Battery pack, disassembly method of the same, and degradation diagnostic method of the same

Abstract

To inhibit scattering of components of a laminated body when a battery pack is disassembled.SOLUTION: A battery pack 1 includes: a battery stack 5 in which battery cells 10 are laminated; a case 40 which is open on an upper surface and in which the battery stack 5 is compressed in a lamination direction D and housed; and through holes 42A which are formed at a bottom plate 42 of the case 40 which correspond to both ends in the lamination direction D of the battery stack 5 and penetrate through the bottom plate 42 in a direction in which the battery stack 5 is removed from the case 40.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a battery pack, a method for dismantling a battery pack, and a method for diagnosing deterioration of a battery pack. [Background technology]

[0002] It is known that in a battery pack, a laminate is housed in a case in a state where the laminate is compressed in the stacking direction (see, for example, Patent Document 1).

[0003] Patent Document 1 discloses a configuration in which multiple battery cells are stacked in the thickness direction to form a battery stack, and then the battery stack is housed in a module case while a compressive force in the stacking direction is applied to the battery stack. This applies pressure to the surfaces of the battery cells and restricts and restrains the relative movement of the multiple battery cells and separators that make up the battery stack.

[0004] Incidentally, it is preferable to prevent components of the laminate from scattering when the battery pack is disassembled. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2018-032519 A Summary of the Invention [Problem to be solved by the invention]

[0006] In consideration of the above, an object of the present invention is to prevent components of a laminate from scattering when a battery pack is disassembled. [Means for solving the problem]

[0007] A battery pack of a first aspect of the present invention comprises a stack of battery cells, a case with an open top in which the stack is housed while being compressed in the stacking direction, and through holes formed in a bottom plate of the case corresponding to both ends of the stack in the stacking direction and extending in the direction in which the stack is removed from the case.

[0008] In a battery pack of a second aspect of the present invention, in the battery pack of the first aspect of the present invention, the contact surfaces of the case with which the laminate at both ends in the stacking direction contact are inclined surfaces that face diagonally upward.

[0009] A battery pack dismantling method of a third aspect of the present invention is a battery pack dismantling method of the first or second aspect of the present invention, which includes a first extrusion step of inserting a pressurizer that presses the laminate into the through hole to extrude the laminate from the case.

[0010] A fourth aspect of the battery pack dismantling method of the present invention is the same as the third aspect of the battery pack dismantling method of the present invention, and includes a clamping step of clamping an upper end of the stack pushed out from the case in the stacking direction by a clamping portion whose clamping width is adjustable in the stacking direction, and a second extrusion step of pushing the stack out of the case by the pressurizer while the stack is clamped in the stacking direction by the clamping portion.

[0011] In a battery pack deterioration diagnosis method utilizing the battery pack disassembly method of the third aspect of the present invention, a pressure load applied by the pressurizer to the stack is measured, and a restraining force of the stack is calculated based on the pressure load. Effect of the Invention

[0012] In the battery pack of the first aspect of the present invention, through holes are formed in the bottom plate of the case corresponding to both ends of the stacking direction of the stack, and penetrate in the direction in which the stack is removed from the case, so that a presser that presses the stack can be inserted into the through hole to press the stack. Therefore, the stack is pressed in the vicinity of the stack where frictional force occurs between the stack and the case. As a result, the center of the stack in the stacking direction is prevented from bending in a convex shape upward. This makes it possible to prevent components of the stack from scattering when the battery pack is disassembled.

[0013] In the battery pack according to the second aspect of the present invention, the contact surfaces of the case that contact the stack at both ends in the stacking direction are inclined so as to face diagonally upward, so that when the stack is pushed out of the case by the presser, the compressive force of the case is gradually released, thereby making it possible to prevent components of the stack from scattering when the battery pack is disassembled.

[0014] In the battery pack disassembly method according to the third aspect of the present invention, a first extrusion step is included in which a presser for pressing the laminate is inserted into the through hole to extrude the laminate from the case, so that the laminate is gradually extruded from the case by the presser in the vicinity of the laminate where frictional force occurs between the laminate and the case. Therefore, scattering of components of the laminate during disassembly of the battery pack can be suppressed.

[0015] In a battery pack disassembly method according to a fourth aspect of the present invention, the stack is clamped in the stacking direction by the clamping section, and the stack is pushed out of the case by the presser, so that the stack is pushed out of the case without the compression force in the stacking direction being released. Then, the clamping force of the pushed-out stack is gradually released by the clamping section, which has an adjustable clamping width. This makes it possible to prevent components of the stack from scattering when the battery pack is disassembled.

[0016] In the battery pack deterioration diagnosis method of the fifth aspect of the present invention, the pressure load applied by a pressurizer to the stack is measured, and the restraining force of the stack is calculated based on the pressure load, thereby making it possible to diagnose the deterioration of the battery pack in a simple manner when disassembling the battery pack. [Brief description of the drawings]

[0017] [Figure 1] FIG. 2 is an exploded perspective view showing the battery pack according to the embodiment. [Diagram 2] FIG. 2 is an exploded perspective view showing a laminate according to an embodiment. [Diagram 3] FIG. 2 is an exploded cross-sectional view showing a schematic view of the battery pack according to the embodiment, showing the cross-section AA in FIG. [Figure 4] 2 is a cross-sectional view showing a battery pack and a disassembly jig according to an embodiment of the present invention. FIG. [Diagram 5] 1A to 1C are cross-sectional views showing a procedure for dismantling a battery pack according to an embodiment, illustrating a first pushing-out step. [Figure 6] 10A to 10C are cross-sectional views showing a procedure for dismantling the battery pack according to the embodiment, illustrating a clamping step. [Figure 7] 10A to 10C are cross-sectional views showing a procedure for dismantling the battery pack according to the embodiment, illustrating a second pushing-out step. [Figure 8] 10A to 10C are cross-sectional views showing a procedure for dismantling the battery pack according to the embodiment, illustrating a clamping release step. [Figure 9] FIG. 11 is a cross-sectional view illustrating a battery pack and a disassembly jig according to another embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, a battery pack according to an embodiment will be described with reference to the drawings. The battery pack according to the embodiment will be described as an example of a lithium ion secondary battery used as an in-vehicle power source for an electric vehicle, a hybrid vehicle, or the like. In each drawing, the arrow UP indicates the upper side in the vertical direction of the battery pack, the arrow FR indicates the front side in the front-rear direction of the battery pack, the arrow LH indicates the left side in the left-right direction of the battery pack, and the arrow D indicates the stacking direction D of the battery stack. In the embodiment, the stacking direction D of the battery stack coincides with the front-rear direction of the battery pack.

[0019] [Battery pack 1 configuration] As shown in FIG. 1, the battery pack 1 includes a battery stack 5 as a laminate, and a case 40 that houses the battery stack 5.

[0020] (Battery stack 5) 1 and 2, the battery stack 5 includes a plurality of battery cells (single cells) 10, a plurality of intermediate members 20 arranged in contact with wide surfaces of the battery cells 10, and bus bars 15 that electrically connect the battery cells 10. In the battery stack 5, the battery cells 10 and the intermediate members 20 are arranged alternately in the stacking direction D.

[0021] <Battery cell 10> 2, the battery cell 10 is formed in a rectangular plate shape with the plate thickness direction being the front-rear direction. The battery cell 10 has two electrode terminals 12 on its top surface 10A. One of the two electrode terminals 12 is a positive terminal and the other is a negative terminal. Note that the battery cell 10 may have multiple pole groups arranged therein.

[0022] <Intermediate member 20> The intermediate member 20 is made of, for example, insulating resin. This suppresses heat transfer between the battery cells 10 and insulates the battery cells 10 from each other. The intermediate member 20 may be formed with comb-like ribs for forming air passages between the intermediate member 20 and the battery cells 10.

[0023] The intermediate member 20 includes a spacer 24 and an end plate 22 .

[0024] The spacers 24 are disposed between the battery cells 10 in the stacking direction D. The spacers 24 are formed in the shape of a rectangular plate with the plate thickness direction extending in the front-rear direction.

[0025] As shown in FIG. 3, the end plates 22 are disposed at both ends of the battery stack 5 in the stacking direction D. The end plates 22 are formed in a rectangular plate shape with the plate thickness direction being the front-rear direction. The stacking direction outer surface 22A of the end plates 22 is inclined so that the thickness decreases toward the bottom. In other words, the stacking direction outer surface 22A of the end plates 22 is inclined so as to face diagonally downward. The stacking direction outer surface 22A of the end plates 22 has approximately the same inclination angle as a contact surface 45A of a first side wall 45 described later.

[0026] <Busbar 15> 2, the busbar 15 is formed, for example, in a rectangular plate shape with its plate thickness direction extending in the up-down direction in the stacking direction D. The busbar 15 electrically connects the electrode terminals 12 of adjacent battery cells 10.

[0027] (Case 40) 1, the case 40 is formed by, for example, aluminum die casting. The case 40 is formed into a rectangular box shape with an open top, by a bottom plate 42 and side walls 44. The case 40 is accommodated in the battery stack 5 so as to cover the side and bottom surfaces thereof.

[0028] 3, the bottom plate 42 is formed in a rectangular plate shape with the plate thickness direction being the up-down direction. The bottom plate 42 is formed with a through hole 42A that penetrates in the direction in which the battery stack 5 is removed from the case 40. In other words, the through hole 42A is formed to penetrate the bottom plate 42 in the up-down direction.

[0029] The through holes 42A are formed in the bottom plate 42 of the case 40 corresponding to both ends of the battery stack 5 in the stacking direction D. The through holes 42A are provided, for example, at positions where the end plates 22 are disposed. The through holes 42A are formed in the bottom plate 42 of the case 40 corresponding to the center of the battery stack 5 in the left-right direction.

[0030] The inner diameter of through hole 42A is smaller than the length of the lower surface of end plate 22 in stacking direction D. Note that the inner diameter of through hole 42A may be larger than the length of the lower surface of end plate 22 in stacking direction D.

[0031] 1, the side wall 44 is formed to rise upward from the periphery of the bottom plate 42. The side wall 44 includes a first side wall 45 formed in a rectangular plate shape with the plate thickness direction extending in the front-rear direction, and a second side wall 46 formed in a rectangular plate shape with the plate thickness direction extending in the left-right direction.

[0032] 3, the contact surfaces 45A of the first side walls 45 that come into contact with the battery stacks 5 at both ends in the stacking direction D are inclined surfaces that are inclined so as to face obliquely upward. The inclination of the contact surfaces 45A may be a draft angle used when releasing the case 40 as a molded product.

[0033] 3, the battery stack 5 is housed in the case 40 in a state compressed in the stacking direction D. The battery stack 5 is housed in the case 40 in a state compressed in the stacking direction D by the first side wall 45.

[0034] At this time, since the outer surface 22A in the stacking direction of the end plate 22 has substantially the same inclination angle as the contact surface 45A of the first side wall 45, a uniform compressive force is applied to the battery stack 5.

[0035] [Configuration of Dismantling Jig 50] Disassembly jig 50 is used when disassembling battery pack 1 configured as described above. As shown in Fig. 4, disassembly jig 50 includes first bracket 51, second bracket 52, a pair of bolts 65 as clamping parts, and a pressing member 70.

[0036] (First bracket 51) The first bracket 51 is formed with a first portion 51A and a second portion 51B so as to have a generally U-shaped cross section.

[0037] The first portion 51A is formed in a rectangular plate shape with the plate thickness direction being the up-down direction. The length of the first portion 51A in the front-rear direction is longer than the length of the battery pack 1 in the stacking direction D.

[0038] The second portion 51B is formed so as to extend in the up-down direction from both ends in the front-rear direction of the first portion 51A. The second portion 51B is formed in a rectangular plate shape with its plate thickness direction being in the front-rear direction.

[0039] (Second bracket 52) The second bracket 52 is formed in a rectangular plate shape with its thickness direction in the front-rear direction. The second bracket 52 is attached to the second portion 51B of the first bracket 51 by a bolt 61 and a nut 62.

[0040] The second portion 51B of the first bracket 51 and the second bracket 52 configure an arm portion 60. The distance between the left and right arm portions 60 is formed to be greater than the length of the battery pack 1 in the stacking direction D. The mounting position of the second bracket 52 in the up-down direction relative to the second portion 51B of the first bracket 51 may be configured to be changeable. This allows the arm portion 60 to be configured to be extendable and contractible.

[0041] (Bolt 65) The bolts 65 are attached to the upper end of each second bracket 52 in a parallel orientation in the front-rear direction. The pair of bolts 65 have their tips disposed opposite each other.

[0042] The position of the tip of the bolt 65 in the front-rear direction can be adjusted by rotating the bolt 65 around the axis. In other words, the distance between the tips of the pair of bolts 65 can be adjusted.

[0043] (Pressing element 70) Two pressing pieces 70 are attached to the upper surface of the first portion 51A of the first bracket 51. The pressing pieces 70 are provided at positions corresponding to the through-holes 42A when the battery pack 1 is set in the disassembly jig 50.

[0044] The presser 70 is formed in a round bar shape extending in the up-down direction. The diameter of the presser 70 is smaller than the inner diameter of the through hole 42A. The diameter of the presser 70 is smaller than the length of the lower surface of the end plate 22 in the stacking direction D. The diameter of the presser 70 may be larger than the length of the lower surface of the end plate 22 in the stacking direction D. The presser 70 may also be formed in a square bar shape.

[0045] A sensor 72 is attached to the pressing member 70 to measure the pressing load applied by the pressing member 70. The sensor 72 may be, for example, a load cell.

[0046] [How to disassemble battery pack 1] The battery pack 1 is disassembled through a setting step, a first pushing step, a clamping step, a second pushing step, a clamping release step, and a removal step.

[0047] (Setting process) 4, in the setting step, the disassembly jig 50 is set on the battery pack 1. Specifically, the disassembly jig 50 is set on the battery pack 1 fixed to a jig (not shown) so that the battery pack 1 is inserted between the arms 60. At this time, the pressing member 70 and the through-hole 42A are set at the same position in the front-rear and left-right directions. In other words, the disassembly jig 50 is set on the battery pack 1 so that the pressing member 70 is inserted into the through-hole 42A when the disassembly jig 50 moves upward.

[0048] (First extrusion process) 5, in the first pushing-out step, the disassembly jig 50 is moved upward and the presser 70 is inserted into the through-hole 42A. Then, the lower surface of the end plate 22 is pressed by the presser 70, thereby pushing the battery stack 5 out of the case 40.

[0049] (Pinching process) 6, in the clamping step, the upper end of the battery stack 5 extruded in the first extrusion step is clamped in the stacking direction D by a pair of bolts 65. Specifically, the pair of opposing bolts 65 are rotated around an axis to adjust the front-rear position of the tip of the bolts 65, thereby clamping the upper end of the battery stack 5 in the stacking direction D. In other words, the clamping width of the pair of bolts 65 that clamps the battery stack 5 in the stacking direction D is adjustable.

[0050] (Second extrusion process) 7, in the second pushing-out step, with the battery stack 5 being clamped in the stacking direction D by the pair of bolts 65, the lower surface of the end plate 22 is pressed by a presser 70 to further push the battery stack 5 out of the case 40. In the second pushing-out step, the battery stack 5 is pushed out of the case 40 until the compressive force of the first side wall 45 on the battery stack 5 in the stacking direction D is released.

[0051] (Pinch release process) As shown in FIG. 8, in the clamping release process, the pair of bolts 65 that have been clamping the upper end of the battery stack 5 are rotated around an axis to move the pair of bolts 65 away from each other, thereby gradually releasing the clamping of the battery stack 5.

[0052] (Removal process) In the removal step, with the clamping by the pair of bolts 65 released, the battery stack 5 is removed from the case 40 using a jig or the like.

[0053] [Battery pack 1 deterioration diagnosis method] However, if a compressive force continues to act on the battery stack 5 in the stacking direction D, creep deformation occurs in the battery stack 5. It can be estimated that the greater the creep deformation of the battery stack 5, the more advanced the deterioration of the battery pack 1 is. Also, the greater the creep deformation, the lower the compressive force on the battery stack 5 in the stacking direction D.

[0054] When the compressive force on the battery stack 5 in the stacking direction D decreases, the pressing force with which the presser 70 presses the battery stack 5 decreases when the battery pack is disassembled.

[0055] Therefore, when dismantling the battery pack 1, the pressure load applied by the presser 70 to the battery stack 5 is measured by the sensor 72. The compressive force of the battery stack 5 is calculated based on the measurement value by the sensor 72. Then, the deterioration of the battery pack 1 is diagnosed based on the compressive force of the battery stack 5.

[0056] [Effect] Incidentally, the frictional force between the battery stack 5 and the case 40 is greater than the frictional force between the components that constitute the battery stack 5. Therefore, when attempting to remove the battery stack 5 from the case 40 by pressing the center of the battery stack 5 in the stacking direction during disassembly of the battery pack 1, the frictional force between the battery stack 5 and the case 40 may cause the center of the battery stack 5 in the stacking direction to bend upward in a convex shape, and ultimately, the components of the battery stack 5 may fly off.

[0057] The battery pack 1 of the embodiment comprises a battery stack 5 in which battery cells 10 are stacked, a case 40 which is open at the top and which houses the battery stack 5 compressed in the stacking direction D so as to cover the side and bottom surfaces of the battery stack 5, and through holes 42A formed in a bottom plate 42 of the case 40 corresponding to both ends of the battery stack 5 in the stacking direction D and which extend in a direction such that the battery stack 5 can be removed from the case 40 (see FIG. 3).

[0058] The through-holes 42A are formed in the bottom plate 42 of the case 40 corresponding to both ends of the battery stack 5 in the stacking direction D, and penetrate in the direction in which the battery stack 5 is removed from the case 40, so that a presser 70 for pressing the battery stack 5 can be inserted into the through-hole 42A to press the battery stack 5. Therefore, the battery stack 5 is pressed in the vicinity of the battery stack 5 where frictional force is generated between the battery stack 5 and the case 40. As a result, the central part of the battery stack 5 in the stacking direction is prevented from bending upward in a convex shape. This makes it possible to prevent components of the battery stack 5 from scattering when the battery pack 1 is disassembled.

[0059] In the battery pack 1 of the embodiment, the contact surfaces 45A of the case 40 with which the battery stacks 5 at both ends in the stacking direction D come into contact are inclined surfaces that are inclined so as to face obliquely upward (see FIG. 3).

[0060] The contact surfaces 45A of the case 40 with which the battery stack 5 contacts at both ends in the stacking direction D are inclined so as to face obliquely upward, so that when the battery stack 5 is pushed out of the case 40 by the presser 70, the compressive force of the case 40 is gradually released. Therefore, it is possible to prevent components of the battery stack 5 from scattering when the battery pack 1 is disassembled.

[0061] The method for disassembling the battery pack 1 of the embodiment includes a first pushing step of inserting a pusher 70 for pressing the battery stack 5 into the through-hole 42A to push the battery stack 5 out of the case 40 (see FIG. 5).

[0062] By including the first pushing-out step of inserting the presser 70 for pressing the battery stack 5 into the through-hole 42A to push the battery stack 5 out of the case 40, the battery stack 5 is gradually pushed out of the case 40 in the vicinity of the battery stack 5 where frictional force is generated between the battery stack 5 and the case 40. Therefore, it is possible to prevent components of the battery stack 5 from scattering when the battery pack 1 is disassembled.

[0063] The method for dismantling the battery pack 1 of the embodiment includes a clamping step in which the upper end of the battery stack 5 pushed out from the case 40 is clamped in the stacking direction D by a pair of bolts 65 whose clamping width in the stacking direction D is adjustable, and a second extrusion step in which, with the battery stack 5 clamped in the stacking direction D by the pair of bolts 65, the battery stack 5 is pushed out of the case 40 by a presser 70 (FIGS. 6 and 7).

[0064] With the battery stack 5 sandwiched in the stacking direction D by the pair of bolts 65, the presser 70 pushes the battery stack 5 out of the case 40, so that the battery stack 5 is pushed out of the case 40 without the compression force in the stacking direction D being released. The sandwiching force of the pushed-out battery stack 5 is gradually released by the pair of bolts 65, the sandwiching width of which can be adjusted. This makes it possible to prevent components of the battery stack 5 from scattering when the battery pack is disassembled.

[0065] In the deterioration diagnosis method for the battery pack 1 according to the embodiment, the pressure load applied by the pressing element 70 to the battery stack 5 is measured, and the restraining force of the battery stack 5 is calculated based on the pressure load.

[0066] By measuring the pressure load applied by the pressing element 70 to the battery stack 5 and calculating the restraining force of the battery stack 5 based on the pressure load, it is possible to diagnose the deterioration of the battery pack in a simple manner when disassembling the battery pack.

[0067] The battery pack, the method for disassembling the battery pack, and the method for diagnosing deterioration of the battery pack according to the present invention have been described above based on the embodiments. However, the specific configurations are not limited to these embodiments, and changes in design are permitted as long as they do not deviate from the gist of the invention according to each claim in the scope of the claims.

[0068] In the embodiment, an example has been shown in which two pressing elements 70 are attached to the upper surface of the first portion 51A of the first bracket 51. However, four or more pressing elements may be attached to the upper surface of the first portion 51A of the first bracket 51. In addition, the pressing elements may be provided so as to straddle components of the battery stack 5.

[0069] 9, a second pressing element 170 may be provided to press the battery cells 10 provided on the outer side of the stacking direction D of the battery stack 5. In this case, by making the length of the second pressing element 170 shorter than the length of the pressing element 70, a pressing force is applied to the outer side of the stacking direction D of the battery stack 5. This makes it possible to prevent components of the battery stack 5 from scattering when the battery pack 1 is disassembled.

[0070] In the example shown, one pressing member 70 is formed at a position corresponding to the center in the left-right direction of the battery stack 5. However, a plurality of pressing members 70 may be formed in the left-right direction of the battery stack 5. In this case, it is possible to suppress rotation of the battery stack 5 when the battery stack 5 is pushed out of the case 40.

[0071] In the embodiment, an example has been described in which the clamping portion is a pair of bolts 65. However, the clamping portion is not limited to this embodiment, and may be any portion that is capable of adjusting the clamping width for clamping the battery stack 5 in the stacking direction D.

[0072] In the embodiment, an example has been shown in which the dismantling jig 50 includes the arm portion 60. However, the dismantling jig does not necessarily have to include the arm portion 60.

[0073] In the embodiment, an example has been shown in which the battery stack 5 includes the intermediate member 20. However, the battery stack does not necessarily need to include the intermediate member 20.

[0074] In the embodiment, an example has been shown in which one battery stack 5 is housed in the case 40. However, the case may house two battery stacks 5 with an intermediate plate therebetween.

[0075] In the embodiment, the contact surface 45A of the first side wall 45 of the case 40 is an inclined surface that is inclined so as to face obliquely upward. However, the contact surface 45A of the first side wall 45 of the case 40 may be a vertical surface with no inclination. In this case, the stacking direction outer surface 22A of the end plate 22 may be a vertical surface with no inclination.

[0076] In the above embodiment and the embodiment, the battery pack is a lithium ion secondary battery pack. However, the battery pack may be a battery pack in which the battery stack is compressed in the stacking direction D and accommodated in a case. [Explanation of symbols]

[0077] 1 Battery pack 5 Battery stack (laminate) 10 Battery Cells 40 cases 42 Bottom plate 45A contact surface 65 A pair of bolts (an example of a clamping part) 70 Presser D Stacking direction

Claims

1. a stack of battery cells; and a case having an open top and in which the stack is accommodated while being compressed in the stacking direction; through holes formed in a bottom plate of the case corresponding to both ends of the stack in a stacking direction, the through holes penetrating in a direction in which the stack is removed from the case; A battery pack comprising:

2. The contact surfaces of the case with which the stacked bodies come into contact at both ends in the stacking direction are inclined surfaces that are inclined so as to face obliquely upward. The battery pack according to claim 1 .

3. A method for dismantling a battery pack according to claim 1 or 2, comprising the steps of: a first extrusion step of inserting a pressing member for pressing the laminate into the through hole to extrude the laminate from the case. How to disassemble the battery pack.

4. a clamping step of clamping an upper end of the stack pushed out from the case in the stacking direction by a clamping unit whose clamping width is adjustable in the stacking direction; and a second extrusion step of extruding the laminate from the case by the presser while the laminate is sandwiched by the sandwiching section in the stacking direction. The method for dismantling a battery pack according to claim 3.

5. A battery pack deterioration diagnosis method utilizing the battery pack disassembly method according to claim 3, comprising: A pressure load applied by the presser to the laminate is measured, and a restraining force of the laminate is calculated based on the pressure load. A method for diagnosing deterioration of a battery pack.

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