Measurement method for detecting strain in battery module

Abstract

The invention relates to a battery module (1) having a plurality of battery cells (2) which are connected to a cooled base plate (4) by means of a thermally conductive adhesive. 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 an optical fiber measuring means (8) for strain measurement. The invention further relates to a measuring method for detecting a strain in the region where the battery cells (2) are joined to the cooled base plate (4).

Description

[0001] The invention relates to a measuring method of the type defined in detail in the preamble of claim 1 for detecting strains in the region of battery cell connections in a battery module having a plurality of battery cells.

[0002] Battery modules are well known in the art. They typically consist of an arrangement (e.g., stack) of battery cells housed in a module housing or battery housing. For example, these battery cells can be prismatic cells stacked in a stack and clamped or pressed together between end plates or hold-down frames.

[0003] Especially when using such battery modules in vehicles, it is important to obtain early information about potential mechanical damage, which could subsequently lead to thermodynamic or electrochemical damage within the battery cells. Reference is made to US 2018 / 0361855 A1, which describes various sensors within a battery module. These sensors are connected to the vehicle systems in such a way that information and warnings can be displayed in the vehicle.

[0004] DE 10 2015 115 102 A1 discloses another system with various sensors for measuring the internal state of a battery. This system can measure, for example, the terminal voltage, current, and temperature of each cell or a representative group of cells. Other properties, such as the number of charge and discharge cycles, are also described, ultimately allowing an estimation of the aging of the battery cells.

[0005] DE 10 2021 006 259 A1 discloses a battery module having a frame member. A cover or a base plate is bonded to the battery module by means of structural adhesive.

[0006] DE 10 2023 000 537 A1 describes a thermally conductive adhesive, which can also be used for in-car batteries. Microspheres are embedded in the adhesive, which expand at a certain temperature, causing the adhesive's thermal conductivity to drop significantly and irreversibly above this critical temperature.

[0007] DE 10 2014 219 720 A1 describes a battery and a method for operating the same. To monitor the battery, a sensor is provided that can be used as a strain sensor and / or temperature sensor. It consists of an optical fiber with two fiber Bragg gratings (FBGs) at different locations on the fiber, which allow for the detection of strain and / or temperature changes.

[0008] EP 4 276 432 A1 discloses an optical temperature sensor that can be used in batteries. It includes at least one glass optical fiber for guiding light. The temperature sensor uses fiber-optic measurement technology with a large number of measuring points per glass optical fiber and a detection unit for multiple glass optical fibers. This allows the temperature to be measured as continuously and distributedly as possible over the battery surface. For this purpose, optical elements are arranged at predetermined intervals, and their spacing from one another changes depending on the temperature due to the expansion or contraction of the glass optical fiber. Ultimately, this can also be interpreted as a fiber Bragg grating in the sense of the aforementioned prior art, but here it is used solely for temperature measurement.

[0009] CN 115 790 891 A1 also discloses a lithium battery safety monitoring system using an adhesive tape with integrated fiber optic measurement technology.

[0010] When these battery modules are used in vehicles, they are typically classified as so-called high-voltage batteries, or HV batteries, according to the definition of ECE 100R. Cooling of the battery cells is often required. A typical design provides for multiple battery cells to be connected to a cooled base plate via thermally conductive adhesive to dissipate the waste heat generated by the cells via the adhesive to the cooled base plate. The thermally conductive adhesive itself fulfills several different, and sometimes conflicting, properties and objectives. On the one hand, it conducts thermal energy; on the other, it performs mechanical tasks, such as securing the battery cells. It also compensates for cell placement tolerances and withstands mechanical loads during use, such as alternating loads caused by vibrations during driving, thermomechanical loads, and crash loads in accidents. A key mechanical load for the thermally conductive adhesive is the cyclical tensile-shear loads caused by the reversible cell thickness increase, which reappear in the battery cell with each charging cycle. These cyclical tensile-shear loads are also superimposed on the tensile-shear loads that increase over time due to the irreversible cell thickness increase during cell aging.

[0011] Meeting these specific mechanical and thermodynamic properties of thermally conductive adhesives requires trade-offs in their development and production, ultimately making them expensive and correspondingly heavier. For example, they must be as "tough" as possible to prevent sudden fracture or failure under tensile-shear loads. At the same time, the required thermodynamic properties require high thermal conductivity, which is typically achieved by incorporating thermally conductive particles. However, the higher the particle content, the more brittle the adhesive becomes after hardening, which is completely contrary to the requirement for a tough thermally conductive adhesive.

[0012] In this system design, mechanical properties and thermal conductivity are directly related to the gap height of the glue used (i.e., the thickness of the glue layer between the cooled base plate and the battery cells). The elongation at break or strain capacity increases proportionally with increasing gap height: the larger the gap, the greater the permissible shear displacement. Gap height also plays a proportional role in thermal conductivity, but in the opposite direction: the smaller the gap, the thinner the thermally conductive glue, and the better the thermal conductivity. Furthermore, the thermal conductivity of the glue is also dependent on the contact pressure.

[0013] As the battery cells age, the mechanical tensile-shear load on the thermal adhesive gradually increases due to the aforementioned irreversible increase in cell thickness. At the same time, the cooling demand increases, because the internal resistance of the battery cells also increases with aging. This waste heat must be discharged safely and reliably through the thermal adhesive. If additional heat accumulates here, the aging process will be accelerated, which in turn causes the thermal adhesive to be subjected to more mechanical loads. In particular, in the new lithium-ion battery technology using silicon-containing anodes, the increase in cell thickness is more pronounced than in traditional graphite anode structures. In this case, usually only a few cycles are sufficient to cause detachment of at least the edge cell area in the battery stack, that is, the thermal adhesive quickly fails in this area.

[0014] After this type of failure of the thermally conductive adhesive, the thermodynamic cooling performance of the corresponding battery cells deteriorates significantly, causing some cells to be continuously heated more severely than others within the battery module. This limits the performance of the entire battery module. Furthermore, this leads to uneven aging of the cells, which in the worst case could endanger safety if subjected to extreme heating and / or severe mechanical loads.

[0015] Various solutions are known from the general prior art, such as improving the mechanical properties of thermally conductive adhesives, but this requires a reduction in thermal conductivity.

[0016] At the same time, similar to the above-mentioned prior art, it is known to provide temperature sensors to detect excessive heating. Although temperature sensors themselves are simple and inexpensive components, they are still associated with high costs. After all, for the number of cells typically found in vehicle traction batteries, providing a temperature sensor for each cell or a small number of cell groups, placing it at a suitable location on the cell and connecting it to a battery control unit or similar device via a cable for reading the measured values involves considerable expense.

[0017] Another commonly known and used solution provides for the use of additional clamping pads or clamping elements to absorb the pressure generated by the increased cell thickness (especially irreversible cell thickness increase). These clamping pads or clamping elements require structural space and add additional weight to the system. They are therefore not only expensive but also reduce the energy density or volumetric energy content of the battery cell, which, particularly in vehicles, results in reduced range and is therefore a serious disadvantage.

[0018] Since all solutions are correspondingly complex, laborious and / or disadvantageous in implementation, a battery module or a measuring method is desired that is capable of detecting thermal detachment of the thermally conductive adhesive so that, after identification of the detachment, the operation of the battery module can be adjusted accordingly via software in order to protect the battery cells, ensure the longest possible service life and reliably avoid safety hazards.

[0019] Accordingly, the object of the present invention is to specify a measuring method suitable for strain detection which allows efficient detection of thermal detachment.

[0020] According to the invention, this object is achieved by a measuring method having the features of claim 1, in particular the features of the characterizing part of claim 1. Advantageous refinements and developments are derived from the dependent claims.

[0021] The battery module used for the measurement method according to the present invention comprises multiple battery cells connected to a cooled base plate via thermally conductive adhesive, as is known in the prior art and is commonly found in battery module designs. Furthermore, at least one optical fiber is embedded in the thermally conductive adhesive as part of a fiber-optic strain measurement mechanism. This fiber-optic strain measurement mechanism, which is generally known and commercially available, allows for simple and efficient measurement of strain at multiple discrete points within the optical fiber. According to an advantageous refinement, a single optical fiber, arranged between the battery cell and the thermally conductive adhesive and embedded in the adhesive, allows for reliable strain detection at multiple locations, particularly within the region of each individual battery cell.

[0022] Fiber-optic measurement systems utilize light input into an optical fiber. This light is then guided through the continuous fiber to an evaluation optical component, or reflected and guided to an evaluation optical component combined with the input optical component, to detect specific strains at specific locations within the optical fiber. Fiber-optic measurement systems of this type are also used, for example, for monitoring buildings. Here, for example, the frequency shift of a reference beam relative to a laser beam traveling through the optical fiber is measured.

[0023] The measuring method according to the invention is used to detect strain in the area of the connection between a battery cell and the cooled base plate of such a battery module. According to the invention, the strain of the optical fiber is detected at a plurality of discrete measuring points. According to the invention, the detachment of the thermally conductive adhesive at one of these measuring points is detected by a drop in strain following a previously measured maximum value. This "quasi-continuous" detection, in the sense of the sampling rate described below, allows the detection of an increase in strain. This is a sign of high mechanical load. If the strain then suddenly drops, the thermally conductive adhesive has detached, so it can now be assumed that the thermal conductivity of the thermally conductive adhesive in the area of the measuring point towards the cooling plate has decreased. In this case, the battery module control software should take appropriate countermeasures in order to continue to maintain the highest possible battery module performance and service life through careful operation.

[0024] The sampling rate (i.e., the time interval between each measurement) can be in the order of one minute. This sampling rate allows for high safety on the one hand and significantly reduces costs compared to higher sampling rates on the other hand, which is extremely advantageous because the computing and storage capacity available in vehicle systems is generally limited, so they can particularly benefit from optimizing the necessary computing power on the one hand and maintaining the required safety on the other.

[0025] In addition to pure mechanical strains, which can ultimately lead to the thermally conductive adhesive detaching (necessitating corresponding measurement), these systems also experience thermally induced strains that are superimposed on the mechanical strains. Therefore, a particularly advantageous refinement of the measurement method according to the present invention provides for the use of a compensation factor to eliminate the thermally induced strains. According to a particularly advantageous embodiment of the measurement method, this compensation factor can be determined experimentally. The measured strains can then be converted back to mechanical strains by calculation.

[0026] According to a very advantageous embodiment of the method according to the invention, the strain fields measured at the measuring points can be combined to form a mean strain value, thereby again saving storage and computing costs associated with the evaluation.

[0027] According to an advantageous refinement of the measurement method according to the present invention, the at least one optical fiber can be sheathed with a tube at a plurality of discrete measuring points. In other words, the optical fiber can be sheathed with a tube at the location where the strain is to be detected. Teflon tubes are typically used for this purpose. These protect the fiber from the thermally conductive adhesive in this area, allowing the fiber to withstand the strains occurring within the tube, and thus at the desired measuring points, largely independently of the adhesive that would otherwise adhere the optical fiber. This allows reliable inferences about the detachment of the thermally conductive adhesive to be drawn from the strain values.

[0028] As already explained, the optical fiber is arranged in the area of the thermally conductive adhesive facing the battery cells. Specifically, it can be connected to the battery cells of the battery module, which are fastened together in groups, by gluing the optical fiber to them. This arrangement is then coated with thermally conductive adhesive and sealed with a cooled base plate. The optical fiber is thus located directly in the transition area between the battery cells and the thermally conductive adhesive, enabling reliable detection of potential detachment at this particularly critical interface.

[0029] The battery cells can be designed as prismatic cells and stacked, with the main course of the at least one optical fiber oriented in the stacking direction. This means that the optical fiber extends at least partially in the stacking direction where the measurement is required. This allows strain measurements to be acquired in the highly loaded direction, i.e., the stacking direction of the prismatic cells.

[0030] Furthermore, use in other possible forms of battery cells is of course also conceivable, ie, for example, use in cell modules having cylindrical battery cells.

[0031] Further advantageous embodiments of the battery module and the measuring method according to the invention can also be obtained according to the exemplary embodiments described in detail below with reference to the accompanying drawings, wherein:

[0032] Figure 1 Schematic diagram of the battery module structure together with the motion and reaction forces caused by the increase in cell thickness;

[0033] Figure 2 It is a schematic diagram of the increase in thickness of a single cell in a battery module at different stages of its life cycle;

[0034] Figure 3 is a bottom-up schematic diagram of a battery module together with the optical fiber of an attached optical fiber measurement mechanism;

[0035] Figure 4 is a schematic diagram of the local strain evaluation caused by temperature load;

[0036] Figure 5 Exemplary evaluation results of three measurement points in the aging test are shown.

[0037] exist Figure 1 In the typical structure of a battery module 1, shown in Figure a, several prismatic battery cells 2 are stacked and clamped together between end plates 3. This structure, also known as a cell pack, is then bonded to a cooled base plate 4 via a layer of thermally conductive adhesive (not visible here). This structure is well known in the industry and is frequently used in traction batteries for vehicle applications, such as those using lithium-ion technology.

[0038] Now in Figure 1 In the diagram of b, we can see again that Figure 1The structure of the battery module shown in a at the beginning of its service life, wherein the upper double arrow 5 indicates the increase in cell thickness, while the lower double arrow 6 indicates the displacement of the individual cells 2 relative to the cooled base plate 4. Figure 1 The reaction force caused by the increase in cell thickness inside the battery module 1 is schematically shown again in c. This displacement now results in tensile and shear loads in the thermal adhesive area, which may cause the thermal adhesive in the area of some battery cells 2 to detach. This is due to the displacement of the cell group relative to the cooled base plate 4, which is caused by the reversible and irreversible increase in cell thickness. The increase in cell thickness, which has been explained in detail in the introduction, mainly leads to an increase in the battery in the thickness direction or stacking direction, that is, from one end plate 3 to the other end plate 3, and Figure 1 In the diagram of a, it is marked with S. Since the end plates are relatively easy to deform, the deformation and reaction force are generated from the outside toward the center of the single cell group, as shown in Figure 1 As shown in the schematic diagram of c, the maximum reaction force F appears at the center of the single cell group.

[0039] Here, the battery cell 2 reversibly grows during the charging cycle. This reversible cell thickness increase, also known as "breathing," causes the cell to be thicker in the charged state than in the discharged state. This type of cell thickness change is almost completely reversible. However, as the battery cell 2 ages, irreversible cell thickness increase occurs. Therefore, the battery cell 2 increases in thickness with aging, which is compounded by the cell "breathing" phenomenon during the charge and discharge processes.

[0040] exist Figure 2 In the diagram, the y-axis represents the cell thickness growth ZDW. The x-axis consists of three segments, A, B, and C. Segment A illustrates the clamping of the battery module 1 between the end plates 3 during installation. The overall thickness of the cell stack decreases during the clamping process and then recovers slightly afterward due to the inherent elasticity of the clamping tool. Segment A spans a few seconds.

[0041] Segment B shows a time span of several hours. This corresponds to a cycle of reversible cell thickness increase during charging. During the transition from segment A to segment B, a certain charge level is present, followed by a maximum charge level, where each battery cell 2 reaches its maximum thickness. Upon discharge to a minimum charge level in the transition region between segments B and C, the cell thickness decreases accordingly. This time span spans several years and schematically illustrates the combined effect of reversible cell thickness increase and irreversible cell thickness increase caused by aging. Therefore, each charging cycle and the resulting changes in reversible cell thickness are determined by the gradual thickening trend caused by aging.

[0042] As explained in the introduction, the increased cell thickness of the battery cells 2 and the resulting displacement of the cells 2 relative to the cooled base plate 4 generate corresponding loads that must be borne by the thermally conductive adhesive. Since the outward reaction force (i.e., toward the end plates 3) is smaller than the reaction force at the center of the cell, the cell displacements are cumulative at these locations, resulting in the greatest displacement toward the two end plates 3. Therefore, when the thermally conductive adhesive cracks, the battery cells 2 located at the edges are most likely to detach. At this point, the thermal adhesive's thermal connection properties are lost. "Detachment" in this sense refers to the formation of microcracks in the thermal adhesive due to the displacement or shear stress. These microcracks lead to a corresponding decrease in thermal conductivity within the assembly. This detachment or cracking of the thermally conductive adhesive now results in a continuously increasing heating of the affected battery cells. Consequently, the aging of these battery cells is accelerated, and ultimately, the irreversible increase in cell thickness caused by aging is also exacerbated.

[0043] However, once the detachment is detected, the control software of the battery module can be adjusted accordingly to alleviate this problem. In order to be able to simply and efficiently identify this detachment, a special measurement method is used to detect detachment or exceeding the permissible strain. For this purpose, Figure 3 The figure shows a view from below of a stack of battery cells 2, wherein an optical fiber 7 is used between the thermally conductive adhesive and the battery cells 2 of the battery module 1. It is glued to the cell stack from below, for example. Figure 3 The fiber-optic measuring device connected to the optical fiber 7 and the electronics for the (laser) light input and its evaluation are indicated by the box denoted by 8. The fiber-optic measuring device 8 can detect strains at several discrete measuring points along the entire length of the optical fiber 7.

[0044] Figure 3 The structure shown is now provided with a heat-conducting adhesive, and the cooled base plate 4 is fixed by means of the heat-conducting adhesive. In order to prevent the optical fiber 7 from being fixed by the heat-conducting adhesive in the measurement-relevant areas, Teflon tubes are arranged in these areas surrounding the optical fiber 7. Figure 3 This can be indicated in the diagram by slightly thicker lines and reference numeral 9. The optical fibers themselves can be laid in any desired manner. Since the main increase in cell thickness occurs in the stacking direction S, at least those areas of the optical fibers 7 that serve as measuring points for the fiber-optic measuring device 8 should extend in the stacking direction S. This is achieved accordingly in the configuration shown here.

[0045] With a sampling rate of the order of one minute, very good strain measurements can be obtained, which allow a good understanding of the displacement caused by the increase in cell thickness. Figure 4In the diagram, different strains can be seen at three measuring points I, II, and III when a corresponding temperature profile test is performed in a test setup. The lines stacked in the y-direction here show the strains at three exemplary measuring points labeled I, II, and III at different temperatures.

[0046] In addition to the electrochemical / mechanical strains that need to be detected due to the increase in cell thickness of the battery cell 2, there are also superimposed thermal strains. Therefore, it can be provided that the strain ε is separated from the thermal strains by means of an expansion coefficient or compensation factor α. In this case, relevant measurements and analyses have shown that the compensation factor is

[0047]

[0048] In order to reduce the storage and evaluation costs of the measurement data, the measurement points representing a strain field are simplified to a strain value. To this end, the strain field ε1(x) is averaged over the measurement points by integrating the following equation:

[0049]

[0050] In order to now compensate for temperature influences, the equation is supplemented with the above-mentioned compensation factor α:

[0051]

[0052] Therefore, the compensation coefficient α multiplied by the temperature difference can be used to calculate the average strain after temperature compensation at each measuring point I, II, and III

[0053] In the subsequent Figure 5 In the figure, the absolute average strain is now shown at the three exemplary measuring points I, II, III with the corresponding curve diagrams of the respective measuring points I, II, III. and the average strain after temperature compensation Here, the y-axis represents the strain and the x-axis represents the number of cycles, for example, the total width of each measuring point here is approximately 150 cycles.

[0054] Here, we can see that the strain ε reaches its maximum after just a few cycles and then decreases accordingly. This suggests that displacement has already caused the thermal adhesive to detach, a hypothesis verified by thermodynamic cooling tests conducted on a test bench using a large number of temperature sensors. The figures also show that the strain ε and the resulting displacement (i.e., microcrack formation) continue to increase after detachment, further exacerbating the problem.

[0055] Therefore, after a detachment phenomenon is detected (identified by a decrease in strain ε after reaching a maximum value), it is ideal to make corresponding adjustments via the software of the battery control device to ensure that each battery cell is adversely affected as little as possible by the detachment phenomenon.

Claims

1. A measuring method for detecting strains in the region of a battery cell (2) joined to a cooled base plate (4) of a battery module (1), the battery module (1) comprising a plurality of battery cells (2) connected to the cooled base plate (4) by means of a thermally conductive adhesive, in, At least one optical fiber (7) is embedded in the thermally conductive adhesive, wherein the optical fiber is part of an optical fiber measuring mechanism (8) for strain measurement. The invention is characterized in that the strain of the optical fiber (7) is detected at a plurality of discrete measuring points (I, II, III), wherein the detachment of the thermally conductive adhesive at one of the measuring points (I, II, III) is detected by a decrease in the strain after a previously determined maximum value.

2. The measuring method according to claim 1, wherein: A sampling rate of the order of one minute is used.

3. The measuring method according to claim 1 or 2, wherein The strain field (ε) measured at the measuring points (I, II, III) is summed up into an average strain value 4. The measuring method according to claim 1, 2 or 3, characterized in that: The optical fiber (7) is sheathed by a tube (8) at the plurality of discrete measuring points (I, II, III).

5. The measuring method according to any one of claims 1 to 4, characterized in that: The measurement is carried out in a battery module (1) whose battery cells (2) are designed as prismatic battery cells and are stacked, wherein the main course of the optical fiber (7) is oriented in the stacking direction (S).

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