JP7509835B2 - Battery pack - Google Patents

Description

The present invention relates to a battery pack.

In recent years, the demand for secondary batteries for vehicles has been increasing due to environmental regulations. Among them, lithium-ion secondary batteries are generally considered promising because they have a higher discharge potential than lead batteries and nickel-metal hydride batteries, and therefore can be made small and have a high energy density. For full-scale application, lithium-ion secondary batteries are required to have higher energy density, higher output density, and longer life. In order to increase the output of a battery, it is effective to input and output a large current from the battery, as well as to increase the potential. However, when a large current is input and output from the battery, heat is generated inside the battery due to the internal resistance of the battery. If the generated heat cannot be sufficiently removed from the battery, the battery temperature will rise. The battery characteristics of lithium-ion batteries, such as the battery capacity and internal resistance, tend to deteriorate depending on the battery temperature, and in particular, the higher the battery temperature, the more the battery characteristics tend to deteriorate. Therefore, technology development to improve the heat dissipation performance of batteries is required.

When multiple lithium-ion cells (hereinafter referred to as cells) are combined and used as a battery group (for example, when used as a battery module or battery pack), it is desirable to reduce the temperature difference between the cells in the battery group. This is because if the temperature difference between the cells is large, differences in deterioration are likely to occur between the cells. Since the characteristics of the battery group tend to be determined by the characteristics of the most deteriorated battery among the cells in the battery group, it is necessary to design the battery group in a way that avoids a structure that deteriorates a specific battery.

Therefore, technology has been developed to reduce the temperature difference between cells in a battery group formed by combining multiple cells. Specifically, Patent Document 1 describes a storage battery in which a battery case containing the cells is formed into a rectangular parallelepiped with narrow short sides and wide long sides, and multiple cells are connected adjacent to each other between the short sides of the battery case to form an assembled battery with the required power capacity.

On the other hand, when large currents are input and output from a battery, the cross-sectional area of the cable connected to the battery must also be large. Metals such as copper are used as materials for the cables, and metals generally have high thermal conductivity and therefore high heat dissipation performance.

JP 2000-164186 A

In the technology described in Patent Document 1, multiple ribs are formed on the long side of the battery case of each unit cell. The unit cells are cooled by forcing air or the like to circulate between these ribs. In such a configuration, if the cooling efficiency decreases (for example, if the flow rate of the forced air is small, or if the input/output current is large and the unit cells generate a lot of heat), a distribution of battery temperatures will occur within the battery group that is configured, and the battery temperature of the unit cells located near the center of the long side of the battery group will be particularly high, and deterioration may progress. The present invention was made in consideration of the above problems, and aims to provide a battery pack with a small temperature difference between battery groups.

The battery pack described in the present invention comprises a first battery group in which a plurality of storage batteries, each having a battery can side surface and a battery can bottom surface connected to the battery can side surface, are stacked with the battery can side surfaces facing each other, a second battery group in which a plurality of storage batteries, each having a battery can side surface and a battery can bottom surface connected to the battery can side surface, are stacked with the battery can side surfaces facing each other, and a housing that houses the first battery group and the second battery group, and the first battery group and the second battery group are characterized in that the opposing surfaces of the first battery group and the second battery group are directly or indirectly thermally connected to each other.

The present invention provides a battery pack that reduces the temperature difference between battery groups.

FIG. 2 is a diagram for explaining an example of a specific configuration of a battery group in the first embodiment. FIG. 11 is a diagram for explaining an example of a specific configuration of a battery group in the second embodiment. FIG. 13 is a diagram for explaining an example of a specific configuration of a battery group in a fifth embodiment. FIG. 23 is a diagram for explaining an example of a specific configuration of a battery group in a sixth embodiment. FIG. 23 is a diagram for explaining an example of a specific configuration of a battery group in Example 7. FIG. 4 is a diagram for explaining an example of a specific configuration of a battery group in Comparative Example 1. FIG. 11 is a diagram for explaining an example of a specific configuration of a battery group in Comparative Example 2. FIG. 11 is a temperature rise ratio diagram showing the temperature rise of each single cell from the environmental temperature in the configurations of Examples 1 and 2 and the conventional configuration, expressed as a ratio from the lowest temperature in the battery group. FIG. 11 is a temperature rise ratio diagram showing the temperature rise of each cell from the environmental temperature in the configurations of Examples 3 and 4 and a conventional configuration, expressed as a ratio from the lowest temperature in the battery group. FIG. 13 is a temperature rise ratio diagram showing the temperature rise of each single cell from the environmental temperature in the configurations of Examples 5 and 6 and the conventional configuration, expressed as a ratio from the lowest temperature in the battery group. FIG. 13 is a temperature rise ratio diagram showing the temperature rise of each single cell from the environmental temperature in the configuration of Example 7 and the conventional configuration, expressed as a ratio from the lowest temperature in the battery group. FIG. 3 is a top view of the battery pack of FIG. 2 . The first modified example of FIG. A second modification of FIG. FIG. 1 is a perspective view of a secondary battery of the present invention.

The following describes the form for implementing the present invention. However, this embodiment is not limited to the following content in any way, and can be modified as desired without departing from the gist of the present invention.

This embodiment will be described in detail. In this embodiment, a lithium ion secondary battery is used as the secondary battery, but this configuration can be applied to other types of storage batteries. In addition, the effect can be obtained regardless of the constituent materials of the lithium ion secondary battery. In other words, in this invention, an electrode made of Al current collecting foil and a positive electrode material having a layered structure is used as the positive electrode, and an electrode made of Cu current collecting foil and a carbon material is used as the negative electrode, but other configurations are also possible. For example, as described in Examples 3 and 4, it is possible to improve heat dissipation even when Al foil is used as the negative electrode. The cooling environment is an example and can be applied to cases where other refrigerants are used. In addition, in this embodiment, a rectangular battery is used as the shape of the lithium ion battery, but the effect can be obtained even with batteries of other shapes known, such as laminated type and cylindrical type.

When the cells are connected to form a battery group, the cells are connected in series or in parallel. In this case, in order to ensure safety, a member that can ensure insulation between the cells may be introduced around the cells, as shown in Example 7, for example. The shape of the member is free, and the material can be freely selected, but it is preferable that a heat transfer member is included. When the cells are connected in series or in parallel, the wiring to be used is not particularly limited, but for example, a bus bar can be used. Regardless of the type of series or parallel connection, the effect can be obtained by using the battery arrangement and external terminal configuration of the present invention. For example, the effect of the present invention can be obtained even with a battery group in which two parallel battery groups are connected in series to six battery groups. In addition to electrically connecting the cells in series or in parallel, it is preferable that the cells are physically restrained using a fixing jig. However, the present invention is not limited to the method of restraint. For example, the effect was obtained whether the two battery groups were fastened using one set of fixing jigs or two sets of fastening jigs.

In the present invention, the battery pack is basically composed of a first battery group and a second battery group connected by the means described above, but the battery pack may also be equipped with a battery control device (e.g., a Battery Management System; BMS) and a safety mechanism (e.g., a fuse), and the effects of the present invention can be obtained by connecting these to the wiring in the battery groups.

The method of contacting the bottom surface of the housing with the battery groups is not particularly limited, and the effects of the present invention can be achieved by, for example, bonding with an adhesive or connecting via a fixing device using bolts and nuts. In this embodiment, the shape of the housing is exemplified as a rectangular parallelepiped, but the shape is not particularly limited. Furthermore, the effects of the present invention are not limited to the conditions for applying current to the battery pack or the cooling conditions.

The present invention will be described in more detail below with reference to examples and comparative examples. Fig. 1 is an exploded perspective view of a battery pack 100 of the present invention. In the following description, the terms up, down, left, right, front, and rear refer to the directions shown in the lower left corner of each drawing.

The battery pack 100 is composed of a first battery group 10A, a second battery group 10B, and a housing 5 (5a, 5b) that houses the first battery group 10A and the second battery group 10B. The housing 5 is composed of a case 5a and a lid 5b that covers the case opening. Note that, although the bottom surface 5b is a separate member in this embodiment, a structure in which the case 5a has a bottom surface and an opening on the top surface may also be used, with the lid 5b disposed on the top surface.

Figure 15 is a diagram showing a storage battery 1 used in the present invention. The storage battery 1 consists of a pair of wide surfaces 1a, a pair of narrow surfaces 1b, a bottom surface 1c, and a lid 1d. The lid 1d is provided with a positive external terminal 2a and a negative external terminal 2b.

Returning to FIG. 1, the first battery group 10A and the second battery group 10B will be described. The first battery group 10A is made by stacking multiple storage batteries 1 (six in this embodiment) with their wide surfaces 1a facing each other. The second battery group 10B is also made by stacking the storage batteries with their wide surfaces facing each other, just like the first battery group 10A.

The storage batteries 1 constituting the first battery group 10A are connected in series to each other by a bus bar 2. Similarly, the storage batteries 1 of the second battery group 10B are connected in series to each other by a bus bar 2. The storage battery 1 on the bottom surface 5b side of the first battery group 10A and the storage battery 1 on the bottom surface 5b side of the second battery group 10B are connected to each other by a bus bar 2. The two battery groups 10A and 10B are each arranged so that an external terminal 3 connected to other electronic components (such as a junction box) housed in the battery pack 100 is arranged on the center side of the upper surface of the battery pack 100. In the present invention, the temperature change of the battery pack 100 was measured in several positions (Figs. 1 to 7).

Example 1
First, a description will be given of Example 1. The first and second battery groups were placed flat as shown in Fig. 1, and then directly thermally connected to each other. Then, a copper HV cable having a diameter of 95 mm was attached to the tip of the positive electrode external terminal and the negative electrode external terminal, and a current was applied. The amount of heat generated by the batteries at this time was 3 W on average.

As for cooling conditions, air was blown at a speed of 5 m/sec only onto the bottom plate of the casing located below the battery group. Figure 1 shows the top surface of the casing. Figure 8 shows the results when the battery pack reached a nearly steady state after these conditions were applied.

Example 2
Next, a description will be given of Example 2. Example 2 differs from Example 1 in that after the first battery group and the second battery group are placed flat, a flat plate-shaped thermally conductive member 4 is interposed between them.

Figure 2 shows the structure of this embodiment. As described above, in this embodiment, a thermally conductive member 4 was placed between the first battery group 10A and the second battery group 10B. Two thicknesses of this thermally conductive member, 3 mm and 15 mm, were prepared, and the temperature rise of the battery pack 100 was measured when each was used. The results are shown in Figure 8. The cooling conditions and current application conditions were the same as in Example 1.

Example 3
Next, a description will be given of Example 3. Example 3 is different from Example 1 in that the material of the negative electrode current collector foil is aluminum current collector foil instead of copper foil.

The specific structure of the battery pack 100 has the same layout as in Fig. 1, so a description thereof will be omitted. The results of the temperature rise of the battery pack 100 are shown in Fig. 9. The cooling conditions and current application conditions are the same as in the first embodiment.

Example 4
Next, Example 4 will be described. Example 4 is different from Example 2 in that the material of the negative electrode current collector foil is changed from copper foil to aluminum current collector foil. The specific structure of the battery pack 100 has the same layout relationship as in FIG. 2, so a description thereof will be omitted. In this embodiment, a member having a thickness of 15 mm is used for the thermal conductive member 4. The result of the temperature rise of the battery pack 100 is shown in FIG. 9. The cooling conditions and current application conditions are the same as those in Example 1.

Example 5
Next, Example 5 will be described. Example 5 is different from Example 1 in that the first battery group 10A and the second battery group 10B are vertically placed as shown in FIG. 3, and the narrow side surfaces of the storage batteries constituting the battery groups are in contact with the bottom surface of the housing 5 of the battery pack 100. The results of the temperature rise of the battery pack 100 are shown in FIG. 10. The cooling conditions and current application conditions are the same as those of Example 1.

Example 6
Next, Example 6 will be described. Example 6 differs from Example 2 in that the first battery group and the second battery group are vertically placed as shown in FIG. 4, and the narrow side surfaces of the storage batteries constituting the battery groups are in contact with the bottom surface of the housing 5 of the battery pack 100. In this example, the thickness of the thermal conductive member 4 is the same as in Example 2, and two thermal conductive members 4 having a thickness of 3 mm and a thickness of 15 mm are prepared, and the measurement results for each are shown in FIG. 10. The cooling conditions and current application conditions are the same as in Example 1.

(Example 7)
Next, Example 7 will be described. This example is different from Examples 1 to 6 in that a first battery group and a second battery group, each of which has six storage batteries 1 in series, are connected in series with each other, and the bottom surface of the storage battery 1 is provided so as to contact the bottom surface 5b of the housing of the battery pack 100. In this embodiment, a thermally conductive member 4 is also disposed between the first battery group 10A and the second battery group 10B. An aluminum plate having a thickness of 15 mm is used as the thermally conductive member 4. The results of the temperature rise of the battery pack 100 are shown in FIG. 11. The cooling conditions and current application conditions are the same as those of Example 1.

(Comparative Example 1)
Next, a description will be given of Comparative Example 1. Comparative Example 1 differs from Examples 1 to 7 in that the battery group is not divided into two, but is stacked as a single battery group.

Figure 6 is a diagram of a battery pack 100 of Comparative Example 1. Twelve batteries 1 are connected in series to form a single battery group, which is arranged so that the wide surface 1a of the single battery 1 contacts the bottom surface 5b of the housing 5. The results of the temperature rise of the battery pack 100 are shown in Figures 8 and 9. The cooling conditions and current application conditions are the same as those of Example 1.

(Comparative Example 2)
Next, a description will be given of Comparative Example 2. Comparative Example 2 differs from Examples 1 to 7 in that the battery group is not divided into two but is stacked as a single battery group, and further in that the narrow surface 1b of the cell 1 is in contact with the bottom surface 5b of the housing 5.

Fig. 7 is a diagram of a battery pack 100 of Comparative Example 2. A single battery group is formed by connecting 12 batteries in series, and is arranged so that the narrow surface 1b of the cell 1 contacts the bottom surface 5b of the housing 5. The results of the temperature rise of the battery pack 100 are shown in Figs. 10 and 11. The cooling conditions and current application conditions are the same as those of Example 1. Below, the effects of the present invention will be explained based on the results of the Example and Comparative Example.

When the conditions shown in the present embodiment and comparative example were applied to the battery group, the battery temperature rose compared to the ambient temperature when the temperature of the battery group reached a nearly steady state. Figures 8 to 11 are diagrams showing, for each battery, the ratio of the temperature rise relative to the ambient temperature for each cell in the battery group to the temperature rise of the cell with the smallest temperature rise. The cell numbers in the figures correspond to the cell numbers shown in the figures corresponding to each embodiment or comparative example. In this case, for the two battery groups in the embodiment, the cells located at opposing positions in the battery groups have been omitted for simplicity, since they had nearly the same temperature. The results of each figure are explained in detail below.

Figure 8 shows the temperature rise ratios of the configurations of Examples 1 and 2, which are examples of applying the present invention to a battery arrangement placed flat like Comparative Example 1. From the figure, it can be seen that in the Comparative Example, which is a conventional configuration, the temperature of cell No. 7, which is the center part of the stacked batteries, is the highest. This is because the heat generated by the surrounding batteries is not dissipated in the battery near cell No. 7, so not only the battery temperature but also the temperature of the surrounding batteries is high, and there is no temperature difference, making it difficult for heat to flow, resulting in a high battery temperature. On the other hand, the temperature rise of the cell No. 1 single battery in contact with the bottom surface of the case and the cell No. 12 battery with an external terminal is suppressed because there is a heat dissipation path. From the above results, there was a difference in the temperature rise between cell No. 1, cell No. 12, and cell No. 7, so the temperature rise ratio changed significantly even within the same battery group in Comparative Example 1. Therefore, it can be seen that the temperature difference between the single batteries is likely to appear in Comparative Example 1.

At the same time, Figure 8 also shows an example of the present invention. In the present invention, the rate of temperature rise between batteries is compared for all cells present in the same arrangement, and it can be seen that the rate of temperature rise can be reduced in all cells compared to the comparative example. Also, as shown in Example 2, the effect is further enhanced by arranging a heat transfer member between the battery groups, and it can be seen that the temperature difference between the batteries is mitigated. This means that by arranging the external terminal on cell No. 6, which has the highest temperature, a heat dissipation path can be effectively secured. In addition, in Example 2, it can be seen that a heat dissipation path can also be secured by introducing an Al plate, and therefore the temperature rise difference between the batteries can be effectively reduced.

Figure 9 shows the temperature rise rate for the configurations of Examples 3 and 4, which are examples in which the present invention is applied to a battery configuration laid flat like Comparative Example 1. As can be seen from the figure, there is a tendency for the temperature difference between batteries to be mitigated, as shown in Figure 8, even when the negative electrode current collector foil is made of Al. The thermal behavior that occurs in Figure 9 is caused by the same phenomenon that occurs in Figure 8.

Figure 10 shows the configurations of Examples 5 to 6, which are examples in which the present invention is applied to a vertically-positioned battery configuration, just like Comparative Example 2. From the figure, it can be seen that in the Comparative Example, which has a conventional configuration, the temperature of cell No. 6, which is the center part of the stacked batteries, is the highest. The temperature rise that occurs here is due to the same phenomenon as the behavior that appeared in Comparative Example 1, shown in Figure 8.

The figure also shows that the temperature difference between the batteries is mitigated when placed vertically, as in Figures 8 and 9. When placed vertically instead of flat, the temperature tends to be higher near the center of the long side. By placing the external terminals in this area, it is possible to obtain the same effect as in Figures 8 and 9, and reduce the temperature difference.

Figure 11 compares the temperature difference when the present invention is applied to the configuration shown in Example 7 with the battery configuration in which it is placed vertically as in Comparative Example 2. From the figure, it can be seen that the present invention also tends to reduce the temperature difference. This is because, in Example 7, the heat can be effectively dissipated to the outside of the battery group by installing an external terminal on cell No. 6, which has the highest battery temperature.

The battery pack according to the present invention comprises a first battery group (10A) in which a plurality of storage batteries (1) having a battery can bottom surface (1c) connected to the battery can side surfaces (1a, 1b) and a battery can side surface (1a, 1b) are stacked with the battery can side surfaces (1a, 1b) facing each other, a second battery group (10B) in which a plurality of storage batteries (1) having a battery can bottom surface (1c) connected to the battery can side surfaces (1a, 1b) and a battery can side surface (1a, 1b) are stacked with the battery can side surfaces (1a, 1b) facing each other, and a housing (5) for housing the first battery group (10A) and the second battery group (10B), and the first battery group (10A) and the second battery group (10B) are directly or indirectly thermally connected to each other at their opposing surfaces. This structure makes it possible to provide a battery pack with a small temperature difference between the battery groups.

In addition, the present invention is structured so that the wide surfaces (1a) of the two battery groups face the bottom surface of the housing (5). This structure increases the cooling area, improving cooling performance compared to when the narrow surfaces 1b are in contact with the housing 5.

In addition, in the battery pack described in the present invention, when the wide surface 1a of the battery 1 is placed downward, the external terminal 3 is located at the center of the battery pack, so that the center of the battery pack, where heat is less likely to escape, can be cooled via the external terminal. This makes it possible to provide a battery pack with improved heat dissipation and reduced temperature differences between battery groups.

As shown in FIG. 12, in the battery pack described in the present invention, a first heat transfer member (6) is disposed between the first battery group (10A) and the second battery group (10B), and the heat transfer member (6) is in close contact with the first battery group (10A) and the second battery group (10B). With this structure, it is possible to provide a battery pack in which the heat transfer member promotes heat diffusion and reduces the temperature difference between the battery groups. Note that FIG. 12 is a top view of FIG. 2.

In addition, in the battery pack described in the present invention, as shown in FIG. 13, a second heat transfer member 61 and a third heat transfer member 62 may be arranged on both sides of the first battery group 10A and the second battery group 10B. In this case, the first battery group 10A is sandwiched between the heat transfer member 6 and the second heat transfer member 61, and the second battery group 10B is sandwiched between the heat transfer member 6 and the third heat transfer member 10B, improving heat dissipation on both sides of the battery groups 10A and 10B, making it possible to provide a battery pack with a smaller temperature difference between the battery groups.

The battery pack described in the present invention is also structured so that the second heat transfer member 61 and the third heat transfer member 62 are both wider than the first heat transfer member 6. This structure allows the housing 5 and the heat transfer members 61 and 62 to be secured together with screws or the like, and the housing 5 and the heat transfer members 61 and 62 can be more firmly attached to each other. This makes it easier to transfer heat from the battery 1 to the housing 5, making it possible to provide a battery pack with improved cooling performance.

In addition, in the battery pack described in the present invention, as shown in FIG. 14, as another embodiment, the first heat transfer member 6 is thicker than the second heat transfer member 61 and the third heat transfer member 62. By adopting such a structure, it is possible to provide a battery pack with improved cooling performance by promoting heat diffusion between the two battery groups where heat dissipation is the worst. In addition, in the present invention, the above-mentioned battery pack in which the wide surface 1a of the battery 1 is in contact with the bottom surface of the housing 5 is very effective when used with natural cooling that does not flow cooling air between the battery groups or between the single cells. Therefore, the present invention is very suitable for a structure in which the battery 1 is placed horizontally. The present invention also provides a battery pack comprising a plurality of storage batteries, each formed in a rectangular parallelepiped having a wide side and a narrow side, stacked with the wide sides facing each other, the battery pack comprising a first battery group and a second battery group, the first battery group and the second battery group arranged side by side and housed in a housing so that the narrow sides of the storage batteries face each other, a first heat transfer member is disposed between the first battery group and the second battery group, the first heat transfer member is in close contact with the first battery group and the second battery group, external terminals are provided on the central side of the battery pack for connecting the first battery group and the second battery group to an electronic circuit within the housing, and heat is dissipated via the external terminals. The present invention further relates to a battery pack having a second heat transfer member and a third heat transfer member, the first battery group being sandwiched between the first heat transfer member and the second heat transfer member, the second battery group being sandwiched between the first heat transfer member and the third heat transfer member, and the second heat transfer member and the third heat transfer member being thicker than the first heat transfer member. The present invention further relates to a battery pack having a first battery group being sandwiched between the first heat transfer member and the second heat transfer member, the second battery group being sandwiched between the first heat transfer member and the third heat transfer member, and the first heat transfer member being thinner than the second heat transfer member and the third heat transfer member.

Although the embodiments of the present invention have been described above in detail, the present invention is not limited to the above-described embodiments, and various design changes can be made without departing from the spirit of the present invention described in the claims. For example, the above-described embodiments have been described in detail to clearly explain the present invention, and are not necessarily limited to those having all of the configurations described. In addition, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Furthermore, it is possible to add, delete, or replace part of the configuration of each embodiment with other configurations.

Reference Signs List 1: Single cell 2: Wiring 3: External terminal 4: Bottom surface of housing 5: Top surface of housing 6, 61, 62: Heat transfer member

Claims (4)

A battery pack in which a first battery group and a second battery group are housed in a housing, the first battery group and the second battery group being arranged with the wide sides facing each other and the plurality of storage batteries formed in a rectangular parallelepiped including a pair of wide sides and a plurality of narrow sides, and physically constrained,
the narrow surface includes a first narrow surface having an external terminal, a second narrow surface facing the first narrow surface, and a third narrow surface other than the first narrow surface and the second narrow surface;
the first battery group and the second battery group are assembled with the third narrow surface of the first battery group facing the third narrow surface of the second battery group so as to form a heat transfer portion therebetween , and the assembled first battery group and second battery group are housed in a case of the housing;
The case is closed with a lid.
Battery pack. A battery pack in which a first battery group and a second battery group are housed in a housing, the first battery group and the second battery group being arranged with the wide sides facing each other and the plurality of storage batteries formed in a rectangular parallelepiped including a pair of wide sides and a plurality of narrow sides, and physically constrained,
the narrow surface includes a first narrow surface having an external terminal, a second narrow surface facing the first narrow surface, and a third narrow surface other than the first narrow surface and the second narrow surface;
the first battery group and the second battery group are accommodated in a case of the housing such that the third narrow surface of the first battery group faces the third narrow surface of the second battery group;
The case is closed with a lid,
a terminal for connecting each of the first battery group and the second battery group to a device other than a battery is provided on a center side of the battery pack for each of the first narrow surface of the first battery group and the first narrow surface of the second battery group, the terminal being separate from the external terminal;
Battery pack. Each of the storage batteries of the first battery group and the second battery group is connected to an adjacent storage battery by a connecting member.
3. The battery pack according to claim 1 or 2. A heat transfer portion is provided.
the heat transfer portion is thermally transferred from the first battery group and the second battery group ;
the first battery group and the second battery group are assembled such that the third narrow surface of the first battery group and the third narrow surface of the second battery group face each other so that the heat transfer portion is formed therebetween, and the assembled first battery group and second battery group are housed in the housing;
3. The battery pack according to claim 2.