Method for generating smoke in cell pack

Abstract

A method for generating smoke in a cell pack by which a very small amount of smoke can be generated from a specific single unit cell among unit cells constituting the cell pack. The method causes smoke generation in a cell pack in which unit cells are connected and which includes a positive electrode output terminal for electrically connecting the positive electrode terminal of the unit cell and an external device, and a negative electrode output terminal for electrically connecting the negative electrode terminal of the unit cell and an external device. By attaching a heating means to the negative electrode output terminal, the electrode body of a specific single unit cell is heated through the negative electrode output terminal to which the heating means is attached. As a result, it is possible to cause the generation of a small amount of smoke from the specific single unit cell.

Claims

1. A method for setting a sensitivity of a smoke sensor based on smoke generated in a cell pack, the cell pack comprising: a plurality of unit cells, each having an electrode body formed by interposing a separator between a positive electrode and a negative electrode, and a cell case for accommodating the electrode body; connecting members for connecting each of the plurality of unit cells; a sealed container for accommodating the plurality of unit cells connected by the connecting members; a positive electrode output terminal for electrically connecting a positive electrode terminal of one of the plurality of unit cells and an external device, a portion of which is exposed to the outside through the sealed container; and a negative electrode output terminal for electrically connecting a negative electrode terminal of one of the plurality of unit cells and the external device, a portion of which is exposed to the outside through the sealed container, the method comprising: attaching a heating means to the portion exposed to the outside of the sealed container on one of the positive electrode output terminal and the negative electrode output terminal to perform heating; heating the electrode body of a specific single unit cell of the plurality of unit cells through the positive electrode output terminal or the negative electrode output terminal to which the heating means has been attached; generating smoke from the specific single unit cell by the short-circuit between the positive electrode and the negative electrode due to melting the separator; and setting a sensitivity of the smoke sensor such that the smoke sensor is configured to detect the smoke generated from only the specific single unit cell among the plurality of unit cells included in the sealed container, wherein the setting of the sensitivity of the smoke sensor comprises: changing the sensitivity of the smoke sensor so that the smoke sensor reacts to the smoke generated from the specific single unit cell; and setting the sensitivity of the smoke sensor based on a sensitivity when the smoke sensor reacted.

2. The method of claim 1, wherein, the plurality of unit cells are electrically connected in series by the connecting members, the positive electrode output terminal is connected to a positive electrode terminal of a unit cell arranged at a first end of the series connection and the negative electrode output terminal is connected to a negative electrode terminal of a unit cell arranged at a second end, and the heating means is attached to one of the positive electrode output terminal and the negative electrode output terminal and heated.

3. The method of claim 1, wherein a heating temperature created by the heating means is set to 200° C. to 250° C.

4. The method of claim 1, wherein the heating means is a heater provided with a heating wire wound around the positive electrode output terminal or the negative electrode output terminal.

5. The method of claim 1, wherein a current interrupting device for blocking a conduction path inside one of the plurality of unit cells when one of the plurality of unit cells is in an overcharged state is provided at at least one of the positive electrode terminal and the negative electrode terminal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view schematically showing an example of a unit cell constituting a cell pack;

(2) FIG. 2 is an explanatory view schematically showing an example of an electrode body used for a unit cell;

(3) FIG. 3 is a plan view illustrating a method for generating smoke in a cell pack according to an embodiment of the present invention; and

(4) FIG. 4 is a plan view illustrating a method for generating smoke in a cell pack according to another embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

(5) Preferred embodiments of the present invention will be described below with reference to the drawings as appropriate. In the following drawings, the same reference numerals are attached to members and parts that exhibit the same action. Further, the dimensional relationship (length, width, thickness, etc.) in each drawing does not reflect the actual dimensional relationship. Matters other than those particularly mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters by a person skilled in the art on the basis of the related art in the pertinent field. The present invention can be carried out based on the contents disclosed in this specification and technical common sense in the pertinent field.

1. Structure of Cell Pack

(6) In the present specification, the structure of the cell pack, which is the object of smoke generation in the method for generating smoke in a cell pack according to the present embodiment, will first be described. Such a cell pack has a plurality of unit cells of the following structure.

(7) (1) Structure of Unit Cell

(8) FIG. 1 is a perspective view schematically showing an example of a unit cell constituting a cell pack, and FIG. 2 is an explanatory view schematically showing an example of an electrode body used for a unit cell.

(9) As shown in FIG. 1, a unit cell 10 includes a flat rectangular cell case 50, and an electrode body 20 (see FIG. 2) is accommodated in the cell case 50.

(10) (A) Cell Case

(11) As shown in FIG. 1, the cell case 50 is composed of a rectangular case main body 52 with an open upper surface, and a lid member 54 which closes the opening of the upper surface. It is preferable that the cell case 50 be made of, for example, metal, resin, or the like.

(12) A pair of electrode terminals (a positive electrode terminal 60 and a negative electrode terminal 62) are provided at the lid member 54 forming the upper surface of the cell case 50. Specifically, as will be described hereinbelow, these positive electrode terminal 60 and negative electrode terminal 62 are elongated conductive members, a first end of which is exposed to the outside of the cell case 50. Although not shown, a second end of the positive electrode terminal 60 (negative electrode terminal 62) is electrically connected to the electrode body 20 inside the cell case 50. The positive electrode terminal 60 is preferably made of aluminum, an aluminum alloy, or the like, and the negative electrode terminal 62 is preferably made of copper, a copper alloy, or the like.

(13) (B) Electrode Body

(14) As shown in FIG. 2, the electrode body 20 is formed by laminating an elongated sheet-shaped positive electrode 21 and an elongated sheet-shaped negative electrode 25, with a separator 29 interposed therebetween, and then winding the laminated body.

(15) The positive electrode 21 of the electrode body 20 is configured by providing a positive electrode mix layer 23 including a positive electrode active material, a conductive agent, and the like to the surface of a positive electrode current collector 22 made of an elongated sheet-shaped aluminum foil or the like. Further, on one side edge portion in the width direction of the positive electrode 21, a positive electrode current collector exposed portion 24, which is not provided with the positive electrode mix layer 23, is formed.

(16) The negative electrode 25 is configured by providing a negative electrode mix layer 27 including a negative electrode active material and the like to the surface of a negative electrode current collector 26 made of an elongated sheet-shaped copper foil or the like. At one side edge portion in the width direction of the negative electrode 25, a negative electrode current collector exposed portion 28, which is not provided with the negative electrode mix layer 27, is formed.

(17) The same materials as those conventionally used for lithium ion secondary cells can be used without particular limitation as the materials for the positive electrode 21 and the negative electrode 25 described above, and the present invention is not particularly limited thereby. Therefore, the explanation of these materials is herein omitted.

(18) A wound core portion 20A in which the positive electrode mix layer 23 and the negative electrode mix layer 27 are wound is formed at the center portion of the electrode body 20 in the winding axis direction, and a charge-discharge reaction is mainly performed in the wound core portion 20A. On one side edge portion of the electrode body 20, the positive electrode current collector exposed portion 24 is wound in the winding axis direction, and a positive electrode terminal connecting portion 20B connected to a second end of the positive electrode terminal 60 (see FIG. 1) is formed. Further, on the other side edge portion, a negative electrode current collector exposed portion 28 is wound, and a negative electrode terminal connecting portion 20C connected to a second end of the negative electrode terminal 62 (see FIG. 1) is formed.

(19) As will be described in detail hereinbelow, in the method for generating smoke according to the present embodiment, the electrode body 20 in the cell case 50 is heated via the positive electrode terminal 60 (negative electrode terminal 62). Therefore, from the viewpoint of efficiently transmitting the heat from the positive electrode terminal 60 (negative electrode terminal 62) to the electrode body 20, it is preferable that the connection portion between the positive electrode terminal 60 and the positive electrode terminal connecting portion 20B (connection portion between the negative electrode terminal 62 and the negative electrode terminal connecting portion 20C) be joined by a joining means having high thermal conductivity, for example, such as ultrasonic welding or laser welding.

(20) Further, a porous sheet (film) having a function of electrically insulating the positive electrode 21 and the negative electrode 25 and passing lithium ions is used for the separator 29. A resin material having a predetermined heat resistance (specifically, a melting point of 100° C. to 300° C., and preferably 170° C. to 180° C.) can be preferably used for such a separator 29. Examples of the resin material suitable for such a separator 29 include polyethylene (PE), polypropylene (PP), polyesters, cellulose, polyamides, and the like. Further, the separator 29 may be formed by laminating a plurality of porous sheets formed of different resins.

(21) Although not shown, an electrolytic solution is also accommodated, in addition to the electrode body 20 described above, in the cell case 50 shown in FIG. 1. Such an electrolytic solution is prepared by dissolving a supporting salt in a predetermined solvent, but since conventionally used solvent and supporting salt can be used without any particular limitation therefor, description thereof is herein omitted.

(22) (2) Structure of Cell Pack

(23) Next, a cell pack including a plurality of the above-described unit cells 10 will be described. FIG. 3 is a plan view for explaining the method for generating smoke in a cell pack according to the present embodiment. As shown in FIG. 3, a cell pack 100, which is the object of the method for generating smoke according to the present embodiment, has a plurality of the unit cells 10 of the above-described structure, and the unit cells 10 are connected to each other by connecting members 40.

(24) The cell pack 100 shown in FIG. 3 is the so-called series-connected cell pack in which a plurality of the unit cells 10 are connected in series by connecting members 40. In such a series-connected cell pack 100, the unit cells 10 are arranged with alternate orientations so that the positive electrode terminal 60 and the negative electrode terminal 62 are brought close to each other between two adjacently arranged unit cells 10. The positive electrode terminal 60 of one of the unit cells 10 and the negative electrode terminal 62 of the other unit cell 10 are sequentially connected by the connecting members 40.

(25) The connecting member 40 is a plate-like conductive member. Although not shown, terminal insertion holes are formed in both end portions of the connecting member 40. After inserting the positive electrode terminal 60 (or the negative electrode terminal 62) of the unit cell 10 through the terminal insertion hole, the connecting member 40 is connected by tightening a fixing nut.

(26) In the cell pack 100, each of the unit cells 10 connected by the connecting members 40 is accommodated in the sealed container 30. Although not particularly limited, it is preferable that the sealed container 30 be made of a metal, a resin, or the like having a predetermined strength.

(27) In the cell pack 100 shown in FIG. 3, the unit cells 10A and 10B disposed at both ends of the series connection are connected to an external device such as a motor of a vehicle. A negative electrode output terminal 46 (total-minus output terminal) which is to be electrically connected to the positive electrode of the external device is connected to the negative electrode terminal 62 of the unit cell 10A at a first end, among the unit cells 10A and 10B arranged at both ends. Further, a positive electrode output terminal 45 (total-plus output terminal) which is to be electrically connected to the negative electrode of the external device is connected to the positive electrode terminal 60 of the unit cell 10B at the second end. Further, the positive electrode output terminal 45 and the negative electrode output terminal 46 penetrate the sealed container 30 and are exposed to the outside.

2. Method for Generating Smoke in Cell Pack

(28) Next, a method for generating smoke in a cell pack according to the present embodiment will be described.

(29) As shown in FIG. 3, in the method for generating smoke according to the present embodiment, a heating means 70 is attached to a portion of the negative electrode output terminal 46 which is exposed to the outside of the sealed container 30, and the negative electrode output terminal 46 is heated by the heating means 70. General heating means can be used, without particular limitation, for the heating means 70, such a heating means being exemplified by a heater having a heating wire wound around the negative electrode output terminal 46. The heater having the heating wire can heat the negative electrode output terminal 46 in a concentrated manner, thereby preventing heat from diffusing to other unit cells 10.

(30) When the negative electrode output terminal 46 is heated by the heating means 70, the electrode body 20 (see FIG. 2) of the unit cell 10A connected to the negative electrode output terminal 46 is heated.

(31) Specifically, in the present embodiment, the heat applied by the heating means 70 is transmitted, via the negative electrode output terminal 46, to the negative electrode terminal 62 of the unit cell 10A arranged in the sealed container 30. As described above, the negative electrode terminal 62 is connected to the negative electrode terminal connecting portion 20C (see FIG. 2) of the electrode body 20 inside the cell case 50. Therefore, the heat transmitted to the negative electrode terminal 62 is further transferred to the electrode body 20 to heat the electrode body 20. In this embodiment, when the temperature of the electrode body 20 is raised to a predetermined temperature, the separator 29 (see FIG. 2) is melted and the positive electrode 21 and the negative electrode 25 are short-circuited, so that the unit cell 10A generates smoke. Thus, according to the method for generating smoke of this embodiment, smoke can be generated by directly heating the electrode body 20 of a specific single unit cell 10A.

(32) Further, unlike the method of piercing with a conductive member such as a nail, the method for generating smoke according to the present embodiment does not damage the cell case 50, and therefore reliably prevents a large amount of smoke from leaking from the damaged portion. Also, unlike the method for generating smoke by overcharging, the abnormality of the unit cell does not rapidly progress to ignition. Therefore, it is possible to safely carry out the test of the cell pack.

(33) As described above, according to the method for generating smoke of the present embodiment, it is possible to generate smoke from a specific single unit cell in a very small amount about the same as at the initial stage of smoke generation in the actual product, so that it is possible to inspect and set a smoke sensor with high accuracy. As a result, it is possible to stably provide a highly accurate notification device capable of accurately detecting a very small amount of smoke at the initial stage of smoke generation and notifying a vehicle occupant thereof, so that it is possible to contribute greatly to the improvement of safety of the vehicle equipped with the cell pack.

(34) Further, in the cell pack 200 shown in FIG. 3, as described above, each unit cell 10 is accommodated in the sealed container 30. A problem associated with the cell pack 200 having such a sealed container 30 is that since the sealed container 30 diffuses heat from the outside, a plurality of unit cells 10 are easily heated at the same time. However, in the method for generating smoke according to the present embodiment, since the electrode body 20 of the specific unit cell 10A can be directly heated by the transfer of heat from the negative electrode output terminal 46, it is possible to appropriately heat and cause only the specific single unit cell 10A to generate smoke even in the cell pack 200 having such a sealed container 30.

(35) Further, unlike the method for generating smoke by overcharging, the method for generating smoke according to the present embodiment can adequately cause smoke generation, without actuating a current interrupting device (CID) even when the CID is provided at the positive terminal (or negative terminal) of the unit cell. Therefore, the method for generating smoke according to the present embodiment can be particularly advantageously used for a cell pack including a unit cell equipped with a CID.

(36) The heating temperature created by the heating means 70 is preferably set so that the unit cell 10A, which is the object to be heated, can be suitably caused to generate smoke. More specifically, since the heat applied from the heating means 70 is cooled while being transmitted through the negative electrode output terminal 46 and the negative electrode terminal 62, the heating temperature to the electrode body 20 becomes somewhat lower than the heating temperature created by the heating means 70.

(37) Therefore, when the objective is to cause smoke generation by melting the separator 29, it is preferable to set the heating temperature created by the heating means 70 to a temperature somewhat higher than the melting temperature of the separator 29. For example, when a resin having a melting temperature of 170° C. to 180° C. is used for the separator 29, it is preferable to set the heating temperature created by the heating means 70 to 200° C. to 250° C. Accordingly, it is possible to appropriately melt the separator 29 so as to appropriately generate smoke due to a short circuit between the positive electrode 21 and the negative electrode 25.

(38) Further, in the above description, the electrode body 20 is caused to generate smoke by short-circuiting the positive and negative electrodes by melting of the separator 29. However, when the smoke generation temperature of the constituent material (the positive electrode 21, the negative electrode 25, and the like) of the electrode body 20 is lower than the melting temperature of the separator 29, where the electrode body 20 is heated to the smoke generation temperature of the constituent material of the electrode body 20, the unit cell 10A generates smoke. Therefore, it is preferable to set the temperature of the heating means 70 in consideration of the smoke generation temperature of the constituent material of the electrode body 20.

3. Another Embodiment

(39) The method for generating smoke in a cell pack disclosed herein is not limited to the above embodiment, and can be changed variously.

(40) For example, in the above-described embodiment, the heating means 70 is attached to the negative electrode output terminal 46, and the electrode body 20 of the unit cell 10A connected to the negative electrode output terminal 46 is heated. However, the heating position in the method for generating smoke disclosed herein may be at one of the positive electrode output terminal 45 and the negative electrode output terminal 46. For example, the heating means 70 may be attached to the positive electrode output terminal 45 in FIG. 3. Even in this case, it is possible to generate a very small amount of smoke same as in the actual product by properly heating the electrode body 20 of the specific single unit cell 10B.

(41) Further, in the above-described embodiment, the object of smoke generation is a series-connected cell pack, but the method for generating smoke in a cell pack disclosed is also applicable to the cell pack 200 with parallel connection such as shown in FIG. 4.

(42) In the parallel-connected cell pack 200, the positive electrode output terminal 45 is connected to the positive electrode terminal 60 of each of the plurality of unit cells 10, and the negative electrode output terminal 46 is connected to the negative electrode terminal 62. When the parallel-connected cell pack 200 is the object of smoke generation, a heat absorbing material 80 is attached close to the unit cell 10 other than the unit cell 10C which is the object of smoke generation (for example, to the negative electrode output terminal 46 close to the unit cell 10) before sealing the sealed container 30. Then, the heating means 70 is attached to the portion of the negative electrode output terminal 46 exposed to the outside of the sealed container 30 and heating is performed. As a result, the electrode body of the unit cell 10C, which is the object of smoke generation, is heated in a concentrated manner. Therefore, the specific single unit cell 10C can be adequately caused to generate smoke.

(43) Further, in all of the above-described embodiments, the object of smoke generation is a cell pack having a sealed container, but the method for generating smoke disclosed herein can naturally be applied to the cell pack not having the sealed container. Further, when a parallel-connected cell pack is not provided with a sealed container, it is possible to cause appropriately smoke generation in a specific single unit cell by simply attaching a heating means to a negative electrode output terminal (or a positive electrode output terminal) in the vicinity of a unit cell which is the object of smoke generation. In this case, by contrast with the cell pack such as shown in FIG. 4, it is not necessary to attach a plurality of heat absorbing materials 80. Therefore, the specific single unit cell can be easily caused to generate smoke.

Test Example

(44) Hereinafter, test examples relating to the present invention will be described, but the following test examples are not intended to limit the present invention.

1. Explanation of Each Test Example

(1) Test Example 1

(45) (A) Construction of Test Cell Pack

(46) An elongated sheet-shaped positive electrode was prepared in which a positive electrode mix layer in which a positive electrode active material (LiNi.sub.1/3Co.sub.1/3Mn.sub.1/3O.sub.2), a conductive material (acetylene black), and a binder (PVDF) were mixed at a mass ratio of 94:3:3 was formed on both sides of a positive electrode current collector (aluminum foil) having a thickness of 12 μm. An elongated sheet-shaped negative electrode was also prepared in which a negative electrode mix layer in which a negative electrode active material (graphite), a thickener (CMC), and a binder (SBR) were mixed at a mass ratio of 98:1:1 was formed on both sides of a negative electrode current collector (copper foil) having a thickness of 10 μm.

(47) Then, the positive electrode and the negative electrode were laminated with a separator having a three-layer structure of PP/PE/PP (melting point: 170° C.) interposed therebetween, and then the laminate was wound to prepare a wound electrode body. The wound electrode body was accommodated together with an electrolytic solution in an angular cell case (width W 148 mm, thickness D 26.4 mm, height H 91 mm) as shown in FIG. 1 to produce a unit cell having a capacity of 25 Ah.

(48) Then, in Test Example 1, a total of 96 of the above-described unit cells were produced, the unit cells were connected in series by using copper connecting members as shown in FIG. 3, a positive electrode output terminal made of copper was connected to a unit cell arranged at the first end of the series connection, and a negative electrode output terminal made of copper was connected to a unit cell arranged at the second end. Then, the unit cells were accommodated in a transparent sealed container, and the positive electrode output terminal and the negative electrode output terminal were exposed to the outside of the sealed container to construct a cell pack for testing.

(49) (B) Smoke Generation Test

(50) Next, in Test Example 1, the cell pack thus produced was charged to state of charge (SOC) 100% by constant-current constant-voltage (CC-CV) charging under a room temperature environment (about 25° C.). Then, a heating wire was wound around a portion of the negative electrode output terminal exposed to the outside of the sealed container, and the negative electrode output terminal was heated until smoke generation at 300° C. Then, the number of unit cells that generated smoke during heating and the amount of generated smoke (smoke generation amount) were visually observed.

(2) Test Example 2

(51) In Test Example 2, the construction of the cell pack and the smoke generation test were carried out and the number of unit cells that generated smoke during heating and the amount of generated smoke were visually observed in the same manner as in Test Example 1, except that a unit cell with a current interrupting device (CID) attached thereto was used to construct the test cell pack.

(3) Test Example 3

(52) In Test Example 3 the construction of the cell pack and the smoke generation test were carried out and the number of unit cells that generated smoke during heating and the amount of generated smoke were visually observed in the same manner as in Test Example 1, except that a method of piercing the cell pack with an iron nail so as to penetrate one of the plurality of unit cells constituting the cell pack was performed.

(4) Test Example 4

(53) In Test Example 4, the construction of the cell pack and the smoke generation test were carried out and the number of unit cells that generated smoke during heating and the amount of generated smoke were visually observed in the same manner as in Test Example 3, except that a unit cell with a current interrupting device (CID) attached thereto was used to construct the test cell pack.

(5) Test Example 5

(54) In Test Example 5, the construction of the cell pack and the smoke generation test were carried out and the number of unit cells that generated smoke during heating and the amount of generated smoke were visually observed in the same manner as in Test Example 1, except that a method of overcharging a single unit cell, among a plurality of unit cells constituting the cell pack, until smoke generation was performed.

(6) Test Example 6

(55) In Test Example 6, the construction of the cell pack and the smoke generation test were carried out in the same manner as in Test Example 5, except that a unit cell with a current interrupting device (CID) attached thereto was used to construct the test cell pack.

(7) Test Example 7

(56) In Test Example 7, the construction of the cell pack and the smoke generation test were carried out and the number of unit cells that generated smoke during heating and the amount of generated smoke were visually observed in the same manner as in Test Example 1, except that a method of heating a plurality of unit cells from the outside of the sealed container of the cell pack with a heater set at 400° C. was carried out in the smoke generation test.

(8) Test Example 8

(57) In Test Example 8, the construction of the cell pack and the smoke generation test were carried out and the number of unit cells that generated smoke during heating and the amount of generated smoke were visually observed in the same manner as in Test Example 7, except that a unit cell with a current interrupting device (CID) attached thereto was used to construct the test cell pack.

2. Evaluation Results

(58) The results of the smoke generation tests conducted in Test Examples 1 to 8 are shown in Table 1 below. In Test Example 6, since no smoke was generated even though the smoke generation test was continued, the test was stopped halfway.

(59) TABLE-US-00001 TABLE 1 Number of Method for cells that Amount of generating Presence generated generated smoke of CID smoke smoke Test Heating of Not present 1 Very small Example 1 negative Test electrode Present 1 Very small Example 2 output terminal Test Piercing Not present 1 Large Example 3 Test Present 1 Large Example 4 Test Overcharging Not present 1 Large Example 5 Test Present 0 None Example 6 Test Heating of Not present 3 Large Example 7 unit cell Test Present 3 Large Example 8

(60) According to the results shown in Table 1, in Test Example 1 in which the method for generating smoke by heating the negative electrode output terminal was performed, it was possible to appropriately generate smoke from a single unit cell connected to the negative electrode output terminal, and the amount of smoke generated from this unit cell was very small. Based on these results, it could be confirmed that, in the case of using the method for generating smoke by heating the negative electrode output terminal, it is possible to generate a very small amount of smoke from a specific single unit cell similarly to the initial stage of smoke generation in the actual product. The same result was obtained also in Test Example 2 using a unit cell having a CID.

(61) Meanwhile, in Test Examples 3 and 4 in which the method for generating smoke by piercing with a nail was carried out, it was possible to generate smoke from a single unit cell pierced with the nail, but since a large amount of smoke generated in the cell case leaked from the damaged portion, the smoke generation amount increased. For this reason, when using the method for generating smoke by piercing with a nail, it was impossible to generate a very small amount of smoke such as at the initial stage of smoke generation in the actual product.

(62) In Test Example 5 in which the method for generating smoke by overcharging was performed, it was possible to generate smoke from a specific single unit cell, but a large amount of smoke was confirmed. This is presumably because an abnormality of the electrode body inside the cell case was not limited to smoke generation and progressed to ignition. In Test Example 6, no smoke was generated at all. This is presumably because the CID was actuated during the smoke generation test.

(63) In Test Examples 7 and 8 in which the smoke generation test by heating was performed, since smoke was generated simultaneously from three cells, the amount of smoke increased. This is apparently because the heat from the heater dispersed and a plurality of cells were simultaneously heated.

(64) Although specific examples of the present invention have been described in detail above, these are merely illustrative and do not limit the scope of the claims. Techniques described in the claims include those in which the concrete examples exemplified above are variously modified and changed.