JP6580391B2 - Fuel cell stack and method of assembling the same - Google Patents

Description

  The present invention relates to a fuel cell stack in which power generation cells are stacked and end plates are disposed at both ends in the stacking direction, and an assembling method thereof.

  For example, a solid polymer fuel cell employs a solid polymer electrolyte membrane made of a polymer ion exchange membrane. A fuel cell has an electrolyte membrane / electrode structure (MEA) in which an anode electrode and a cathode electrode each having an electrode catalyst (electrode catalyst layer) and porous carbon (gas diffusion layer) are disposed on both sides of a solid polymer electrolyte membrane. ). The electrolyte membrane / electrode structure constitutes a power generation cell (fuel cell) by being sandwiched between separators (bipolar plates). A predetermined number of power generation cells are stacked to form a fuel cell stack, and the fuel cell stack is used as an in-vehicle fuel cell stack mounted on, for example, a fuel cell electric vehicle.

  In an in-vehicle fuel cell stack, a considerable number of power generation cells are stacked. For this reason, it is necessary to accurately position each power generation cell and to secure a desired sealing property. For this reason, in the assembly operation of the fuel cell stack, usually, knock pins (positioning pins) are integrally inserted into positioning holes formed in the respective power generation cells to position the power generation cells. In this state, a pair of end plates are fastened and fixed with a tie rod or the like while applying a compressive force to the stack of power generation cells in the stacking direction.

  For example, in the fuel cell assembling method disclosed in Patent Document 1, a knock pin is inserted into the positioning hole of the power generation cell, and the power generation cell is stacked along the knock pin. Next, by applying a load in the stacking direction to the stacked power generation cells and compressing the power generation cells, the extension portions of the knock pins project from the end portions in the stacking direction of the power generation cells. Removed from the main unit.

  Therefore, by removing the extension portion of the positioning rod, the end portion of the positioning rod protruding from the fuel cell stack after the compression process can be easily removed, and a fuel cell stack without unnecessary protrusion of the positioning rod can be easily formed. It can be done.

JP 2013-196849 A

  By the way, in said patent document 1, while extending | stacking a power generation cell, while the extension part is attached to the knock pin main-body part, the said extension part is removed from the said knock-pin main-body part at the time of compression of the said power generation cell. For this reason, it is necessary to attach and detach the extension portion, and workability may be reduced.

  The present invention solves this type of problem, and an object of the present invention is to provide a fuel cell stack capable of assembling the fuel cell stack easily and quickly and a method for assembling the fuel cell stack.

  The present invention relates to a fuel cell stack having a power generation cell that generates power by an electrochemical reaction between a fuel gas and an oxidant gas, wherein the plurality of power generation cells are stacked and end plates are disposed at both ends in the stacking direction. Is.

The fuel cell stack includes positioning pins that are inserted into the positioning holes provided in the plurality of power generation cells and are held between the end plates. The positioning pin is disposed in the hollow pin main body portion, in the pin main body portion, the expansion / contraction portion inserted so as to be able to advance and retreat in the pin axial direction of the pin main body portion, and in the pin main body portion, An elastic body that advances and retreats in the axial direction, a guide portion that guides the expansion and contraction portion with respect to the pin main body portion, and the expansion and contraction portion is held at the advanced tip position, and the expansion and contraction portion is detached from the pin main body portion And a locking portion for preventing the above.

  Furthermore, the present invention has a power generation cell that generates power by an electrochemical reaction between a fuel gas and an oxidant gas, and a plurality of the power generation cells are stacked and end plates are disposed at both ends in the stacking direction. It relates to a stack assembly method.

  In the assembling method, one end plate holds one end of a positioning pin that can be expanded and contracted in the pin axis direction. A plurality of power generation cells are positioned and stacked by inserting positioning pins into positioning holes provided in the power generation cells. Next, a compression force is applied in the stacking direction to the plurality of power generation cells stacked between the other end plate and the one end plate, and the positioning pins are reduced in the pin axis direction. In this state, the space between one end plate and the other end plate is fastened and fixed.

  Furthermore, the positioning pin preferably has a hollow pin main body portion and an expansion / contraction portion that is inserted into the pin main body portion and can be moved forward and backward in the pin axial direction with respect to the pin main body portion by the elastic force of the elastic body. . When the power generation cells are stacked, the expansion / contraction portion protrudes outward from the pin main body portion. On the other hand, when the power generation cell is compressed, the expansion / contraction portion is preferably housed inside the pin main body portion.

  According to the present invention, the plurality of power generation cells are positioned by integrally inserting the positioning pins into the positioning holes of the respective power generation cells. Then, when a compressive force is applied to the plurality of power generation cells in the stacking direction and the plurality of power generation cells are compressed, the positioning pins can be reduced in the pin axis direction. For this reason, it is not necessary to divide the positioning pins, and the fuel cell stack can be assembled easily and quickly.

1 is a schematic perspective view of a fuel cell stack according to an embodiment of the present invention. It is a partial cross section side view of the said fuel cell stack. It is a principal part disassembled perspective explanatory drawing of the fuel cell which comprises the said fuel cell stack. FIG. 4 is a perspective explanatory view of a knock pin constituting the fuel cell stack. It is a cross-sectional side view of the knock pin. It is principal part explanatory drawing of the assembly apparatus applied to the assembly method of the fuel cell stack which concerns on embodiment of this invention. It is explanatory drawing of the state by which the end plate was arrange | positioned at the said assembly apparatus. It is explanatory drawing of the state by which the power generation cell was laminated | stacked on the said assembly apparatus. It is explanatory drawing of the state which gave the clamping load to the said power generation cell with the said assembly apparatus. It is explanatory drawing at the time of the said knock pin being pressed and reduced. It is explanatory drawing of the state by which the said knock pin was pressed to the reduction end.

  As shown in FIG. 1, in the fuel cell stack 10 according to the embodiment of the present invention, a plurality of power generation cells 12 are stacked in the horizontal direction (arrow A direction) in a standing posture. The fuel cell stack 10 constitutes, for example, an in-vehicle fuel cell stack and is mounted on a fuel cell vehicle (fuel cell electric vehicle) (not shown).

  As shown in FIGS. 1 and 2, a terminal plate 14 a, an insulator 16 a, and an end plate 18 a are sequentially arranged at one end in the stacking direction of the power generation cells 12 toward the outside. At the other end of the power generation cell 12 in the stacking direction, a terminal plate 14b, an insulator 16b, and an end plate 18b are sequentially disposed outward. Terminal portions 20a and 20b extending outward in the stacking direction are provided at substantially the center of the terminal plates 14a and 14b. The terminal portions 20a and 20b project outside the end plates 18a and 18b. The terminal plates 14a and 14b are accommodated in recesses 16ad and 16bd formed on the plate surfaces of the insulators 16a and 16b.

  The end plates 18a and 18b have a horizontally long (or vertically long) rectangular shape, and a connecting bar 22 is disposed between the sides. Each connection bar 22 is fixed at both ends to the inner surfaces of the end plates 18a, 18b via bolts 24, and applies a tightening load in the stacking direction (arrow A direction) to the plurality of stacked power generation cells 12. Note that the fuel cell stack 10 may include a housing having end plates 18a and 18b as end plates, and a plurality of power generation cells 12 may be accommodated in the housing.

  As shown in FIGS. 2 and 3, the power generation cell 12 includes an electrolyte membrane / electrode structure 26, and an anode separator 28 and a cathode separator 30 that sandwich the electrolyte membrane / electrode structure 26. The anode separator 28, the electrolyte membrane / electrode structure 26, and the cathode separator 30 are stacked in the horizontal direction.

  The anode separator 28 and the cathode separator 30 are made of, for example, a steel plate, a stainless steel plate, an aluminum plate, a plated steel plate, or a metal plate obtained by performing a surface treatment for corrosion prevention on the metal surface. The anode separator 28 and the cathode separator 30 have a rectangular planar shape and are formed into a concavo-convex shape. For example, a carbon separator may be used as the anode separator 28 and the cathode separator 30 instead of the metal separator.

  The anode separator 28 and the cathode separator 30 have a horizontally long shape, and are configured such that the long side extends in the horizontal direction (arrow B direction) and the short side extends in the gravity direction (arrow C direction).

  As shown in FIG. 3, one end edge of the power generation cell 12 in the long side direction (arrow B direction) communicates with each other in the arrow A direction, and the oxidant gas supply communication hole 32a, the cooling medium supply communication hole 34a, and Fuel gas discharge communication holes 36b are provided above and below. The oxidant gas supply communication hole 32a supplies an oxidant gas, for example, an oxygen-containing gas. The cooling medium supply communication hole 34a supplies a cooling medium, while the fuel gas discharge communication hole 36b discharges a fuel gas, for example, a hydrogen-containing gas.

  A fuel gas supply communication hole 36a, a cooling medium discharge communication hole 34b, and an oxidant gas discharge communication hole 32b are provided vertically at the other end edge in the long side direction of the power generation cell 12 so as to communicate with each other in the arrow A direction. . The fuel gas supply communication hole 36a supplies fuel gas, the cooling medium discharge communication hole 34b discharges the cooling medium, and the oxidant gas discharge communication hole 32b discharges the oxidant gas.

  As shown in FIGS. 2 and 3, the electrolyte membrane / electrode structure 26 includes, for example, a solid polymer electrolyte membrane (cation exchange membrane) 38 in which a perfluorosulfonic acid thin film is impregnated with water. The solid polymer electrolyte membrane 38 is sandwiched between the cathode electrode 40 and the anode electrode 42. The solid polymer electrolyte membrane 38 may use an HC (hydrocarbon) electrolyte in addition to the fluorine electrolyte.

  The cathode electrode 40 and the anode electrode 42 are formed by uniformly applying a gas diffusion layer (not shown) made of carbon paper or the like and porous carbon particles carrying a platinum alloy on the surface thereof to the surface of the gas diffusion layer. And an electrode catalyst layer (not shown) to be formed. The electrode catalyst layers are formed on both surfaces of the solid polymer electrolyte membrane 38.

  An oxidant gas flow path 44 that connects the oxidant gas supply communication hole 32a and the oxidant gas discharge communication hole 32b is formed on the surface 30a of the cathode separator 30 facing the electrolyte membrane / electrode structure 26. The oxidant gas channel 44 is formed by a plurality of wavy channel grooves (or linear channel grooves) extending in the direction of arrow B.

  A fuel gas flow path 46 that connects the fuel gas supply communication hole 36 a and the fuel gas discharge communication hole 36 b is formed on the surface 28 a of the anode separator 28 facing the electrolyte membrane / electrode structure 26. The fuel gas channel 46 is formed by a plurality of wave-like channel grooves (or linear channel grooves) extending in the direction of arrow B.

  Between the surface 28b of the anode separator 28 and the surface 30b of the adjacent cathode separator 30, a cooling medium flow path 48 is formed which communicates with the cooling medium supply communication hole 34a and the cooling medium discharge communication hole 34b. The cooling medium channel 48 is formed by a plurality of wave-like channel grooves (or linear channel grooves) extending in the direction of arrow B.

  A first seal member 50 is integrally formed on the surfaces 30 a and 30 b of the cathode separator 30 around the outer peripheral edge of the cathode separator 30. A second seal member 52 is integrally formed on the surfaces 28 a and 28 b of the anode separator 28 around the outer peripheral edge of the anode separator 28.

  As the first seal member 50 and the second seal member 52, for example, EPDM, NBR, fluorine rubber, silicone rubber, fluorosilicone rubber, butyl rubber, natural rubber, styrene rubber, chloroprene or acrylic rubber or the like, cushion material Alternatively, an elastic seal member such as a packing material is used.

  As shown in FIG. 3, in the power generation cell 12, positioning hole portions (knock holes) 54a and 54b are formed at one diagonal position (near the fuel gas supply communication hole 36a and the fuel gas discharge communication hole 36b). . The positioning holes 54a and 54b may be formed in the other diagonal position of the power generation cell 12 (near the oxidant gas supply communication hole 32a and the oxidant gas discharge communication hole 32b).

  As shown in FIG. 2, positioning holes 54a and 54b are formed in the insulators 16a and 16b, and holes 56a and 56b are formed coaxially with the positioning holes 54a in the end plates 18a and 18b. Is done. Hole portions 58a and 58b are formed on the end plates 18a and 18b coaxially with the positioning hole portion 54b.

  Both ends of a knock pin (positioning pin) 60a extending through the positioning hole 54a of each power generation cell 12 in the stacking direction are fitted into the holes 56a and 56b. Both ends of knock pins (positioning pins) 60b extending through the positioning holes 54b of the respective power generation cells 12 in the stacking direction are fitted into the holes 58a and 58b.

  As shown in FIG. 2, the knock pin 60a is set to an axial length corresponding to the distance between the bottoms of the holes 56a, 56b of the end plates 18a, 18b when the fuel cell stack 10 is fastened. Similarly, when the fuel cell stack 10 is fastened, the knock pin 60b is set to an axial length corresponding to the distance between the bottoms of the holes 58a, 58b of the end plates 18a, 18b. The knock pins 60a and 60b are preferably made of SUS (stainless steel), aluminum, iron, resin such as PPS (polyphenylene sulfide), carbon, or the like.

  As shown in FIGS. 4 and 5, the knock pin 60 a includes a hollow pin body portion 62. The pin main body 62 has a cylindrical shape, is integrally inserted into the positioning hole 54a of each power generation cell 12, and a hole 64 has a predetermined depth on one end side in the pin axial direction (arrow A direction). It is formed over. One end in the pin axis direction of the pin main body 62 is opened, and four (any number) slits are provided in the circumferential portion at one end in the pin axis direction with a predetermined angle (for example, equal angle) interval. 66 is formed over a predetermined length.

  In the pin main body 62, an extendable / contracting portion 68 that can be advanced and retracted in the pin axial direction of the pin main body 62 is disposed. The expansion / contraction part 68 has a cylindrical shape, is provided with a flange part 68a at one end inserted into the hole part 64, and is provided with a spherical part 68b at the other end part (tip part) opposite to the one end part. A plurality of, for example, four guide plate portions 70 extending in the pin axis direction between one end portion and the other end portion are provided on the outer peripheral surface of the expandable portion 68. Each guide plate portion 70 is disposed in each slit 66 of the pin main body portion 62, and inclined portions 70 a and 70 b that are inclined inward in the radial direction are formed at both ends of the guide plate portion 70.

  In the pin main body 62, an elastic body, for example, a coil spring 72, for moving the expansion / contraction part 68 back and forth in the pin axis direction is disposed. One end 72 a of the coil spring 72 is fixed to the bottom surface 64 s of the hole portion 64, while the other end 72 b of the coil spring 72 is fixed to the flange portion 68 a of the extendable portion 68. The coil spring 72 has a function of preventing the expansion / contraction part 68 from coming off, and also has a function of storing the expansion / contraction part 68 in the pin main body part 62 when the fuel cell stack 10 is fastened. The elastic body may be a disc spring or rubber.

  The knock pin 60b is configured in the same manner as the above-described knock pin 60a, and the same components are denoted by the same reference numerals, and detailed description thereof is omitted.

  FIG. 6 is an explanatory view of a main part of an assembling apparatus 80 applied to the assembling method of the fuel cell stack 10.

  In the assembling apparatus 80, a fixed plate 84 is disposed on a base 82, and the fixed plate 84 has a mounting surface 84s on which the end plate 18a (or 18b) is mounted. Although not shown, an opening for accommodating the terminal portion 20a (or 20b) is formed on the mounting surface 84s. Four guide bars 86 are erected on the fixed plate 84, and the guide bar 86 supports the movable plate 88.

  The movable plate 88 has a fixed surface 88s for fixing the end plate 18b (or 18a). Although not shown, an opening for accommodating the terminal portion 20b (or 20a) is formed in the fixed surface 88s. The rod portion 90a of the elevating cylinder 90 is fixed to the movable plate 88 downward.

  Next, an operation of assembling the fuel cell stack 10 using the assembling apparatus 80 will be described below.

  As shown in FIG. 7, for example, the end plate 18 a is fixed to the mounting surface 84 s of the fixed plate 84, and for example, the end plate 18 b is fixed to the fixed surface 88 s of the movable plate 88. In the end plate 18a, one ends of the knock pins 60a and 60b are inserted into the holes 56a and 58a.

  Further, as shown in FIG. 8, an insulator 16a and a terminal plate 14a are disposed on the end plate 18a, and a predetermined number of power generation cells 12 are provided with knock pins 60a and 60b and positioning holes 54a and 54b. It is positioned and placed under the guiding action. A terminal plate 14b and an insulator 16b are placed on the uppermost power generation cell 12. At that time, the tips of the knock pins 60a and 60b are exposed upward.

  Accordingly, when the lifting cylinder 90 is driven and the rod portion 90a is lowered, the movable plate 88 supported by the rod portion 90a is lowered integrally with the end plate 18b. Therefore, the end plate 18b presses the insulator 16b and the terminal plate 14b downward, and applies a compressive force to the stacked power generation cells 12 (see FIG. 9).

  At that time, the distal ends of the knock pins 60a and 60b, that is, the expansion and contraction portion 68 are inserted into the holes 56b and 58b of the end plate 18b, and then a pressing force is applied from the bottom surfaces of the holes 56b and 58b. Therefore, as shown in FIG. 10, the telescopic part 68 enters the pin main body part 62 against the elastic force of the coil spring 72. Further, when the end plate 18b is lowered to a predetermined fastening position, the entire extension / contraction part 68 is accommodated in the pin main body part 62 as shown in FIG.

  In this state, a connecting bar 22 is disposed between the sides of the end plates 18a and 18b, and the connecting bar 22 is fixed to the inner surfaces of the end plates 18a and 18b via bolts 24. Thereby, the assembly operation of the fuel cell stack 10 is completed.

  In this case, in the present embodiment, as shown in FIG. 7, one end of knock pins 60a and 60b that are extendable in the pin axis direction is held on the end plate 18a. The knock pins 60a and 60b are inserted into the positioning holes 54a and 54b provided in each power generation cell 12, whereby the plurality of power generation cells 12 are positioned and stacked (see FIG. 8).

  Next, as shown in FIG. 9, compressive force is applied in the stacking direction to the plurality of power generation cells 12 stacked between the end plate 18b and the end plate 18a, and the knock pins 60a and 60b are reduced in the pin axis direction. I am letting. Specifically, as shown in FIG. 11, the expansion / contraction portions 68 constituting the knock pins 60 a and 60 b enter the pin main body portion 62 against the elastic force of the coil spring 72. In this state, the end plates 18 a and 18 b are fastened and fixed via the connecting bar 22.

  As described above, in the present embodiment, the knock pins 60 a and 60 b are integrally inserted into the positioning holes 54 a and 54 b of each power generation cell 12, thereby positioning the plurality of power generation cells 12. When the plurality of power generation cells 12 are compressed in the stacking direction and the plurality of power generation cells 12 are compressed, the knock pins 60a and 60b can be reduced in the pin axis direction. For this reason, it is not necessary to divide the knock pins 60a and 60b, and the effect that the fuel cell stack 10 can be assembled easily and quickly is obtained.

  Further, when the fuel cell stack 10 is disassembled, conventionally, when the compressive force in the stacking direction is released, it has been necessary to attach the knock pin main body portion to the extension portion again. On the other hand, in this embodiment, it is not necessary to attach an extension part, and the effect that the work efficiency of disassembly can be improved effectively is also acquired.

  Further, in the present embodiment, as shown in FIGS. 4 and 5, four guide plate portions 70 are provided on the outer peripheral surface of the expansion / contraction portion 68, and each guide plate portion 70 has a pin body portion 62. It arrange | positions at each slit 66 of this. Therefore, the expansion / contraction part 68 has an advantage that it can easily and reliably advance and retract under the guide action of the guide plate part 70. In addition, as long as it is a structure which guides the expansion-contraction part 68, it is not limited to the guide board part 70, A various guide structure is employable.

  The operation of the fuel cell stack 10 configured as described above will be described below.

  First, as shown in FIG. 1, an oxidant gas such as an oxygen-containing gas is supplied to the oxidant gas supply communication hole 32a of the end plate 18a, and a hydrogen-containing gas or the like is supplied to the fuel gas supply communication hole 36a. The fuel gas is supplied. Further, a coolant such as pure water, ethylene glycol, or oil is supplied to the coolant supply passage 34a.

  Therefore, as shown in FIG. 3, the oxidant gas is introduced into the oxidant gas flow path 44 of the cathode separator 30 from the oxidant gas supply communication hole 32a. The oxidant gas moves in the direction of arrow B along the oxidant gas flow path 44 and is supplied to the cathode electrode 40 of the electrolyte membrane / electrode structure 26.

  On the other hand, the fuel gas is supplied to the fuel gas passage 46 of the anode separator 28 from the fuel gas supply communication hole 36a. The fuel gas moves in the direction of arrow B along the fuel gas flow path 46 so as to face the flow direction of the oxidant gas, and is supplied to the anode electrode 42 of the electrolyte membrane / electrode structure 26.

  Therefore, in the electrolyte membrane / electrode structure 26, the oxidizing gas supplied to the cathode electrode 40 and the fuel gas supplied to the anode electrode 42 are consumed by an electrochemical reaction in the electrode catalyst layer to generate power. Is called.

  Next, the oxidant gas consumed by being supplied to the cathode electrode 40 of the electrolyte membrane / electrode structure 26 is discharged in the direction of arrow A along the oxidant gas discharge communication hole 32b. On the other hand, the fuel gas supplied to and consumed by the anode electrode 42 of the electrolyte membrane / electrode structure 26 is discharged in the direction of arrow A along the fuel gas discharge communication hole 36b.

  The cooling medium supplied to the cooling medium supply communication hole 34 a is introduced into the cooling medium flow path 48 between the cathode separator 30 and the anode separator 28. The cooling medium moves in the direction of arrow B to cool the electrolyte membrane / electrode structure 26. The cooling medium is discharged in the direction of arrow A along the cooling medium discharge communication hole 34b.

DESCRIPTION OF SYMBOLS 10 ... Fuel cell stack 12 ... Power generation cell 18a, 18b ... End plate 22 ... Connection bar 26 ... Electrolyte membrane and electrode structure 28 ... Anode separator 30 ... Cathode separator 32a ... Oxidant gas supply communication hole 32b ... Oxidant gas discharge communication Hole 34a ... Cooling medium supply communication hole 34b ... Cooling medium discharge communication hole 36a ... Fuel gas supply communication hole 36b ... Fuel gas discharge communication hole 38 ... Solid polymer electrolyte membrane 40 ... Cathode electrode 42 ... Anode electrode 44 ... Oxidant gas flow Channel 46 ... Fuel gas channel 48 ... Cooling medium channel 54a, 54b ... Positioning holes 56a, 56b, 58a, 58b, 64 ... Holes 60a, 60b ... Knock pin 62 ... Pin body 66 ... Slit 68 ... Extendable part 70 ... Guide plate 72 ... Coil pulling 80 ... Assembly device 84 ... Fixed plate 88 ... Movable pre Doo 90 ... lifting cylinder

Claims (3)

A fuel cell stack having a power generation cell that generates power by an electrochemical reaction between a fuel gas and an oxidant gas, wherein the plurality of power generation cells are stacked, and end plates are disposed at both ends in the stacking direction,
A positioning pin that is inserted into each positioning hole provided in the plurality of power generation cells and is held between the end plates,
The positioning pin includes a hollow pin body portion,
A telescopic part inserted into the pin body part so as to be able to advance and retreat in the pin axial direction of the pin body part;
An elastic body disposed in the pin main body and moving the telescopic portion back and forth in the pin axial direction;
A guide portion that guides the stretchable portion in the advancing and retracting direction with respect to the pin body portion;
A locking portion that holds the stretchable portion at the advanced tip position and prevents the stretchable portion from being detached from the pin body portion;
A fuel cell stack comprising: An assembly method of a fuel cell stack comprising a power generation cell that generates power by an electrochemical reaction between a fuel gas and an oxidant gas, wherein the plurality of power generation cells are stacked and end plates are disposed at both ends in the stacking direction. And
A step of holding one end of a positioning pin that is extendable in the pin axis direction on one end plate;
Positioning and laminating the plurality of power generation cells by inserting the positioning pins into positioning holes provided in the power generation cells; and
A compression force is applied to the plurality of power generation cells stacked between the other end plate and the one end plate in the stacking direction, and the positioning pin is contracted in the pin axial direction, Tightening and fixing between the other end plate and the other end plate;
A method for assembling a fuel cell stack, comprising: 3. The assembly method according to claim 2 , wherein the positioning pin is inserted into the pin main body portion and a hollow pin main body portion, and can advance and retreat in the pin axial direction with respect to the pin main body portion by an elastic force of an elastic body. With an elastic part,
The fuel cell is characterized in that, when the power generation cells are stacked, the expansion / contraction portion protrudes outward from the pin main body portion, and when the power generation cell is compressed, the expansion / contraction portion is accommodated in the pin main body portion. Battery stack assembly method.

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