranged between them. In addition, "cell" is also understood to mean the joint arrangement of several such individual cells, in which the layer structure of cathode - separator - anode is repeated periodically. If several such individual cells are used to construct a cell, the individual cells contained therein are preferably connected in parallel. The cell therefore has the voltage of a respective individual cell. An individual cell typically has a voltage in the range of a few volts, for example in the range between 1 and 5 volts. Typical values for a single lithium cell are - depending on the materials used - between 2 and 4.5 volts. In this case, "cell" is understood to mean, in particular, an individual cell.

The housing of a cell, also referred to as the cell casing, consists, for example, of an inherently rigid and dimensionally stable, typically cuboid-shaped solid body. Alternatively, the cell casing is made of a flexible material, such as a (metal) foil.

In the present case, “cell module” is understood to mean, in particular, a cell module in which, for example, 4 to 20, and in particular 4 to 14 cells are connected and arranged in series.

Individual monitoring of each cell by measuring the change in distance between two adjacent cells in the transverse direction has the particular advantage of high sensitivity with regard to a malfunction of an individual cell, i.e., with regard to the potential occurrence of a thermal runaway within a cell. In contrast to state-of-the-art monitoring systems, which, for example, only determine the expansion of the entire cell module (assembly of cells) to check the state of charge, the sensitivity is significantly higher. Only this ensures reliable and safe predictive detection of a thermal runaway before it occurs.

In a practical embodiment, each sensor and the evaluation unit are designed to detect a change in distance between two cells in the range between 1 µm and 100 µm, and in particular between 1 µm and 50 µm. This ensures high sensitivity, allowing even the smallest expansions due to increasing temperature to be reliably detected.

If we speak of a "change" in this case, for example, of a measured value from the sensor or a value derived from it, such as distance, the term "change" always refers to a base or reference value in an initial state of the battery. A suitable initial state is, in particular, a load-free state at a reference temperature, in particular of 20°C, for example. This reference or base value is recorded and stored, in particular, by a calibration measurement. Such a calibration measurement is carried out, for example, for each individual battery or, alternatively, solely on the basis of a reference battery to which the respective battery is identical in construction. Therefore, the sensor and/or the evaluation unit specifically performs/determines only a relative measurement or relative change, without a precise absolute determination being required or performed.

The sensor also has a thickness in the transverse direction in the range of 150µm to 500µm, and in particular a thickness in the range of 200 to 400µm, and especially a thickness of 300µm. This only slightly increases the required installation space for the cell module and still ensures a compact design.

The sensors are preferably flat, capacitive sensors. The sensor area corresponds, for example, to more than 5%, preferably more than 20%, of the base area of the respective cells, which is oriented perpendicular to the transverse direction.

In a preferred embodiment, a sensor is arranged between each pair of cells, so that a sensor is arranged between each cell pair. In general, the distance between two adjacent cells of a cell module is monitored using the measuring arrangement.

In general, the evaluation unit is designed to compare the measured values, specifically the (capacitance) changes of the various sensors. Furthermore, the evaluation unit is designed to monitor these measured values of the sensors for any irregular changes (compared to the aforementioned baseline value). An irregular change in the measured values is interpreted as an exceedance of the critical value, and appropriate countermeasures are initiated.

The individual measured values from the sensors are therefore compared relative to one another. This is based on the idea that, for example, a significant change in one sensor indicates that a cell adjacent to that sensor is faulty. In contrast, sensors assigned to non-faulty cells would experience a significantly smaller change, so that a conclusion about a defective cell can be drawn from this relative comparison. If an "uneven change" is mentioned here, this means that the changes between two sensors differ from one another by more than a permissible tolerance range, where the tolerance range is, for example, 10%, 20%, or 30% of the change.

Through the individual evaluation of the individual sensors, it is also possible to localize the defective cell and this is preferably carried out by the evaluation unit.

According to the invention, a detection unit, in particular a temperature sensor, is additionally arranged for detecting a current operating temperature of the battery. The evaluation unit is further configured such that the critical value is set depending on the current operating temperature. The at least one temperature sensor is placed at a suitable measuring point directly on or in the battery. Preferably, several temperature sensors are provided, and in this case, a value derived from the several measured temperatures, for example, an average value, is determined as the current operating temperature.

The consideration of the current operating temperature and the setting of the critical value, i.e. the value for the permissible change in the sensor's measured value compared to its reference value, is based on the fact that the individual cells experience a permissible expansion during operation as a result of normal, operating temperatures. High load conditions, especially during charging, typically lead to temperature increases and corresponding expansion of the individual cells. These are therefore already taken into account by the evaluation unit through the change in the critical value. In the case of high temperatures, higher values for expansion are permissible overall, i.e. the critical value for the permissible change compared to the reference value increases with increasing operating temperature. For example, a change of 20%, relative to the reference value, can be caused solely by a normal operating temperature. However, if a change of 20% occurs when the battery is "cold", this can already be an indication of a defective cell. Accordingly, as the operating temperature increases, a higher value is set for the critical value, above which a fault is predictively concluded.

In a practical embodiment, each sensor is designed as a differential capacitor with two opposing base electrodes and a measuring electrode arranged between them. This design enables high sensitivity even for the smallest distance changes.

For convenience, the base electrodes and the measuring electrodes are designed as flat electrodes and arranged parallel to each other.

In an alternative embodiment, the base electrode and the measuring electrode are arranged in a Z-shape. Such a configuration of a Z-shaped differential capacitor is described, for example, in WO 2006/015565 A1 described.

In a practical design, the base electrodes are connected to ground potential (earth), and the measuring electrode is connected to a measuring terminal of the evaluation unit. Such a differential capacitor is generally formed by two capacitors connected in series. This increases the overall capacitance and thus the sensitivity. By connecting the opposing, outer base electrodes to ground potential, shielding is also achieved by these base electrodes.

The previously described parallel arrangement of the electrodes is the preferred configuration. This allows for a particularly compact design and reliably detects even the smallest expansion of a cell and the resulting pressure increase, which leads to minimal compression of the sensor.

The evaluation unit includes an oscillator, specifically an RC oscillator. Its resonant frequency shifts as a result of a change in the capacitance of the capacitor formed by the sensor, thus ensuring accurate detection of any change.

In a preferred embodiment, an elastic, dielectric layer (made of a non-gaseous material) is arranged between the base electrode and the measuring electrode. This layer simultaneously defines an insulating spacer between the two electrodes and allows compression due to thermally induced cell expansion.

The dielectric layer preferably has a thickness in the range of 100µm to 200µm.

By choosing the thickness and/or density of the dielectric layer, the properties of the capacitor (sensor) are influenced and can be adjusted as required.

The electrodes, especially the base electrodes, are formed from metal foils, specifically copper foils. These typically have a thickness in the range of only 50 to 80 µm.

In a preferred embodiment, the evaluation unit is further arranged within a module housing that surrounds the multiple cells combined to form a cell module. This integral arrangement within the module housing enables direct monitoring. The individual evaluation units of the respective cell modules of the battery are connected in particular to a central unit, for example, a battery management system. This is also a central evaluation unit, which is typically arranged directly on the battery or integrated into it.

• 1 ein Zellmodul mit mehreren innerhalb eines Modulgehäuses integrierten Zellen mit dazwischen angeordneten Sensoren,
• 2 eine Batterie mit mehreren der in 1 dargestellten Zellmodule,
• 3 eine Messanordnung mit einer ersten Variante eines kapazitiven Sensors und
• 4 eine Messanordnung mit einer zweiten Variante eines kapazitiven Sensors.

Embodiments of the invention are explained in more detail below with reference to the figures, each of which shows simplified representations.

• 1 a cell module with several cells integrated within a module housing with sensors arranged in between,
• 2 a battery with several of the 1 cell modules shown,
• 3 a measuring arrangement with a first variant of a capacitive sensor and
• 4 a measuring arrangement with a second variant of a capacitive sensor.

In the figures, parts with the same function are shown with the same reference symbols.

The 1 The cell module 2 shown has a module housing 4, within which a plurality of cells 6 are arranged. The cell module 2 extends in a transverse direction 8, and the individual cells, which are preferably approximately cuboid-shaped, are arranged next to one another in this transverse direction 8. The module housing 4 has side walls 10, viewed in the transverse direction 8, which clamp the cells 6 between them in the transverse direction.

Viewed in the transverse direction 8, a sensor 12 is arranged between each two adjacent cells 6. This is designed in particular as a capacitive sensor and specifically as a differential capacitor, as is available in different variants to the 3 and 4 is described in more detail.

The individual sensors 12 are therefore clamped between two adjacent cells 6. The pressure exerted by the module housing 4 exerts a pressure on the sensors 12, represented by first arrows 14.

In the exemplary embodiment, a total of four cells 6 are shown. Each cell 6 has a housing 16. A respective sensor 12 is therefore arranged and clamped between walls of adjacent housings 16.

The respective sensor 12 has a thickness of preferably 300µm, viewed in the transverse direction 8. This thickness also corresponds to a distance a between adjacent cells 6

Within the respective module housing 4, an evaluation unit 18 is also arranged, which is connected to each of the sensors 12 in order to record its measured values. 1 An evaluation circuit comprising an RC oscillator is schematically indicated for the middle sensor 12. Each sensor 12 is connected to a measuring terminal 20 of the evaluation unit 18. A change in the distance a causes a change in the sensor's capacitance. This changes the frequency of a periodic, for example, sinusoidal AC voltage signal from the oscillator.

In the 1 the sensor 12 is designed as a differential capacitor with two opposing, flat base electrodes 22, a measuring electrode 24 arranged therebetween and a dielectric layer 26 arranged between each two adjacent electrodes 22, 24. The two base electrodes 22 form outer electrodes, which preferably lie directly against the wall of the respective housing 16, if necessary only with an insulating layer interposed.

The base electrodes 22 are flat, foil-like elements made of metal, in particular copper. The two base electrodes 22 are connected to a common base or ground potential. In the exemplary embodiment, the two base electrodes 22 are formed as one piece and are connected to one another by a cross-connecting piece 28, which in the exemplary embodiment of the 1 is shown curved. Viewed in cross section, the two base electrodes 22 therefore form a "U". The measuring electrode 24 arranged between them runs parallel to the two base electrodes 22 and extends along a flat plane perpendicular to the transverse direction 8 and can therefore - viewed in cross section - be viewed as an "I". The sensor 12 shown is therefore also referred to as a "UI" sensor 12. The dielectric layers 26 are preferably formed in one piece, just like the base electrodes 22, and have a cross-connecting piece. Therefore, viewed in cross section, they are also U-shaped. Base electrodes 22 and dielectric layers are therefore inserted into one another as (viewed in cross section) two U-shaped, one-piece structures.

As in the 2 As shown, to construct a battery 30, several such cell modules 2 are combined within a battery housing 32 and suitably interconnected. The battery 30 has two connection poles 31 for connection, in particular, to a motor vehicle electrical system. The battery 30 is assigned a central evaluation unit 34, which is preferably arranged within the battery housing 32. This central evaluation unit 34 is, in particular, a so-called battery management system. This central evaluation unit 34 is, in particular, connected to the individual evaluation units 18. The individual module-specific evaluation units 18 together with the central evaluation unit 34 functionally form an evaluation unit that is designed to monitor the battery, in particular with regard to early detection of a thermal runaway.

The individual sensors 12 together with the individual evaluation units 18 and possibly together with the central evaluation unit 34 form a measuring arrangement 36 which is designed for the predictive detection of an undesired thermal runaway.

A thermal runaway occurs due to a malfunction within an individual cell 6, for example, due to overload or an internal short circuit. This leads to thermal heating. If a limit temperature is reached, a chemical reaction occurs within cell 6, further increasing the cell temperature. This reaction intensifies as the temperature increases. An unacceptable temperature change is indicated by a thermally induced expansion of cell 6.

Such an expansion of a cell 6 leads to an increased pressure or to an expansion in the direction of the second arrows 38, as shown in one of the 1 shown cells 6. This (minor) expansion is detected by this cell 6, hereinafter referred to as the faulty cell 6a. The expansion of this faulty cell 6a leads to a reduction in the distance a to the neighboring cells and thus to a compression of the sensors 12 that are applied to this faulty cell 6a. This compression leads to a change in capacitance, which is detected by the evaluation unit 18, 34.

If this change in capacity and thus the change in distance between two adjacent cells 6 exceeds a critical value, which indicates the potential occurrence of a thermal runaway, the evaluation unit 18, 34 emits a corresponding warning signal in order to initiate a countermeasure. This countermeasure consists, for example, in switching off the battery or at least in a reduction of the load on the battery, for example a reduction of a charging current or a current currently delivered.

The evaluation unit 18, 34 particularly evaluates the measurement results of the individual sensors 12 of a respective cell module 2 in parallel. The evaluation unit 18, 34 detects an exceedance of the critical value and triggers the countermeasure if the relative change (change in distance, change in capacitance) of the individual sensors 12 deviates from one another beyond a tolerance range. The tolerance range is, for example, +/-10%, +/-20%, or +/-30%. This takes into account the fact that in the case of a single defective cell 6a, initially only this cell 6a expands disproportionately (compared to the non-defective cells 6) and therefore, in particular, the sensors 12 adjacent to this defective cell 6a experience excessive compression. Different changes are therefore an indication of the defective cell 6a. This cell is also specifically localized because the sensors 12 adjacent to it show a disproportionate increase in capacitance.

The evaluation unit 18,34 is further designed to take into account the temperature of the battery 30. For this purpose, at least one temperature sensor 40 is provided, which is 2 shown by way of example as a component of the central evaluation unit 34. Alternatively, a temperature sensor 40 is arranged, for example, on or in each cell module 2 or each cell 6.

The critical value at which a malfunction is concluded varies depending on the temperature detected by the temperature sensor 40. This means that the evaluation of whether a faulty cell 6a is present generally depends on the detected current operating temperature of the battery 30 or at least the current operating temperature of the respective cell module 2 or the respective cell 6.

Such a battery 30 is designed, in particular, as a high-voltage battery specifically for supplying electrical power to an electric drive motor (traction motor) of an electric or hybrid vehicle (motor vehicle) and is arranged in such a vehicle. The voltage of the battery 30 is typically in the range of several hundred volts.

The 3 shows, in a highly simplified manner, a measuring arrangement 36 with the previously described UI sensor 12 (differential capacitor) with an evaluation unit 18 connected to it. In this schematic representation, the dielectric layer 26 arranged between the electrodes 22, 24 is not shown. The two base electrodes 22 and the measuring electrode 24 are designed as flat electrodes and arranged parallel to one another. The arrows shown illustrate the pressure exerted on the sensor 12. The two base electrodes 22 are connected to ground potential. The measuring electrode 24 is connected to an evaluation circuit. This evaluation circuit outputs a periodic alternating voltage signal as an output signal, the period of which depends on the capacitance. When the sensor 12 is compressed, its capacitance increases, which leads to a change in the period.

In the 4 An alternative embodiment of the capacitive sensor 12 is shown. In contrast to the 3 The intermediate measuring electrode 24 is not arranged parallel but rather diagonally, so that, viewed in cross-section, the three electrodes are arranged in an approximately Z-shaped configuration. The base electrodes 22 are electrically decoupled from the measuring electrode 24. This means that the connecting piece between the measuring electrode 24 and the two adjacent base electrodes 22 is formed by an electrical insulator.

The 3 The variant of the UI sensor shown is the preferred design, since it is narrower when viewed in the transverse direction 8 and is therefore more suitable for the desired compact arrangement.

The present invention is not limited to the described embodiment. Modifications are possible within the scope defined by the claims. In particular, other sensors, especially distance sensors, can be used instead of the capacitive sensors described here. However, the differential capacitors described here have proven to be particularly suitable and sensitive.

List of reference symbols

2

Cell module

4

Module housing

6

cells

6a

faulty cell

8

transverse direction

10

side walls

12

sensor

14

first arrows

16

Housing

18

Evaluation unit

20

Measuring connection

22

Base electrodes

24

measuring electrode

26

dielectric layer

28

Cross connector

30

battery

31

Connection pole

32

Battery case

34

central evaluation unit

36

Measuring arrangement

38

second arrows

d

thickness

a

Distance

Claims (13)

Device for monitoring a battery (30), in particular a lithium-ion battery, to prevent thermal runaway, with the battery (30) having a plurality of cells (6) arranged next to one another within a housing (16) under a prestress in a transverse direction (8), wherein a measuring arrangement (36) is provided for detecting an expansion of at least one of the cells in the transverse direction (8), and the measuring arrangement (36) has a plurality of sensors (12) and an evaluation unit (18, 34), characterized in that the sensors (12) are each arranged between two cells (6) adjacent in the transverse direction (8) and are connected to the evaluation unit (18, 34), so that an expansion of one of the cells (6) in the transverse direction (8) is detected by the measuring arrangement (36) and monitored with regard to exceeding a critical value for the expansion, wherein the critical value indicates a potential occurrence of a thermal runaway, characterized in that a detection unit (40) is provided for detecting a current operating temperature of the battery (30) and the evaluation unit (18,34) is arranged such that the critical value is set as a function of the current operating temperature. Device according to Claim 1 , in which a respective sensor (12) and the evaluation unit (18, 34) are designed to detect a change in distance between two cells (6) in the range between 1µm and 50µm. Device according to one of the preceding claims, in which the sensor (12) has a thickness (d) in the transverse direction (8) in the range from 150 µm to 500 µm and in particular in the range from 200 µm to 400 µm. Device according to one of the preceding claims, in which a sensor (12) is arranged between each pair of cells (6). Device according to one of the preceding claims, in which the evaluation unit (18, 34) is designed to compare measured values of the sensors (12) with one another, wherein an uneven change in the measured values of the various sensors (12) is evaluated as an exceeding of the critical value. Device according to one of the preceding claims, in which a respective sensor (12) is designed as a differential capacitor with two opposite base electrodes (22) and with a measuring electrode (24) arranged between them. Device according to the preceding claim, in which the base electrodes (22) and the measuring electrode (24) are arranged parallel to one another. Device according to Claim 6 in which the base electrodes (22) and the measuring electrode (24) are arranged in a Z-shape. Device according to one of the Claims 6 until 8 , in which the base electrodes (22) are at ground potential and the measuring electrode (24) is connected to a measuring connection (20) of the evaluation unit (18). Device according to one of the Claims 6 until 9 in which an elastic dielectric layer (26) is arranged between a base electrode (22) and the measuring electrode (24). Device according to the preceding claim, wherein the dielectric layer (26) has a thickness in the range of 100µm to 200µm Device according to one of the preceding claims, in which the cells (6) are combined to form a cell module (2) and are arranged within a module housing (4), wherein the evaluation unit (18) is arranged within the module housing (4). Method for monitoring a battery (30), in particular a lithium-ion battery (30) to avoid a thermal runaway, wherein the battery (30) has a plurality of cells (6) which are arranged next to one another within a housing (16) under a prestress in a transverse direction (8), and an extension of at least one of the cells (6) in the transverse direction (8) is detected with the aid of a measuring arrangement (36), wherein for this purpose a respective distance (a) between adjacent cells is monitored, and wherein in the event of an over exceeding a critical value for the expansion, which indicates a potential occurrence of a thermal runaway, a countermeasure is initiated, characterized in that a current operating temperature of the battery (30) is detected and the critical value is set as a function of the current operating temperature.