Accelerated fatigue evaluation device for welded joints of automotive battery module frames

Abstract

In one example, an accelerated fatigue evaluation device for module frame welds includes a module frame having a top plate and end frames welded to the upper surface and both end surfaces of the ∪-frame, respectively, and a test module frame in which the bottom surface of the ∪-frame, which is not welded, is open, a center jig connected to support the front and rear surfaces of the test module frame, a guide block jig supporting the end frames of the test module frame from below, a piston that enters the test module frame and repeatedly applies a load to the inner surface of the top plate, at least one battery cell interposed between the piston and the inner surface of the top plate, and a displacement sensor that measures deformation at the center of the outer surface of the top plate.

Description

Technical Field

[0001] The present invention relates to an accelerated fatigue evaluation apparatus for a welded portion of a module frame, which can obtain reliable results for accelerated fatigue failure of the welded portion by applying a repeated load simulating the swelling behavior of battery cells to the welded portion of an automotive battery module frame.

[0002] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0180259 filed on December 21, 2022, and all the contents disclosed in the document of the Korean Patent Application are included as part of this specification.

Background Art

[0003] Unlike primary batteries, secondary batteries can be recharged and have been extensively researched and developed in recent years due to the possibility of miniaturization and large capacity. With the increasing technological development and demand for mobile devices, as well as the emergence of electric vehicles and energy storage systems in line with the contemporary requirements of environmental protection, the demand for secondary batteries as an energy source has been increasing even more rapidly.

[0004] When a large capacity of secondary batteries is required, such as in electric vehicles, a necessary capacity condition is satisfied by arranging a large number of battery cells in a structure assembled through series and / or parallel connections. A module frame is provided as a case for housing a large number of secondary batteries, and a large number of battery cells are aligned and stacked in a storage space formed in a hexahedron shape inside the module frame.

[0005] Although it is common for a module frame to be manufactured by welding several plates, an example of such a module frame is shown in FIG. 1. The module frame is completed by welding end frames to the left and right of a U-frame bent in a U-shape, and welding a top plate to the remaining one surface forming the upper surface of the module frame.

[0006] Multiple battery cells housed within a module frame experience a swelling phenomenon in their electrode assemblies during repeated charging and discharging cycles. This swelling, caused by the repeated expansion and contraction of the electrode assemblies during charging and discharging, leads to localized expansion of the battery cells themselves. This expansion of the battery cells then places continuous pressure on the module frame.

[0007] Since the module frame must maintain its durability for the entire lifespan of the battery module, it is essential to evaluate the structural stability of the module frame in the early stages of battery module design. However, in practice, evaluating the structural stability of the module frame while repeatedly charging and discharging battery cells takes a long time, so simulated experiments are conducted to shorten the test time.

[0008] In the module frame shown in Figure 1, the area of ​​greatest concern regarding fatigue failure is the welded region of the top plate. When properly stacked battery cells expand, the force due to their deformation acts in the vertical direction. However, since the underside of the module frame is a single, integrated folded structure, the welded region of the top plate is relatively structurally weak. Therefore, it is anticipated that fatigue failure due to repeated loading may cause cracks to form in the welded region of the top plate, propagate along the weld, and cause the top plate to break. We will proceed with evaluation to address this possibility.

[0009] Traditionally, testing has been conducted by repeatedly applying a constant load to the top plate using piston hydraulics. However, in order to simulate the characteristic that the central part of a battery cell expands the most, the piston has a convex shape with a gently protruding central region. We discovered that the size and pattern of this convex shape make a big difference in the experimental results.

[0010] Furthermore, although the hydraulic pressure applied to the piston followed the load conditions at the end of life (EOL), it was difficult to obtain reliable results regarding the actual lifespan of the modular frame because testing was conducted under harsh conditions from the beginning. [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] The present invention aims to provide an accelerated fatigue evaluation device that can obtain reliable results for accelerated fatigue fracture of welded joints by applying a repeated load that simulates the swelling behavior of battery cells housed in a module frame under conditions similar to those in actual operation.

[0012] However, the technical problems that the present invention aims to solve are not limited to those described above, and other problems not mentioned can be clearly understood by an ordinary person of the art from the description of the invention below. [Means for solving the problem]

[0013] The present invention relates to an accelerated fatigue evaluation device for a module frame weld, and in one example, includes a module frame in which a top plate and end frames are welded to the top surface and both end surfaces of a U-frame, respectively, a test module frame in which the bottom surface of the U-frame that is not welded is left open, a center fixture connected to support the front and rear surfaces of the test module frame, a guide block fixture that supports the end frames of the test module frame from below, a piston that enters the test module frame and repeatedly applies a load to the inner surface of the top plate, at least one battery cell interposed between the piston and the inner surface of the top plate, and a displacement sensor that measures the deformation at the center of the outer surface of the top plate.

[0014] In one embodiment of the present invention, the battery cell may have the same specifications as the battery cell mounted on the module frame.

[0015] For example, the battery cell may be a pouch cell, and it is preferable that the pouch cell is interposed between the piston and the inner surface of the top plate in a fully discharged state.

[0016] Furthermore, it is preferable that the guide block jig is supported from below so as not to be located at the welded portion of the end frame.

[0017] On the other hand, the repeated load applied by the piston to the top plate via the battery cell can be applied as a profile that simulates the swelling expansion and contraction displacements that occur as the charge-discharge cycle of the battery cell progresses.

[0018] For example, the repeated load applied by the piston to the top plate via the battery cell may be applied as a load-cycle profile, which converts the swelling expansion and contraction displacements corresponding to the progression of the charge-discharge cycle of the battery cell into a load.

[0019] The above load-cycle profile may exhibit a pattern where the load during charging and the load during discharging increase as the cycle progresses.

[0020] Furthermore, the above load-cycle profile may exhibit a characteristic where the difference between the load during charging and the load during discharging increases as the cycle progresses.

[0021] Furthermore, if the deformation amount of the top plate measured by the displacement sensor deviates from a linear increasing pattern and increases rapidly instantaneously, it can be determined that the welded portion of the top plate has suffered fatigue failure, and at this point, the repeated application of the load can be stopped. [Effects of the Invention]

[0022] The accelerated fatigue evaluation device for module frame welds of the present invention, having the above configuration, simulates the constraint conditions of a module frame mounted on a vehicle using a center jig and a guide block jig, while the piston repeatedly applies a load to the inner surface of the top plate, thereby simulating the pressurizing conditions under which battery cells exhibit swollen behavior inside the module frame. This allows for a reliable evaluation of the accelerated fatigue of the module frame welds.

[0023] Also, the repeated load of the piston acts on the welded portion of the top plate through the battery cell interposed between the inner surfaces of the top plates. As a result, it becomes possible to approach conditions similar to those where the battery cell applies force to the top plate under actual conditions, and it is possible to prevent a deviation in the evaluation result due to the shape of the piston.

[0024] Then, by analyzing the actual swelling behavior of the battery cell according to the charge / discharge cycle and applying it as the cycle of the repeated load applied to the piston, it is possible to deviate from the overestimation due to applying the load conditions from the initial stage to the end-of-life stage of the test, and it becomes possible to derive more realistic and reliable test results.

[0025] However, the technical effects that can be obtained by the present invention are not limited to the above-described effects, and other effects not mentioned can be clearly understood by those skilled in the art from the description of the invention described below.

[0026] The following drawings attached to this specification illustrate preferred embodiments of the present invention and serve to further understand the technical idea of the present invention together with the detailed description of the invention to be described later. Therefore, the present invention should not be construed as being limited only to the matters described in such drawings.

Brief Description of the Drawings

[0027] [Figure 1] It is a drawing showing an example of an automotive battery module frame. [Figure 2] It is a drawing showing a configuration in which a center jig and a guide block jig are installed on a test module frame. [Figure 3] It is a drawing showing a state in which a center jig and a guide block jig are installed on a test module frame. [Figure 4] It is a drawing showing an accelerated fatigue evaluation device for a module frame welded portion including the test module frame of FIG. 2. [Figure 5]This diagram shows an example of the swelling behavior of a battery cell in response to charge and discharge cycles. [Figure 6] This diagram shows an example of converting the swelling behavior in Figure 5 into a load-cycle profile. [Figure 7] Figure 4 shows an example of fatigue test results using an accelerated fatigue evaluation device for the welded section of a module frame. [Modes for carrying out the invention]

[0028] The present invention can be modified in various ways and may have a variety of embodiments; therefore, specific embodiments will be described in detail below.

[0029] However, this is not intended to limit the present invention to any particular embodiment, but rather should be understood to include all modifications, equivalents, or substitutions that fall within the spirit and technical scope of the present invention.

[0030] In the present invention, terms such as "includes" and "have" are intended to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof as described in the specification, and do not preemptively exclude the presence or possibility of adding one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0031] Furthermore, in this invention, when a part such as a layer, film, region, or plate is described as being "on top" of another part, this includes not only the case where it is "directly on top" of the other part, but also the case where another part is located in between. Conversely, when a part such as a layer, film, region, or plate is described as being "below" another part, this includes not only the case where it is "directly below" the other part, but also the case where another part is located in between. Also, in this application, being "on top" may include being located not only at the top but also at the bottom.

[0032] The present invention relates to an accelerated fatigue evaluation device for a module frame weld, and in one example, to a module frame in which a top plate and end frames are welded to the top surface and both end surfaces of a U-frame, respectively, the device includes a test module frame in which the bottom surface of the U-frame that is not welded is left open, a center fixture connected to support the front and rear surfaces of the test module frame, a guide block fixture that forms the end frame of the test module frame, a piston that enters the test module frame and repeatedly applies a load to the inner surface of the top plate, at least one battery cell interposed between the piston and the inner surface of the top plate, and a displacement sensor that measures the deformation at the center of the outer surface of the top plate.

[0033] The accelerated fatigue evaluation device for module frame welds of the present invention, having the above configuration, simulates the constraint conditions of a module frame mounted on a vehicle using a center jig and a guide block jig, while the piston repeatedly applies a load to the inner surface of the top plate, thereby simulating the pressurizing conditions under which battery cells exhibit swollen behavior inside the module frame. This allows for a reliable evaluation of the accelerated fatigue of the module frame welds.

[0034] Furthermore, the repeated load of the piston acts on the welded portion of the top plate via the battery cell interposed between the inner surfaces of the top plate. This allows the system to more closely approximate the conditions under which the battery cell applies force to the top plate in real-world conditions, preventing deviations in evaluation results due to the piston's shape.

[0035] Specific embodiments of the present invention will be described in detail below with reference to the attached drawings. For reference, the forward / backward and up / down / left / right directions used to specify relative positions in the following description are for the purpose of aiding the understanding of the invention, and unless otherwise defined, the directions shown in the drawings are used as the reference.

[0036] (First Embodiment) The accelerated fatigue evaluation device 10 for a module frame weld according to the present invention (hereinafter simply referred to as "accelerated fatigue evaluation device for a weld") includes a test module frame 100', a center jig 200 for supporting it, a guide block jig 300, a hydraulically operated piston 400, a battery cell 500, and a displacement sensor 600.

[0037] The test module frame 100', provided for fatigue failure of the welded joint, is made by processing the module frame 100 shown in Figure 1. The module frame 100 is completed by welding end frames 120 to the left and right sides of a U-shaped bent U-frame 110, and welding a top plate 130 to the remaining surface that forms the top surface of the module frame 100. Here, the area of ​​greatest concern for fatigue failure in the module frame 100 is the welded area of ​​the top plate 130, so the test module frame 100' is prepared by cutting and leaving open the bottom surface of the U-frame 110 that is not welded.

[0038] The test module frame 100' is mounted on a test surface plate (not shown) by a centering fixture 200 and a guide block fixture 300. In particular, it is preferable that the test module frame 100' be mounted as a restraining structure that simulates the state in which the actual module frame 100 is mounted on a vehicle.

[0039] Looking at the module frame 100 in Figure 1, mounting brackets 122 are provided on the end frames 120 at both ends. Therefore, it is possible to install it on a test platen by utilizing the mounting brackets 122 on the end frames 120. However, in an actual vehicle, the module frame 100 is installed lying down with its front and rear surfaces facing vertically, so installing it vertically using the mounting brackets 122 does not match the actual situation. Furthermore, if we proceed with accelerated fatigue evaluation to shorten the test time by significantly reducing the time taken per cycle, there is a high risk that the test will fail because the mounting brackets 122 will deform and the load on the piston 400 will act unevenly.

[0040] Taking these circumstances into consideration, as shown in Figure 2, the present invention is configured to support the front and rear surfaces of the test module frame 100' using a center jig 200, while supporting the end frame 120 of the test module frame 100' from below as a guide block jig 300.

[0041] Here, if the center fixture 200, which acts as the four legs supporting the test module frame 100', is directly attached to the front and rear surfaces of the test module frame 100', the force acting on the test module frame 100' (reaction force against piston load) is concentrated in a narrow area, causing deformation of the U-frame 110, which could affect the weld strength of the top plate 130. This does not align with the purpose of simulating fatigue failure of the weld due to the swelling behavior of the battery cell 500, as would occur in a real-world situation.

[0042] Taking these points into consideration, the center jig 200 is connected to the front and rear surfaces of the test module frame 100' via a reinforcing plate 210. The reinforcing plate 210 covers an area equivalent to more than 60% of the front and rear surface area of ​​the test module frame 100' and is attached to the front and rear surfaces of the test module frame 100' by numerous evenly spaced bolts 240 and nuts 250. As shown in Figure 2, the reinforcing plate 210 and the U-frame 110 are firmly joined together by fastening them with bolts 240 and nuts 250 to numerous through holes 220 formed on the front and rear surfaces of the U-frame 110, respectively.

[0043] Corresponding fastening holes 230 are formed in the center jig 200 and the reinforcing plate 210, and the support structure for the center jig 200 is created by fastening the center jig 200 and the reinforcing plate 210 together with bolts 240 via the fastening holes 230. In this way, the center jig 200 supports the front and rear surfaces of the U-frame 110 via the reinforcing plate 210, so that the repeated load of the piston 400 is fully applied to the top plate 130.

[0044] Then, to suitably distribute and bear the force acting on the center jig 200, the end frame 120 of the test module frame 100' is supported from below as a guide block jig 300. Here, as shown in Figure 3, the guide block jig 300 supports the edge of the end frame 120 from below so as not to be located on the weld of the end frame 120. Since the weld of the end frame 120 and the weld of the top plate 130 are attached to each other on the upper surface of the test module frame 100', if the guide block jig 300 were located on the weld of the end frame 120, it would consequently support a portion of the weld of the top plate 130. In this case, the guide block jig 300 would affect the fatigue failure of the weld of the top plate 130 due to the swelling behavior of the battery cell 500, so to avoid this, the position of the guide block jig 300 is set so as to support a portion of the outer edge of the weld of the end frame 120 from below.

[0045] Here, the reference for the top and bottom surfaces is the module frame 100 in Figure 1. As a result, the test module frame 100' is installed upside down, as shown in Figures 2 and 3, but the surface of the top plate 130 to which the repeated load of the piston 400 is applied is referred to as the top surface.

[0046] Then, the piston 400 connected to the hydraulic cylinder 410 enters the interior through the bottom surface of the open test module frame 100' and repeatedly applies a load to the inner surface of the top plate 130. In this case, the accelerated fatigue evaluation device 10 of the welded joint of the present invention has at least one battery cell 500 interposed between the piston 400 and the inner surface of the top plate 130.

[0047] The repeated load of the piston 400 acts on the welded portion of the top plate 130 via the battery cell 500 interposed between the inner surfaces of the top plate 130, thereby allowing the battery cell 500, which has actually expanded due to the swelling phenomenon, to approach the top plate 130 under conditions similar to those under which force is applied. Furthermore, the intervention of the battery cell 500 in the load transmission path of the piston 400 prevents deviations in the evaluation results due to the shape of the piston 400.

[0048] In this regard, in one embodiment of the present invention, it is preferable that the battery cells 500 placed inside the test module frame 100' are of the same specifications as the battery cells 500 mounted on the actual module frame 100. This is because it is possible to evaluate the accelerated fatigue of the weld under conditions closer to actual conditions, and the size of the actual battery cells 500 accurately matches the test module frame 100'. The number of battery cells 500 used in the test can be suitably selected, and generally, a number that is slightly less than half the height of the test module frame 100' can be used.

[0049] For example, the battery cell 500 used in accelerated fatigue testing may be a pouch cell. Since the pouch cell is housed in a pouch made of a flexible laminate sheet, the repeated load of the piston 400 will act evenly on the welded portion of the top plate 130. In this case, it is preferable that the pouch cell is interposed between the piston 400 and the inner surface of the top plate 130 in a completely discharged state. This is because, in the event of electrolyte leakage from the pouch cell due to the load applied by the piston 400, the risk of fire must be considered.

[0050] Furthermore, the accelerated fatigue evaluation device 10 of the welded joint according to the present invention includes a displacement sensor 600 that measures the deformation at the center of the outer surface of the top plate 130. The sensing part 610 of the displacement sensor 600 is in contact with the center of the outer surface of the top plate 130, and the amount of deformation of the top plate 130 is measured via the displacement of the sensing part 610. Generally, if the precision of the displacement sensor 600 is at a level of about 1 / 100 mm, the amount of deformation of the top plate 130 can be determined with sufficient accuracy.

[0051] The control unit 700 controls the hydraulic cylinder 410 so that the repeated load on the piston 400 is applied according to a defined strategy, while also receiving measurement signals from the displacement sensor 600 to observe and store the deformation of the top plate 130 in real time. The control unit 700 can be implemented using a conventional computing device.

[0052] (Second Embodiment) In the first embodiment of the present invention, the hardware configuration of the accelerated fatigue evaluation device 10 for welded joints will be described in detail, and below, it will be explained how it is preferable to set the repeated load applied to the piston 400.

[0053] The object of the present invention is to obtain reliable results regarding accelerated fatigue fracture of welded joints by applying a repeated load that simulates the swelling behavior of the battery cells 500 housed in the module frame 100 under conditions similar to those in actual testing. Therefore, it is preferable that the repeated load applied by the piston 400 to the welded joint of the top plate 130 simulates the deformation of the battery cells 500 associated with the actual swelling behavior of the battery cells 500.

[0054] Figure 5 is a diagram showing an example of the swelling behavior of battery cell 500 according to the charge-discharge cycle. In the graph of Figure 5, the solid line at the top shows the change in the total width (mm) of the battery cell 500 module when the battery cell 500 is fully charged (SOC 98.3%), as the charge-discharge cycle progresses, while the dashed line at the bottom shows the change in the total width (mm) of the battery cell 500 module when it is completely discharged (SOC 0%), as the charge-discharge cycle progresses.

[0055] A battery cell 500 module refers to multiple battery cells 500 housed within a single module frame 100, and can be understood as the total width change when the swelling behavior of individual battery cells 500 is extended to the module level. Roughly speaking, the total width change of a battery cell 500 module tends to increase as the charge-discharge cycle progresses.

[0056] Therefore, the present invention makes it possible to obtain more reliable accelerated fatigue test results by configuring the piston 400 to apply a repeated load to the top plate 130 via the battery cell 500 as a profile that simulates the swelling expansion and contraction displacements corresponding to the progression of the charge-discharge cycle of the battery cell 500 as shown in Figure 5. In other words, the repeated load of the piston 400 corresponds to the change in the total width of the battery cell 500 module in Figure 5, and by repeatedly increasing and decreasing the load while moving back and forth between two change curves as the cycles accumulate, the charge-discharge cycle is simulated to be as close to reality as possible.

[0057] However, in actual fatigue failure tests, it is simpler to control the load than to control the displacement, so the repeated load on the piston 400 can be applied as a load-cycle profile that converts the swelling expansion and contraction displacements corresponding to the progression of the charge-discharge cycle of the battery cell 500 into load. An example of such a load-cycle profile for repeated load on the piston 400 is shown in Figure 6.

[0058] As can be seen in Figure 6, the load-cycle profile shows that the load during charging and the load during discharging both increase as the cycle progresses. Furthermore, it can be confirmed that the difference between the load during charging and the load during discharging in the load-cycle profile can also increase as the cycle progresses. Here, the time required for one cycle can be suitably selected considering the trade-off relationship between shortening the overall test time as an accelerated fatigue test and the risk of errors in the test results due to a large difference from the actual swelling behavior if cycles are repeated at short intervals. For example, the load can be repeatedly applied for a time of 20 seconds (0.05 Hz) per cycle.

[0059] Figure 7 is a graph showing the deformation (mm) of the top plate 130 measured by the displacement sensor 600 during the accelerated fatigue evaluation process described above. The deformation generally shows a linear increase when the load during charging and the load during discharging are applied as the cycle progresses, but after a certain number of cycles, it deviates from the linear increase and the deformation increases instantaneously and rapidly in a step-like manner. At this point, it can be determined that the welded joint of the top plate 130 has suffered fatigue failure, and the test can be stopped. By converting the number of cycles at which the welded joint failed, derived as a result of the accelerated fatigue test, into the actual charge-discharge cycle time, the fatigue failure life of the module frame 100 can be predicted.

[0060] Thus, the accelerated fatigue evaluation device 10 for welded joints according to the present invention analyzes the actual swelling behavior of the battery cell 500 in accordance with the charge-discharge cycle and applies it as the cycle of repeated load applied to the piston 400. This allows for a departure from overestimation caused by applying load conditions from the beginning to the end of the test life, and enables the deriving of more realistic and reliable test results.

[0061] The present invention has been described in more detail above through the drawings and embodiments. However, the configurations described in the drawings or embodiments described herein are merely one embodiment of the present invention and do not represent the entire technical concept of the present invention. Therefore, there may be various equivalents and modifications that can substitute for them at the time of filing. [Explanation of Symbols]

[0062] 10: Accelerated fatigue evaluation device for welded joints 100: Module Frame 100': Test module frame 110: U-frame 120: End frame 122: Mounting bracket 130: Top plate 200: Centering jig 210: Reinforced plate 220: Through Hole 230: Fastening Hole 240: Bolt 250: Nut 300: Guide block jig 400: Piston 410: Hydraulic cylinder 500: Battery cell 600: Displacement sensor 610: Sensing Unit 700: Control Unit

Claims

1. A module frame in which a top plate and end frames are welded to the top and both end faces of a U-frame, respectively, is compared to a test module frame in which the bottom face of the U-frame, which is not welded, is left open. A centering fixture connected to support the front and rear surfaces of the aforementioned test module frame, A guide block jig that supports the end frame of the test module frame from below, A piston that enters the aforementioned test module frame and repeatedly applies a load to the inner surface of the top plate, At least one battery cell interposed between the piston and the inner surface of the top plate, The system includes a displacement sensor that measures deformation at the center of the outer surface of the top plate, The aforementioned guide block jig supports the edge of the end frame from below so as not to be located in the welded portion of the end frame, and is an accelerated fatigue evaluation device for module frame welds.

2. The aforementioned battery cell is The accelerated fatigue evaluation device for a module frame weld according to claim 1, wherein the battery cell mounted on the module frame has the same specifications as the battery cell.

3. The accelerated fatigue evaluation apparatus for a module frame weld, according to claim 2, wherein the battery cell is a pouch cell.

4. The accelerated fatigue evaluation apparatus for a module frame weld according to claim 3, wherein the pouch cell is in a fully discharged state.

5. The repeated load applied by the piston to the top plate via the battery cell is, The accelerated fatigue evaluation apparatus for a module frame weld according to any one of claims 1 to 4, which applies a profile that simulates the swelling expansion and contraction displacements corresponding to the progression of the charge-discharge cycle of the battery cell.

6. The repeated load applied by the piston to the top plate via the battery cell is, The accelerated fatigue evaluation device for a module frame weld according to claim 5, wherein the load-cycle profile is applied, which converts the swelling expansion and contraction displacements corresponding to the progression of the charge-discharge cycle of the battery cell into a load.

7. The aforementioned load-cycle profile is The accelerated fatigue evaluation apparatus for a module frame weld according to claim 6, wherein the load during charging and the load during discharging increase as the cycle progresses.

8. The aforementioned load-cycle profile is The accelerated fatigue evaluation apparatus for a module frame weld according to claim 7, wherein the difference between the load during charging and the load during discharging increases as the cycle progresses.

9. The accelerated fatigue evaluation device for a module frame weld according to claim 8, wherein the device determines that the weld of the top plate has fatigue-fractured in a cycle in which the amount of deformation of the top plate measured by the displacement sensor deviates from a linear increasing pattern and increases rapidly instantaneously, and interrupts the application of the repeated load.

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