Battery pack and method of manufacturing the same

Abstract

In a battery pack provided with a first battery and a second battery, the first battery includes a negative electrode terminal having a first metal portion and a second metal portion formed of a metal different from the first metal portion, the second metal portion is dissimilarly joined to the first metal portion, a joining surface and a concave-shaped portion are formed in the second metal portion, and a manufacturing method of the battery pack includes: a step of inserting a welding auxiliary member into the concave-shaped portion; and a step of disposing a bus bar on the joining surface of the negative electrode terminal into which the welding auxiliary member is inserted, and joining the bus bar on the joining surface of the negative electrode terminal by laser welding.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a battery pack and a manufacturing method thereof. BACKGROUND

[0002] As disclosed in Japanese Patent Application Publication No. 2016-192322 and Japanese Patent Application Publication No. 2020-191230 described below, a battery pack is configured by stacking a plurality of secondary batteries and electrically connecting secondary batteries adjacent to each other. For example, positive and negative terminals of secondary batteries adjacent to each other are electrically connected by a bus bar (a plate made of metal), and the plurality of secondary batteries are connected in series.

[0003] The positive and negative terminals are sometimes formed of dissimilar metals, in other words, metals of different kinds from each other. For example, the positive terminal is formed of an aluminum alloy, and the negative terminal is formed of a copper alloy.

[0004] The bus bar is formed of, for example, an aluminum alloy, and the bus bar is connected to the positive and negative terminals by welding. In this case, the welding of the bus bar to the positive terminal is welding of metals of the same kind to each other, and the welding of the bus bar to the negative terminal is welding of metals of dissimilar kinds to each other. In the case where metals of dissimilar kinds are welded to each other, it is difficult to obtain a connection strength compared to the case where metals of the same kind are welded to each other. SUMMARY

[0005] As a method for improving the connection strength of the bus bar to the negative terminal, it is considered that the negative terminal is formed of two kinds of dissimilar metals (for example, a copper alloy and an aluminum alloy). For example, a lower portion of the negative terminal is formed of a copper alloy, and an upper portion of the negative terminal is formed of an aluminum alloy. The negative terminal is configured by joining the copper alloy and the aluminum alloy in advance. The negative terminal thus obtained (specifically, the aluminum alloy in the negative terminal) is joined to the bus bar formed of an aluminum alloy. The portion of the aluminum alloy in the negative terminal and the bus bar made of an aluminum alloy can be firmly joined.

[0006] However, in the case where the negative terminal is formed of two kinds of dissimilar metals (for example, a copper alloy and an aluminum alloy), it is necessary to appropriately join the two kinds of dissimilar metals to configure the negative terminal, for example, in order to be able to withstand long-term use. In the case where the two kinds of dissimilar metals are joined by ultrasonic welding, resistance welding, or friction stir spot welding, or the like, the thickness of the negative terminal is thinned at a welding processed portion, and a concave-shaped portion is formed. The concave-shaped portion is formed in, for example, the aluminum alloy joined to the upper portion of the copper alloy.

[0007] When laser welding a busbar to a negative electrode terminal with a concave portion, the laser irradiation position may shift from the intended position due to misalignment during assembly of the components, as well as manufacturing tolerances in the busbar, terminal, and cell. When this misalignment occurs and the laser irradiates the concave portion, heat easily reaches the dissimilar metal joint at the negative electrode terminal due to its thinness. When excessive heat is applied to the dissimilar metal joint at the negative electrode terminal, intermetallic compounds may form between the dissimilar metals, potentially reducing the joint strength.

[0008] The present disclosure provides a battery pack having a structure capable of appropriately connecting a bus bar to a negative electrode terminal even when the negative electrode terminal is formed of mutually joined dissimilar metals, and a method for manufacturing the battery pack.

[0009] A first aspect of the present invention relates to a method for manufacturing a battery pack comprising a first battery and a second battery. The first battery includes a negative electrode terminal having a first metal portion and a second metal portion, the first metal portion comprising a first metal, the second metal portion comprising a second metal different from the first metal, the second metal portion forming a dissimilar metal bond to an upper portion of the first metal portion, the second metal portion including a bonding surface and a concave portion on its upper surface, the concave portion being recessed relative to the bonding surface. The second battery includes a positive electrode terminal electrically connected to the negative electrode terminal via a busbar. The method comprises: inserting a welding auxiliary member into the concave portion; placing the busbar on the bonding surface of the negative terminal after the welding auxiliary member has been inserted into the concave portion; and bonding the busbar to the bonding surface of the negative terminal by laser welding.

[0010] In the above-mentioned method of manufacturing a battery pack, the welding auxiliary member may include the same metal as the second metal of the second metal portion of the negative electrode terminal.

[0011] In the above-mentioned method for manufacturing a battery pack, the concave portion may have a shape recessed relative to the joining surface by joining the second metal portion to an upper portion of the first metal portion using dissimilar metals.

[0012] In the above-mentioned method for manufacturing a battery pack, the second metal portion may be joined to an upper portion of the first metal portion using dissimilar metals by friction stir spot welding, ultrasonic welding, or resistance welding.

[0013] A second aspect of the present disclosure relates to a battery pack including a first battery, a second battery, and a bus bar. The first battery includes a negative terminal having a first metal portion and a second metal portion. The first metal portion includes a first metal, and the second metal portion includes a second metal different from the first metal. The second metal portion is dissimilarly metal bonded to an upper portion of the first metal portion. An upper surface of the second metal portion includes a bonding surface and a concave portion. The concave portion has a shape that is recessed with respect to the bonding surface. The second battery includes a positive terminal electrically connected to the negative terminal via the bus bar. A welding auxiliary member is inserted into the concave portion. The bus bar is disposed on the bonding surface of the negative terminal after the welding auxiliary member is inserted into the concave portion. The bus bar is bonded to the bonding surface of the negative terminal by laser welding.

[0014] In the battery pack, the welding auxiliary member can include the same metal as the second metal of the second metal portion of the negative terminal.

[0015] According to the present disclosure, a battery pack having a structure in which a bus bar can be properly connected to a negative terminal formed of dissimilar metals that are bonded to each other, and a method of manufacturing such a battery pack, can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0016] Features, advantages, and technical and industrial significance of embodiments of the application will be described below with reference to the accompanying drawings, wherein the same reference numerals denote the same elements, and wherein:

[0017] Figure 1 is a perspective view showing a battery pack.

[0018] Figure 2 is a cross-sectional view showing an internal structure of a secondary battery.

[0019] Figure 3 is a cross-sectional view showing a state in which a second metal portion is dissimilarly metal bonded to an upper portion of a first metal portion by a friction stir spot welding method.

[0020] Figure 4 is a cross-sectional view showing a state in which a second metal portion is dissimilarly metal bonded to an upper portion of a first metal portion by a friction stir spot welding method.

[0021] Figure 5 is a cross-sectional view showing a state in which a welding auxiliary member is inserted into a concave portion provided in a second metal portion.

[0022] Figure 6 is a cross-sectional view showing a state in which a welding auxiliary member is inserted into a concave portion provided in a second metal portion.

[0023] Figure 7 This is a cross-sectional view showing a state in which a bus bar is joined to a joining surface of a negative electrode terminal (second metal portion) by laser welding.

[0024] Figure 8 This is a cross-sectional view showing a state in which a bus bar is joined to a joining surface of a negative electrode terminal (second metal portion) by laser welding in the case of Comparative Example 1.

[0025] Figure 9 This is a cross-sectional view showing a state in which a bus bar is joined to a joining surface of a negative electrode terminal (second metal portion) by laser welding in the case of Comparative Example 2.

[0026] Figure 10 Modification 1 of the embodiment is a cross-sectional view showing a state in which a second metal portion and an upper portion of a first metal portion are joined using dissimilar metals by ultrasonic welding.

[0027] Figure 11 Modification 2 of the embodiment is a cross-sectional view showing a state in which a second metal portion and an upper portion of a first metal portion are joined using dissimilar metals by resistance welding.

[0028] Figure 12 Modification 3 of the embodiment is a cross-sectional view showing a state in which a welding auxiliary member is inserted into a concave portion provided in the second metal portion. DETAILED DESCRIPTION

[0029] Implementation Method

[0030] The following describes a battery pack 100 and a method for manufacturing the same according to an embodiment with reference to the accompanying drawings. Where numbers, quantities, materials, and the like are mentioned, the scope of the present disclosure is not necessarily limited to such numbers, quantities, materials, and the like, unless otherwise specified. Identical and equivalent components are denoted by the same reference numerals, and repeated descriptions may not be repeated. It was intended from the outset that the structures described in the embodiments be used in appropriate combinations. For the sake of clarity and simplification of the drawings, the dimensional relationships of length, width, thickness, depth, and the like have been appropriately modified and do not represent actual dimensional relationships.

[0031] Battery Pack 100

[0032] Figure 1 1 is a perspective view showing a battery pack 100 . The battery pack 100 includes a secondary battery 11 as a first battery, a secondary battery 12 as a second battery, a bus bar 70 , and end plates 18 and 19 .

[0033] In battery pack 100, multiple secondary batteries, including secondary batteries 11 and 12, are stacked in one direction, with end plates 18 and 19 positioned at both ends in the stacking direction. Unillustrated restraining members maintain the stacked state, forming a single battery pack 100. Secondary batteries 11 and 12 are positioned adjacent to each other. Battery pack 100 is used, for example, in hybrid vehicles, plug-in hybrid vehicles, fuel cell vehicles, and electric vehicles as a power source for these vehicles.

[0034] The multiple secondary batteries included in the battery pack 100 are, for example, non-aqueous electrolytic secondary batteries such as lithium-ion secondary batteries. Each of the multiple secondary batteries has a positive terminal and a negative terminal. Among adjacent secondary batteries, the negative terminal 50 of one secondary battery 11 is electrically connected to the positive terminal 80 of another secondary battery 12 via a busbar 70. The technical concepts disclosed herein are not limited to battery packs including multiple non-aqueous electrolytic secondary batteries and can also be applied to battery packs including other secondary batteries and their manufacturing methods. The battery packs are also not limited to automotive applications and can be applied to a variety of technical fields.

[0035] Secondary battery 11

[0036] Figure 2 1 is a cross-sectional view showing the internal structure of the secondary battery 11. The secondary battery 11 includes an electrode assembly 20, an electrolyte 22, a frame 30, a gasket 40, a negative electrode terminal 50, a positive electrode terminal (not shown), and a welding auxiliary member 60.

[0037] The electrode assembly 20 is formed by laminating or winding a positive electrode, a negative electrode, and a separator, and functions as a power generation element. The electrode assembly 20 is immersed in an electrolyte 22 inside the frame 30 .

[0038] The frame 30 includes a case member 31 and a cover member 32. The case member 31 has an opening 31H and is generally square in shape. The case member 31 houses the electrode assembly 20 and the electrolyte 22. The cover member 32 is flat and has an outer shape (e.g., rectangular) that corresponds to the opening 31H of the case member 31.

[0039] The cover member 32 is joined to the case member 31 by welding and is arranged so as to close the opening 31H of the case member 31 . Figure 2 A state is shown in which the cover member 32 is engaged with a portion of the opening 31H of the case member 31 and the cover member 32 and the case member 31 are integrated.

[0040] The negative electrode terminal 50 is electrically connected to the electrode body 20 disposed inside the case member 31 through a conductive structure (not shown). A gasket 40 is disposed between the cover member 32 of the frame 30 and the negative electrode terminal 50. The gasket 40 electrically insulates the negative electrode terminal 50 from the cover member 32. Although detailed description is omitted, the positive electrode terminal ( Figure 2 The positive electrode terminal (not shown) is also electrically connected to the electrode body 20 and has the same conductive structure as the negative electrode terminal 50. The positive electrode terminal (not shown) is formed of, for example, an aluminum alloy.

[0041] The negative electrode terminal 50 includes a first metal portion 51 and a second metal portion 52 formed of a different metal from the first metal portion 51. The first metal portion 51 constitutes the lower portion of the negative electrode terminal 50, that is, the portion of the negative electrode terminal 50 on the side close to the electrode body 20. The second metal portion 52 constitutes the upper portion of the negative electrode terminal 50, that is, the portion of the negative electrode terminal 50 on the side away from the electrode body 20.

[0042] The first metal portion 51 is formed of, for example, a copper alloy, and the second metal portion 52 is formed of, for example, an aluminum alloy. The second metal portion 52 is bonded to the upper portion of the first metal portion 51 using dissimilar metals. The method for manufacturing the battery pack 100 may also include a step of bonding the second metal portion 52 to the upper portion of the first metal portion 51 using dissimilar metals.

[0043] The upper surface of the second metal part 52 is formed with a joining surface 53 and a concave portion 54, which is recessed relative to the joining surface 53. The concave portion 54 is formed, for example, by performing dissimilar metal bonding on the upper portion of the first metal part 51 and the second metal part 52, so as to have a recessed shape relative to the joining surface 53. This will be described in more detail below. Alternatively, the concave portion 54 may be recessed relative to the joining surface 53 before dissimilar metal bonding is performed.

[0044] Manufacturing method

[0045] The method for manufacturing the battery pack 100 may include a step of joining the second metal portion 52 to the upper portion of the first metal portion 51 using dissimilar metals by, for example, friction stir spot welding, ultrasonic welding, or resistance welding. Figure 3 This is a cross-sectional view showing a state where the second metal portion 52 and the upper portion of the first metal portion 51 are joined by dissimilar metals by friction stir spot welding.

[0046] like Figure 3As shown, in the case of friction stir spot welding, the second metal part 52 and the first metal part 51 are joined using an FSW rod 91 (FSW). An effective connection portion 98 having a joint strength exceeding a predetermined value is formed on the joint surface between the second metal part 52 and the first metal part 51. Prior to joining the second metal part 52 and the first metal part 51, the joint surface 53 has no concave portion 54 formed thereon, and the joint surface 53 has an overall flat surface shape.

[0047] Figure 4 1 is a cross-sectional view showing a state in which a second metal portion 52 is joined to a dissimilar metal on an upper portion of a first metal portion 51 by friction stir spot welding. Figure 4 As shown, the second metal portion 52 is joined to the upper portion of the first metal portion 51 by dissimilar metal bonding to form the negative electrode terminal 50. The concave portion 54 is formed by dissimilar metal bonding to the second metal portion 52 on the upper portion of the first metal portion 51 to have a shape recessed relative to the bonding surface 53.

[0048] Figure 5 It is a cross-sectional view showing a state in which the welding auxiliary member 60 is inserted into the concave portion 54 provided in the second metal portion 52 . Figure 6 5 is a cross-sectional view showing a state where the welding auxiliary member 60 is inserted into the concave portion 54 provided in the second metal portion 52. Figure 5 and Figure 6 As shown, the welding auxiliary member 60 is inserted into the concave portion 54 . That is, the method for manufacturing the battery pack 100 includes the step of inserting the welding auxiliary member 60 into the concave portion 54 .

[0049] The welding auxiliary member 60 is made of, for example, metal. For example, the welding auxiliary member 60 is formed of the same metal as the second metal portion 52 of the negative electrode terminal 50. Here, as an example, the welding auxiliary member 60 is formed of an aluminum alloy. The welding auxiliary member 60 is not limited to being formed of the same metal as the second metal portion 52 and may be formed of a member that melts at the same or similar melting point as the second metal portion 52 during laser welding.

[0050] Welding auxiliary member 60 is inserted from its bottom side into the inner side of concave portion 54. Welding auxiliary member 60 may have a width W2, diameter, or bottom area that is larger than width W1 of concave portion 54, or the diameter of the hole or the area of ​​the opening of concave portion 54. Welding auxiliary member 60 can be securely fixed inside concave portion 54 by press-fitting or interlocking (interference fit).

[0051] Figure 7This is a cross-sectional view showing a bus bar 70 being laser-welded to the bonding surface 53 of the negative electrode terminal 50 (second metal portion 52). The bonding surface 53 of the second metal portion 52 constitutes the portion of the negative electrode terminal 50 that protrudes outward from the secondary battery 11. The bus bar 70 is electrically connected to the bonding surface 53.

[0052] That is, the manufacturing method of the battery pack 100 includes the following steps: placing the bus bar 70 on the joint surface 53 of the negative electrode terminal 50 after the welding auxiliary member 60 is inserted into the concave portion 54, and irradiating the battery pack 100 with the laser beam L( Figure 7 ) is laser welded to join the bus bar 70 to the joining surface 53 of the negative terminal 50. The bus bar 70 and the negative terminal 50 function as a current path for extracting the power stored in the electrode body 20 to the outside or for taking the power from the outside into the electrode body 20.

[0053] Function and effect

[0054] The effects and functions of this embodiment are compared with those of Comparative Example 1 ( Figure 8 ) and Comparative Example 2 ( Figure 9 ) for comparison to illustrate. Figure 8 This is a cross-sectional view showing a state in which the bus bar 70 is joined to the joining surface 53 of the negative electrode terminal 50 (second metal portion 52 ) by laser welding in the case of Comparative Example 1.

[0055] Comparative Example 1

[0056] like Figure 8 As shown, in the case of Comparative Example 1, the welding auxiliary member 60 is not inserted into the concave portion 54. When the bus bar 70 and the negative terminal 50 are laser welded, the laser L may be irradiated to the concave portion 54 ( Figure 8 The portion of the negative electrode terminal 50 (second metal portion 52) where the concave portion 54 is formed is thinner than the portion of the negative electrode terminal 50 (second metal portion 52) where the concave portion 54 is not formed.

[0057] As mentioned at the beginning of this specification, heat easily reaches the joint surface between the dissimilar metals of the negative electrode terminal 50 (in other words, the effective connection portion 98 where the first metal portion 51 and the second metal portion 52 are joined). When a large amount of heat is supplied to the joint surface between the dissimilar metals of the negative electrode terminal 50, intermetallic compounds 85 are formed between the dissimilar metals, resulting in a decrease in the joint strength of the dissimilar metal joint surface, particularly the joint strength of the effective connection portion 98. Furthermore, when the laser light L is irradiated on the concave portion 54, sparks may be generated, and smoke (unwanted substances) may be produced.

[0058] In contrast to the case of Comparative Example 1, in the case of the above embodiment (refer to Figure 7 ), for example, the following functions and effects [1] to [5] can be obtained.

[0059] [1] Since the welding auxiliary member 60 is present inside the concave portion 54, even if the irradiation position of the laser L is shifted, the laser L is hardly or not irradiated to the concave portion 54, and consequently, sparks are hardly or not generated and smoke is hardly or not formed.

[0060] [2] Since the welding auxiliary member 60 is present inside the concave portion 54, even if the irradiation position of the laser L is shifted, heat is effectively suppressed from reaching the joint surface between the dissimilar metals of the negative terminal 50 (in other words, the effective connection portion 98 where the first metal portion 51 and the second metal portion 52 are joined to each other). Furthermore, intermetallic compounds 85 are hardly formed between the dissimilar metals. As a result, the bonding strength of the bonding surface between the dissimilar metals, especially the bonding strength of the effective connection portion 98, is hardly reduced.

[0061] [3] Since there is a welding auxiliary member 60 on the inner side of the concave portion 54, even if the irradiation position of the laser L is shifted and the laser L is irradiated to the welding auxiliary member 60, the welding of the busbar 70 itself can be carried out and / or welding can be continued, which can reduce the welding failure rate and thus improve the manufacturing yield.

[0062] [4] The weld marks on the dissimilar metal surfaces between the first metal portion 51 and the second metal portion 52 are likely to become the starting point of cracks that may develop over time. However, in the present embodiment, the welding auxiliary member 60 melts and fills the weld marks, thereby also suppressing the occurrence of such cracks.

[0063] [5] By filling the concave portion 54 with the welding auxiliary member 60 , it is possible to suppress the condensation of water in the concave portion 54 or the accumulation of dust in the concave portion 54 , thereby improving the corrosion resistance of the negative electrode terminal 50 .

[0064] In the above embodiment, the welding auxiliary member 60 and the second metal portion 52 of the negative electrode terminal 50 melt at the same melting point. For example, the welding auxiliary member 60 and the second metal portion 52 of the negative electrode terminal 50 are formed of the same metal, thereby facilitating laser welding.

[0065] Comparative Example 2

[0066] Figure 9This is a cross-sectional view showing a state in which the busbar 70 is joined to the joining surface 53 of the negative electrode terminal 50 (second metal portion 52) by laser welding in Comparative Example 2. In Comparative Example 2, the second metal portion 52 is formed thicker than in Comparative Example 1. As a result, even if the irradiation position of the laser light L shifts and the laser light L strikes a concave portion, heat is suppressed from reaching the joining surface between the dissimilar metals of the negative electrode terminal 50 (in other words, the portion where the first metal portion 51 and the second metal portion 52 are joined).

[0067] In the case of Comparative Example 2, the formation of intermetallic compound 85 ( Figure 8 ), as a result, it is possible to suppress a decrease in the joint strength of the joint surface between dissimilar metals. However, in order to join the second metal part 52 having such a large thickness to the first metal part 51, it may be necessary to adopt a limited method, such as providing a mechanical fitting structure at the interface between the second metal part 52 and the first metal part 51, or using a method such as friction welding (solid phase welding) generated by the action of rotation and pressure welding.

[0068] In other words, when joining the second metal part 52 having such a large thickness to the first metal part 51, it is difficult to use methods such as ultrasonic welding, resistance welding, or friction stir spot welding, making it difficult to increase the degree of freedom in manufacturing. In addition, the overall size of the device increases accordingly with the increase in the thickness of the second metal part 52.

[0069] Compared with the case of Comparative Example 2, in the case of the above embodiment, for example, by implementing the FSW rod 91 ( Figure 3 ) friction stir spot welding, the second metal portion 52 can be joined to the first metal portion 51, almost without manufacturing restrictions. Compared with the case of Comparative Example 2, the overall volume of the device will not be increased due to the thickness of the second metal portion 52.

[0070] Modification 1

[0071] Figure 10 The first modification of the embodiment is a cross-sectional view showing a state where the second metal portion 52 and the upper portion of the first metal portion 51 are joined by dissimilar metals by ultrasonic welding. Not limited to the friction stir spot welding in the above embodiment, the first metal portion 51 may be joined to the second metal portion 52 by ultrasonic welding using a horn 92 and anvil 93. Even in the case of adopting this structure, by forming the concave portion 54 (see Figure 4 ), and weld the auxiliary member 60 (refer to Figure 6 ) is inserted into the concave portion 54, and the same function and effect as the above-mentioned embodiment can be obtained.

[0072] Modification 2

[0073] Figure 11 The second modification example of the embodiment is a cross-sectional view showing a state where the second metal portion 52 is joined to the upper portion of the first metal portion 51 by resistance welding. The first metal portion 51 may be joined to the second metal portion 52 by resistance welding, rather than friction stir spot welding in the above embodiment or ultrasonic welding in the first modification example. Even in the case of adopting this structure, by forming the concave portion 54 (see FIG. Figure 4 ), and weld the auxiliary member 60 (refer to Figure 6 ) is inserted into the concave portion 54, and the same function and effect as the above-mentioned embodiment can be obtained.

[0074] Modification 3

[0075] Figure 12 Modification 3 of the embodiment is a cross-sectional view showing a state in which a welding auxiliary member 60 is inserted into a concave portion 54 provided in a second metal portion 52 . Figure 12 The welding auxiliary member 60 shown is also Figure 5 The welding auxiliary member 60 is inserted from its bottom side into the inner side of the concave portion 54. The welding auxiliary member 60 may have a tapered shape (or a wedge shape) with a width that narrows on the bottom side relative to the concave portion 54. The welding auxiliary member 60 can be securely fixed to the inner side of the concave portion 54 by a press-fit or interlocking (interference fit) structure.

[0076] While the embodiments of the present disclosure have been described above, the contents of this disclosure are illustrative in all aspects and are not restrictive. The technical scope of the present disclosure is indicated by the claims, and includes all modifications within the meaning and scope equivalent to the claims.

Claims

1. A method for manufacturing a battery pack, the battery pack comprising a first battery and a second battery, the first battery including a negative electrode terminal, the negative electrode terminal including a first metal portion and a second metal portion, the first metal portion comprising a first metal, the second metal portion comprising a second metal different from the first metal, the second metal portion being joined to an upper portion of the first metal portion by dissimilar metals, the upper surface of the second metal portion comprising a joining surface and a concave portion, the concave portion being recessed relative to the joining surface, the second battery including a positive electrode terminal electrically connected to the negative electrode terminal via a bus bar, the method for manufacturing the battery pack comprising: a step of inserting a welding auxiliary member into the concave portion; and The step of arranging the bus bar on the joint surface of the negative electrode terminal after the welding auxiliary member is inserted into the concave portion, and joining the bus bar to the joint surface of the negative electrode terminal by laser welding.

2. The method for manufacturing a battery pack according to claim 1, wherein: The welding auxiliary member contains the same metal as the second metal of the second metal portion of the negative electrode terminal.

3. The method for manufacturing a battery pack according to claim 1 or 2, wherein: When the second metal portion is joined to an upper portion of the first metal portion by dissimilar metals, the concave portion has a shape that is recessed relative to the joining surface.

4. The method for manufacturing a battery pack according to claim 3, wherein: The second metal portion is joined to an upper portion of the first metal portion by friction stir spot welding, ultrasonic welding, or resistance welding.

5. A battery pack, characterized in that: have: A first battery including a negative electrode terminal having a first metal portion and a second metal portion, the first metal portion comprising a first metal, the second metal portion comprising a second metal different from the first metal, the second metal portion being bonded to an upper portion of the first metal portion in a dissimilar metal bond, the upper surface of the second metal portion including a bonding surface and a concave portion, the concave portion being recessed relative to the bonding surface; busbar; as well as a second battery including a positive terminal electrically connected to the negative terminal via the bus bar, A welding auxiliary member is inserted into the concave portion, and The bus bar is arranged on the joint surface of the negative electrode terminal after the welding auxiliary member is inserted into the concave portion, and the bus bar is joined to the joint surface of the negative electrode terminal by laser welding.

6. The battery pack according to claim 5, characterized in that The welding auxiliary member contains the same metal as the second metal of the second metal portion of the negative electrode terminal.

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