JP4590910B2 - Battery - Google Patents

Description

  The present invention relates to an assembled battery, and more particularly to an assembled battery in which a plurality of columnar batteries each having an electrode group in which a separator is interposed between a positive electrode and a negative electrode and an electrolytic solution are contained in a cylindrical container are combined.

  Conventionally, in order to construct an assembled battery by combining a plurality of cylindrical batteries, a cylindrical battery is held in a battery holder (see, for example, Patent Document 1), or a cylindrical battery having an inner diameter larger than the diameter of the cylindrical battery. A technique is disclosed in which a cylindrical battery is inserted into a holder, metal pieces that electrically connect the cylindrical batteries are welded or screwed to both end faces of the cylindrical battery holder, and the battery is fixed with this joining force. Yes.

JP-A-10-270006 (FIG. 8, paragraph numbers “0032” to “0040”)

  However, the technique disclosed in Patent Document 1 is expensive because the battery holder is complicated. In addition, the battery holder portion of the battery holder is curved in the same shape as the battery side surface, which complicates battery assembly. Furthermore, when recycling the assembled battery to which the battery is fixed, there arises a problem that the dismantling work becomes complicated and the amount of discarded members increases.

  Further, in the technique of inserting a cylindrical battery into the cylindrical battery holder, not only the battery holder becomes expensive, but also a metal piece for fixing the cylindrical battery by the joining force of the battery holder and carrying out electrical connection. The volume of the battery increases, and the weight of the assembled battery increases accordingly.

  In view of the above-described case, an object of the present invention is to provide an assembled battery excellent in assembling property, volume, and weight efficiency.

In order to solve the above-described problems, the present invention provides an assembled battery in which a plurality of columnar batteries each containing an electrode group in which a separator is interposed between a positive electrode and a negative electrode and an electrolytic solution are housed in a cylindrical container. A first spacer having a flat surface other than one surface in contact with the outer peripheral side surface is disposed on the outer peripheral side surface of the battery, and the first spacer has only one surface in contact with the outer peripheral side surface. In addition, the facing surfaces are all flat, and the cylindrical battery is combined with at least the facing surface as a fixed surface.

In the present invention, the first spacer is disposed on the outer peripheral side surface of the cylindrical battery, and the first spacer is composed of only one surface in contact with the outer peripheral side surface, and all the facing surfaces are flat . Since the facing surface other than the one surface in contact with the cylindrical battery is flat, at least the facing surface of the first spacer can be a fixed surface, so that the assemblability of the cylindrical battery can be improved and the first spacer The volume and weight increase occupied by the battery can be suppressed, and the volume and weight efficiency of the assembled battery can be improved.

In the present invention, a second spacer having the same shape as the first spacer is provided on the opposite side surface of the outer peripheral side surface where the first spacer of the cylindrical battery is disposed, and the opposite surface other than the one surface in contact with the opposite side surface is planar. Furthermore, if it arrange | positions, a cylindrical battery can be fixed and combined with the fixed surface of a 1st and 2nd spacer. In addition, if the first spacer has a substantially U-shaped cross section and a recess for positioning the cylindrical battery is formed on a surface in contact with the outer peripheral side surface of the cylindrical battery, the cylindrical battery and the first spacer Integration can be performed easily and reliably. Furthermore, if the first spacer is a frame having a substantially rectangular cross section and a cylindrical battery is inserted into the frame, the number of members is as small as one per cylindrical battery, so that the cost can be reduced and the cylindrical shape can be reduced. Four fixed surfaces can be formed on the battery.

  In addition, the first spacer has a substantially triangular cross section, a depression is formed on a surface in contact with the outer peripheral side surface of the cylindrical battery, and at least one of the two surfaces of the cylindrical battery other than the surface where the depression is formed is fixed. In addition, a second spacer having the same shape as the first spacer is further arranged on the side surface opposite to the outer peripheral side surface where the first spacer of the cylindrical battery is arranged. Further, the cylindrical battery may be combined with at least one of the two surfaces other than the surface where the recess of the second spacer is formed as a fixed surface. In these forms, it is possible to further reduce the size of the assembled battery. If the spacer and the cylindrical battery are covered with a heat-shrinkable cylindrical body, the cylindrical battery and the spacer can be integrated by shrinking the cylindrical body that is deformed by heating. Can be improved.

According to the present invention, the first spacer is disposed on the outer peripheral side surface of the cylindrical battery, and the first spacer is composed of only one surface in contact with the outer peripheral side surface, and all the facing surfaces are flat. Since the facing surface of the spacer other than the one in contact with the cylindrical battery is flat, at least the facing surface of the first spacer can be a fixed surface, so that the assembly of the cylindrical battery can be improved and the first The increase in volume and weight occupied by the spacers can be suppressed, and the volume and weight efficiency of the assembled battery can be improved.

(First embodiment)
Hereinafter, a first embodiment of an assembled battery according to the present invention will be described with reference to the drawings.

(Constitution)
As shown in FIGS. 1 (A) and 1 (B), the assembled battery 28 of this embodiment includes eight individual batteries 5c arranged in two rows and four columns.

  As shown in FIGS. 2 and 3, each individually bound battery 5 c includes a cylindrical lithium ion secondary battery (hereinafter abbreviated as a secondary battery) 1 and a plate-like synthetic resin spacer 4 a on the outer peripheral side surface thereof. The secondary battery 1 and the spacer 4a are made of synthetic resin, covered with a cylindrical heat-shrinkable tube 9, and are individually bound and closely attached. In the spacer 4 a, an arcuate recess 24 a for positioning the secondary battery 1 is formed on the surface in contact with the outer peripheral side surface of the secondary battery 1. Further, the surface of the spacer 4a where the recess 24a is formed is a flat surface, and both corners are rounded. For this reason, the individual battery 5c has one flat fixed adhesive surface 6c formed on the opposite surface and two flat fixed adhesive surfaces 6d intersecting the fixed adhesive surface 6c. Yes.

  The secondary battery 1 includes an electrode group in which a positive electrode and a negative electrode are wound around a polypropylene hollow rod-shaped shaft core mixed with 30% glass filler through a polyethylene microporous thin film separator that allows lithium ions to pass through. The non-aqueous electrolyte is accommodated in a cylindrical bottomed battery can, and when the internal pressure of the battery reaches a predetermined pressure, the upper lid cap 2 having a gas release valve that releases the gas is caulked and sealed through an insulator, and the caulking portion 3 Is formed (see FIG. 2). The upper lid cap 2 and the bottom surface of the battery can serve as positive and negative terminals of the secondary battery 1, respectively.

In the positive electrode, 100 parts by weight of an active material lithium manganate (LiMn ₂ O ₄ ) powder, 10 parts by weight of scaly graphite having an average particle diameter of 20 μm as a conductive agent, and 10 parts by weight of polyvinylidene fluoride (PVDF) as a binder are used. The slurry obtained by adding, kneading, and kneading the dispersion solvent N-methylpyrrolidone was applied to both surfaces of the aluminum foil, dried, pressed and cut.

  On the other hand, the negative electrode was added to 10 parts by weight of PVDF as a binder to 100 parts by weight of graphite powder, and N-methylpyrrolidone as a dispersion solvent was added to this, and a slurry obtained by kneading was applied to both sides of the rolled copper foil. It was obtained by drying, pressing and cutting.

In the non-aqueous electrolyte, for example, lithium hexafluorophosphate (LiPF ₆ ) or lithium tetrafluoroborate was dissolved in an organic solvent such as ethylene carbonate (EC) or dimethyl carbonate (DMC) at about 1 mol / liter. An electrolyte is used.

  As shown in FIG. 1, the bound battery 5 c is formed by adhering the positive and negative terminals of the adjacent secondary battery 1 alternately in the vertical direction and adhering the fixed adhesive surfaces 6 c and the adjacent fixed adhesive surfaces 6 d to each other. ing. That is, the fixed adhesive surfaces 6c and 6d are fixed surfaces between the individually bound batteries 5c.

  The positive and negative terminals of the secondary battery 1 are connected to different polarities of the adjacent secondary batteries by a plate-shaped metal connection bus bar 7, connected in series as the entire secondary battery 1, and the assembled battery 28 is It is configured. A cross-shaped welding through hole (projection) (not shown) is formed at both ends of the connection bus bar 7, and the positive and negative terminals are spot welded to the connection bus bar 7 through the through holes, respectively.

(Production of assembled battery)
Next, a manufacturing procedure of the assembled battery 28 of the present embodiment will be described.

  A spacer 4 a is arranged on the outer peripheral side surface of the secondary battery 1, and the secondary battery 1 and the spacer 4 a are covered with a heat shrinkable tube 9. Next, the heat-shrinkable tube 5a is heat-shrinked so that the secondary battery 1 and the spacer 4a are tied and adhered. Thereby, the individual battery 5c is produced.

  After producing a plurality of individually bound batteries 5c, an adhesive is applied to one of the fixed adhesive surfaces 6c and 6d of the individually bound batteries 5c to be bonded, and then the adjacent batteries in the fixing jig are turned upside down. Then, the fixed adhesive surfaces 6c and 6d are bonded together to combine the individually bound batteries 5c. The positive and negative terminals of the individually bound battery 5c and the adjacent different electrodes of the individually bound battery 5c are spot-welded through the welding through holes of the connection bus bar 7 to complete the production of the assembled battery 28.

(Function)
Next, the operation and the like of the assembled battery 28 of the present embodiment will be described.

  In the assembled battery 28 of this embodiment, the spacer 4a is arrange | positioned at the outer peripheral side surface of the secondary battery 1, and the fixed adhesion surfaces 6c and 6d of the individually bound battery 5c are planar. For this reason, since the fixed adhesion surfaces 6c and 6d can be used as a fixed surface between the bound batteries 5c, the assemblability of the secondary battery 1 can be improved. Further, since the spacer 4a has a plate-like shape and the spacer 4a and the insulating tube 9 are both made of synthetic resin and light in weight, an increase in the volume and weight occupied by the spacer 4a can be suppressed. , Weight efficiency can be improved. Furthermore, since the battery holder which is complicated and expensive is unnecessary, the cost of the assembled battery 28 can be reduced.

  Further, in the assembled battery 28 of the present embodiment, when the assembled battery 28 is disassembled for recycling or when the series-parallel connection is changed, the heat-shrinkable tube 9 can be simply disassembled by simply breaking. it can. Moreover, since the spacer 4a can be repeatedly used after the dismantling operation, the assembled battery 28 having excellent recyclability can be obtained.

  Furthermore, in the assembled battery 28 of this embodiment, the heat shrinkable tube 9 is used to cover the spacer 4a and the secondary battery 1. For this reason, since the secondary battery 1 and the spacer 4a can be united by heating and shrinking the heat-shrinkable tube 9, assembly property can be further improved. Moreover, since the spacer 4a and the heat shrinkable tube 9 are made of synthetic resin, the secondary batteries 1 can be insulated from each other.

  Furthermore, in the assembled battery 28 of the present embodiment, an arcuate recess 24a is formed on the surface of the spacer 4a that contacts the outer peripheral side surface of the secondary battery 1. For this reason, the secondary battery 1 and the spacer 4a can be integrated easily and reliably at the time of assembly.

  And in the assembled battery 28 of this embodiment, since each secondary battery 1 is adhere | attached by the fixed adhesion | attachment surfaces 6c and 6d, since the connection bus bar 7 is spot-welded and connected in series to a positive / negative electrode terminal, connection work is easy. Become.

  In the present embodiment, the fixed adhesive surfaces 6c and 6d are flat and the fixed surfaces of the bound batteries 5c are shown. However, if at least the fixed adhesive surface 6c is flat. Two secondary batteries 1 can be combined.

  Further, in the present embodiment, the assembled battery 28 in which the eight secondary batteries 1 are combined and connected in series is illustrated, but the combination and connection of the secondary batteries 1 are not limited to these, for example, two batteries The secondary batteries 1 may be connected in parallel, and eight batteries connected in parallel may be connected in series.

(Second Embodiment)
Next, a second embodiment in which the present invention is applied to an assembled battery will be described. In the present embodiment, a spacer 14a having the same shape as the spacer 4a shown in the first embodiment is further arranged on the opposite side of the outer peripheral side surface of the secondary battery 1 and is covered with an insulating tube 9 to form an individual battery. is there. In the following embodiments, the same members as those in the first embodiment described above are denoted by the same reference numerals, description thereof is omitted, and only different portions will be described.

  As shown in FIG. 4, the assembled battery 8 of the present embodiment includes four individually bound batteries 5 a arranged in one row and four columns.

  As shown in FIG. 5, in the individual battery 5a, a spacer 14a is further arranged on the opposite side of the outer peripheral side surface of the secondary battery 1, and the secondary battery 1 is connected to the heat shrinkable tube 9 via the spacers 4a and 14a. Covered individually and tied tightly. For this reason, the individual bound battery 5a has a flat fixed adhesive surface 6a on each side of the surfaces of the spacers 4a and 14a that are in contact with the secondary battery 1. In the bound battery 5a, the positive and negative terminals of the adjacent secondary battery 1 are combined in a line by adhering the fixed adhesive surfaces 6a with the vertical direction alternately. That is, the fixed adhesive surface 6a is a fixed surface between the individually bound batteries 5a.

  Spacers 4 a and 14 a are disposed on the side opposite to the outer peripheral side surface of the secondary battery 1, and the secondary battery 1 and the spacers 4 a and 14 a are covered with the heat shrinkable tube 9. Next, the heat-shrinkable tube 5a is heat-shrinked so that the secondary battery 1 and the spacers 4a and 14a are tied and adhered. Thereby, the individual bound battery 5a is produced.

  After producing a plurality of individually bound batteries 5a, an adhesive is applied to one fixed adhesive surface 6a of the individually bound batteries 5a to be bonded, and then the adjacent batteries in the fixing jig are turned upside down. Then, the fixed adhesive surfaces 6a are bonded together to combine the individually bound batteries 5a, and the connection bus bar 7 is connected to produce the assembled battery 8.

  In the assembled battery 8 of this embodiment, the spacer 14a is arrange | positioned on the opposite side surface of the outer peripheral side surface where the spacer 4a of the secondary battery 1 is arrange | positioned. For this reason, the individually bound batteries 5a can be bonded and combined with each other at the fixed adhesive surfaces 6a of the spacers 4a and 14a. Since the fixed adhesive surface 6a is larger than the fixed adhesive surface 6d and the bonding force is large, the assembled battery 8 that is resistant to external forces such as vibration can be obtained.

  In the above (first and second) embodiments, the example in which the spacer 4a (14a) whose surface in contact with the secondary battery 1 is arc-shaped is shown, but as shown in FIGS. 8 (A) and 8 (B). In addition, one or two plate-like spacers 4b made of synthetic resin in which a substantially U-shaped depression 24b for positioning on a surface in contact with the outer peripheral side surface of the secondary battery 1 is formed per secondary battery 1 are used. May be. For example, as shown in FIGS. 7A and 7B, a spacer 4b is arranged on the outer peripheral side surface of the secondary battery 1, and a spacer 14b having the same shape as the spacer 4b is arranged on the opposite side surface of the outer peripheral side surface. Alternatively, the tied battery 5b may be configured. As shown in FIG. 6, the bound battery 5b has two substantially rectangular planar fixed adhesive surfaces 6b on the outer surface. For this reason, the individually bound battery 5b can be combined with the fixed adhesive surface 6b, and the weight of the spacer can be reduced as compared with the case where the spacers 4a and 14a are used. Therefore, the weight efficiency of the assembled battery 8 is further improved. be able to.

  Further, in the present embodiment, the example in which the spacers 4a and 14a having the same shape are disposed on the outer peripheral side surface and the opposite side surface of the secondary battery 1 is shown, but similarly, the spacer 4b may be disposed on the opposite side surface. Individually bound batteries can be combined.

(Third embodiment)
In this embodiment, as shown in FIG. 9, a frame 6e having a substantially rectangular cross section is used instead of using the spacers 4a and 14a. In the individually bound battery 5d, the secondary battery 1 is inserted in the frame 6e. Four planar fixed adhesive surfaces 6e are formed on the outer surface of the individually bound battery 5d.

  In the assembled battery of the present embodiment, the individually bound battery 5d is configured by inserting the secondary battery 1 into a frame 6e. For this reason, since it is not necessary to heat-shrink the heat-shrinkable tube 9, a heat curing time is not required, so that the assemblability can be further improved.

  Moreover, in the assembled battery of this embodiment, it has the four planar fixed adhesion surfaces 6e on the outer surface of the bound battery 5d. For this reason, it is possible to arrange the combined batteries 5d not only in one direction but also in two directions and combine them, and the fixed adhesive surface 6e is larger than the fixed adhesive surface 6d and has a larger bonding force as in the second embodiment. The assembled battery can withstand external forces such as vibration.

  Furthermore, in the assembled battery according to the present embodiment, the single battery 6d is constituted by one frame 6e per secondary battery 1. For this reason, since the number of members per secondary battery 1 can be reduced, cost can be reduced.

(Fourth embodiment)
In the present embodiment, as shown in FIG. 10, the assembled battery 38 includes eight individually bound batteries 5e arranged in two rows and four columns.

  As shown in FIG. 11, columnar synthetic resin spacers 4 e having a substantially triangular cross section are arranged along the outer peripheral side surface of the secondary battery 1, and the secondary battery 1 and the spacer 4 e are covered with the heat shrinkable tube 9. To do. Next, the heat-shrinkable tube 9 is heat-shrinked so that the secondary battery 1 and the spacer 4e are tied and adhered. Thereby, the individually bound battery 5e is produced.

  Therefore, in the present embodiment, the arcuate recess 24e for positioning the secondary battery 1 is formed on the spacer 4e and the surface in contact with the outer peripheral side surface of the secondary battery 1. The individually bound battery 5e has two planar fixed adhesive surfaces 6f on the outer peripheral extension portion by the spacer 4e and the heat shrinkable tube 9.

  In the bound battery 5e, the positive and negative terminals of the adjacent secondary battery 1 are alternately adjacent in the vertical direction, and the fixed adhesive surface 6f is opposed to the fixed adhesive surface 6f of the secondary battery forming one row. Be placed. At the time of this arrangement work, an adhesive is applied to the opposing fixed adhesive surface 6f, and is held in an array on a fixing jig and fixed and bonded. After fixing and bonding, the positive and negative terminals of the secondary battery 1 are assembled by connecting the different polarities of adjacent secondary batteries with a plate-shaped metal connection bus bar 7 and connecting them in series as the entire secondary battery 1. The battery 38 is completed.

  As shown in FIG. 10, in the present embodiment, the fixed adhesive surface 6 f is located in the internal space of the assembled battery 38, so that the assembled battery 38 can be further downsized.

(Fifth embodiment)
In the present embodiment, a spacer 14e having the same shape as the spacer 4e shown in the fourth embodiment is further arranged on the opposite side of the outer peripheral side surface of the secondary battery 1 and covered with an insulating tube 9 to form an individual battery. is there. In the present embodiment, the same members as those in the fourth embodiment described above are denoted by the same reference numerals, the description thereof is omitted, and only different portions will be described.

  As shown in FIG. 12, the assembled battery 48 of the present embodiment includes six individually bound batteries 5e and three individually bound batteries 5f arranged in 3 rows and 3 columns.

  As shown in FIG. 13, in the individual battery 5f, a spacer 14e is further disposed on the side surface opposite to the outer peripheral side surface of the secondary battery 1, and the secondary battery 1 is connected to the heat-shrinkable tube 9 via the spacers 4e and 14e. After covering with, heat shrink to tie up and stick. For this reason, the individually bound battery 5f has four flat fixed adhesive surfaces 6g on the outer peripheral extension of the secondary battery 1 by the spacers 4e and 14e and the heat shrinkable tube 9.

  In the individually bound battery 5e, the positive and negative terminals of the adjacent secondary battery 1 are alternately arranged in the vertical direction, and in the same way, the individual battery 5f in the adjacent row is also alternately positive and negative in the vertical direction. Then, again, the individually bound batteries 5e are arranged in the same manner as described above to constitute the third row. At this time, an adhesive is applied in advance to the surface where the fixed adhesive surface 6g and the fixed adhesive surface 6f face each other, and the fixed adhesive is completed by being held in an array on a fixing jig. Note that this arrangement order is merely an example, and it goes without saying that the assembled battery 48 can be manufactured without following this arrangement order.

  The opposite surfaces of the fixed adhesive surface 6g and the fixed adhesive surface 6f are fixed surfaces of the individually bound battery 5e and the individually bound battery 5f. After fixing and bonding, the positive and negative terminals of the secondary battery 1 are assembled by connecting the different polarities of adjacent secondary batteries with a plate-shaped metal connection bus bar 7 and connecting them in series as the entire secondary battery 1. The battery 48 is completed.

  In the assembled battery 48 of this embodiment, the spacer 14e is arrange | positioned on the opposite side surface of the outer peripheral side surface where the spacer 4e of the secondary battery 1 is arrange | positioned. For this reason, the individually bound battery 5e and the individually bound battery 5f can be bonded and combined with the fixed adhesive surfaces 6f and 6g of the spacers 4e and 14e, respectively. Further, since the fixed adhesive surfaces 6f and 6g are located in the internal space of the assembled battery 48, it is possible to reduce the size of the assembled battery of three or more rows.

  The present invention provides an assembled battery excellent in assemblability, volume and weight efficiency and contributes to the manufacture and sale of the assembled battery, and thus has industrial applicability.

The assembled battery of 1st Embodiment which can apply this invention is shown, (A) is a top view, (B) is a front view. 1 shows an individual-bound battery according to a first embodiment, where (A) is a plan view and (B) is a front view. The spacer of the bound battery of 1st Embodiment is shown, (A) is a top view, (B) is a front view. The assembled battery of 2nd Embodiment is shown, (A) is a top view, (B) is a front view. FIG. 2 shows an individually bound battery according to a second embodiment, wherein (A) is a plan view and (B) is a front view. The assembled battery of other embodiment which can apply this invention is shown, (A) is a top view, (B) is a front view. The bound battery of other embodiment is shown, (A) is a top view, (B) is a front view. The spacer of the bound battery of other embodiment is shown, (A) is a top view, (B) is a front view. The individually bound battery of 3rd Embodiment is shown, (A) is a top view, (B) is a front view. It is a top view of the assembled battery of 4th Embodiment. It is a disassembled perspective view of the individually bound battery of 4th Embodiment. It is a top view of the assembled battery of 5th Embodiment. It is a disassembled perspective view of the individually tied battery of 5th Embodiment.

Explanation of symbols

1 Cylindrical lithium ion secondary battery (cylindrical battery)
4a, 4b, 4e Spacer (first spacer)
14a, 14b, 14e Spacer (second spacer)
4d Frame 5a, 5b, 5c, 5d, 5e, 5f Individual battery 6a, 6b, 6c, 6d, 6e, 6f, 6g Fixed adhesive surface (fixed surface)
9 Heat shrinkable tube (cylindrical body)
24a, 24b, 24c, 24d Indentation 8, 18, 28, 38, 48 Collective battery

Claims (7)

In an assembled battery in which a plurality of columnar batteries each containing an electrode group in which a separator is interposed between a positive electrode and a negative electrode and an electrolytic solution are contained in a cylindrical container, the outer peripheral side surface of the cylindrical battery is in contact with the outer peripheral side surface. A first spacer having a planar shape other than the first spacer is disposed, and the first spacer is composed of only one surface in contact with the outer peripheral side surface, and all the facing surfaces are planar. An assembled battery, wherein the battery is combined with at least the facing surface as a fixed surface.   A second spacer having the same shape as the first spacer having a plurality of opposite surfaces other than one surface in contact with the opposite side surface is further provided on the opposite side surface of the outer peripheral side surface where the first spacer of the cylindrical battery is disposed. The assembled battery according to claim 1, wherein the cylindrical battery is combined with the second spacer facing the fixed surface as a fixed surface.   2. The first spacer according to claim 1, wherein the first spacer has a substantially U-shaped cross section, and a recess for positioning the cylindrical battery is formed on a surface in contact with an outer peripheral side surface of the cylindrical battery. Item 3. The assembled battery according to Item 2.   The assembled battery according to claim 1, wherein the first spacer is a frame having a substantially rectangular cross section, and the cylindrical battery is inserted into the frame.   The first spacer has a substantially triangular cross section, and a recess is formed on a surface in contact with an outer peripheral side surface of the cylindrical battery, and the cylindrical battery has at least one of two surfaces other than the surface where the recess is formed. The assembled battery according to claim 1, wherein the fixed surfaces are combined.   A second spacer having the same shape as the first spacer is further disposed on the side surface of the cylindrical battery opposite to the outer peripheral side surface on which the first spacer is disposed, and the cylindrical battery further includes the second spacer. The assembled battery according to claim 5, wherein at least one of the two surfaces other than the surface on which the spacer recess is formed is combined as a fixed surface.   The assembled battery according to claim 1, wherein the spacer and the cylindrical battery are covered with a heat-shrinkable cylindrical body.