Battery pack

Abstract

This battery pack comprises an exterior case that houses a battery holder in which a plurality of secondary battery cells are held. The secondary battery cell is a cylindrical battery, and the secondary battery cell, which is arranged at a position where the number of adjacent secondary battery cells is the smallest, is used as a target battery cell A. In the battery holder, a plurality of holding cylinder parts for holding a plurality of secondary battery cells in a parallel orientation are integrally molded. The plurality of holding cylinder parts are provided with: a target partition wall part in which a partition wall part, which is formed between secondary battery cells adjacent to each other, is formed between the target battery cell A and an adjacent secondary battery cell; and non-target partition wall parts formed between the secondary battery cells excluding the target battery cell A. The thickness of the target partition wall part is made larger than the average value of the thickness of all the partition wall parts to suppress heat conduction with the adjacent secondary battery cells.

Description

Battery pack

[0001] The present invention relates to a battery pack in which a plurality of secondary battery cells are held by a battery holder and housed in an exterior case.

[0002] Battery packs containing multiple secondary battery cells housed in an exterior case are used as power sources for portable electrical devices such as medical equipment, electric cleaners, and power tools. These battery packs house multiple secondary battery cells housed in a battery holder in a predetermined arrangement within the exterior case. In battery packs with multiple secondary battery cells, if an unsafe event such as an internal short circuit occurs in one of the secondary battery cells, the secondary battery cell may overheat and ignite. This heat may then heat adjacent secondary battery cells, resulting in a chain reaction and fire. In particular, if there is no route for dissipating the generated heat to the outside, the trapped heat may locally concentrate in adjacent secondary battery cells, causing overheating and potentially resulting in a chain reaction and fire. If an unsafe event occurs in one of the secondary battery cells within the battery pack and generates heat, the heat from the abnormal cell may spread to other secondary battery cells, potentially resulting in a fire, creating an extremely dangerous situation.

[0003] International Publication No. 2013 / 076770

[0004] The present invention was developed with the aim of preventing a chain reaction of fire spreading to adjacent secondary battery cells due to heat generated from an abnormal battery cell in a battery pack having multiple secondary battery cells, even if an unsafe event occurs in one of the secondary battery cells, such as smoke or fire.

[0005] One objective of one embodiment of the present disclosure is to provide a battery pack that can effectively prevent a chain reaction of fires among adjacent secondary battery cells when a secondary battery cell housed in an exterior case becomes abnormal and generates heat by controlling the propagation of heat from the abnormal battery cell to adjacent secondary battery cells. Note that the description of this objective and problem of the present disclosure does not preclude the existence of other objectives and problems. Furthermore, one embodiment of the present disclosure does not necessarily solve all of these problems. Furthermore, other problems can be extracted from the description of the specification, drawings, and claims of the present disclosure.

[0006] A battery pack according to one embodiment of the present disclosure includes a plurality of secondary battery cells, a battery holder that houses the plurality of secondary battery cells, and an exterior case that houses the battery holder. The secondary battery cells are cylindrical batteries, and among the plurality of secondary battery cells housed in the battery holder, a secondary battery cell located at a position with the fewest adjacent secondary battery cells is defined as a target battery cell. The battery holder is integrally formed with a plurality of holding cylindrical portions that house the plurality of secondary battery cells and hold them parallel to each other. The plurality of holding cylindrical portions include partitions formed between adjacent secondary battery cells and a peripheral wall portion that forms the outer surface of the battery holder. The partitions include a target partition portion formed between a secondary battery cell adjacent to the target battery cell and the target battery cell, and non-target partition portions formed between secondary battery cells other than the target battery cell. The thickness (T1) of the target partition portion is greater than the average thickness (Ta) of all partition portions to suppress heat conduction between adjacent secondary battery cells.

[0007] According to the battery pack disclosed herein, when a secondary battery cell housed in an outer case becomes abnormal and generates heat, the transfer of heat from the abnormal battery cell to the surrounding area can be controlled, thereby effectively preventing a chain reaction of fire spreading to adjacent secondary battery cells.

[0008] 1 is a perspective view of a battery pack according to a first embodiment of the present disclosure. FIG. 1 is an exploded perspective view of the battery pack shown in FIG. 2. FIG. 3 is an exploded perspective view of the battery pack shown in FIG. 3, viewed from below. FIG. 1 is a vertical cross-sectional view taken along line V-V of the battery pack shown in FIG. 1. FIG. 5 is a vertical cross-sectional view taken along line VI-VI of the battery pack shown in FIG. 5. FIG. 5 is a vertical cross-sectional view taken along line VII-VII of the battery pack shown in FIG. 5. FIG. 5 is an exploded cross-sectional view corresponding to the cross section taken along line VIII-VIII of the battery pack shown in FIG. 5. FIG. 6 is a cross-sectional view of a battery holder in which secondary battery cells are housed. FIG. 7 is an enlarged cross-sectional view of a main portion of the battery pack shown in FIG. 5. FIG. 7 is an exploded cross-sectional view of a battery pack according to a second embodiment. FIG. 8 is a cross-sectional view showing a state in which a secondary battery is housed in a battery holder according to a comparative example.

[0009] First, we will explain one of the key points of this disclosure. In a battery pack in which multiple rechargeable battery cells are housed in a battery holder, and this battery holder is housed in an exterior case, if an unsafe event such as an internal short circuit occurs in one of the rechargeable battery cells and ignites, the heat generated by the battery pack can heat nearby rechargeable battery cells, causing a chain reaction of fire and fire spread. To solve this problem, a structure that controls the conduction of heat from an abnormal battery cell to adjacent rechargeable battery cells has been studied.

[0010] Battery holders that store multiple secondary battery cells in a predetermined orientation include those with a structure consisting of multiple connected cylindrical holding tubes that separate and separate each secondary battery cell. This structure allows for control of heat conduction between adjacent secondary battery cells by adjusting the thickness of the partitions formed between the adjacent secondary battery cells. For example, this battery holder can suppress heat conduction from an abnormal battery cell to the adjacent secondary battery cell by thickening the partitions between adjacent secondary battery cells, thereby preventing harmful effects such as fire from the abnormal battery cell. However, a structure with thick partitions between adjacent secondary battery cells has the disadvantage of increasing the external dimensions of the battery holder when storing multiple secondary battery cells aligned diametrically. Ensuring sufficient inter-cell distance is particularly difficult within the limited pack volume of a small battery pack. Furthermore, thicker partitions between secondary battery cells increase the amount of resin used, increasing manufacturing costs, and also necessitating measures to prevent sink marks when molding thick partitions into resin. For this reason, the structure of thickening the partition walls between the secondary battery cells is not necessarily an effective method for all battery packs.

[0011] In light of the above, the inventor conducted an experiment to determine whether fire would spread to adjacent secondary battery cells if an abnormality occurred in a secondary battery cell placed in a specific position in a battery holder while maintaining a uniform thickness of the partitions between adjacent secondary battery cells. In this experiment, as shown in Fig. 12 , a battery holder 92 was used in which eight secondary battery cells 91 were arranged in two tiers, four cells in a row, one above the other, in a rice-bag-like configuration. The thickness of the partitions 82 between adjacent secondary battery cells 91 was maintained uniform, and an abnormality was caused in the secondary battery cells 91 placed in positions A to D in Fig. 12 . The experiment showed that when the thickness of all partitions 82 was set to a specific value, fire spread to the adjacent secondary battery cells 91 placed in positions B and C when an abnormality occurred in the secondary battery cell 91 placed in positions B to D. However, fire did not spread to any of the adjacent secondary battery cells 91 when an abnormality occurred in the secondary battery cell 91 placed in positions B to D. From the results of this experiment, the inventor hypothesized that if an abnormality occurs in the secondary battery cell 91 located at the position indicated by A, the heat generated from the abnormal battery cell will be concentrated on the two secondary battery cells 91 located at positions B and C, whereas if an abnormality occurs in a secondary battery cell 91 located at the positions indicated by B to D, the heat generated from the abnormal battery cell will be divided and conducted to three to four adjacent secondary battery cells 91, and the heat capacity of the adjacent secondary battery cells 91 will allow these secondary battery cells 91 to absorb the heat generated from the abnormal battery cell, thereby preventing the spread of fire.

[0012] Furthermore, based on the above hypothesis, the inventors investigated various structures through repeated trial and error, and as a result, they discovered that, among multiple secondary battery cells housed in a battery holder, the secondary battery cell located in the position with the fewest adjacent secondary battery cells (corresponding to the secondary battery cell at position A in Figure 12) is designated as the target battery cell, and by making the thickness of the partition wall provided between this target battery cell and the adjacent secondary battery cells greater than the aforementioned specific value, it is possible to prevent fire from spreading to the adjacent secondary battery cells located at positions B and C. Furthermore, even if the thicknesses of the partition walls provided between the other secondary battery cells excluding the target battery cell are made smaller than the aforementioned specific value, fire does not spread to any of the secondary battery cells. In other words, the present invention provides a battery pack that, in a battery holder that houses multiple secondary batteries, designates the secondary battery cell located in the position with the fewest adjacent secondary battery cells as the target battery cell, and by making the partition wall between this target battery cell and the adjacent secondary battery cells thicker while making the partition wall between the secondary battery cells excluding the target battery cell thinner, it is possible to effectively prevent fire from spreading even if an abnormality occurs in any of the secondary battery cells while suppressing the external dimensions of the battery holder from becoming larger.

[0013] A battery pack according to an embodiment of the present disclosure includes a plurality of secondary battery cells, a battery holder that houses the plurality of secondary battery cells, and an exterior case that houses the battery holder. The secondary battery cells are cylindrical batteries, and a target battery cell is a secondary battery cell that is located in a position among the plurality of secondary battery cells housed in the battery holder and has the fewest number of adjacent secondary battery cells. The battery holder is integrally formed with a plurality of holding cylindrical portions that house the plurality of secondary battery cells and hold them parallel to each other. The plurality of holding cylindrical portions include partitions formed between adjacent secondary battery cells and a peripheral wall portion that forms the outer surface of the battery holder. The partitions include a target partition portion formed between the secondary battery cell adjacent to the target battery cell and the target battery cell, and non-target partition portions formed between secondary battery cells other than the target battery cell. The thickness (T1) of the target partition portion is greater than the average thickness (Ta) of all partition portions to suppress heat conduction between adjacent secondary battery cells.

[0014] According to the above configuration, of the multiple secondary battery cells stored in the battery holder, the secondary battery cell that is located in a position with the fewest number of adjacent secondary battery cells is set as the target battery cell, and the thickness (T1) of the target partition portion formed between the target battery cell and a secondary battery cell adjacent to the target battery cell is made larger than the average thickness (Ta) of all the target partition portions.Therefore, even if an abnormality occurs in a target battery cell that is located in a position that is most susceptible to fire spreading when heat is generated, the thick target partition portion provides a sufficient gap, thereby suppressing heat conduction between the target battery cell and adjacent secondary battery cells and effectively preventing fire spreading.

[0015] In this specification, the thickness of the partition wall refers to the thickness of the partition wall on a straight line connecting the centers of adjacent secondary battery cells when viewed in cross section of the battery holder, and in the case of a partition wall having a curved portion that follows the outer peripheral surface of a cylindrical battery, refers to the thickness at the part where the thickness is smallest.

[0016] In a battery pack according to another embodiment of the present disclosure, the non-target partition portion having the smallest thickness among the non-target partition portions is designated as the specific partition portion, and the thickness (T2) of the specific partition portion can be set to be less than the average thickness (Tb) of all the non-target partition portions.

[0017] According to the above configuration, the thickness (T2) of the specific partition section that has the smallest thickness among the non-target partition sections formed between secondary battery cells excluding the target battery cell is made smaller than the average thickness (Tb) of all the non-target partition sections. This prevents the external dimensions of the battery holder from becoming larger, promotes heat conduction between adjacent secondary battery cells via the specific partition section, and suppresses the spread of fire due to the thermal capacity of the secondary battery cells.

[0018] In another embodiment of the battery pack of the present disclosure, a battery holder is provided with a plurality of retaining tube portions arranged to store battery rows in multiple stages, each row having a plurality of secondary battery cells arranged diametrically, and the battery rows stacked in multiple stages are arranged such that the secondary battery cells of one battery row are located in the valleys formed between adjacent secondary battery cells of the other battery row, between the battery row arranged in the lower stage and the battery row arranged in the upper stage, and the battery holder can designate the non-target partition portions formed between secondary battery cells other than the target battery cell as the specific partition portions in each battery row.

[0019] According to the above configuration, the battery holder is arranged with multiple retaining tube portions so as to store battery rows in multiple stages, each row consisting of multiple secondary battery cells arranged diametrically, and in each battery row, the non-target partition portions formed between the secondary battery cells other than the target battery cell are designated as specific partition portions. Therefore, by offsetting the thickness (T1) of the target partition portion and the thickness (T2) of the specific partition portion, the overall length of the battery row can be effectively prevented from increasing, and the external dimensions of the battery holder can be kept from increasing.

[0020] In a battery pack according to another embodiment of the present disclosure, a battery holder is arranged with multiple retaining tube portions so as to store battery rows in two tiers, each row having multiple secondary battery cells arranged diametrically, and the battery rows stacked in two tiers have a target battery cell positioned at one end of the lower tier battery row and the other end opposite the upper tier battery row, so that the left-right positional deviation between the lower tier battery row and the upper tier battery row can be less than the radius of the secondary battery cell.

[0021] According to the above configuration, a battery row in which a plurality of secondary battery cells are arranged in the diameter direction is stored in two stages, and the spacing between the secondary battery cells stored in the battery holder is optimized to prevent the spread of fire, while effectively preventing the overall length of the battery holder from increasing.

[0022] In a battery pack according to another embodiment of the present disclosure, the battery holder has a plurality of retaining tube portions arranged to store battery rows in two tiers, each row having a plurality of secondary battery cells arranged diametrically, and the angle formed by the center line connecting the centers of adjacent left and right battery cells adjacent to the target battery cell in the left-right direction and the center line connecting the centers of adjacent upper and lower battery cells diagonally above and below the target battery cell is greater than 60 degrees, the non-target partition portions formed between the adjacent left and right battery cells and the adjacent upper and lower battery cells are restraining partition portions, and the thickness (T3) of the restraining partition portion can be greater than the thickness (T1) of the target partition portion.

[0023] According to the above configuration, the structure stores battery rows in two stages, each row having multiple secondary battery cells aligned diametrically. The spacing between the secondary battery cells stored in the battery holder is optimized to prevent fire spread, while effectively preventing the overall length of the battery holder from increasing. In addition, by making the thickness (T3) of the suppression partition wall portion formed between adjacent left and right battery cells and adjacent upper and lower battery cells positioned diagonally above and below larger than the thickness (T1) of the target partition wall portion, interference between adjacent left and right battery cells and adjacent upper and lower battery cells that are heated by the target battery cell when it generates heat is suppressed, thereby preventing fire spread.

[0024] In another embodiment of the battery pack of the present disclosure, a battery holder is arranged with multiple retaining tube portions so as to accommodate a single battery row in which multiple secondary battery cells are arranged in the diameter direction, and the secondary battery cells located at both ends of the battery row are designated as target battery cells, and non-target partition portions formed between secondary battery cells other than the target battery cells are designated as specific partition portions.

[0025] According to the above configuration, the battery holder houses a single battery row in which multiple secondary battery cells are arranged in the diameter direction, and has target battery cells at both ends of the battery row, with specific partition walls disposed between the secondary battery cells excluding the target battery cells. Therefore, by offsetting the thickness (T1) of the target partition wall and the thickness (T2) of the specific partition wall, the overall length of the battery row can be effectively prevented from increasing, and the external dimensions of the battery holder can be prevented from becoming larger.

[0026] In a battery pack according to another embodiment of the present disclosure, the difference (T1-T2) between the thickness (T1) of the target partition portion and the thickness (T2) of the specific partition portion can be 3.5% or more of the radius (R) of the secondary battery cell.

[0027] According to the above configuration, by changing the difference between the thickness (T1) of the target partition portion and the thickness (T2) of the specific partition portion in accordance with the radius (R) of the secondary battery cell, the battery holder ensures attenuation of heat conduction to prevent ignition for the target battery cell facing the thick target partition portion, while effectively utilizing the thermal capacity of the secondary battery cells other than the target battery cell, particularly for the secondary battery cell facing the thinnest specific partition portion.

[0028] In a battery pack according to another embodiment of the present disclosure, the thickness (T1) of the target partition wall portion can be set to 1.5 mm or more, and the thickness (T2) of the specific partition wall portion can be set to 1.1 mm or less.

[0029] In another embodiment of the battery pack of the present disclosure, the battery holder is provided with a heat capacity portion that is connected to the outside of the retaining cylindrical portion and absorbs heat from the secondary battery cells that is conducted from the retaining cylindrical portion, and the heat capacity portion is composed of a plurality of connecting ribs that are located in the valley portions formed between adjacent retaining cylindrical portions and are arranged in the extension direction of the valley portions to connect adjacent retaining cylindrical portions, and groove portions are formed between the adjacent connecting ribs, and the groove portions can be used as recesses to prevent sink marks when the heat capacity portion is resin-molded.

[0030] According to the above configuration, a heat capacity section is provided that is connected to the outside of the cylindrical holding section and absorbs heat from the secondary battery cells conducted from the cylindrical holding section. The heat capacity section is formed as multiple connecting ribs located in the valleys formed between adjacent cylindrical holding sections and arranged in the extension direction of the valleys, connecting adjacent peripheral wall sections. This allows for a large heat capacity section that effectively utilizes the space between the valleys to effectively absorb heat from heated secondary battery cells. The connecting ribs connecting adjacent cylindrical holding sections allow for efficient heat conduction from the cylindrical holding section to the heat conduction section, regardless of whether the secondary battery cells housed in either cylindrical holding section are generating heat. The multiple connecting ribs connecting adjacent peripheral wall sections also function as reinforcing ribs, improving the rigidity of the battery holder. Furthermore, the heat capacity section has grooves formed between adjacent connecting ribs that serve as sink mark prevention recesses during resin molding of the heat capacity section. The space in the valleys is effectively utilized to form deep sink mark prevention recesses, effectively preventing sink marks during resin molding.

[0031] In another embodiment of the present disclosure, a battery pack includes an exterior case having a connected heat capacity portion on its inner surface that is thermally coupled to a battery holder and absorbs heat conducted from the battery holder, the connected heat capacity portion being configured as a plurality of heat-conducting protrusions protruding from the interior surface of the exterior case, the plurality of heat-conducting protrusions being positioned opposite a plurality of grooves provided in the battery holder, and the plurality of grooves of the battery holder serving as fitting recesses into which the heat-conducting protrusions are fitted. In this battery pack, when the battery holder is housed in the exterior case, the heat-conducting protrusions and the fitting recesses are fitted to thermally couple the battery holder and the exterior case, and heat generated by the secondary battery cells is conducted from the battery holder to the exterior case and dissipated to the outside.

[0032] According to the above configuration, the outer case has a plurality of heat-conducting protrusions protruding from the inner surface as a connected heat capacity portion that is thermally coupled to the battery holder and absorbs heat conducted from the battery holder, and the battery holder has a plurality of grooves that serve as fitting recesses into which the heat-conducting protrusions are fitted.With the battery holder housed in the outer case, the heat-conducting protrusions and fitting recesses are fitted together, and the heat generated by the cylindrical secondary battery is absorbed by the heat capacity portion and the connected heat capacity portion while being conducted from the battery holder to the outer case, allowing for efficient heat dissipation to the outside, effectively preventing the heat generated by the secondary battery cells from overheating adjacent secondary battery cells and causing them to catch fire.

[0033] Embodiments of the present disclosure will be described below with reference to the drawings. However, the embodiments described below are merely examples for embodying the technical concepts of the present disclosure, and the present disclosure is not limited to the following. Furthermore, this specification does not in any way specify the components set forth in the claims to be those of the embodiments. The dimensions, materials, shapes, relative positions, etc. of components described in the embodiments are not intended to limit the scope of the present disclosure, and are merely illustrative examples, unless otherwise specified. The size and relative positions of components shown in the drawings may be exaggerated for clarity. Furthermore, in the following description, the same names and symbols indicate components that are identical or of the same quality, and detailed descriptions will be omitted as appropriate. Furthermore, the elements constituting the present invention may be configured with the same components, such that multiple elements are served by a single component, or conversely, the functions of a single component may be shared by multiple components.

[0034] The battery pack according to the present disclosure can be suitably used as a battery pack used as a power source for portable medical equipment, electric cleaners, power tools, and other portable electrical equipment. It can also be used as a power source for mobile objects such as power-assisted bicycles and electric carts. Hereinafter, as one embodiment of the present disclosure, a battery pack used as a power source for a pulse oximeter in medical equipment will be described.

[0035] [Embodiment 1] A battery pack according to Embodiment 1 of the present disclosure is shown in Figures 1 to 9. In these figures, Figure 1 is a perspective view showing a battery pack according to Embodiment 1, Figure 2 is an exploded perspective view of the battery pack of Figure 1, Figure 3 is an exploded perspective view further disassembled of the battery pack of Figure 2, Figure 4 is an exploded perspective view of Figure 3 as viewed from below, Figure 5 is a vertical cross-sectional view of the battery pack of Figure 1 along line V-V, Figures 6 and 7 are vertical cross-sectional views of the battery pack of Figure 5 along line VI-VI and line VII-VII, respectively, Figure 8 is an exploded cross-sectional view corresponding to the cross section of the battery pack of Figure 5 along line VIII-VIII, Figure 9 is a cross-sectional view of a battery holder in which secondary battery cells are housed, and Figure 10 is an enlarged cross-sectional view of a main portion of Figure 5.

[0036] The battery pack 100 shown in these figures includes multiple secondary battery cells 1, a battery holder 2 that houses these secondary battery cells 1, and an exterior case 3 that houses the battery holder 2. The secondary battery cells 1 are cylindrical batteries, and of the multiple secondary battery cells 1 housed in the battery holder 2, the secondary battery cell 1 located in a position with the fewest adjacent secondary battery cells 1 is designated as the target battery cell A. The battery holder 2 is integrally formed with multiple holding cylindrical portions 21 that house the multiple secondary battery cells 1 therein and hold them parallel to each other. The multiple holding cylindrical portions 21 include partitions 22 formed between adjacent secondary battery cells 1 and an outer peripheral wall portion 23 that forms the outer surfaces of the multiple holding cylindrical portions 21. The partitions 22 include a target partition portion 22A formed between the secondary battery cell 1 adjacent to the target battery cell A and the target battery cell A, and non-target partition portions 22B formed between the secondary battery cells 1 other than the target battery cell A. The battery holder 2 has a thickness (T1) of the target partition wall portion 22A that is greater than the average thickness (Ta) of all the partition wall portions 22 to suppress heat conduction between adjacent secondary battery cells 1. Furthermore, the battery holder 2 designates the non-target partition wall portion 22B with the smallest thickness as the specific partition wall portion 22x, and sets the thickness (T2) of the specific partition wall portion 22x to be equal to or less than the average thickness (Tb) of all the non-target partition wall portions 22B.

[0037] 2 shows a battery pack 100 in which a battery holder 2 containing multiple rechargeable battery cells 1 in a predetermined arrangement is housed in an exterior case 3 as a battery assembly 10. The multiple rechargeable battery cells 1 housed in a predetermined arrangement in the battery holder 2 have lead plates 13 connected to both end faces for a predetermined connection. Furthermore, a circuit board 12 is stacked on the top surface of the battery holder 2, and each lead plate 13 is connected to the circuit board 12 to form the battery assembly 10.

[0038] (Secondary Battery Cell 1) The secondary battery cell 1, which is a cylindrical battery, houses an electrode assembly in a cylindrical metal outer can with a bottom and is filled with an electrolyte. The opening of the outer can is crimped and hermetically sealed with a sealing body. In this secondary battery cell 1, the sealing body and the outer can serve as positive and negative electrodes. The end face of the secondary battery cell 1 sealed by the sealing body is designated as a first end face 1A, and the bottom face of the outer can is designated as a second end face 1B. A highly energy-efficient nonaqueous electrolyte secondary battery, such as a lithium-ion secondary battery, can be suitably used for this secondary battery cell 1. However, the battery pack of the present disclosure does not limit the secondary battery cells to lithium-ion secondary batteries. Any rechargeable battery, such as a nickel-metal hydride battery, a nickel-cadmium battery, or an all-solid-state battery, can also be used for the secondary battery cell.

[0039] The battery pack 100 shown in Figures 3 and 4 contains eight rechargeable battery cells 1. The number of rechargeable battery cells 1 contained in the battery pack 100 is determined to be an optimum number taking into consideration the application and capacity of the battery pack 100, the capacity of each rechargeable battery cell 1, and other factors, and can be, for example, 3 to 20. Furthermore, the battery pack 100 shown in the figures has eight rechargeable battery cells 1 connected in four parallel and two series via lead plates 13. However, the connection state of the multiple rechargeable battery cells 1 can be changed in various ways depending on the application and purpose. The battery pack 100 can increase the output voltage by increasing the number of rechargeable battery cells 1 connected in series, and can increase the maximum current supplied to a load and the charge / discharge capacity of the entire battery pack by increasing the number of rechargeable battery cells 1 connected in parallel.

[0040] (Battery Holder 2) The battery holder 2 holds multiple rechargeable battery cells 1 in a fixed position with a predetermined orientation. The battery holder 2 shown in Figures 2 to 4 holds multiple rechargeable battery cells 1 parallel to one another, with both longitudinal end faces of the rechargeable battery cells 1 aligned flush with one another. The battery holder 2 shown in the figures includes multiple holding tubular sections 21 that house multiple rechargeable battery cells 1 and hold them parallel to one another, and a heat capacity section 4 connected to the outside of the holding tubular sections 21 and absorbing heat conducted from the holding tubular sections 21 to the rechargeable battery cells 1. The battery holder 2 is made of resin, and the multiple holding tubular sections 21 are integrally molded, and the heat capacity section 4 is integrally molded with the holding tubular sections 21. The resin used to form the battery holder 2 is preferably a material with excellent thermal conductivity, insulation, and heat resistance, such as polycarbonate or ABS.

[0041] (Retaining tubular portion 21) As shown in Figures 3 and 4, the battery holder 2 has a structure in which multiple retaining tubular portions 21 are integrally connected and each accommodates a respective rechargeable battery cell 1. Each retaining tubular portion 21 is cylindrical with an inner shape that follows the outer shape of the rechargeable battery cell 1, and defines a battery storage section 20 inside that accommodates the rechargeable battery cell 1. As shown in Figures 6 to 9, the multiple retaining tubular portions 21 are integrally connected at the boundaries between adjacent rechargeable battery cells 1 to form partition walls 22. Furthermore, in the outer peripheral region where there are no adjacent rechargeable battery cells 1, an outer peripheral wall portion 23 forms the outer surface of the battery holder 2 and follows the outer periphery of the rechargeable battery cell 1.

[0042] The battery holder 2 is provided with multiple integrally molded retaining cylindrical portions 21 so that it can store multiple rechargeable battery cells 1 aligned in the diameter direction (left-right direction in FIGS. 6 to 9 ) in one or more tiers. A battery holder 2 that stores multiple tiers of left-right aligned rechargeable battery cells 1 has multiple retaining cylindrical portions 21 molded in a bale-like arrangement so that vertically arranged rechargeable battery cells 1 are located in the valleys between the left-right arranged rechargeable battery cells 1. The battery holder 2 shown in the figures has four rechargeable battery cells 1 aligned linearly in the left-right direction to form a battery string 11, and two battery strings 11 stacked in two tiers, storing eight rechargeable battery cells 1 in a bale-like arrangement with the secondary battery cells 1 of one battery string 11 located in the valleys between the secondary battery cells 1 of the other battery string 11. The battery holder 2 shown in the figure has eight holding tube portions 21 integrally connected together so that it has a generally parallelogram shape in cross section.

[0043] As shown in Fig. 9 , in the secondary battery cells 1 where battery rows 11 arranged in the diameter direction (left-right direction in the figure) are stacked in multiple stages, the secondary battery cell 1 located in the position with the fewest number of adjacent secondary battery cells 1 is the target battery cell A that is most likely to catch fire in the event of an abnormality. In the example of Fig. 9 , the secondary battery cell 1 located at the leftmost position in the lower battery row 11 and the secondary battery cell 1 located at the rightmost position in the upper battery row 11 are the two with the fewest number of adjacent secondary battery cells 1, and are therefore the target battery cells A. In the figure, the vertically adjacent battery cells C adjacent to the target battery cell A in the vertical direction have three adjacent secondary battery cells 1, while the horizontally adjacent battery cells B adjacent to the target battery cell A and the middle battery cell D adjacent to these horizontally adjacent battery cells B have four adjacent secondary battery cells 1. Therefore, in this arrangement, for the left and right adjacent battery cells B, the top and bottom adjacent battery cells C, and the middle battery cell D, which have three to four adjacent secondary battery cells 1, even if an abnormality occurs, the large number of adjacent secondary battery cells 1 increases the thermal capacity of the adjacent secondary battery cells 1, and these secondary battery cells 1 absorb the heat, thereby suppressing the spread of fire. In contrast, the target battery cell A, which has the fewest number of adjacent secondary battery cells 1 (two), has a small number of adjacent secondary battery cells 1 and a small thermal capacity, making it the secondary battery cell 1 most susceptible to the spread of fire. Therefore, in the battery pack 100 of the present disclosure, the secondary battery cell 1 arranged in such a position with the fewest number of adjacent secondary battery cells 1 is designated as the target battery cell A, and the thickness of the partition wall 22 formed between the secondary battery cell 1 adjacent to this target battery cell A and the target battery cell A is increased to suppress heat conduction from the target battery cell A to the adjacent secondary battery cell 1.

[0044] The partition wall portions 22 of the battery holder 2 shown in the figure include target partition wall portions 22A formed between the target battery cell A and the secondary battery cells 1 adjacent to the target battery cell A (left-right adjacent battery cells B, and above-bottom adjacent battery cells C), and non-target partition wall portions 22B formed between the secondary battery cells 1 other than the target battery cell A (left-right adjacent battery cells B, above-bottom adjacent battery cells C, and middle battery cell D). The thickness (T1) of the target partition wall portion 22A is made larger than the average thickness (Ta) of all partition wall portions 22 to suppress heat conduction between the target battery cell 1 and the adjacent secondary battery cell 1. In this way, by increasing the thickness (T1) of the target partition wall portion 22A provided between the target battery cell A located in a position most susceptible to fire spread when heated and the adjacent secondary battery cell 1, even if an abnormality occurs in the target battery cell A, the thick target partition wall portion 22A can provide a sufficient gap between the target battery cell A and the adjacent secondary battery cell 1, suppressing heat conduction from the target battery cell A to the adjacent secondary battery cell 1 and effectively preventing fire spread. In particular, by making the thickness (T1) of the target partition wall portion 22A greater than the average thickness (Ta) of all the partition wall portions 22, heat conduction can be more reliably suppressed than in the non-target partition wall portion 22B, effectively preventing fire spread. Here, in a battery pack 100 in which the radius (R) of the secondary battery cell 1 is 9 mm, the thickness (T1) of the target partition wall portion 22A can be set to a ratio of 16% or more to the radius (R) of the secondary battery cell 1. For example, the thickness (T1) of the target partition wall portion 22A can be set to 1.44 mm or more, preferably 1.5 mm or more. Furthermore, when specifying the thickness (T1) of the target partition wall portion 22A as a ratio to the radius (R) of the secondary battery cell 1, it is also preferable to change the ratio based on the radius (R) of the secondary battery cell 1. In a battery pack 100 in which the radius (R) of the secondary battery cell 1 is 11 mm, the thickness (T1) of the target partition wall 22A can be 13.5% or more of the radius (R) of the secondary battery cell 1. For example, the thickness (T1) of the target partition wall 22A can be 1.485 mm or more, preferably 1.5 mm or more. Therefore, in a battery pack in which the radius (R) of the secondary battery cell 1 is 9 to 11 mm, the thickness (T1) of the target partition wall 22A provided on the battery holder 2 can be 1.5 mm or more to effectively suppress heat conduction between adjacent secondary battery cells 1.

[0045] Furthermore, the battery pack 100 according to the present disclosure not only prevents fire spread by increasing the thickness of the target partition wall portion 22A between the target battery cell A and the adjacent secondary battery cell 1 to widen the gap between them, but also prevents the external dimensions of the battery holder from becoming larger by reducing the thickness of the non-target partition wall portion 22B. For example, the non-target partition wall portion 22B with the smallest thickness can be designated as the specific partition wall portion 22x, and the thickness (T2) of this specific partition wall portion 22x can be set to be equal to or less than the average thickness (Tb) of all the non-target partition wall portions 22B. This structure, in which the non-target partition portions 22B formed between the secondary battery cells 1 other than the target battery cell A are thinned and the thickness (T2) of the specific partition portion 22x, which has the smallest thickness, is made smaller than the average thickness (Tb) of all the non-target partition portions 22B, prevents the external dimensions of the battery holder 2 from increasing. This promotes heat conduction between adjacent secondary battery cells 1 via the specific partition portion 22x, and suppresses fire spread due to the thermal capacity of the secondary battery cells 1. Here, in a battery pack 100 in which the radius (R) of the secondary battery cell 1 is 9 mm, the thickness (T2) of the specific partition portion 22x can be set to a ratio of 12% or less to the radius (R) of the secondary battery cell 1. For example, the thickness (T2) of the specific partition portion 22x can be set to 1.08 mm or less, preferably 1.05 mm or less. Furthermore, when specifying the thickness (T2) of the specific partition portion 22x as a ratio to the radius (R) of the secondary battery cell 1, it is also preferable to change the ratio based on the radius (R) of the secondary battery cell 1. In a battery pack 100 in which the radius (R) of the secondary battery cell 1 is 11 mm, the thickness (T2) of the specific partition wall 22x can be 10% or less of the radius (R) of the secondary battery cell 1. For example, the thickness (T2) of the specific partition wall 22x can be 1.1 mm or less, preferably 1.05 mm or less. Therefore, in a battery pack in which the radius (R) of the secondary battery cell 1 is 9 to 11 mm, the thickness (T2) of the specific partition wall 22x provided on the battery holder 2 is 1.1 mm or less, which promotes heat conduction between adjacent asymmetric battery cells via the specific partition wall and suppresses spreading of fire due to the thermal capacity of the secondary battery cells.

[0046] Furthermore, the difference (T1-T2) between the thickness (T1) of the target partition portion 22A and the thickness (T2) of the specific partition portion 22x can be set to 3.5% or more of the radius (R) of the secondary battery cell 1, and preferably 4% to 7% of the radius (R) of the secondary battery cell 1. In this way, by specifying the difference (T1-T2) between the thickness (T1) of the target partition portion and the thickness (T2) of the specific partition portion as a ratio to the radius (R) of the secondary battery cell, heat conduction to the secondary battery cell 1 facing the target battery cell can be suppressed according to the outer diameter of the secondary battery cell 1, while heat conduction between adjacent asymmetric battery cells via the specific partition portion 22x can be promoted, making effective use of heat capacity.

[0047] Furthermore, in a battery string 11 in which multiple secondary battery cells 1 are arranged in the diametrical direction, the non-target partitions 22B formed between the secondary battery cells 1 except for the target battery cell A can be designated as specific partitions 22x. For example, in the battery pack shown in FIG. 9 , among the multiple secondary battery cells 1 constituting each battery string 11, the target partitions 22A formed between the target battery cell A and its left-right adjacent battery cells B can have a thick thickness (T1), while the non-target partitions 22B formed between the left-right adjacent battery cells B and the middle battery cell D and the non-target partitions 22B formed between the middle battery cell D and its upper and lower adjacent battery cells C can be designated as specific partitions 22x with a thin thickness (T2). With this structure, the thicknesses (T1) of the target partitions 22A and the thicknesses (T2) of the specific partitions 22x between the secondary battery cells 1 constituting the same battery string 11 are offset, effectively preventing the overall length of the battery string 11 from increasing, and thus preventing the external dimensions of the battery holder 2 from increasing.

[0048] Furthermore, in a structure in which multiple battery strings 11 are stacked vertically as shown in Figure 9, the target partition wall 22A located between a target battery cell A arranged diagonally above and below and its adjacent vertical battery cells C is thickened to widen the gap between them, but in this case, the vertically arranged battery strings 11 are arranged in a unique manner to prevent the overall length of the battery holder 2 from increasing. To clearly explain the features of the arrangement of the battery strings 11 arranged above and below the battery holder 2 shown in Figure 9, we will first explain the arrangement of the rechargeable battery cells 91 in a battery holder 92 of Comparative Example 1.

[0049] Comparative Example 1 Figure 12 is a cross-sectional view of a battery holder 92 of Comparative Example 1 that accommodates eight rechargeable battery cells 91. The battery holder 92 shown in this figure has four rechargeable battery cells 91, each with an outer diameter of 18 mm, stacked horizontally in two rows, one above the other. The battery holder 92 in this figure has multiple integrally molded retaining tubes 81 that hold the rechargeable battery cells 91 parallel to one another. The partition walls 82 formed between each rechargeable battery cell 91 are all 1.2 mm thick, and the outer peripheral walls 83 are all 1.0 mm thick. Of the eight rechargeable battery cells 91 accommodated in this battery holder 92, the leftmost rechargeable battery cell 91 in the lower battery row 90 and the rightmost rechargeable battery cell 91 in the upper battery row 90 are designated as target battery cells A, since they have the fewest adjacent rechargeable battery cells 91 (two in the figure).

[0050] The total length (L) of this battery holder 92 is the distance between a vertical tangent line S1 drawn to the outer peripheral wall 83 covering the left side of the target battery cell A located at the leftmost position in the lower battery row 90 and a vertical tangent line S2 drawn to the right side of the outer peripheral wall 83 of the target battery cell A located at the rightmost position in the upper battery row 90. In this figure, this total length (L) is 87.2 mm. Consider the case where, from this state, the distance between the target battery cell A and its adjacent horizontal battery cells B is increased to 1.6 mm, and the distance between the target battery cell A and its adjacent vertical battery cells C is also increased to 1.6 mm. In this case, if the distance between the target battery cell A and the vertically adjacent battery cells C is increased to 1.6 m while maintaining the angle θ between the center line N connecting the centers of the target battery cell A and the vertically adjacent battery cells C and the center lines M1, M2 of the battery string 90 at 60 degrees, i.e., if the upper and lower battery strings 90 are moved 0.4 mm in a 60-degree diagonal direction, the external dimensions of the battery holder 92 will increase by 0.6 mm left and right and 0.35 mm up and down. In other words, if the target battery cell A is moved 0.4 mm left and right relative to the horizontally adjacent battery cell B, the overall length of the battery string 90 will increase by 0.4 mm, and if the target battery cell A is moved 0.4 mm in a 60-degree diagonal direction relative to the vertically adjacent battery cells C, the horizontal positions of the upper and lower battery strings will increase by 0.2 mm, resulting in an overall increase in the overall length of the battery holder 92 by 0.6 mm.

[0051] To solve this problem, in the battery holder 2 shown in Fig. 9 , in order to prevent the overall length (L) from increasing, the thickness of the partition wall 22 is adjusted by moving only the left and right adjacent battery cells B and the middle battery cell D in each battery string 11 without changing the positions of the target battery cell A and the upper and lower adjacent battery cells C, which are the secondary battery cells 1 located at the left and right ends. That is, in Fig. 9 compared to Fig. 12 , in each of the upper and lower battery strings 11, the middle battery cell D is moved 0.2 mm laterally closer to the upper and lower adjacent battery cells C, and the left and right adjacent battery cells B are moved 0.4 mm laterally away from the target battery cell A. As a result, the distance between the target battery cell A and the left and right adjacent battery cells B is 1.6 mm, and the distances between the left and right adjacent battery cells B and the middle battery cell D and between the middle battery cell D and the upper and lower adjacent battery cells C are all 1.0 mm. This allows the distance between the target battery cell A and the left and right adjacent battery cells B to be increased by 0.4 mm without changing the overall length of each battery string 11.

[0052] Furthermore, for vertically arranged secondary battery cells 1, if the distance between a target battery cell A and its vertically adjacent battery cells C, located diagonally above and below, is increased by 0.4 mm in a 60-degree diagonal direction, the relative positions of the vertically arranged battery strings 11 will shift by 0.2 mm left and right and 0.35 mm up and down, resulting in an increase in the overall external size of the battery holder 2. Regarding the external size of the battery pack 100, the following can be said: The vertical size of the battery pack 100 is determined not only by the vertical size of the battery holder 2, but also by the circuit board 12 stacked on the battery holder 2, the battery components mounted on the circuit board 12, and the shape and connection structure of the exterior case 3. Therefore, even if the vertical external size of the battery holder 2 increases slightly, this has little effect on the change in the external size of the battery pack 100 as a whole. In contrast, the horizontal size of the battery pack 100, i.e., its overall length, is determined by the horizontal size of the battery holder 2. Therefore, reducing the overall length (L) of the battery holder 2 is extremely important in reducing the external size of the battery pack 100.

[0053] Therefore, to prevent the overall length (L) of the battery holder 2 shown in Fig. 9 from increasing, the gap between the target battery cell A and its adjacent vertical battery cells C is increased by 0.4 mm by moving the upper and lower battery rows 11 vertically from the state shown in Fig. 12 without moving the upper and lower battery rows 11 diagonally at 60 degrees. For example, in the battery holder 92 shown in Fig. 12 , in which all partition walls 82 are 1.2 mm thick, the distance (D) between the center line M1 passing through the center of the lower battery row 90 and the center line M2 passing through the center of the upper battery row 90 is 16.6 mm. In contrast, in the battery holder 2 shown in Fig. 9 , the gap between the lower and upper battery rows 11 is increased by 0.5 mm in the vertical direction so that the distance (D) between the center line M1 and the center line M2 is 17.1 mm. This allows the gap between the target battery cell A and its adjacent vertical battery cells C, which are positioned diagonally above and below, to be increased by 0.4 mm. Compared to the battery holder 92 shown in Figure 12, this structure of the battery holder 2 allows for a wider gap between the target battery cell A and the adjacent secondary battery cells 1 (the adjacent battery cells B on the left and right and the adjacent battery cells C on the top and bottom) without increasing the overall length (L) in the left-right direction.

[0054] To achieve the above structure, in the battery holder 2 shown in FIG. 9 , the angle θ between the center line N connecting the centers of the target battery cell A and the vertically adjacent battery cells C, which are diagonally adjacent to each other, and the center lines M1, M2 of the battery rows 11, is set to be greater than 60 degrees. In particular, the battery holder 2 shown in FIG. 9 is designed so that the left-right misalignment between the lower battery row 11 and the upper battery row 11 is less than the radius (R) of the secondary battery cell 1. Specifically, in the battery holder 2 shown in FIG. 9 , the distance between the vertical tangent line S3 drawn to the outer peripheral wall 23 covering the right side of the rightmost vertically adjacent battery cell C in the lower battery row 11 and the vertical tangent line S2 drawn to the outer peripheral wall 23 covering the right side of the target battery cell A, which is the rightmost right in the upper battery row 11, is equal to or less than the radius of the secondary battery cell 1. In other words, the battery holder 2 is positioned so that a perpendicular line S4 drawn from the center of the rightmost target battery cell A in the upper battery row 11 to the center line M1 of the lower battery row 11 touches or intersects with the rightmost adjacent battery cell C in the lower battery row 11. This structure provides the advantage of preventing the overall length (L) of the battery holder 2 from becoming too long while maintaining the optimal spacing between the secondary battery cells 1 housed in the battery holder to prevent the spread of fire.

[0055] 9 , the distance between vertically adjacent left and right battery cells B and C, and between vertically adjacent middle battery cells D, is 1.8 mm, and the distance between vertically adjacent left and right battery cells B and D is 1.3 mm. Therefore, restricting partitions 22y with a thickness (T3) of 1.8 mm are formed between the vertically adjacent left and right battery cells B and C, and between the vertically adjacent middle battery cells D, and intermediate partitions 22z with a thickness (T4) of 1.3 mm are formed between the vertically adjacent left and right battery cells B and D.

[0056] As described above, in a battery holder 2 in which the angle θ formed by the center line N connecting the centers of the target battery cell A and the vertically adjacent battery cells C that are diagonally adjacent in the vertical direction and the center line M1, M2 of the battery string 11 is greater than 60 degrees, the thickness (T3) of the restraining partition wall portion 22y formed between the diagonally adjacent left-right battery cells B and the vertically adjacent battery cells C can be made greater than the thickness (T1) of the target partition wall portion 22A. This structure has the advantage of being able to widen the gap between the horizontally adjacent battery cells B and the vertically adjacent battery cells C that are heated by the target battery cell A when it generates heat, and is able to suppress the spread of fire caused by interference between the horizontally adjacent battery cells B and the vertically adjacent battery cells C. Furthermore, for the left and right adjacent battery cells B and the top and bottom adjacent battery cells C that are heated when the target battery cell A generates heat, a specific partition section 22x is provided between each of them and the middle battery cell D adjacent to them in the left and right direction to narrow the gap.This allows effective heat conduction from the left and right adjacent battery cells B to the middle battery cell D, and from the top and bottom adjacent battery cells C to the middle battery cell D, thereby suppressing the spread of fire due to the heat capacity of the middle battery cell D.

[0057] (Holder Tube Portion 2A, Holder Lid Portion 2B) The battery holder 2 shown in Figures 3 to 5 is composed of three components: a holder tube portion 2A that holds the longitudinal middle of the rechargeable battery cell 1, and a pair of holder lid portions 2B connected to both ends of the holder tube portion 2A and holding both ends of the rechargeable battery cell 1. As shown in Figures 4 and 5, the holder tube portion 2A is integrally connected to multiple holding tube portions 21 in a predetermined arrangement. The holder tube portion 2A, which is composed of multiple holding tube portions 21, has openings at both ends, each of which is closed by a pair of holder lid portions 2B. When the battery storage section 20, which is the internal space of each holding tube portion 21, is molded using a mold, a draft angle is provided so that the inner diameter gradually increases from the center toward the opening edge, thereby making the inner diameter at the center the smallest. As a result, the rechargeable battery cell 1 stored in the holding tube portion 21 is held with the center portion closest to the inner surface of the holding tube portion 21. Normally, the central portion of a secondary battery cell 1 is the most likely to expand and generate heat, so a structure that tightly contacts this portion with the inner surface of the holding tube portion 21 has the advantage of being able to efficiently conduct heat from the secondary battery cell 1 to the holding tube portion 21.

[0058] As shown in Figures 3 to 5, the holder lid 2B has an opposing surface shaped to match the end shape of the holder tubular portion 2A, and is structured to cover and hold the ends of the rechargeable battery cells 1 that protrude from the opening of the holder tubular portion 2A. In the example shown in Figure 5, the holder lid 2B has tubular holding portions 41 that hold the ends of the rechargeable battery cells 1. These holding portions 41 hold the ends of the rechargeable battery cells 1 and are inserted inside the edge of the opening of the holder tubular portion 2A to hold the ends of the rechargeable battery cells 1 in place. Furthermore, the holder lid 2B has electrode windows 42 that expose the end surface electrodes of the rechargeable battery cells 1. Lead plates 13 are connected to the first end surface 1A and second end surface 1B exposed through the electrode windows 42, connecting the multiple rechargeable battery cells 1 in series and parallel.

[0059] Although not shown, a battery holder consisting of multiple holding tubes can also be configured as two separate holders split in half at the middle. In this battery holder, the ends of the rechargeable battery cells are inserted into the openings of the two separate holders, and the rechargeable battery cells are stored inside with the two holders sandwiching the rechargeable battery cells from both sides.

[0060] (Heat Capacity Section 4) Furthermore, the battery holder 2 shown in the figures includes a heat capacity section 4 on the outside of the cylindrical holding section 21 that absorbs heat conducted from the rechargeable battery cells 1 to effectively absorb and dissipate the heat from the stored rechargeable battery cells 1. This structure, combined with the control of heat conduction by adjusting the thickness of the partition section 22, has the advantage of more effectively preventing fire spread. As shown in Figures 3 to 7 , the heat capacity section 4 is integrally molded with the cylindrical holding section 21 and has the heat capacity to absorb heat conducted from the rechargeable battery cells 1 through the cylindrical holding section 21. The heat capacity section 4 is located in the valley section 24 formed between adjacent cylindrical holding sections 21 and is integrally molded across the cylindrical holding sections 21 to effectively absorb heat conducted from the cylindrical holding sections 21. This structure, which provides the heat capacity section 4 in the valley section 24 between adjacent cylindrical holding sections 21, allows the heat capacity section 4 to be provided by effectively utilizing dead space in the battery holder 2. In particular, by molding the battery holder 2 so that it spans adjacent cylindrical holding sections 21, heat generated by any of the secondary battery cells 1 housed in the cylindrical holding sections 21 can be reliably transferred to the heat capacity section 4 and absorbed. Increasing the volume of the heat capacity section 4 located in the valley section 24 between adjacent cylindrical holding sections 21 increases the heat capacity and effectively absorbs heat conducted from the cylindrical holding sections 21. However, a large-volume heat capacity section 4 can become lumpy and cause sink marks during resin molding. To address this issue, the battery holder 2 shown in the figure has a sink mark prevention recess 25 adjacent to the heat capacity section 4 to prevent sink marks during molding of the heat capacity section 4. This sink mark prevention recess 25 also serves as the fitting recess 5 into which the heat conduction protrusion 7 of the exterior case 3, described below, fits.

[0061] (Connecting Ribs 26, Grooves 27) The heat capacity portion 4 shown in Figures 3 to 10 is composed of multiple connecting ribs 26 connecting adjacent holding tubular portions 21. The multiple connecting ribs 26 are arranged in multiple rows parallel to each other at predetermined intervals along the extension direction of the valley portions 24 between adjacent holding tubular portions 21. This battery holder 2 divides the heat capacity portion 4 formed in the valley portions 24 into multiple rows, providing gaps between adjacent heat capacity portions 4 and sink mark prevention recesses 25. Each connecting rib has a predetermined thickness and is integrally connected to the holding tubular portion 21 along the valley portions 24, which are generally V-shaped in cross section and formed between adjacent holding tubular portions 21. The connecting ribs 26 in this structure also function as reinforcing ribs that reinforce adjacent holding tubular portions 21. This structure, in which multiple connecting ribs 26 connect adjacent holding tubular portions 21 and reinforce them, increases the rigidity of the battery holder 2 and improves its resistance to impacts such as dropping. 10, the thickness (t) of each connecting rib 26 is a thickness that can maintain sufficient strength while ensuring heat capacity, and is, for example, 3 mm or more, preferably 4 mm or more. The thickness (t) of each connecting rib 26 is a thickness that can prevent sink marks from occurring during resin molding, and is, for example, 10 mm or less, preferably 8 mm or less.

[0062] The heat capacity portion 4, which is made up of a plurality of connecting ribs 26, has grooves 27 with a predetermined width (m) formed between adjacent connecting ribs 26, and these grooves 27 serve as sink mark prevention recesses 25. The grooves 27 formed in the valley portions 24 are formed deep enough to reach the bottom of the valley portions 24, and by making the opposing surfaces between these sink mark prevention recesses 25 and the connecting ribs 26 deep and wide, adjacent connecting ribs 26 are separated over a wider area, reliably preventing sink marks during molding of the heat capacity portion 4. Furthermore, by forming the grooves 27 deep and wide, when the heat conduction protrusions 7 are fitted into the grooves 27 as fitting recesses 5, they can be connected to the heat conduction protrusions 7 over a wider area while maintaining a reliable thermal coupling state. Furthermore, as shown in Figure 8, the groove portion 27 partially exposes the outer peripheral surface of the holding tube portion 21, and as shown in Figure 7, the heat conduction protrusions 7 of the outer case 3 are guided into the groove portion 27, and the heat conduction protrusions 7 are brought close to or in close contact with the holding tube portion 21, thereby achieving thermal coupling.

[0063] The battery holder 2 shown in Figures 3, 4, and 6-8 has multiple cylindrical holding sections 21 stacked in two stages in a bale-like configuration, with the multiple cylindrical holding sections 21 connected to form a roughly parallelogram in cross section. This battery holder 2 has multiple rows of valleys 24 on its underside, with multiple rows of connecting ribs 26 serving as heat capacity sections 4, and grooves 27 between adjacent connecting ribs 26 to form sink-prevention recesses 25. These sink-prevention recesses 25 serve as mating recesses 5 that guide multiple heat-conducting protrusions 7 protruding from the bottom surface of the exterior case 3 in which the battery holder 2 is housed. Furthermore, the battery holder 2 shown in Figures 6 and 7 has an undercut surface 28 on the side of the battery holder 2 formed by stacking vertically stacked retaining cylindrical sections 21. The battery holder 2 has multiple retaining cylindrical sections 21 connected to form a parallelogram in cross section. The undercut-shaped sloped side (the right side in Figures 6 and 7) of the battery holder 2 is also provided with a heat capacity section 4 and a sink mark prevention recess 25. The sink mark prevention recess 25 on the undercut surface 28 serves as a mating recess 5 for guiding multiple heat conduction protrusions 7 protruding from the sloped side of the exterior case 3. Furthermore, the battery holder 2 shown in Figures 3, 6, and 7 has multiple rows of connecting ribs 26 as heat capacity sections 4 in the multiple rows of valleys 24 formed on the top surface, and grooves 27 between adjacent connecting ribs 26 to form sink mark prevention recesses 25. The grooves 27 formed on the top surface do not double as fitting recesses 5 because there are no opposing heat-conducting protrusions 7, and instead function as sink-mark prevention recesses 25. By providing a heat capacity portion 4 on the top surface as well, this structure can absorb heat from the secondary battery cells 1 arranged in the upper tier while increasing rigidity by using the connecting ribs 26 as reinforcing ribs. In particular, by dividing the heat capacity portion 4 into multiple connecting ribs 26 and providing grooves 27 between them, the surface area of ​​the heat capacity portion 4 can be increased, improving heat dissipation characteristics.

[0064] The battery holder 2 described above has multiple connecting ribs 26 formed on the outer periphery of the holder tubular portion 2A, which function as the heat capacity portion 4. This structure, which provides the heat capacity portion 4 on the holder tubular portion 2A, is characterized by its ability to effectively absorb and dissipate heat near the center of the rechargeable battery cell 1, a region prone to heat generation. In particular, grooves 27 formed on the holder tubular portion 2A function as mating recesses 5, which thermally couple to the heat-conducting protrusions 7 on the exterior case 3, thereby more effectively dissipating heat from the rechargeable battery cell 1 to the outside. Furthermore, the battery holder 2 shown in the figure also has connecting ribs 26 formed on the holder lid 2B as the heat capacity portion 4. While the connecting ribs 26 formed on the holder lid 2B are not thermally coupled to the heat-conducting protrusions 7 on the exterior case 3, they reinforce the holder lid 2B, increasing its strength, and their increased surface area allows for more effective dissipation of heat conducted from the holder lid 2B. However, it is also possible to provide a mating recess adjacent to the connecting rib formed on the holder lid and provide a heat-conducting protrusion on the exterior case that fits into this mating recess. This structure allows the heat from the holder lid to be effectively conducted to the exterior case and dissipated.

[0065] As described above, by dividing the heat capacity portion 4 formed on the battery holder 2 into multiple connecting ribs 26 and providing grooves 27 between adjacent connecting ribs 26, sink marks during resin molding can be prevented, but this reduces the volume of the heat capacity portion 4 and therefore the heat capacity. To solve this problem, the battery pack 100 shown in the figures has a connecting heat capacity portion 6 on the inside surface of the exterior case 3 that is thermally coupled to the battery holder 2.

[0066] (External Case 3) As shown in Figures 1 to 6, the external case 3 has a box-shaped exterior and a hollow interior to house the battery assembly 10. The external case 3 shown in the figures is made of resin and includes a box-shaped main case 3A with an opening at the top and a lid case 3B that closes the top opening of the main case 3A. The main case 3A includes a bottom plate 31 that faces the underside of the battery holder 2, side plate portions 32 connected to both sides of the bottom plate 31 and facing the side surfaces of the battery holder 2, and end plate portions 33 connected to both ends of the bottom plate 31 and the side plate portion 32 and facing the end surfaces of the battery holder 2. The lid case 3B includes a top plate 35 and a low peripheral wall portion 36 that runs along the outer periphery of the top plate portion 35. The external cases 3 are connected together with the edge of the opening of the main case 3A facing the leading edge of the peripheral wall portion 36 of the lid case 3B. With the battery assembly 10 housed inside the exterior case 3, the upper opening of the main case 3A is closed by the cover case 3B, and the battery holder 2 is housed inside.

[0067] (Linked Heat Capacity Section 6) The exterior case 3 has a linked heat capacity section 6 on its inner surface that is thermally coupled to the battery holder 2 housed therein and absorbs heat conducted from the battery holder 2. As shown in Figures 3 to 10 , the linked heat capacity section 6 is integrally molded with the exterior case 3 and has the thermal capacity to absorb heat conducted from the secondary battery cells 1 by the battery holder 2. The linked heat capacity section 6 is composed of multiple heat-conducting protrusions 7 protruding from the interior surface of the exterior case 3. The linked heat capacity section 6, which is integrally molded with the exterior case 3, is divided into multiple heat-conducting protrusions 7, providing gaps 34 between adjacent heat-conducting protrusions 7 to prevent sink marks during resin molding of the linked heat capacity section 6. The multiple divided heat-conducting protrusions 7 are fitted into fitting recesses 5 formed in the battery holder 2 and thermally coupled to the battery holder 2 to effectively absorb heat conducted from the battery holder 2.

[0068] The illustrated exterior case 3 has multiple heat-conducting protrusions 7 on the inner surface of the main case 3A, namely, the inner surface of the bottom plate 31 facing the underside of the battery holder 2 housed in the main case 3A, and the inner surface of the side plate 32 facing the undercut surface 28 of the battery holder 2, forming a connected heat capacity section 6. The multiple heat-conducting protrusions 7 on the bottom plate 31 are located in an area facing the valleys 24 between the multiple holding cylindrical sections 21 arranged on the underside of the battery holder 2, and are positioned opposite the multiple fitting recesses 5 formed in the valleys 24. The multiple heat-conducting protrusions 7 on the side plate 32 are located in an area facing the valleys 24 between the holding cylindrical sections 21 arranged above and below the undercut surface 28 of the battery holder 2, and are positioned opposite the multiple fitting recesses 5 formed in the valleys 24. The exterior case 3 shown in the figures has a side plate portion 32 facing the undercut surface 28, which has an inclined portion 32A that follows the undercut surface 28 and a vertical portion 32B that is connected to the upper end of the inclined portion 32A, and a plurality of heat-conducting protrusions 7 are provided on the inclined portion 32A that faces the undercut surface 28.

[0069] (Heat Conduction Protrusions 7) The heat conduction protrusions 7 shown in Figures 3 and 4 are formed in a rib shape that fits into the mating recesses 5, which are the grooves 27 of the battery holder 2. The rib-shaped heat conduction protrusions 7 are arranged in parallel rows along the extension direction of the opposing valleys 24, with gaps 34 of a predetermined width between them. The thickness (w) of the rib-shaped heat conduction protrusions 7, as shown in Figure 10, is a thickness that ensures sufficient heat capacity while maintaining an upright position with sufficient strength, and is, for example, 3 mm or more, preferably 4 mm or more. The thickness (w) of the heat conduction protrusions 7 is also a thickness that prevents sink marks during resin molding, and is, for example, 10 mm or less, preferably 8 mm or less.

[0070] As shown in Figure 10 , the thickness (w) and width (s) of the gaps 34 of the multiple heat-conducting protrusions 7 are adjusted so that they can fit into the multiple mating recesses 5 formed along the valleys 24 of the battery holder 2. The gaps 34 formed between adjacent heat-conducting protrusions 7 serve as insertion gaps for guiding the connecting ribs 26 of the battery holder 2. The thickness (w) of the heat-conducting protrusions 7 and the width (s) of the gaps 34 are determined so that the heat-conducting protrusions 7 can fit into the grooves 27 formed between the connecting ribs 26 and the connecting ribs 26 can fit into the gaps 34 formed between the heat-conducting protrusions 7. The thickness (w) of the heat-conducting protrusions 7 is made slightly smaller than or approximately equal to the groove width (m) of the grooves to ensure reliable thermal coupling while smoothly inserting into the grooves 27 that form the mating recesses 5. Similarly, the width (s) of the gap 34 between adjacent heat-conducting protrusions 7 is made slightly larger than or approximately equal to the thickness (t) of the connecting rib 26 so that the connecting rib 26 of the battery holder 2 can be inserted smoothly and thermally bonded reliably.

[0071] Furthermore, the rib-shaped heat-conducting protrusions 7 can have a sloped surface so that their thickness gradually decreases in the direction of protrusion. The inner surface of the grooves 27 of the battery holder 2, which form the mating recesses 5, can have a sloped surface so that the groove width (m) gradually increases toward the opening edge. This structure allows the heat-conducting protrusions 7 to be inserted into the grooves 27, with the sloped opposing surfaces approaching each other for mating. The heat-conducting protrusions 7 can be easily and reliably guided into the grooves 27, and once the heat-conducting protrusions 7 are fitted into the grooves 27, the opposing surfaces can be brought closer together for reliable thermal coupling. Here, a structure in which the grooves 27 formed between the connecting ribs 26 have a gradient on the inner surface so that the groove width (m) gradually increases toward the opening edge results in a structure in which the thickness (t) of adjacent connecting ribs 26 gradually decreases from the bottom of the valley portion 24 toward the opening edge, and a structure in which the surface of the rib-shaped heat conducting protrusions 7 has a gradient so that the thickness gradually decreases in the protruding direction results in a structure in which the width (s) of the gaps 34 formed between the heat conducting protrusions 7 gradually increases from the base side toward the tip side of the heat conducting protrusions 7. Therefore, even when the connecting rib 26 is inserted into the gaps 34 between adjacent heat conducting protrusions 7, the connecting rib 26 can be easily and reliably guided, and when the connecting rib 26 is fitted into the gaps 34, the opposing surfaces of the connecting rib 26 can be brought close together to ensure reliable thermal coupling.

[0072] As described above, a structure in which both the heat-conducting protrusions 7 and the grooves 27 have sloped surfaces allows for more effective mating. However, it is also possible to slope the surface of only one of the heat-conducting protrusions 7 (or gaps 34) or the grooves 27 (or connecting ribs 26), while leaving the other surface unsloped, for example, vertical. Even in this case, providing a slope on one of the surfaces simplifies and facilitates the mating of the heat-conducting protrusions 7 and the grooves 27, and the mating of the connecting ribs 26 and the gaps 34, while ensuring reliable thermal coupling. Furthermore, as shown in FIG. 10 , the heat-conducting protrusions 7 can also have chamfered edges 7a on both sides of their leading edge, at the boundaries with the side surfaces. This structure allows the heat-conducting protrusions 7 to fit more smoothly into the mating recesses 5.

[0073] 10, the height (H) of the heat-conducting protrusions 7 is preferably smaller than the depth (K) of the grooves 27 so that the tip surfaces of the heat-conducting protrusions 7 do not contact the bottom surface of the grooves when fitted into the recessed fitting portions 5. With this structure, when the heat-conducting protrusions 7 are inserted all the way into the recessed fitting portions 5, the tip surfaces of the connecting ribs 26 can be connected in surface contact with the inner surface of the outer case 3. This allows heat conducted from the retaining tube portion 21 to the connecting ribs 26 to be conducted directly to the outer case 3, allowing it to be effectively dissipated to the outside.

[0074] The rib-shaped heat-conducting protrusions 7 fitted into the grooves 27 have a surface shape that conforms to the outer circumferential surface of the holding tubular portion 21. When fitted into the grooves 27, the heat-conducting protrusions 7 are thermally coupled by bringing both opposing surfaces close to or in close contact with the outer surfaces of the opposing holding tubular portion 21. The heat-conducting protrusions 7 shown in the figure are shaped like Mount Fuji when viewed from the front. The top surface is close to or in close contact with the partition wall 22 of the battery holder 2, and both side surfaces are curved surfaces 7A that conform to the outer circumferential surface of the outer wall 23 of the holding tubular portion 21, also close to or in close contact with the holding tubular portion 21. This structure allows the outer circumferential surface of the holding tubular portion 21 to be thermally coupled directly to the heat-conducting protrusions 7 of the outer case 3, allowing for efficient thermal conduction and dissipation of heat generated by the rechargeable battery cells 1 from the holding tubular portion 21 to the outer case 3. Furthermore, the heat-conducting protrusions on the inclined portion 32A of the side plate 32 have a vertical surface 7B facing the left side in Fig. 8. This structure, in which the facing surface of the heat-conducting protrusions 7 formed on the inclined portion 32A is vertical surface 7B, has the advantage that the battery holder 2 can be inserted smoothly into the main case 3A without contact with the retaining tube portion 21.

[0075] Furthermore, in the fitting structure shown in FIG. 10 , thermally conductive paste 15 is disposed between the fitting recess 5 and the thermally conductive protrusion 7. This structure improves the thermal conductivity between the fitting recess 5 and the thermally conductive protrusion 7, enabling effective heat conduction. In particular, even if the height (H) of the thermally conductive protrusion 7 is smaller than the depth (K) of the groove 27, creating a gap between the tip surface of the thermally conductive protrusion 7 and the bottom surface of the groove 27, applying thermally conductive paste 15 to this gap provides a more effective thermal coupling between the battery holder 2 and the exterior case 3. Silicone oil, thermally conductive grease, or the like can be used as this thermally conductive paste 15. Furthermore, the structure in which thermally conductive paste 15 is disposed between the fitting recess 5 and the thermally conductive protrusion 7 also has the advantage of allowing the fitting recess 5 and the thermally conductive protrusion 7 to fit together more smoothly with low resistance during the assembly process of the battery pack 100.

[0076] (Second Linked Heat Capacity Portion 8) The exterior case 3 also includes a second linked heat capacity portion 8 that is thermally coupled to the holding tube portion 21 of the battery holder 2 along the boundary between the bottom plate portion 31 and the side plate portion 32 of the main case 3A. The second linked heat capacity portion 8 is composed of multiple opposing ribs 38 that are integrally connected across the bottom plate portion 31 and the side plate portion 32. The illustrated second linked heat capacity portion 8 is divided into multiple opposing ribs 38, providing gaps between adjacent opposing ribs 38 to prevent sink marks during resin molding of the second linked heat capacity portion 8. Furthermore, the opposing surface of each opposing rib 38 is shaped to conform to the surface of the holding tube portion 21. The opposing surface of the illustrated opposing rib 38 is an arc-shaped curved surface 8A that conforms to the curved surface of the holding tube portion 21. This allows for close surface contact with the holding tube portion 21, enabling effective heat conduction. In this way, the multiple opposing ribs 38 integrally formed across the bottom plate portion 31 and the side plate portion 32 have the advantage of being able to increase the rigidity and reinforce the corner portions of the box-shaped main body case 3A.

[0077] In the battery pack 100 with the above structure, the heat capacity portion 4 on the battery holder 2 and the connecting heat capacity portion 6 on the exterior case 3 are connected by a fitting structure at the opposing portion between the outer peripheral surface of the battery holder 2 and the inner surface of the exterior case 3. This effectively utilizes the excess space between the battery holder 2 and the exterior case 3 to form a resin mass that would not be possible with either component alone, enabling efficient heat conduction and dissipation of the heat from the generated secondary battery cells 1 from the battery holder 2 to the exterior case 3. The heat transfer that occurs in the battery pack 100 when the heat from the generated secondary battery cells 1 is conducted from the battery holder 2 to the exterior case 3 for heat dissipation is described in detail in FIGS. 5 to 7 and 10 . Note that FIG. 5 is a cross-sectional view corresponding to the cross section along line V-V in FIGS. 6 and 7 , and FIG. 10 is an enlarged cross-sectional view of a key portion of FIG. 5 .

[0078] In these figures, arrows a to h indicate the transfer of heat from the target battery cell A located at the lower left in Figures 6 and 7 when the target battery cell A generates heat due to an abnormality such as thermal runaway. A portion of the heat generated in the target battery cell A is conducted from the holding cylindrical portion 21 to the connecting rib 26, which is the heat capacity portion 4, as shown by arrow a, and then thermally conducted from the connecting rib 26 to the exterior case 3 as shown by arrow b, where it is dissipated to the outside. A portion of the heat generated in the target battery cell A is conducted from the holding cylindrical portion 21 to the heat conductive protrusions 7, which are the connecting heat capacity portion 6, as shown by arrow c, and then thermally conducted from the heat conductive protrusions 7 to the exterior case 3 as shown by arrow d, where it is dissipated to the outside. Furthermore, a portion of the heat conducted from the holding cylindrical portion 21 to the connecting rib 26, which is the heat capacity portion 4, as shown by arrow a, is conducted from the connecting rib 26 to the heat conductive protrusions 7 as shown by arrow e (see Figure 10 ), and then thermally conducted from the heat conductive protrusions 7 to the exterior case 3 as shown by arrow d, where it is dissipated to the outside. In this way, the structure in which the heat capacity section 4 provided on the battery holder 2 and the connecting heat capacity section 6 provided on the exterior case are thermally coupled by an interlocking structure, as shown in Figure 5, thermally couples the underside of the battery holder 2 and the bottom plate section 31 of the exterior case 3 in a tight, tight contact state, allowing efficient heat conduction from the battery holder 2 to the exterior case 3 and heat dissipation.

[0079] 6 and 7 , where the retaining cylindrical portion 21 of the battery holder 2 makes direct contact with the inner surface of the exterior case 3, some of the heat generated by the target battery cell A is thermally conducted from the retaining cylindrical portion 21 to the exterior case 3 as shown by arrow f, and then dissipated to the outside. Also, where the retaining cylindrical portion 21 of the battery holder 2 makes contact with the second linked heat capacity portion 8 provided at the corner of the exterior case 3, some of the heat generated by the target battery cell A is conducted from the retaining cylindrical portion 21 to the opposing rib 38, which is the second linked heat capacity portion 8, as shown by arrow g, and then from the opposing rib 38 to the exterior case 3 as shown by arrow h, and then dissipated to the outside.

[0080] The above battery pack is assembled in the following steps.

[0081] (1) The rechargeable battery cells 1 are stored in the battery holder 2. The battery holder 2 shown in Figures 3 and 4 is composed of three components. After the holder lid 2B is fixed to one opening of the holder tube portion 2A, the rechargeable battery cells 1 are inserted into the battery storage portions 20 of each holding tube portion 21. After the rechargeable battery cells 1 (eight rechargeable battery cells 1 in the figure) are inserted in the correct orientation into all holding tube portions 21 of the holder tube portion 2A, the holder lid 2B is fixed to the opening on the opposite side to close the opening.

[0082] (2) Lead plates 13 are connected to the electrodes at both ends of the rechargeable battery cells 1 housed in the battery holder 2. The electrodes at both ends of the rechargeable battery cells 1 are connected via the lead plates 13, creating a predetermined connection. In the figure, eight rechargeable battery cells 1 are connected via the lead plates 13 in a four-parallel, two-series configuration.

[0083] (3) The circuit board 12 is placed on the top surface of the battery holder 2, and the lead plates 13 connected to the secondary battery cells 1 are connected to the circuit board 12. In this state, the circuit board 12 is fixed to the battery holder 2 via the lead plates 13, completing the battery assembly 10.

[0084] (4) Apply thermally conductive paste 15 to the multiple mating recesses 5 formed in the battery holder 2. The battery holder 2 shown in the figure has grooves 27 formed on the undersurface and grooves 27 formed in the undercut surface 28 as mating recesses 5, so apply thermally conductive paste 15 to these mating recesses.

[0085] (5) The battery assembly 10 is placed inside the main case 3A. At this time, the multiple heat-conducting protrusions 7 on the bottom plate 31 of the main case 3A are guided into the mating recesses 5 on the underside of the battery holder 2, while the multiple heat-conducting protrusions 7 on the inclined portions 32A of the side plate 32 are guided into the mating recesses 5 on the undercut surfaces 28 of the battery holder 2, until the heat-conducting protrusions 7 and mating recesses 5 are mated. In this state, the battery assembly is placed in its designated position in the main case.

[0086] (6) The lid case 3B is connected to the edge of the opening of the main case 3A, and the upper opening of the main case 3A is closed with the lid case 3B.

[0087] Second Embodiment In the above example, a plurality of secondary battery cells 1 are arranged in a single row in the diameter direction to form a battery row 11, and two battery rows 11 are arranged one above the other with the rows offset left and right so as to form a stacked posture like rice bales. However, a battery pack can also be configured with a single battery row 11 consisting of a plurality of secondary battery cells 1 arranged in a single row in the diameter direction. A battery pack 300 with this structure is shown in FIG. 11 . The battery pack 300 shown in FIG. 11 has five secondary battery cells 1 arranged in a single row in the diameter direction, parallel to one another, housed in a battery holder 2, and this battery holder 2 is housed in an exterior case 3 as a battery assembly 10. In this second embodiment, the same components as those in the battery pack of the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

[0088] The battery holder 2 shown in Fig. 11 is integrally formed with five holding tube portions 21 arranged in a diametrical direction and connected together so that it can accommodate five rechargeable battery cells 1. As shown in Fig. 11 , in a battery row 11 in which multiple rechargeable battery cells 1 are arranged in a diametrical direction (left-right direction in the figure), the rechargeable battery cells 1 at both ends are designated as target battery cells A, which have the fewest number of adjacent rechargeable battery cells 1 (one in the figure). The left-right adjacent battery cells B adjacent to the target battery cell A in the left-right direction and the middle battery cell D located in the center each have two adjacent rechargeable battery cells 1. Therefore, in this arrangement, even if an abnormality occurs in the left-right adjacent battery cells B and the middle battery cell D, which have two adjacent rechargeable battery cells 1, the larger number of adjacent rechargeable battery cells 1 increases the thermal capacity of the adjacent rechargeable battery cells 1. These rechargeable battery cells 1 absorb heat, thereby preventing fire from spreading. In contrast, the target battery cell A, which has the fewest number of adjacent secondary battery cells 1 at one, is the secondary battery cell 1 that is most susceptible to fire spread because of the small number of adjacent secondary battery cells 1 and the small heat capacity of the secondary battery cell 1. Therefore, in the battery pack 300, the thickness of the partition wall 22 formed between the target battery cell A and the secondary battery cell 1 adjacent to the target battery cell A is increased to suppress heat conduction from the target battery cell A to the adjacent secondary battery cell 1.

[0089] The partitions 22 of the battery holder 2 shown in Fig. 11 include target partitions 22A formed between a target battery cell A and its adjacent left and right battery cells B, and non-target partitions 22B formed between the secondary battery cells 1 (the left and right adjacent battery cells B and the middle battery cell D) other than the target battery cell A. The thickness (T1) of the target partitions 22A is greater than the average thickness (Ta) of all partitions 22 to suppress heat conduction between adjacent secondary battery cells 1. The battery pack 100 prevents fire spread by thickening the target partitions 22A between the target battery cell A and the adjacent secondary battery cell 1 to widen the gap between them, and reduces the thickness of the non-target partitions 22B to suppress an increase in the external size of the battery holder. The battery holder 2 shown in Fig. 11 defines the non-target partitions 22B as specific partitions 22x, and the thickness (T2) of these specific partitions 22x is equal to or less than the average thickness (Tb) of all non-target partitions 22B. In the battery pack 300 described above, the thickness (T1) of the target partition wall portion 22A formed between the target battery cell A and the left-right adjacent battery cells B is increased, while the thickness (T2) of the non-target partition wall portion 22B formed between the left-right adjacent battery cells B and the intermediate battery cell D is reduced as the specific partition wall portion 22x. This offsets the thickness (T1) of the target partition wall portion 22A and the thickness (T2) of the specific partition wall portion 22x, thereby preventing the external dimensions of the battery holder 2 from becoming larger. Note that the thickness (T1) of the target partition wall portion 22A and the thickness (T2) of the specific partition wall portion 22x in the battery holder 2 shown in FIG. 11 can be within the aforementioned ranges.

[0090] Furthermore, like the battery holder 2 described above, the battery holder 2 has multiple rows of connecting ribs 26 as heat capacity sections 4 in multiple rows of valley sections 24 formed on the underside, and grooves 27 between adjacent connecting ribs 26 to form sink-mark prevention recesses 25. These sink-mark prevention recesses 25 also serve as mating recesses 5 that guide multiple heat-conducting protrusions 7 protruding from the bottom surface of the exterior case 3 that houses the battery holder 2. By fitting the heat-conducting protrusions 7 on the exterior case 3 into the mating recesses 5 on the underside of the battery holder 2, the connecting heat capacity sections 6 on the exterior case 3 and the heat capacity sections 4 of the battery holder 2 can be thermally coupled without any gaps. In this structure, all of the retaining cylindrical portions 21 in which the secondary battery cells 1 are housed in a row housed in the battery holder 2 can be thermally coupled to the outer case 3 via the heat capacity portion 4 and the connecting heat capacity portion 6, so that even when any of the secondary battery cells 1 is generating heat, the heat from the secondary battery cells 1 can be effectively conducted to the outer case 3 and dissipated.

[0091] 11 also has multiple rows of connecting ribs 26 formed as heat capacity sections 4 in multiple rows of valley sections 24 formed on the top surface, and grooves 27 are formed between adjacent connecting ribs 26 to form sink-mark prevention recesses 25. Because there are no opposing heat-conducting protrusions 7, the grooves 27 formed on the top surface do not also function as mating recesses 5, but instead function as sink-mark prevention recesses 25. By dividing the heat capacity section 4 into multiple connecting ribs 26 and providing grooves 27 between them, the surface area of ​​the heat capacity section 4 can be increased, improving heat dissipation characteristics.

[0092] As described above, battery holders 2 that house a single battery row 11 have multiple cylindrical retaining sections 21 connected in a flush configuration, which can make them vulnerable to impacts. However, the battery holder 2 shown in the figure has connecting ribs 26 on both the top and bottom of the valleys 24 between adjacent cylindrical retaining sections 21. This reinforces the adjacent cylindrical retaining sections 21 with the connecting ribs 26, increasing the rigidity of the battery holder 2 and improving its impact resistance. Furthermore, on the underside of the battery holder 2, the heat capacity section 4 of the battery holder 2 and the connecting heat capacity section 6 of the exterior case 3 are connected by a fitting structure, further reinforcing the battery holder 2 while ensuring thermal coupling between the battery holder 2 and the exterior case 3 for heat dissipation.

[0093] Furthermore, although not shown, in an embodiment of the present disclosure, a battery row in which a plurality of secondary battery cells are aligned in the diameter direction can be stacked in three or more stages and housed in a battery holder. In this case, too, the secondary battery cell located in the position with the fewest adjacent secondary battery cells is designated as the target battery cell, and the thickness (T1) of the target partition wall portion formed between the target battery cell and the adjacent secondary battery cell is increased to prevent adverse effects on the target battery cell.

[0094] The battery pack according to the present disclosure can be suitably used as a power source for portable medical equipment, electric cleaners, power tools, and other portable electrical equipment, or as a battery pack for mobile objects such as power-assisted bicycles and electric carts.

[0095] DESCRIPTION OF SYMBOLS 100, 300... Battery pack 1... Secondary battery cell 1A... First end face 1B... Second end face 2... Battery holder 2A... Holder cylindrical portion 2B... Holder lid portion 3... Outer case 3A... Main body case 3B... Lid case 4... Heat capacity portion 5... Fitting recess 6... Linking heat capacity portion 7... Heat conductive protrusion 7A... Curved surface 7B... Vertical surface 7a... Chamfered portion 8... Second linking heat capacity portion 8A... Curved surface 10... Battery assembly 11... Battery string 12... Circuit board 13... Lead plate 15... Heat conductive paste 20... Battery storage portion 21... Retaining cylindrical portion 22... Partition portion 22A... Target partition portion 22B... Non-target partition portion 22x... Specific partition portion 22y... Restraining partition portion 22z... Intermediate partition portion 23... Outer wall portion 24... Valley portion 25...Sink mark prevention recess 26...Connecting rib 27...Groove portion 28...Undercut surface 31...Bottom plate portion 32...Side plate portion 32A...Inclined portion 32B...Vertical portion 33...End plate portion 34...Gap 35...Top plate portion 36...Peripheral wall portion 38...Opposing rib 41...Retaining portion 42...Electrode window 81...Retaining cylinder portion 82...Partition wall portion 83...Peripheral wall portion 90...Battery row 91...Secondary battery cell 92...Battery holder A...Target battery cell B...Left and right adjacent battery cells C...Up and down adjacent battery cells D...Intermediate battery cell

Claims

1. A battery pack comprising: a plurality of secondary battery cells; a battery holder that houses the plurality of secondary battery cells; and an exterior case that houses the battery holder, wherein each of the plurality of secondary battery cells is a cylindrical battery, and a target battery cell is a secondary battery cell that is arranged at a position among the plurality of secondary battery cells housed in the battery holder where the number of adjacently arranged secondary battery cells is the smallest, the battery holder is integrally formed with a plurality of retaining cylindrical portions that house the plurality of secondary battery cells respectively and hold them parallel to each other, the plurality of retaining cylindrical portions include partition portions formed between the adjacent secondary battery cells and an outer peripheral wall portion that forms the outer surface of the battery holder, and the partition portion includes a target partition portion formed between the secondary battery cell adjacent to the target battery cell and the target battery cell, and a non-target partition portion formed between the secondary battery cells other than the target battery cell, A battery pack in which a thickness of the target partition portion is made larger than an average value of thicknesses of all the partition portions, thereby suppressing thermal conduction between adjacent secondary battery cells among the plurality of secondary battery cells.

2. A battery pack as described in claim 1, wherein the non-target partition sections that have the smallest thickness are designated as specific partition sections, and the thickness of the specific partition section is set to be equal to or less than the average thickness of all the non-target partition sections.

3. A battery pack as described in claim 2, wherein the battery holder arranges the multiple retaining tube portions so as to store battery rows in which the multiple secondary battery cells are aligned in the diameter direction in multiple stages, and the battery rows stacked in multiple stages are arranged such that the secondary battery cells of one battery row are located in the valleys formed between adjacent secondary battery cells of the other battery row in the battery row arranged in the lower stage and the battery row arranged in the upper stage, and the battery holder defines the non-target partition portions formed between the secondary battery cells excluding the target battery cell in each battery row as the specific partition portions.

4. A battery pack as described in claim 3, wherein the battery holder is arranged with the multiple retaining tube portions so as to store the battery rows in which the multiple secondary battery cells are aligned in the diameter direction in two tiers, the battery rows stacked in two tiers have the target battery cell disposed at one end of the lower tier battery row and at the other end opposite the upper tier battery row, and the left-right positional deviation between the lower tier battery row and the upper tier battery row is less than the radius of the secondary battery cell.

5. A battery pack as described in claim 3, wherein the battery holder is arranged with the multiple retaining tube portions so as to store the battery string in which the multiple secondary battery cells are aligned in the diameter direction in two stages, and an angle formed by a center line connecting the centers of left and right adjacent battery cells adjacent to the target battery cell in the left-right direction and a center line connecting the centers of upper and lower adjacent battery cells adjacent to the target battery cell in the diagonal directions above and below the target battery cell is greater than 60 degrees, and the non-target partition portions formed between the left and right adjacent battery cells and the upper and lower adjacent battery cells are restraining partition portions, and the thickness of the restraining partition portions is greater than the thickness of the target partition portions.

6. A battery pack as described in claim 2, wherein the battery holder is arranged with the multiple retaining tube portions so as to accommodate a single battery row in which a plurality of the secondary battery cells are aligned in the diameter direction, the battery row defines each of the multiple secondary battery cells located at both ends as the target battery cell, and the battery holder defines the non-target partition portions formed between the multiple secondary battery cells excluding the target battery cells as the specific partition portions.

7. A battery pack as described in any one of claims 2 to 6, wherein the difference in thickness between the target partition portion and the specific partition portion is 3.5% or more of the radius of the secondary battery cell.

8. A battery pack as claimed in any one of claims 2 to 6, wherein the target partition portion has a thickness of 1.5 mm or more, and the specific partition portion has a thickness of 1.1 mm or less.

9. A battery pack as claimed in any one of claims 1 to 6, wherein the battery holder is provided with a heat capacity section connected to the outside of the plurality of retaining cylindrical sections and absorbing heat of the secondary battery cells conducted from the plurality of retaining cylindrical sections, the heat capacity section being composed of a plurality of connecting ribs located in valleys formed between adjacent retaining cylindrical sections of the plurality of retaining cylindrical sections and arranged in the extension direction of the valleys to connect the adjacent retaining cylindrical sections, and a plurality of grooves being formed between adjacent connecting ribs of the plurality of connecting ribs, each of the plurality of grooves serving as a sink mark prevention recess during resin molding of the heat capacity section.

10. A battery pack as described in claim 9, wherein the outer case has on its inner surface a connected heat capacity portion that is thermally coupled to the battery holder and absorbs heat conducted from the battery holder, the connected heat capacity portion being composed of a plurality of heat conductive protrusions protruding from the inner surface of the outer case, the plurality of heat conductive protrusions being provided at positions facing the plurality of grooves provided in the battery holder, the plurality of grooves of the battery holder serving as a plurality of fitting recesses into which the plurality of heat conductive protrusions are fitted, and when the battery holder is stored in the outer case, the plurality of heat conductive protrusions are fitted into the plurality of fitting recesses to thermally couple the battery holder and the outer case, and heat generated by the secondary battery cells is thermally conducted from the battery holder to the outer case and dissipated to the outside.

11. A battery pack comprising: a plurality of secondary battery cells; a battery holder that houses the plurality of secondary battery cells; and an exterior case that houses the battery holder, wherein each of the plurality of secondary battery cells is a cylindrical battery, and a secondary battery cell that is located at a position among the plurality of secondary battery cells housed in the battery holder and that has the fewest number of adjacently arranged secondary battery cells is defined as a target battery cell, the battery holder is integrally formed with a plurality of holding tube portions that house the plurality of secondary battery cells respectively and hold them parallel to each other, the plurality of holding tube portions include partition portions formed between adjacent secondary battery cells among the plurality of secondary battery cells, and a peripheral wall portion that forms the outer surface of the battery holder, and the partition portions include a target partition portion formed between the secondary battery cell adjacent to the target battery cell and the target battery cell, and a non-target partition portion formed between the secondary battery cells other than the target battery cell, and the thickness of the target partition portion is greater than the average thickness of all the partition portions, The thickness of the partition portion is the thickness of the partition portion on a straight line connecting the centers of adjacent secondary battery cells in a cross-sectional view of the battery holder, and in the partition portion having a curved portion that follows the outer peripheral surface of the secondary battery cell, the thickness is the thickness at the part where the thickness is smallest.

12. A battery pack as described in claim 11, wherein the non-targeted partition portion having the smallest thickness among the non-targeted partition portions is the specific partition portion.

13. A battery pack as described in any one of claims 12, wherein the difference in thickness between the target partition portion and the specific partition portion is 3.5% or more of the radius of the secondary battery cell.

14. A battery pack as claimed in any one of claims 12, wherein the thickness of the target partition wall is 1.44 mm or more, and the thickness of the specific partition wall is 1.1 mm or less.

15. A battery pack as described in claim 11, wherein the battery holder has a heat capacity portion connected to the outside of the multiple retaining cylindrical portions and absorbing heat of the multiple secondary battery cells conducted from the multiple retaining cylindrical portions, each of the multiple heat capacity portions has a plurality of connecting ribs, each of the multiple connecting ribs is located in a valley portion formed between adjacent ones of the retaining cylindrical portions, the multiple heat capacity portions are arranged along the axes of the multiple retaining cylindrical portions and connect adjacent ones of the retaining cylindrical portions, and a space having a depth reaching to the bottom of the valley portion is provided between each of the multiple adjacent connecting ribs.

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