Battery pack

Abstract

Disclosed in the present application is a battery pack, comprising a plurality of batteries. Each battery comprises a housing, a cover plate, terminal posts, conductive busbars and a battery cell; the housing is provided with an opening; the cover plate covers the opening, and the cover plate is provided with through holes; each terminal post is at least partially provided within a through hole; each conductive busbar is electrically connected to the side of a terminal post facing away from the battery cell; the battery cell comprises a battery cell body and tabs; the battery cell body is provided within the housing; each tab comprises a first connecting portion, a fusing portion and a second connecting portion that are sequentially connected, the cross-sectional areas of the first connecting portion and the second connecting portion being both greater than the cross-sectional area of the fusing portion, the first connecting portion being connected to the battery cell body, and the second connecting portion being connected to the end of a terminal post located inside the housing; the minimum cross-sectional area of the fusing portion is S1, the surface area of each conductive busbar is S2, and the thickness of each conductive busbar is T1, where 9600≤S1*S2*T1≤320000. The present application enables the fusing portion to fuse at a preset current threshold, thereby ensuring the safety of the battery pack.

Description

battery pack

[0001] Related applications

[0002] This application claims priority to Chinese Patent Application No. CN202411785016.8, filed on December 6, 2024, entitled "Battery Pack", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of battery technology, and in particular to a battery pack. Background Technology

[0004] Since the commercialization of lithium-ion power batteries, they have been rapidly and widely used in various electrical devices due to their many advantages, such as light weight, high voltage, small size, no memory effect, high specific energy, long cycle life and environmental friendliness.

[0005] To ensure battery safety, conventional batteries have a fuse at the tab. In the event of a short circuit, the short circuit current is greater than the fuse's breaking current, causing the fuse to melt and disconnect from the battery cell.

[0006] However, in the battery pack, since the tabs are connected to the terminals and the heat sink is connected to the terminals, the heat at the tabs will be transferred to the heat sink. Under the heat dissipation of the heat sink, the fuse may not melt at the originally set current threshold, thus affecting the safety of the battery pack. Summary of the Invention

[0007] The purpose of this application is to provide a battery pack that ensures the fuse of the electrode tab can more accurately melt at the originally set current threshold, thereby ensuring safety.

[0008] To address the aforementioned technical problems, this application provides the following technical solutions:

[0009] A battery pack includes multiple batteries, each battery comprising:

[0010] The shell has an opening;

[0011] A cover plate is placed over the opening, and a through hole is provided on the cover plate;

[0012] The electrode post is at least partially disposed within the through hole;

[0013] The battery cell includes a battery cell body and electrode tabs. The battery cell body is located inside the housing. The electrode tabs include a first connecting part, a fuse part, and a second connecting part connected in sequence. The cross-sectional areas of the first connecting part and the second connecting part are both larger than the cross-sectional area of ​​the fuse part. The first connecting part is connected to the battery cell body, and the second connecting part is connected to the end of the electrode post located inside the housing.

[0014] The conductive busbar is electrically connected to the side of the electrode that faces away from the battery cell.

[0015] The minimum cross-sectional area of ​​the fuse is S1, the surface area of ​​the busbar is S2, and the thickness of the busbar is T1, where 9600≤S1*S2*T1≤320000.

[0016] Compared with the prior art, the battery pack of this application has the following advantages:

[0017] In this application, the smaller the minimum cross-sectional area of ​​the fuse portion of the electrode tab, the greater its resistance and the greater the heat generated under the same overcurrent. Therefore, the minimum cross-sectional area of ​​the fuse portion of the electrode tab determines the fusing current threshold of the fuse portion. The first connecting part of the electrode tab is connected to the main body of the battery cell, the second connecting part of the electrode tab is welded to the end of the electrode post located inside the housing, and the busbar is connected to the end of the electrode post located outside the housing. Therefore, the heat of the main body of the battery cell can be transferred to the busbar through the electrode tab. Since the busbar has a heat dissipation function and can absorb the heat at the fuse portion of the electrode tab, the fuse portion may not melt within the originally set current threshold. At the same time, the larger the volume of the busbar, the greater the heat absorbed by the busbar at the fuse portion. Generally, the material, specific heat capacity, and density of the busbar are fixed, so the heat that the busbar can absorb mainly depends on the volume of the busbar. Therefore, in order to ensure that the fuse portion melts within the set current threshold, the minimum cross-sectional area of ​​the fuse portion is larger when the volume of the busbar is larger. The smaller the cross-sectional area, the better to compensate for the increased maximum current that the fuse can withstand due to heat dissipation from the conductive busbar. Therefore, the minimum cross-sectional area of ​​the fuse is set as S1, the surface area of ​​the conductive busbar as S2, and the thickness of the conductive busbar as T1. Where 9600≤S1*S2*T1≤320000, when the value of S1*S2*T1 is greater than 320000, it indicates that the conductive busbar absorbs a large amount of heat, and the minimum cross-sectional area of ​​the fuse is also large, which may cause the fuse to melt outside the set current threshold. When the value of S1*S2*T1 is less than 9600, it indicates that the conductive busbar absorbs a small amount of heat, and the minimum cross-sectional area of ​​the fuse is also small, which may cause the fuse to melt easily before reaching the set current threshold, and the melting time may not meet the requirements. Therefore, keeping S1*S2*T1 between 9600 and 320000 allows the fuse to melt within the originally set current threshold, thus ensuring the safety of the battery pack. Attached Figure Description

[0018] Figure 1 is a schematic diagram of the battery structure according to an embodiment of this application;

[0019] Figure 2 is a schematic diagram of one embodiment of the battery cell, cover plate and conductive busbar of this application;

[0020] Figure 3 is another schematic diagram of the battery cell, cover plate and conductive bus of an embodiment of this application;

[0021] Figure 4 is a front view of the battery cell according to an embodiment of this application;

[0022] Figure 5 is a side view of the battery cell according to an embodiment of this application;

[0023] Figure 6 is a schematic diagram showing the relationship between the battery cell body, tabs, terminals and busbars in an embodiment of this application.

[0024] Figure 7 is a schematic diagram showing the dimensions of one of the battery cell body, tab, terminal post and conductor bus according to an embodiment of this application.

[0025] Figure 8 is another dimensional schematic diagram of the battery cell body, tabs, terminals and busbars in an embodiment of this application;

[0026] Figure 9 is a top view of the conductive bus according to an embodiment of this application;

[0027] Figure 10 is a front view of the conductive bus according to an embodiment of this application;

[0028] Figure 11 is a schematic diagram showing the relationship between the flexible circuit board and the conductive busbar in an embodiment of this application;

[0029] Figure 12 is a schematic diagram showing the relationship between the insulating film and the tab in an embodiment of this application.

[0030] In the diagram, 100 is the casing; 200 is the cover plate; 300 is the first solder mark; 400 is the second solder mark; 500 is the third solder mark; 1 is the main body of the battery cell; 2 is the electrode tab; 21 is the first connection part; 22 is the fuse part; 23 is the second connection part; 3 is the pole; 4 is the conductive busbar; 41 is the buffer groove; 5 is the flexible circuit board; 51 is the circuit trace; 52 is the fuse; 53 is the cantilever part; 6 is the temperature acquisition unit; 7 is the conductive connecting piece; and 8 is the insulating film. Detailed Implementation

[0031] The specific embodiments of this application will be described in further detail below with reference to the accompanying drawings and examples. The following examples are used to illustrate this application, but are not intended to limit the scope of this application.

[0032] In the description of this application, it should be understood that the term "comprising" as used in this specification means the presence of features, integers, steps, operations, parts, and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, parts, components, and / or groups thereof. It should be understood that when we say a part is "connected" to another part, it can be directly connected to the other part, or there may be intermediate parts. The term "and / or" as used herein includes all or any unit and all combinations of one or more associated listed items.

[0033] As shown in Figures 1 to 12, this application relates to a battery pack, which includes multiple batteries. Each battery includes: a casing 100, a cover plate 200, a terminal post 3, a conductive busbar 4, and a battery cell.

[0034] The housing 100 has an opening; a cover plate 200 is placed over the opening and has a through hole; the pole post 3 is at least partially disposed in the through hole; and the conductive bus 4 is electrically connected to the side of the pole post 3 that is away from the battery cell.

[0035] The battery cell includes a cell body 1 and a tab 2. The cell body 1 is disposed inside a housing 100. The tab 2 includes a first connecting part 21, a fuse part 22, and a second connecting part 23 connected in sequence. The cross-sectional areas of the first connecting part 21 and the second connecting part 23 are both larger than the cross-sectional area of ​​the fuse part 22. The first connecting part 21 is connected to the cell body 1, and the second connecting part 23 is connected to the end of the electrode post 3 located inside the housing 100. The minimum cross-sectional area of ​​the fuse part is S1, where S1 = [4, 20] mm. 2 S1 can be 4mm or 6mm 2 8mm 2 10mm 2 12mm 2 14mm 2 16mm, 18mm or 20mm 2 The surface area of ​​the busbar is S2, where S2 = [1200, 4000] mm. 2 S2 can be 1200mm 2 1600mm 2 2000mm 2 2400mm 2 2800mm 2 3200mm 2 3600mm 2 Or 4000mm 2 The thickness of the conductive busbar is T1, T1 = [2,4] mm. T1 can be 2 mm, 2.5 mm, 3 mm, 3.5 mm or 4 mm, where 9600 ≤ S1 * S2 * T1 ≤ 320000.

[0036] In this application, the smaller the minimum cross-sectional area of ​​the fuse portion 22 of the tab 2, the greater its resistance and the greater the heat generated under the same overcurrent. Therefore, the minimum cross-sectional area of ​​the fuse portion 22 of the tab 2 determines the fusing current threshold of the fuse portion 22. The first connecting portion 21 of the tab 2 is connected to the cell body 1, and the second connecting portion 23 of the tab 2 is welded to the end of the pole 3 located inside the housing 100. The conductive bus 4 is connected to the end of the pole 3 located outside the housing 100. Therefore, the heat of the cell body 1 can be transferred to the conductive bus 4 through the tab 2. Because the conductive busbar 4 has a heat dissipation function and can absorb heat from the fuse portion 22 of the tab 2, the fuse portion 22 may not melt within the originally set current threshold. Furthermore, the larger the volume of the conductive busbar 4, the more heat it absorbs from the fuse portion 22. Since the material, specific heat capacity, and density of the conductive busbar 4 are generally fixed, the amount of heat it can absorb mainly depends on its volume. Therefore, to ensure that the fuse portion 22 melts within the set current threshold, the volume of the conductive busbar 4... When the current is larger, the minimum cross-sectional area of ​​the fuse 22 should be smaller to compensate for the error caused by the increased maximum current that the fuse 22 can withstand due to heat dissipation from the busbar 4. Therefore, the minimum cross-sectional area of ​​the fuse 22 is set as S1, the surface area of ​​the busbar is set as S2, and the thickness of the busbar is set as T1, where 9600≤S1*S2*T1≤320000. When the value of S1*S2*T1 is greater than 320000, it indicates that the heat absorption of the busbar 4 is large, and the minimum cross-sectional area of ​​the fuse 22 is also large. This causes the fuse 22 to melt outside the set current threshold. When the value of S1*S2*T1 is less than 9600, it indicates that the heat absorption of the conductive busbar 4 is small and the minimum cross-sectional area of ​​the fuse 22 is also small. This makes it easy for the fuse 22 to melt before reaching the set current threshold, and the melting time does not meet the requirements. Therefore, keeping S1*S2*T1 between 9600 and 320000 allows the fuse 22 to melt within the original set current threshold, thereby ensuring the safety of the battery pack.

[0037] In this embodiment, the distance between the fuse part 22 and the conductive busbar 4 on the path of heat transfer from the battery cell to the conductive busbar 4 is L1, wherein 6mm≤L1≤18mm, and L1 can be 6mm, 8mm, 10mm, 12mm, 14mm, 16mm or 18mm.

[0038] When the fuse part 22 is too close to the conductive busbar 4, the heat transfer path between them is short, resulting in the conductive busbar 4 absorbing more heat from the fuse part 22 and achieving higher heat dissipation efficiency. Furthermore, when the conductive busbar 4 vibrates, it is easy to transfer the energy of the vibration to the fuse part 22, making the fuse part 22 prone to breakage. When the fuse part 22 is too far from the conductive busbar 4, the distance between the fuse part 22 and the battery cell body 1 becomes even closer, and the heat from the fuse part 22 is likely to damage the battery cell body 1. Therefore, keeping L1 between 6-18mm can ensure the stability of the fuse part 22's own structure and avoid affecting the battery cell body 1.

[0039] In this embodiment, the minimum cross-sectional area of ​​the conductive busbar 4 is S3, where S3 = [60, 150] mm. 2 S3 can be 60mm 2 80mm 2 100mm 2 120mm 2 140mm 2 Or 150mm 2 Where 3≤S3 / S1≤37.5.

[0040] When S3 / S1 is less than 3, that is, the ratio of the minimum cross-sectional area of ​​the conductive busbar 4 to the minimum cross-sectional area of ​​the fuse part 22 is too small, the conductive busbar 4 is likely to melt before the fuse part 22, so that the fuse part 22 cannot play its role and the battery pack cannot be protected.

[0041] In this embodiment, the conductive busbar 4 and the terminal post 3 are welded together to form a first solder mark 300. The area of ​​the first solder mark 300 is S4, where S4 = [60, 150] mm. 2 S4 can be 60mm 2 80mm 2 100mm 2 120mm 2 140mm 2 Or 150mm 2 , where 240≤S4*S1≤3000.

[0042] The conductive busbar 4 and the terminal post 3 are welded together, forming a first weld mark 300 between them. Since heat transfer is easier at the welded location of the conductive busbar 4 and the terminal post 3, a larger area of ​​the first weld mark 300 results in higher heat absorption efficiency for the conductive busbar 4. Therefore, the minimum cross-sectional area of ​​the fuse 22 should be designed to be as small as possible; otherwise, the fuse 22 will not easily melt at the set current threshold. Thus, keeping S4*S1 between 240-3000 ensures that the fuse 22 melts at the originally set current threshold, guaranteeing the safety of the battery pack.

[0043] In this embodiment, the contact area between the conductive busbar 4 and the terminal post 3 is S5, where S5 = [100, 500] mm. 2 The S5 can be 100mm 2 150mm 2 200mm 2 250mm 2 300mm 2 350mm 2 400mm 2 450mm 2 Or 500mm 2 Where 400≤S5*S1≤10000.

[0044] Since both the contact surfaces of the conductive busbar 4 and the electrode post 3 conduct heat, and the larger the contact area between the two, the larger the cross-sectional area of ​​the heat transfer path and the higher the heat transfer efficiency, in order to ensure that the fuse part 22 melts at the originally set current threshold, the minimum cross-sectional area of ​​the fuse part 22 should be designed to be smaller. Therefore, S5*S1 is kept between 400-10000 to ensure the safety of the battery pack.

[0045] In this embodiment, the conductive busbar 4 is connected to multiple terminals 3. A buffer groove 41 is provided on the conductive busbar 4 in the area between two adjacent terminals 3. The width of the buffer groove 41 is W1, and the depth of the buffer groove 41 is D, where W1*D=[6,12]mm. 2 , 0.3≤(W1*D) / S1≤3.

[0046] By providing a buffer groove 41 on the conductive busbar 4, and placing the buffer groove 41 between two adjacent terminals 3 (i.e., between two adjacent cells), a buffering effect can be achieved when the cell vibrates. This reduces the vibration transmitted to the tab 2, making the fuse 22 less susceptible to vibration and thus less likely to break. Since the smaller the minimum cross-sectional area of ​​the fuse 22, the more fragile it is, the greater the buffering capacity required. Furthermore, the buffering capacity of the buffer groove 41 is related to its width and depth; the greater the width and depth, the stronger the buffering capacity. Therefore, (W1*D) / S1 is kept between 0.3 and 3 to ensure that the fuse 22 is not easily broken.

[0047] In this embodiment, the battery pack further includes a flexible circuit board 5 and a temperature acquisition unit 6 disposed on the conductive busbar 4. The temperature acquisition unit 6 is electrically connected to the flexible circuit board 5. On the path of heat transfer from the battery cell to the temperature acquisition unit 6, the distance between the temperature acquisition unit 6 and the fuse part 22 is L2, wherein 6.5mm≤L2≤20mm. L2 can be 6.5mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm or 20mm.

[0048] By setting the temperature acquisition unit 6, the temperature of the conductive busbar 4 can be acquired. Furthermore, on the path of heat transfer from the cell to the temperature acquisition unit 6, the distance between the temperature acquisition unit 6 and the fuse part 22 is 6.5mm≤L2≤20mm, so that the temperature acquisition unit 6 is closer to the fuse part 22. This allows the temperature monitored by the temperature acquisition unit 6 to be closer to the temperature of the fuse part 22, facilitating timely control by the battery management system. The temperature acquisition unit 6 can be a thermistor sensor.

[0049] Preferably, on the path of heat transfer from the battery cell to the temperature acquisition unit 6, the distance between the temperature acquisition unit 6 and the electrode 3 is L3, wherein 0.5mm≤L3≤12mm. L3 can be 0.5mm, 2mm, 4mm, 6mm, 8mm, 10mm or 12mm.

[0050] Along the path of heat transfer from the battery cell to the temperature acquisition unit 6, the distance between the temperature acquisition unit 6 and the electrode 3 should be 0.5mm≤L3≤12mm to avoid the temperature acquisition unit 6 and the electrode 3 being too close, so that the conductor bus 4 and the electrode 3 are less likely to interfere when assembled, and also to avoid the temperature acquisition unit 6 and the electrode 3 being too far apart, which would affect the accuracy of temperature acquisition.

[0051] Furthermore, the battery pack also includes a conductive connecting piece 7, which is connected to the conductive busbar 4. A circuit trace 51 is provided on the flexible circuit board 5, and the conductive connecting piece 7 is connected to the circuit trace 51. A fuse 52 is provided on the circuit trace 51. Along the path of heat transfer from the battery cell to the flexible circuit board 5, the distance between the fuse 52 and the fusible link 22 is L4, where 6.5mm ≤ L4 ≤ 20mm. L4 can be 6.5mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm, or 20mm.

[0052] Since the heat from the fuse 22 can be transferred through the electrode 3, conductive busbar 4, conductive connecting piece 7, and circuit trace 51 to the flexible circuit board 5, if the fuse 52 is too close to the fuse 22 in the heat transfer path from the battery cell to the flexible circuit board 5, the fuse 52 on the flexible circuit board 5 is likely to melt first due to heat. If the fuse 52 is too far from the fuse 22, the heat transfer path between the fuse 22 and the fuse 52 is too long, and more heat is lost from the fuse 22 in the heat transfer path, which can easily affect the melting of the fuse 52. Therefore, the distance L4 between the fuse 52 and the fuse 22 should be maintained between 6.5-20mm to ensure the normal use of the fuse 52. The conductive connecting piece 7 can be a nickel sheet.

[0053] Furthermore, the flexible circuit board 5 extends outward with a cantilever portion 53, and the circuit trace 51 extends to the cantilever portion 53. One end of the conductive connecting piece 7 is attached to the conductive busbar 4, and the other end is disposed on the cantilever portion 53 and connected to the circuit trace 51 on the cantilever portion 53. The fuse 52 is disposed on the circuit trace 51 of the cantilever portion 53.

[0054] That is, by placing the fuse 52 in the cantilever portion 53, the fuse 52 can avoid contact with the heated conductor 4 and the terminal 3, thereby minimizing the heat transfer to the fuse 52.

[0055] In this embodiment, the number of battery cell bodies 1 is n, and the conductive busbar 4 is connected to the pole post 3 corresponding to the n battery cell bodies 1, where n=[2,400], 24≤((S2*T1) / n)*S1≤160000.

[0056] Since the conductive busbar 4 is electrically connected to the terminals 3 corresponding to the n battery cell bodies 1, one conductive busbar 4 absorbs the heat transferred by the fuse part 22 of the tabs 2 corresponding to the n battery cell bodies 1 at the same time. Therefore, the volume of the conductive busbar 4 can be divided into multiple parts according to the number of battery cell bodies 1. Therefore, in order to ensure that the fuse part 22 can melt under the originally set current threshold, it is necessary to make 24≤S1*(V1 / n)≤160000.

[0057] In one embodiment, the battery cell is a wound battery cell, and the tab 2 includes a plurality of tab pieces stacked together. Each tab piece includes a first sub-connection portion, a sub-fuse portion, and a second sub-connection portion connected in sequence. All the first sub-connection portions of the tab 2 are stacked together to form a first connection portion 21, and all the second sub-connection portions of the tab 2 are stacked together to form a second connection portion 23. All the sub-fuse portions of the tab 2 are welded together to form a fuse portion 22, and a second solder mark 400 is formed on the side of the tab 2.

[0058] Since tab 2 comprises multiple tabs stacked together, each tab includes a first sub-connection portion, a sub-fuse portion, and a second sub-connection portion connected in sequence. The first sub-connection portion is connected to the cell body 1, and the second sub-connection portion is welded to the terminal post 3. The second sub-connection portions of multiple tabs are welded together and then welded to the terminal post 3. Furthermore, the multiple sub-fuse portions of the multiple tabs are welded together. Therefore, when a set current threshold is reached, the multiple sub-fuse portions can ensure simultaneous melting, thus successfully cutting off the electrical connection between the cell body 1 and the terminal post 3. If the sub-fuse portions of the multiple tabs are separated, when the fuse 22 melts, some sub-fuse portions may not melt. The melted sub-fuse portions may re-attach to the unmelted tabs, meaning the electrical connection between the cell body 1 and the terminal post 3 is not severed, and the fuse 22 cannot provide the high-current protection function.

[0059] In one embodiment, portions of all the sub-fuse portions of the tab 2 are welded together, and the area of ​​the second weld mark 400 is smaller than the area of ​​the side surface of the fuse portion 22. This also prevents the problem of overlapping and adhesion between the fused and unfused sub-fuse portions.

[0060] In another embodiment, all areas of the sub-fuse portions of the tab 2 are welded together, and the area of ​​the second weld mark 400 is equal to the area of ​​the side surface of the fuse portion 22. This also prevents the problem of overlapping and adhesion between the fused and unfused sub-fuse portions.

[0061] In other embodiments, all areas of the sub-fuse portions of the tab 2 are welded together, the area of ​​the second weld mark 400 is larger than the area of ​​the side of the fuse portion 22, and the second weld mark 400 completely covers the fuse portion 22. This also prevents the problem of overlapping and adhesion between the fused and unfused sub-fuse portions.

[0062] Specifically, the area between the first sub-connecting part and the second sub-connecting part of the electrode tab can be welded together first, and then the area of ​​welding can be cut or punched to reduce the cross-sectional area of ​​the welding area, thereby forming the fused part 22 at the position with the smallest cross-sectional area, so as to improve the assembly efficiency and avoid the need to cut multiple electrode tabs individually.

[0063] Preferably, a second solder mark 400 is formed on the side of the outermost tab. Along the heat transfer path from the battery cell to the busbar 4, the distance between the end of the second solder mark 400 near the second sub-connection and the end of the sub-fuse portion of the tab forming the second solder mark 400 near the second sub-connection is L5, where 1mm ≤ L5 ≤ 8mm. L5 can be 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, or 8mm.

[0064] If the distance between the end of the second solder mark 400 near the second sub-connection and the end of the sub-fuse portion of the tab forming the second solder mark 400 near the second sub-connection is too large, it indicates that the second solder mark 400 is too close to the solder mark between the tab 2 and the post 3. Assembly tolerances may cause overlap between the two solder marks, resulting in double welding. The strength at the point of double welding will decrease, making the tab 2 prone to breakage. Furthermore, it will also cause a deviation in the maximum current threshold of the fuse portion 22. If the distance between the end of the second sub-connection portion of electrode 2 near the second sub-connection portion and the end of the sub-fuse portion of the electrode tab forming the second weld mark 400 near the second sub-connection portion is too small, the multiple sub-fuse portions of the multiple electrode tabs far from the welding equipment area may not be completely welded together, resulting in partial separation. Consequently, when the multiple sub-fuse portions of the multiple electrode tabs reach the originally set current threshold and melt, the multiple sub-fuse portions may still stick together. This means that the electrical connection between the cell body 1 and the terminal post 3 is not severed, and the high-current protection function of the fuse portion 22 cannot be achieved. Therefore, maintaining the distance L5 between the end of the second weld mark 400 near the second sub-connection portion and the end of the sub-fuse portion of the electrode tab forming the second weld mark 400 near the second sub-connection portion is beneficial to ensuring the safety of the battery pack.

[0065] Furthermore, a second solder mark 400 is formed on the side of the outermost tab. Along the path of heat transfer from the cell to the busbar 4, the distance between the end of the second solder mark 400 near the first sub-connection and the end of the sub-fuse portion of the tab forming the second solder mark 400 near the first sub-connection is L6, where 1mm ≤ L6 ≤ 8mm. L6 can be 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm, or 8mm.

[0066] If the distance between the end of the second solder mark 400 near the first sub-connection and the end of the sub-fuse portion of the tab forming the second solder mark 400 near the first sub-connection is too large, it indicates that the second solder mark 400 is too close to the cell body 1. This is equivalent to the high temperature generated by welding when the sub-fuse portions of multiple tabs are welded together, which may burn the cell body 1, causing electrical performance loss or thermal safety risk to the cell body 1. If the distance between the end of the second solder mark 400 near the first sub-connection and the end of the sub-fuse portion of the tab forming the second solder mark 400 near the first sub-connection is too small, it may cause the multiple sub-fuse portions of the multiple tabs far from the welding equipment area of ​​the tab 2 to not be completely welded together, resulting in partial separation. As a result, when the multiple sub-fuse portions of the multiple tabs reach the originally set current threshold and melt, the multiple sub-fuse portions may still stick together. This is equivalent to the cell body 1 and the pole 3 not being cut off from the electrical connection, and the high current protection function of the fuse portion 22 cannot be achieved. Therefore, keeping the distance L6 between the end of the second solder mark 400 near the first sub-connection portion and the end of the sub-fuse portion of the tab forming the second solder mark 400 near the first sub-connection portion helps to ensure the safety of the battery pack.

[0067] The second connecting part 23 is welded to the end of the pole post 3 located inside the housing 100, forming a third solder mark 500. On the path of heat transfer from the cell to the conductive busbar 4, the distance between the second solder mark 400 and the third solder mark 500 is L7, where 1mm≤L7≤5mm. L7 can be 1mm, 2mm, 3mm, 4mm or 5mm.

[0068] That is, the third solder mark 500 formed by connecting the second connecting part 23 of the tab 2 and the pole post 3 is kept at a distance from the second solder mark 400 on the tab 2. This makes the multiple tab pieces of the tab 2 located between the second solder mark 400 and the third solder mark 500 dispersed, which can play a buffering role to ensure that vibration is not easily transmitted to the fuse part 22 of the tab 2 through the pole post 3, making the fuse part 22 less likely to break. Moreover, the distance L7 between the second solder mark 400 and the third solder mark 500 is kept between 1-5mm to prevent the third solder mark 500 from being too close to the second solder mark 400, so as to provide a buffering effect. In addition, the second solder mark 400 is not too close to the cell body 1 to avoid the temperature during welding from affecting the cell body 1.

[0069] Preferably, on the path of heat transfer from the battery cell to the conductive busbar 4, the distance between the end of the fuse part 22 away from the first connection part 21 and the end of the fuse part 22 close to the first connection part 21 is L8, L8 = [2,8] mm, and L8 can be 2 mm, 3 mm, 5 mm, 7 mm or 8 mm, wherein 0.4 ≤ L8 / L7 ≤ 8.

[0070] On the heat transfer path from the tab 2 to the conductive busbar 4, if the ratio of L8, the distance between the end of the fusible link 22 furthest from the first connecting part 21 and the end of the fusible link 22 closest to the first connecting part 21, to L7, the distance between the second solder mark 400 and the third solder mark 500, is too large, it indicates that the length of the fusible link 22 is too large, making it prone to breakage due to vibration. Conversely, if the distance between L8 and L7 is too small, it indicates that the length of the fusible link 22 is too small, and the distance between the second solder mark 400 and the third solder mark 500 is too large, causing the fusible link 22 to fail to meet the fusing time requirements. Therefore, maintaining L8 / L7 between 0.4 and 8 ensures the smooth operation of the fusible link 22.

[0071] In other embodiments, the second connecting portion 23 is welded to the end of the pole post 3 located inside the housing 100 to form a third weld mark 500, and the second weld mark 400 and the third weld mark 500 are connected together.

[0072] That is, a strip is welded directly onto the tab 2, and a long strip is welded when welding the tab 2 and the pole post 3. Then, the area to be welded is cut to form the fused part 22, which can improve the efficiency of assembly.

[0073] In other embodiments, the battery cell is a stacked battery cell, and the electrode 2 includes multiple electrode pieces stacked together. Each electrode piece includes a first sub-connection portion, a sub-fuse portion, and a second sub-connection portion connected in sequence, and the sub-fuse portions of the multiple electrode pieces are spaced apart from each other.

[0074] Since the multiple tabs of the laminated cell are individually connected to the multiple electrode plates within the cell body 1, the sub-fuse parts of the multiple tabs can be set separately, allowing the tabs on the problematic electrode plate to be individually melted.

[0075] In this embodiment, the second connecting part 23 is welded to the end of the pole post 3 located inside the housing 100 to form a third weld mark 500. On the path of heat transfer from the cell to the conductive busbar 4, the distance between the fused part 22 and the third weld mark 500 is L9, where 1mm≤L9≤10mm. L9 can be 1mm, 2mm, 4mm, 6mm, 8mm or 10mm.

[0076] On the heat transfer path from the tab 2 to the conductive busbar 4, if the distance between the fused portion 22 and the third solder mark 500 is too small, the heat from welding the tab 2 to the terminal post 3 may affect the fused portion 22, causing partial melting. Furthermore, if the fused portion 22 is too close to the terminal post 3, it is also susceptible to breakage due to pulling or vibration, affecting its performance. Conversely, if the distance between the fused portion 22 and the third solder mark 500 is too large, it is equivalent to the fused portion 22 being far from the terminal post 3 and close to the cell body 1, which can easily affect the performance of the cell body 1. Therefore, maintaining the distance L9 between the fused portion 22 and the third solder mark 500 between 1-10 mm ensures the performance of the battery pack.

[0077] In this embodiment, the second connecting portion 23 is welded to the end of the pole post 3 located inside the housing 100, forming a third weld mark 500. The area of ​​the third weld mark 500 is S6, where S6 = [10, 80] mm. 2 The S6 can be 10mm. 2 20mm 2 40mm², 60mm², or 80mm², where 0.00062≤S6 / (S²*T1)≤0.033.

[0078] When welding tab 2 and post 3, in order to avoid more heat being transferred to the fusion section 22 and causing the fusion section 22 to melt, the heat can be dissipated through the conductive busbar 4. The volume of the conductive busbar 4 is related to the heat dissipation efficiency. Therefore, the ratio of the area of ​​the third solder mark 500 of tab 2 and post 3 to the volume of the conductive busbar 4 is between 0.00062 and 0.033. Under the premise that the volume of the conductive busbar 4 meets the assembly conditions, the volume of the conductive busbar 4 is increased to achieve a better heat dissipation effect, thereby reducing the impact on the fusion section 22.

[0079] In this embodiment, the battery pack also includes an insulating film 8, and the tab 2 is provided with an insulating film 8 on at least one side in the first direction, thereby protecting the tab 2.

[0080] Preferably, the insulating film 8 covers the area of ​​the fuse portion 22 of the tab 2. This protects the fuse portion 22 from corrosion by the electrolyte or other impurities, ensuring the accuracy of the fusing threshold of the fuse portion 22.

[0081] Furthermore, the insulating film 8 extends to the area of ​​the first connection portion 21, that is, the insulating film 8 extends to the root of the tab 2, thereby further preventing the tab 2 from tearing.

[0082] Furthermore, the insulating film 8 extends to the area of ​​the second connection 23, that is, the insulating film 8 extends to the area where the tab 2 and the post 3 are welded, so as to play a role in preventing corrosion of the third solder mark 500.

[0083] In this embodiment, the distance between the two sides of the insulating film 8 in the second direction is W2, that is, the width of the insulating film 8 is W2, and the distance between the two sides of the tab 2 in the second direction is W3, that is, the width of the tab 2 is W3, wherein 1.1≤W2 / W3≤1.5, and the second direction is perpendicular to the first direction.

[0084] That is, the ratio of the width of the insulating film 8 to the width of the tab 2 is between 1.1 and 1.5. Therefore, the width of the insulating film 8 can be greater than the width of the tab 2, so that the insulating film 8 can provide comprehensive protection for the tab 2 and prevent the tab 2 from breaking and being corroded. At the same time, the width of the insulating film 8 will not be too large and take up too much space.

[0085] In one embodiment, the two opposite sides of the tab 2 are covered with an insulating film 8 to provide more comprehensive protection for the tab 2 and prevent it from being torn or corroded.

[0086] Preferably, one side of the tab 2 in the first direction is a welding surface, and a welding area is provided on the welding surface. The area on the other side of the tab 2 in the first direction corresponding to the welding area is not covered by the insulating film 8.

[0087] Currently, during the assembly of the insulating film 8, an insulating film 8 is first attached to the side of the tab 2 away from the welding surface, and then the tab 2 is welded. After the welding is completed, another insulating film 8 is attached to the welding surface of the tab 2, thus completing the attachment of the insulating film 8 on both sides of the tab 2 in the first direction. However, because the insulating film 8 is attached to the side of the tab 2 away from the welding surface, it is easy to create holes during welding, resulting in poor welding of the tab 2. Therefore, the insulating film 8 located on the side of the tab 2 away from the welding surface and in the area corresponding to the welding area needs to be hollowed out.

[0088] In this embodiment, the distance between the two sides of the tab 2 in the first direction is T2, that is, the thickness of the tab 2 is T2, 0.004mm≤T2≤0.02mm, and T2 can be 0.004mm, 0.008mm, 0.012mm, 0.016mm or 0.02mm. The resistivity of the tab 2 is ρ, 1.5*10^-8Ω·m≤ρ≤3*10^-8Ω·m. On the connection path between the tab 2 and the battery cell body 1, the distance between the end of the fuse part 22 near the first connection part 21 and the end away from the first connection part 21 is L. 10 That is, the length of the fuse section 22 is L. 10 2mm≤L 10 ≤8mm. L 10 It can be 2mm, 3mm, 5mm, 7mm or 8mm.

[0089] In this embodiment, the distance between the fuse part 22 and the battery cell body 1 on the connection path between the tab 2 and the battery cell body 1 is L.11 Where 5mm≤L 11 ≤20mm. L 11 It can be 5mm, 7mm, 9mm, 11mm, 13mm, 15mm, 17mm, 19mm or 20mm.

[0090] Along the connection path between the tab 2 and the cell body 1, the distance L between the fuse part 22 and the cell body 1 is... 11 The distance between the fuse part 22 and the main body 1 is kept between 5-20mm, so that the distance between the fuse part 22 and the main body 1 of the battery cell is not too close, thus avoiding damage to the main body 1 of the battery cell from the heat of the fuse part 22. Also, the distance between the fuse part 22 and the terminal post 3 is not too close, thus avoiding the fuse part 22 from being easily pulled when the terminal post 3 vibrates, causing the fuse part 22 to tear.

[0091] Preferably, the cell body 1 has an active material layer inside, and the distance between the fuse part 22 and the active material layer inside the cell body 1 along the connection path between the electrode 2 and the cell body 1 is L. 12 Where 6.5mm≤L 12 ≤21.5mm. L 12 It can be 6.5mm, 9mm, 12mm, 15mm, 18mm or 21.5mm.

[0092] Along the connection path between the tab 2 and the cell body 1, the distance L between the fuse part 22 and the active material layer inside the cell body 1 is... 12 Maintaining a distance between 6.5-21.5mm serves two purposes. First, the fuse 22 will not be too close to the active material layer of the cell body 1, preventing excessive temperature differences between the active material layer near the fuse 22 and the active material layer not near the fuse 22. This would prevent local resistance differences in the cell body 1, resulting in uneven lithium intercalation and different aging conditions. It would also prevent the fuse 22 from overheating and causing some active materials to become deactivated, thus losing their ability to intercalate and extract lithium ions, leading to problems such as lithium plating and black spots. Second, the distance between the fuse 22 and the electrode post 3 will not be too close, preventing the fuse 22 from being easily pulled apart and torn when the electrode post 3 vibrates.

[0093] Furthermore, the battery cell body 1 has tab adhesive inside, and the distance between the fuse part 22 and the tab adhesive inside the battery cell body 1 on the connection path between the tab 2 and the battery cell body 1 is L. 13 Where 7mm≤L 13 ≤22mm. L 13 It can be 7mm, 9mm, 11mm, 13mm, 15mm, 17mm, 19mm, 21mm or 22mm.

[0094] The tab adhesive is made of PVDF and Al2O3, so it also has a certain degree of high-temperature resistance. However, the instantaneous temperature at which the fuse 22 melts is also high, with a melting point reaching 660 degrees Celsius. Therefore, if the fuse 22 is too close to the tab adhesive in the connection path between the tab 2 and the cell body 1, it will cause the tab adhesive to burn and fail, resulting in a short circuit between the positive electrode foil and the negative electrode material area. This greatly increases the risk of thermal runaway of the cell body 1. If the distance between the fuse 22 and the electrode post 3 is too close, the vibration of the electrode post 3 can easily pull on the fuse 22, causing it to tear. Therefore, the distance L between the fuse 22 and the tab adhesive in the cell body 1 is increased. 13 Maintaining a thickness between 7-22mm is necessary to ensure the performance of the cell body 1.

[0095] Furthermore, the cell body 1 has a diaphragm inside, and the distance between the fuse part 22 and the diaphragm inside the cell body 1 along the connection path between the electrode tab 2 and the cell body 1 is L. 14 Where 5mm≤L 14 ≤20mm. L 14 It can be 5mm, 7mm, 9mm, 11mm, 13mm, 15mm, 17mm, 19mm or 20mm.

[0096] Because the separator inside the cell body 1 is not heat-resistant, when the distance between the fuse part 22 and the separator is too close along the connection path between the tab 2 and the cell body 1, the separator is prone to ablation, resulting in a short circuit between the positive electrode foil and the negative electrode material area. This greatly increases the risk of thermal runaway of the cell body 1. Similarly, when the distance between the fuse part 22 and the electrode post 3 is too close along the connection path between the tab 2 and the cell body 1, vibration of the electrode post 3 can easily pull on the fuse part 22, causing it to tear. Therefore, the distance L between the fuse part 22 and the separator inside the cell body 1 is crucial. 14 Maintain a thickness between 5-20mm to ensure the performance of the cell body 1.

[0097] It should be noted that, since this application aims to reduce the impact of the heat from the fuse 22 on the performance of the cell body 1, the connection path between the tab 2 and the cell body 1 can also be considered as the heat transfer path between the tab 2 and the cell body 1.

[0098] It should be emphasized that, in this application, the minimum cross-sectional area of ​​the fused section is the area of ​​the cross-section formed by breaking the fused section at its minimum size along a direction perpendicular to the fused section; the surface area of ​​the conductive busbar is the sum of the areas of the outer surfaces of the conductive busbar; the volume of the conductive busbar refers to the size of the space it occupies; the minimum cross-sectional area of ​​the conductive busbar is the area of ​​the cross-section formed by breaking the conductive busbar at its minimum size along a direction perpendicular to the conductive busbar; the areas of the first weld mark, the second weld mark, and the third weld mark refer to the area of ​​the welding area.

[0099] The following are the relevant testing methods:

[0100] A test method is provided to determine whether the fusing portion of the electrode tab meets the fusing time requirement:

[0101] The electrode includes a first connecting part, a fusible part, and a second connecting part connected in sequence. The first connecting part is a free end. The second connecting part is welded to one end of the electrode post, and the other end of the electrode post is welded to the busbar. One electrical connection terminal of the current source testing device is connected to the busbar, and the other electrical connection terminal of the current source testing device is connected to the first connecting part. If the current source testing device is powered on with a current of 800A and meets the requirement that the fusible part melts within 10 minutes, it meets the product safety standards.

[0102] If the current is less than 800A and the fuse blows within 10 minutes, it means that the fuse does not meet the overcurrent requirement and is deemed unqualified.

[0103] If the current is greater than 800A and the fuse breaks after 10 minutes, it means that the fuse does not meet the fusing requirements and is deemed unqualified.

[0104] The results obtained using the test method described above are shown in the table below:

[0105] The above are merely preferred embodiments of this application. It should be noted that, for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of this application, and these improvements and substitutions should also be considered within the scope of protection of this application.

Claims

1. A battery pack characterized by comprising: Includes multiple batteries, said batteries comprising: A housing having an opening; A cover plate, which is disposed over the opening, and has a through hole; An electrode post, wherein at least a portion of the electrode post is disposed within the through hole; A battery cell, comprising a battery cell body and electrode tabs, wherein the battery cell body is disposed within the housing, and the electrode tabs comprise a first connecting portion, a fusible portion, and a second connecting portion connected in sequence, wherein the cross-sectional areas of the first connecting portion and the second connecting portion are both larger than the cross-sectional area of ​​the fusible portion, the first connecting portion is connected to the battery cell body, and the second connecting portion is connected to the end of the electrode post located within the housing; A conductive busbar is electrically connected to the side of the electrode post facing away from the battery cell; The minimum cross-sectional area of ​​the fuse is S1, the surface area of ​​the conductive busbar is S2, and the thickness of the conductive busbar is T1, wherein 9600≤S1*S2*T1≤320000.

2. The battery pack of claim 1, wherein, Along the path of heat transfer from the battery cell to the conductive busbar, the distance between the fused portion and the conductive busbar is L1, where 6mm≤L1≤18mm.

3. The battery pack of claim 1, wherein, The minimum cross-sectional area of ​​the conductive busbar is S3, where 3≤S3 / S1≤37.

5.

4. The battery pack of claim 1, wherein, The conductive busbar and the electrode are welded together to form a first weld mark, the area of ​​which is S4, where 240≤S4*S1≤3000.

5. The battery pack of claim 1, wherein, The contact area between the conductive busbar and the electrode post is S5, where 400≤S5*S1≤10000.

6. The battery pack of claim 1, wherein, The conductive busbar is connected to a plurality of the poles. A buffer groove is provided on the conductive busbar in the area between two adjacent poles. The width of the buffer groove is W1 and the depth of the buffer groove is D, wherein 0.3≤(W1*D) / S1≤3.

7. The battery pack of claim 1, wherein, The battery pack also includes a flexible circuit board and a temperature acquisition unit disposed on the conductive busbar. The temperature acquisition unit is electrically connected to the flexible circuit board. On the path of heat transfer from the battery cell to the temperature acquisition unit, the distance between the temperature acquisition unit and the fuse is L2, wherein 6.5mm≤L2≤20mm.

8. The battery pack of claim 7, wherein, Along the path of heat transfer from the battery cell to the temperature acquisition unit, the distance between the temperature acquisition unit and the electrode is L3, where 0.5mm≤L3≤12mm.

9. The battery pack of claim 7, wherein, The battery pack also includes a conductive connecting piece connected to the conductive busbar. The flexible circuit board has circuit traces, and the conductive connecting piece is connected to the circuit traces. A fuse is provided on the circuit traces. On the path of heat transfer from the battery cell to the flexible circuit board, the distance between the fuse and the fusible part is L4, where 6.5mm≤L4≤20mm.

10. The battery pack of claim 9, wherein, The flexible circuit board extends outward with a cantilever portion, the circuit trace extends to the cantilever portion, one end of the conductive connecting piece is attached to the conductive busbar, the other end is disposed on the cantilever portion and connected to the circuit trace on the cantilever portion, and the fuse is disposed on the circuit trace of the cantilever portion.

11. The battery pack of claim 1, wherein, The number of battery cell bodies is n, and the conductive busbar is connected to the poles corresponding to the n battery cell bodies, wherein 24≤((S2*T1) / n)*S1≤160000.

12. The battery pack according to any one of claims 1 to 11, characterized by, The battery cell is a wound battery cell. The electrode includes multiple stacked electrode pieces. Each electrode piece includes a first sub-connection portion, a sub-fuse portion, and a second sub-connection portion connected in sequence. All the first sub-connection portions of the electrode are stacked together to form the first connection portion. All the second sub-connection portions of the electrode are stacked together to form the second connection portion. All the sub-fuse portions of the electrode are welded together to form the fuse portion, and a second solder mark is formed on the side of the electrode.

13. The battery pack of claim 12, wherein, The portions of all the sub-fuse parts of the electrode tab are welded together.

14. The battery pack of claim 12, wherein, All areas of the sub-fuse portions of the electrode tab are welded together, and the area of ​​the second weld mark is equal to or greater than the area of ​​the side of the fuse portion.

15. The battery pack of claim 14, wherein, The second solder mark is formed on the side of the outermost tab. On the path of heat transfer from the cell to the busbar, the distance between the end of the second solder mark near the second sub-connection and the end of the sub-fuse portion of the tab forming the second solder mark near the second sub-connection is L5, where 1mm≤L5≤8mm.

16. The battery pack of claim 14, wherein, The second solder mark is formed on the side of the outermost tab. On the path of heat transfer from the cell to the busbar, the distance between the end of the second solder mark near the first sub-connection and the end of the sub-fuse portion of the tab forming the second solder mark near the first sub-connection is L6, where 1mm≤L6≤8mm.

17. The battery pack of claim 12, wherein, The second connection part is welded to the end of the electrode located inside the housing to form a third weld mark. On the path of heat transfer from the cell to the busbar, the distance between the second weld mark and the third weld mark is L7, where 1mm≤L7≤5mm.

18. The battery pack of claim 17, wherein, On the path of heat transfer from the battery cell to the busbar, the distance between the end of the fuse portion away from the first connection portion and the end of the fuse portion close to the first connection portion is L8, where 0.4≤L8 / L7≤8.

19. The battery pack of claim 12, wherein, The second connecting part is welded to the end of the pole located inside the housing to form a third weld mark, and the second weld mark and the third weld mark are connected together.

20. The battery pack of any one of claims 1-11, wherein, The battery cell is a stacked battery cell, and the electrode tab includes multiple electrode tabs stacked together. Each electrode tab includes a first sub-connection part, a sub-fuse part, and a second sub-connection part connected in sequence, and the sub-fuse parts of the multiple electrode tabs are spaced apart from each other.

21. The battery pack of any one of claims 1-11, wherein, The second connection part is welded to the end of the electrode located inside the housing to form a third weld mark. On the path of heat transfer from the cell to the busbar, the distance between the fused part and the third weld mark is L9, where 1mm≤L9≤10mm.

22. The battery pack of any one of claims 1-11, wherein, The second connecting part is welded to the end of the pole located inside the housing to form a third weld mark, the area of ​​which is S6, wherein 0.00062≤S6 / (S2*T1)≤0.

033.

23. The battery pack of any one of claims 1-11, wherein, The battery pack also includes an insulating film, and the insulating film is provided on at least one side of the electrode in a first direction.

24. The battery pack of claim 23, wherein, The insulating film covers the area of ​​the fused portion of the electrode.

25. The battery pack of claim 24, wherein, The insulating film extends to the region of the first connection portion and / or the region of the second connection portion.

26. The battery pack of claim 23, wherein, The distance between the two sides of the insulating film in the second direction is W2, and the distance between the two sides of the tab in the second direction is W3, wherein 1.1≤W2 / W3≤1.5, and the second direction is perpendicular to the first direction.

27. The battery pack of claim 23, wherein, The electrode tab has a welding surface on one side in the first direction, and a welding area is provided on the welding surface. The area on the other side of the electrode tab in the first direction corresponding to the welding area is not covered by the insulating film.

28. The battery pack of any one of claims 1-11, wherein, The distance between the two sides of the electrode in the first direction is T2, 0.004mm≤T2≤0.02mm; the resistivity of the electrode is ρ, 1.5*10^-8Ω·m≤ρ≤3*10^-8Ω·m; and the distance between the end of the fuse portion near the first connection portion and the end away from the first connection portion on the connection path between the electrode and the cell body is L. 10 2mm≤L 10 ≤8mm.

29. The battery pack of any one of claims 1-11, wherein, On a connection path of the tab and the cell main body, a distance between the fusing portion and the cell main body is L 11 wherein 5mm≤L 11 ≤20mm.

30. The battery pack of any one of claims 1-11, wherein, The inside of the cell body has an active material layer, and the distance between the fuse portion and the active material layer on the connection path of the tab and the cell body is L 12 wherein 6.5 mm ≤ L 12 ≤ 21.5 mm.

31. The battery pack of any one of claims 1-11, wherein, The inside of the cell body has a tab rubber, and the distance between the fuse portion and the tab rubber on the connection path of the tab and the cell body is L 13 wherein 7 mm ≤ L 13 ≤ 22 mm.

32. The battery pack of any one of claims 1-11, wherein, The inside of the cell body has a separator, and a distance between the fuse portion and the separator on a connection path of the tab and the cell body is L 14 where 5 mm ≤ L 14 ≤ 20 mm.

Images