Measurement method for detecting strain in a battery module

Abstract

The invention relates to a battery module (1) having a plurality of individual battery cells (2) that are connected using a thermally conductive adhesive to a cooled base plate (4). The battery module according to the invention is characterized in that at least one optical fiber (7) is embedded in the thermally conductive adhesive, said optical fiber being part of a fiber-optic measurement device (8) for strain measurement. The invention also relates to a measurement method for detecting the strain in the region of the connection of the individual battery cells (2) to the cooled base plate (4).

Description

[0001] Measurement method for detecting strain in a battery module

[0002] The invention relates to a measuring method for detecting the strain in the region of the connection of individual battery cells in a battery module with several individual battery cells according to the type defined in more detail in the preamble of claim 1.

[0003] Battery modules are known from the prior art. They typically consist of an arrangement—such as a stack—of individual battery cells arranged in a module housing or a battery housing. For example, these individual battery cells can be prismatic cells stacked in a stack and clamped or pressed together between end plates or pressure plates.

[0004] Especially when using such battery modules in vehicles, it is crucial to have early information about potential mechanical deterioration, which can subsequently lead to thermal or electrochemical deterioration within the individual battery cell. In this context, reference can be made to US 2018 / 0361855 A1, which shows a multitude of different sensors within a battery module. These sensors are connected to a vehicle system in such a way that information and warnings can be displayed in the vehicle.

[0005] Another system with a multitude of sensors for measuring the internal state of a battery is known from DE 102015 115 102 A1. This system can measure, for example, the terminal voltages, current, and temperature of each cell or a representative subset of cells. In addition, other properties such as the number of charge and discharge cycles and thus ultimately an estimate of the aging of the individual battery cell are described. DE 102021 006259 A1 discloses a battery module with a frame element. A cover element or a base plate is bonded to this battery module using a structural adhesive.

[0006] DE 102023 000 537 A1 describes a thermally conductive adhesive that can also be used for a battery in a vehicle. Embedded within this thermally conductive adhesive are microspheres that thermally expand at a certain temperature level, thus irreversibly reducing the thermal conductivity of the adhesive beyond this threshold temperature.

[0007] DE 102014219 720 A1 describes a battery and a method for its operation. A sensor is provided to monitor the battery, which can be used as a strain sensor and / or temperature sensor. It consists of an optical fiber with two fiber Bragg gratings at different positions along the fiber, which enable the detection of strain and / or temperature changes.

[0008] EP 4276 432 A1 discloses an optically operating temperature sensor that can be used in batteries. It comprises at least one fiber optic cable for guiding light beams. The temperature sensor uses optical fiber measurement technology with a large number of measuring points per fiber optic cable and a detection unit for multiple fiber optic cables. This is intended to measure the temperature as quasi-continuously as possible across the surface of the battery. For this purpose, optical elements arranged at a predetermined distance from one another are used, which change their distance from one another depending on the temperature due to the expansion or contraction of the fiber optic cable. Ultimately, this can also be interpreted as a fiber Bragg grating in the sense of the aforementioned prior art, but here it serves exclusively for temperature measurement.

[0009] CN 115 790 891 A1 also discloses a safety monitoring system for a lithium battery that uses an adhesive tape with integrated optical fiber measurement technology. When such battery modules are used in vehicles, they are typically so-called high-voltage batteries or HV batteries as defined in ECE 100R. In these cases, it is common practice for the individual battery cells to require cooling. A typical design involves several of the individual battery cells being connected to a cooled base plate via a thermally conductive adhesive in order to dissipate waste heat from the individual battery cells via this thermally conductive adhesive into the cooled base plate. The thermally conductive adhesive itself fulfills a variety of, sometimes competing, properties and purposes. On the one hand, it conducts heat energy; on the other, it performs mechanical tasks such as securing the individual battery cells.It also compensates for tolerances in the placement of the individual battery cells and is subjected to mechanical loads during operation, such as oscillating loads caused by vibrations while driving, thermomechanical loads, and even crash loads in the event of an accident. A crucial factor in the mechanical stress on the thermally conductive adhesive is the pulsating tensile-shear stress caused by reversible cell thickness growth, which recurs in the individual battery cells with each charging cycle. Furthermore, these pulsating tensile-shear stresses are superimposed by tensile-shear stresses that increase over time due to irreversible cell thickness growth as the individual battery cells age.

[0010] These specific mechanical and thermal properties, which the thermally conductive adhesive must fulfill, compete with each other during its development and production, ultimately making it more expensive and correspondingly heavier. For example, it must be as "tough" as possible to prevent abrupt tearing or failure under tensile and shear stress. At the same time, the required thermal properties require high thermal conductivity, which is typically achieved by adding thermally conductive particles. However, the higher the quantity of these particles, the more brittle the thermally conductive adhesive becomes after curing, thus contradicting the requirement for a tough thermally conductive adhesive.

[0011] In system design, these mechanical properties, like thermal conductivity, are directly related to the gap height of the adhesive used, i.e., the thickness of the adhesive layer between the cooled base plate and the individual battery cells. The elongation at break, or elongation capacity, increases proportionally with increasing gap height; thus, the thicker the gap, the higher the permissible shear displacement. The gap height also has a proportional effect on thermal conductivity, but in the opposite direction, so the smaller the gap and thus the thickness of the thermally conductive adhesive, the better the thermal conductivity. In addition, the thermal conductivity properties of the adhesive also depend on the contact pressure.

[0012] As the individual battery cells age, the mechanical tensile and shear stresses increase due to the aforementioned irreversible cell thickness growth on the thermally conductive adhesive. However, the need for cooling also increases, as the internal electrical resistance of the individual battery cells also increases with age. This waste heat must then be safely and reliably dissipated via the thermally conductive adhesive. If additional heat buildup occurs here, the aging process is accelerated, which in turn leads to increased mechanical stress on the thermally conductive adhesive. Especially with innovative cell technologies for lithium-ion cells that use silicon-containing anodes, cell thickness growth is even more pronounced than with conventional designs using graphite anodes.Often, just a few cycles are enough to cause detachment, at least in the area of ​​the edge cells in a cell stack, so the thermally conductive adhesive fails very quickly in this area.

[0013] The thermal cooling performance of the individual battery cells deteriorates significantly after such a failure of the thermally conductive adhesive, resulting in certain cells continuously heating up more than other individual battery cells in the battery module. This limits the performance of the entire battery module. Furthermore, this leads to inhomogeneous aging of the cells and, in the most serious cases, can even be safety-critical if extreme heating and / or severe mechanical stress occur.

[0014] Various solutions are known from the general state of the art, such as improving the mechanical properties of the thermally conductive adhesive. However, this requires accepting a reduction in thermal conductivity. In parallel, it is known, analogous to the above-mentioned state of the art, to provide temperature sensors in order to detect excessive heating. Although temperature sensors are simple and inexpensive components in themselves, with the usual number of cells in a traction battery for a vehicle it is associated with a great deal of effort and expense to provide a temperature sensor for each individual cell or small groups of a few cells, to place this temperature sensor at the appropriate location on the cell and to retrieve the measured values, for which these must be transmitted via a cable connection, for example to a battery control system or the like. This is an extremely high level of effort.

[0015] Another widely known and widely used solution involves installing additional tensioning mats or tensioning elements to absorb the pressure caused by cell thickness growth, especially irreversible cell thickness growth. These tensioning mats or tensioning elements require additional installation space and add weight to the system. They are therefore not only expensive but also reduce the energy density or volumetric energy content of the individual battery cell, which, especially in vehicles, leads to a reduction in vehicle range and therefore represents a serious disadvantage.

[0016] Since all solution concepts are correspondingly complex, time-consuming and / or disadvantageous in their implementation, it would be desirable to obtain a battery module or a measuring method that is capable of detecting a thermal breakdown of the thermally conductive adhesive, so that after the breakdown is detected via the software, the operation of the battery module can be adjusted accordingly in order to protect the individual battery cells, ensure the longest possible service life and reliably avoid safety-critical situations.

[0017] Accordingly, the object of the present invention is to provide a suitable measuring method for strain detection, which enables efficient detection of a thermal breakdown.

[0018] According to the invention, this object is achieved by a measuring method having the features in claim 1, and here in particular in the characterizing part of claim 1. Advantageous embodiments and further developments emerge from the dependent subclaims. The battery module for the measuring method according to the invention comprises a plurality of individual battery cells which are connected to a cooled base plate via a thermally conductive adhesive, as is also known from the prior art and generally customary in the construction of battery modules. Furthermore, at least one optical fiber, which is part of a fiber-optic measuring device for strain measurement, is embedded in the thermally conductive adhesive. Using such a fiber-optic measuring device for strain measurement, which is known in principle and available on the market, the strain can now be measured simply and efficiently at a plurality of discrete points within the optical fiber.With a single fiber, which according to an advantageous development is arranged between the individual battery cells and the thermally conductive adhesive and is embedded in the thermally conductive adhesive, the strain can be reliably recorded at a large number of locations, in particular in the area of ​​each of the individual battery cells.

[0019] The fiber optic measuring device uses light that is coupled into the fiber and then guided through the fiber to an evaluation optic or reflected and then guided to the evaluation optic combined with the coupling optic to measure the specific strain at individual points within the fiber optic cable. Such fiber optic measuring devices are also used, for example, in the monitoring of buildings and similar applications. For example, the frequency shift of a reference beam relative to a laser light beam traveling through the fiber optic cable is measured.

[0020] The measurement method according to the invention serves to detect the strain in the area where the individual battery cells are connected to the cooled base plate of such a battery module. According to the invention, the strain of the optical fiber is measured at several specific measuring points. According to the invention, a break in the thermally conductive adhesive at one of the measuring points is detected by a drop in the occurring strain after a previously detected maximum. This makes it possible to determine that the strain is increasing through "quasi-continuous" monitoring in the sense of the sampling rate described below. This is a sign of high mechanical stress.If it then suddenly decreases, the thermal adhesive has broken off, so that in the area of ​​this measuring point, a reduced thermal conductivity of the thermal adhesive in the direction of the cooling plate can now be assumed and this should be counteracted accordingly via the control software of the battery module in order to be able to keep the performance and service life of the battery module as high as possible through gentle operation.

[0021] The sampling rate, i.e., the time span within which each measurement is performed, can be on the order of one minute. Such a sampling rate enables high security while significantly reducing the effort required compared to higher sampling rates. This is a decisive advantage, as the available computing and storage capacities in vehicle systems are typically limited, so they particularly benefit from optimization in terms of the necessary computing capacity while maintaining the required security.

[0022] In addition to the purely mechanical strains, which can ultimately result in the thermally conductive adhesive breaking off, which is to be detected accordingly here, thermally induced strains also occur in such a system, which are superimposed on the mechanical strains. According to an exceptionally advantageous development of the measuring method according to the invention, it is therefore provided that thermally induced strains are eliminated using a compensation factor. According to a very advantageous embodiment of the measuring method, such a compensation factor can be previously determined experimentally. This allows the detected strain to be mathematically recalculated to the purely mechanical strain.

[0023] According to a very advantageous embodiment of the method according to the invention, the strain fields recorded at the measuring points can be combined to form an average strain value, which also allows saving storage and computing effort with regard to the evaluation.

[0024] For the measuring method according to the invention, according to an advantageous development, the at least one optical fiber can be encased in a tube at several discrete measuring points. The optical fiber can therefore be encased in a tube at those points where the strain is to be measured. Teflon tubes can typically be used for this purpose. They protect the fiber in this area from the thermally conductive adhesive, so that the fiber can absorb the strain occurring within the tube and thus at the desired measuring point largely independently of the adhesive itself, which would otherwise be bonded to the optical fiber. This allows the strain value to be used to reliably determine whether the thermally conductive adhesive has broken down.

[0025] As already mentioned, the optical fiber is positioned in the area of ​​the thermally conductive adhesive facing the individual battery cells. It can be connected, in particular, to the individual battery cells of the battery module, which are clamped together to form a block, by gluing the optical fiber to them. This assembly is then coated with the thermally conductive adhesive and sealed with the cooled base plate. The optical fiber is thus located directly in the transition area between the individual battery cells and the thermally conductive adhesive, allowing it to reliably detect any potential breakage at this particularly critical boundary layer.

[0026] The individual battery cells can be designed as prismatic battery cells and stacked, with the primary running direction of the at least one optical fiber aligned in the stacking direction. "Primary" in this context means that wherever the measurement is to be taken, the optical fiber runs at least partially in the stacking direction. This makes it possible to measure the strain in the particularly highly stressed direction along the stacking direction of the prismatic battery cells.

[0027] In addition, it is of course also conceivable to use it with other possible forms of individual battery cells, for example a cell module with cylindrical individual battery cells.

[0028] Further advantageous embodiments of the battery module and the measuring method according to the invention also emerge from the exemplary embodiment, which is described in more detail below with reference to the figures.

[0029] Fig. 1 shows a schematic representation of a battery module structure with the resulting movements and counterforces due to cell thickness growth;

[0030] Fig. 2 shows a schematic representation of cell thickness growth over different life phases of a battery module;

[0031] Fig. 3 is a schematic view of the battery module from below with an optical fiber of a fiber optic measuring device attached;

[0032] Fig. 4 is a schematic representation of the evaluation of the local strain due to temperature loading; and

[0033] Fig. 5 shows an exemplary evaluation of three measuring points in an aging test.

[0034] In a typical battery module structure 1, shown in Figure 1a, individual prismatic battery cells 2 are stacked and clamped together between end plates 3. This structure, also referred to as a cell block, is then bonded to a cooled base plate 4 using a layer of thermally conductive adhesive (not visible here). This structure is generally known and is commonly used in traction batteries for vehicle applications, for example, those using lithium-ion technology.

[0035] In Figure 1b, the structure of the battery module shown at the beginning of its service life in Figure 1a can be seen again, with the upper double arrow 5 representing the cell thickness growth, while the lower double arrow 6 represents the displacement of individual cells 2 relative to the cooled base plate 4. Figure 1c again schematically shows the counterforces within the battery module 1 resulting from this cell thickness growth. These displacements lead to tensile and shear loads in the area of ​​the thermally conductive adhesive, which can cause the thermally conductive adhesive to tear off in the area of ​​individual battery cells 2. The reason for this lies in the displacement of the cell block relative to the cooled base plate 4, which results from both the reversible and irreversible cell thickness growth.This cell thickness growth, which has already been explained in detail at the beginning, essentially leads to a growth of the cells in the thickness direction or stacking direction, which here runs from one end plate 3 to the other end plate 3 and is designated S in the illustration in Figure 1a. Since the end plates are comparatively flexible, the deformations and counterforces result from the outside in the direction of the center of the cell block, where, as can be seen in the schematic illustration in Figure 1c, the greatest counterforces F are present.

[0036] The individual battery cells 2 grow reversibly over the charging cycle. This reversible cell thickness growth, also known as "breathing," results in the cells being thicker in the charged state than in the discharged state. This type of cell thickness growth is almost reversible. However, as the individual battery cells 2 age, irreversible cell thickness growth also occurs, so that the individual battery cells 2 become thicker with increasing age, which overlaps with the breathing of the cells during charging and discharging.

[0037] In Figure 2, the cell thickness growth ZDW is plotted along the y-axis. The x-axis encompasses sections A, B, and C. Section A depicts the clamping of the battery module 1 between the end plates 3 during assembly. The total thickness of the cell block decreases during clamping and then increases slightly again after clamping due to the inherent elasticity of the clamping devices. The time span for section A is in the range of a few seconds.

[0038] Section B shows a time span of several hours. This shows a cycle of reversible cell thickness growth during charging. During the transition from section A to section B, a certain state of charge exists, which then increases to the maximum, with each of the individual battery cells 2 having its maximum thickness. During discharging to a minimum state of charge at the transition between sections B and C, the cell thickness then drops again. The time period covers several years and schematically shows the superposition of the reversible cell thickness growth with the age-related irreversible cell thickness growth, so that the individual charging cycles and the reversible cell thickness changes caused by them are determined by an increasing age-related increase in thickness.

[0039] As already mentioned at the beginning, the growth in cell thickness of the individual battery cells 2 and the resulting displacement of the individual battery cells 2 relative to the cooled base plate 4 leads to corresponding loads which must be absorbed by the thermally conductive adhesive. Since the counterforces towards the outside, i.e. towards the end plates 3, are lower than in the middle of the cell, the displacements of the individual cells add up accordingly, so that the displacements towards the two end plates 3 are greatest. This means that the individual battery cells 2 at the edges, in particular, can detach accordingly if the thermally conductive adhesive tears. The thermal connection to the thermally conductive adhesive is then lost. Detachment in this sense refers to microcracks which form in the thermally conductive adhesive as a result of the displacement or shear stress. These microcracks lead to a corresponding deterioration in the thermal conductivity in the composite. Such detachment orTearing off the thermally conductive adhesive causes the affected individual battery cells to continuously heat up. This increases aging and ultimately the age-related, irreversible cell thickness growth of these individual battery cells.

[0040] However, a detected tear can be cushioned accordingly via the battery module's control software. In order to detect such a tear simply and efficiently, a special measuring method is used to record a tear or an exceeding of the permissible elongation. For this purpose, an optical fiber 7, visible in the illustration in Figure 3, which shows a view from below of a stack of individual battery cells 2, is used between the thermally conductive adhesive and the individual battery cells 2 of the battery module 1. This fiber optic measuring device is glued to the cell block from below, as can be seen in the illustration in Figure 3. The fiber optic measuring device connected to the optical fiber 7 and the electronics for coupling in the (laser) light and for evaluating it are indicated by the box labeled 8.The fiber optic measuring device 8 is capable of detecting the strain at individual discrete measuring points along the length of the optical fiber 7.

[0041] The structure shown in Figure 3 is now provided with the thermally conductive adhesive, and the cooled base plate 4 is attached using this thermally conductive adhesive. To prevent the optical fiber 7 from getting stuck in the thermally conductive adhesive in the areas relevant for the measurement, small Teflon tubes are arranged around the optical fiber 7 in this area. These can be identified in the illustration in Figure 3 by the slightly thicker line and the reference number 9. The optical fiber itself can be laid in any desired manner. Since the primary cell thickness growth occurs in the stacking direction S, at least those areas that represent the measuring points within the optical fiber 7 of the fiber optic measuring device 8 should run in this stacking direction S. This is realized accordingly in the structure shown here.

[0042] With a sampling rate of approximately one minute, very accurate strain measurements can be achieved, allowing the displacements resulting from cell thickness growth to be clearly captured. Figure 4 shows the various strains at three measuring points I, II, and III when the corresponding temperature profiles are run in a test setup. The individual lines stacked in the y-direction show the strains at different temperatures at the three exemplary measuring points labeled I, II, and III.

[0043] In addition to the electrochemical / mechanical strains resulting from the cell thickness growth of the single battery cell 2, which are to be recorded here, thermally induced strains also occur superimposed on these. Therefore, it can be provided that the strain £ is adjusted from the thermally induced strains using an expansion or compensation factor a. In corresponding measurements and analyses, it has been shown that this compensation factor lies.

[0044] To reduce the effort required to store and evaluate measurement data, the measurement points, each of which represents a strain field, are reduced to a single strain. To do this, the strain fields £i(x) are averaged over the measurement point using the integral of the following equation:

[0045] In order to compensate for the temperature, this equation is supplemented by the compensation factor a mentioned above

[0046] The compensation factor a multiplied by the temperature difference can therefore be used to calculate a thermally compensated average strain to be calculated at the respective measuring point I, II, III.

[0047] In the following illustration in Figure 5, the absolute mean strain is shown at the three exemplary measuring points I, II, III £ i and the temperature-compensated mean strain e ix®wp is shown in a diagram for each of the measuring points I, II, and III. The strain is plotted on the y-axis, while the x-axis shows the number of cycles, which in this example is approximately 150 cycles across the entire width of each measuring point.

[0048] It is noticeable that the strain £ reaches a maximum after just a few cycles and then decreases accordingly. This indicates that the displacements led to a tear in the thermally conductive adhesive, and this assumption could indeed be verified by a corresponding thermal cooling test on a test bench using numerous temperature sensors.

[0049] Furthermore, it can be seen from the figures that the strains £ and thus ultimately the displacements continue to increase after the tear, i.e. the formation of microcracks, and thus intensify the problem accordingly.

[0050] It would therefore be desirable to counteract the detected rupture, which is detected as a drop in the strain £ after reaching a maximum, via the battery control software in order to ensure that the respective individual battery cells are affected as little as possible by this rupture.

Claims

Patent claims 1. Measuring method for detecting the strain in the region of the connection of individual battery cells (2) to a cooled base plate (4) of a battery module (1) with a plurality of individual battery cells (2) which are connected to the cooled base plate (4) via a thermally conductive adhesive, wherein at least one optical fiber (7) is embedded in the thermally conductive adhesive, which is part of a fiber-optic measuring device (8) for strain measurement, characterized in that the strain of the optical fiber (7) is detected at a plurality of specific measuring points (I, II, III), wherein a tear in the thermally conductive adhesive at one of the measuring points (I, II, III) is detected by a drop in the occurring strain after a previously detected maximum.

2. Measuring method according to claim 1, characterized in that a sampling rate of the order of one minute is used.

3. Measuring method according to one of claims 1 or 2, characterized in that the strain fields (E) recorded at the measuring points (I, II, III) are combined to form an average strain value ( ).

4. Measuring method according to one of claims 1, 2 or 3, characterized in that the optical fiber (7) at the several discrete measuring points (I, II, III) is tube (8) is coated.

5. Measuring method according to one of claims 1 to 4, characterized in that the measurement is carried out in a battery module (1) whose individual battery cells (2) are designed as prismatic individual battery cells and are stacked, the primary running direction of the optical fiber (7) being aligned in the stacking direction (S).

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