CN117691285A - Multifunctional cross member beam for traction battery pack - Google Patents

Abstract

The present disclosure provides a multi-functional cross member beam for a traction battery pack. A multi-functional cross member beam for use within a traction battery pack is provided. An exemplary cross-member beam may include features for interfacing with and supporting various sub-components of a cell stack of a traction battery pack (e.g., battery cells, cell tab terminals, bus bars, thermal barriers, etc.). The cross-member beam may also include features for facilitating the venting of battery cell exhaust byproducts during a battery thermal event.

Description

Multifunctional cross member beam for traction battery pack

Cross Reference to Related Applications

This disclosure claims priority from U.S. provisional application No. 63/403,445, filed on month 9 and 2 of 2022, which is incorporated herein by reference.

Technical Field

The present disclosure relates generally to traction battery packs and, more particularly, to a multi-functional cross-member beam for interfacing with and supporting various sub-components of a cell stack of a traction battery pack.

Background

Motorized vehicles include a traction battery pack for powering the vehicle's motor and other electrical loads. The traction battery pack includes a plurality of battery cells and various other battery internal components that support propulsion of the electric vehicle.

Disclosure of Invention

A traction battery pack according to an exemplary aspect of the present disclosure includes, among other things, a cell stack including: a first cross member beam; a first battery cell including a first cell tab terminal extending through a first cell tab opening of the first cross member beam; and a second battery cell including a second cell tab terminal extending through a second cell tab opening of the first cross member beam. The first cell tab terminal overlaps the second cell tab terminal.

In another non-limiting embodiment of the foregoing traction battery pack, the first battery cell and the second battery cell are part of a cell pouch supported between the first cross-member beam and the second cross-member beam.

In another non-limiting embodiment of any of the foregoing traction battery packs, the battery cell pouch further comprises a structural thermal barrier between the first battery cell and the second battery cell.

In another non-limiting embodiment of any of the foregoing traction battery packs, the first cross member beam further comprises a channel sized to receive the protruding barb of the structural thermal barrier.

In another non-limiting embodiment of any of the foregoing traction battery packs, a third cross member beam is adjacent to the first cross member beam. The first and third cross member beams establish a cross member assembly disposed between the cell stack and a second cell stack of the traction battery pack.

In another non-limiting embodiment of any of the foregoing traction battery packs, the vent passage is disposed between the first cross member beam and the third cross member beam.

In another non-limiting embodiment of any of the foregoing traction battery packs, the housing cover provides a vertically upper side of the exhaust passage and the housing tray or heat exchanger plate provides a vertically lower side of the exhaust passage.

In another non-limiting embodiment of any of the foregoing traction battery packs, the vent opening formed through the first cross member beam establishes a vent path between the cell stack and the vent passageway.

In another non-limiting embodiment of any of the foregoing traction battery packs, the backing tab of the first cross member beam separates the first cell tab opening from the second cell tab opening.

In another non-limiting embodiment of any of the foregoing traction battery packs, the backing tab establishes a backing surface for joining the first and second cell tab terminals together.

In another non-limiting embodiment of any of the foregoing traction battery packs, the first cell tab terminal overlaps the second cell tab terminal on a side of the first cross member beam opposite the first and second battery cells.

In another non-limiting embodiment of any of the foregoing traction battery packs, the weld bead joins the first cell tab terminal to the second cell tab terminal.

In another non-limiting embodiment of any of the foregoing traction battery packs, the first cross-member beam includes a beam body, a first reinforcing section establishing a first pultrusion, and a second reinforcing section establishing a second pultrusion.

In another non-limiting embodiment of any of the foregoing traction battery packs, the first pultrusion is disposed within an upper portion of the beam body and the second pultrusion is disposed within a lower portion of the beam body.

In another non-limiting embodiment of any of the foregoing traction battery packs, the first cross member beam further includes a slot sized to receive a bus bar.

A traction battery pack according to another exemplary aspect of the present disclosure includes, among other things, a first cell stack, a second cell stack, and a cross member assembly disposed between the first cell stack and the second cell stack. The cross member assembly includes a first cross member beam supporting the first cell stack and a second cross member beam supporting the second cell stack.

In another non-limiting embodiment of the aforementioned traction battery pack, the vent passage is disposed between the first cross member beam and the second cross member beam.

In another non-limiting embodiment of any of the foregoing traction battery packs, the housing cover provides a vertically upper side of the exhaust passage and the housing tray or heat exchanger plate provides a vertically lower side of the exhaust passage.

In another non-limiting embodiment of any of the foregoing traction battery packs, the first and second cross member beams each include an exhaust opening fluidly connectable to the exhaust passage.

In another non-limiting embodiment of any of the foregoing traction battery packs, the first cross-member beam includes a cell tab opening for receiving a cell tab terminal of a cell of the first cell stack.

The embodiments, examples and alternatives of the foregoing paragraphs, claims or the following description and drawings (including any of their various aspects or corresponding individual features) may be employed independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments unless such features are incompatible.

Various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

Drawings

Fig. 1 schematically shows an electrically powered vehicle.

Fig. 2 is an exploded perspective view of a traction battery pack for an electrically powered vehicle.

Fig. 3 is a cross-sectional view through section 3-3 of fig. 2.

Fig. 4 illustrates an exemplary cell stack of the traction battery pack of fig. 2 and 3.

Fig. 5 is a partially exploded view of the cell stack of fig. 4.

Figure 6 shows selected portions of the cross-member beams of the cell stack.

Fig. 7 is a cross-sectional view of a portion of a traction battery pack.

Fig. 8 is a cross-sectional view of another portion of the traction battery pack.

Detailed Description

The present disclosure details a multi-functional cross member beam for use within a traction battery pack. An exemplary cross-member beam may include features for interfacing with and supporting various sub-components of a cell stack of a traction battery pack (e.g., battery cells, cell tab terminals, bus bars, thermal barriers, etc.). The cross-member beam may also include features for facilitating the venting of battery cell exhaust byproducts during a battery thermal event. These and other features are discussed in more detail in the following paragraphs of this detailed description.

Fig. 1 schematically illustrates an electrically powered vehicle 10. The motorized vehicle 10 may include any type of motorized driveline. In one embodiment, the motorized vehicle 10 is a Battery Electric Vehicle (BEV). However, the concepts described herein are not limited to BEVs and are extendable to other motorized vehicles, including, but not limited to, hybrid Electric Vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), fuel cell vehicles, and the like. Thus, although not specifically shown in the exemplary embodiment, the powertrain of the motorized vehicle 10 may be equipped with an internal combustion engine that may be employed alone or in combination with other power sources to propel the motorized vehicle 10.

In the illustrated embodiment, the motorized vehicle 10 is depicted as an automobile. However, the motorized vehicle 10 may alternatively be a Sport Utility Vehicle (SUV), van, pick-up truck, or any other vehicle configuration. Although specific component relationships are shown in the drawings of the present disclosure, the illustrations are not intended to limit the disclosure. The placement and orientation of the various components of the motorized vehicle 10 are schematically illustrated and may vary within the scope of the present disclosure. Furthermore, the various figures attached to this disclosure are not necessarily drawn to scale, and some features may be exaggerated or minimized to emphasize certain details of particular components or systems.

In the illustrated embodiment, the motorized vehicle 10 is a pure electric vehicle propelled solely by electric power (such as by one or more electric machines 12) without the assistance of an internal combustion engine. The electric machine 12 may operate as an electric motor, a generator, or both. The electric machine 12 receives electrical power and may convert the electrical power into torque for driving one or more wheels 14 of the motorized vehicle 10.

The voltage bus 16 may electrically couple the motor 12 to a traction battery pack 18. Traction battery pack 18 is an exemplary motorized vehicle battery. Traction battery pack 18 may be a high voltage traction battery pack assembly including a plurality of battery cells configured to output power to power motor 12 and/or other electrical loads of electric vehicle 10. Other types of energy storage devices and/or output devices may alternatively or additionally be used to power the motorized vehicle 10.

The traction battery pack 18 may be secured to an underbody 20 of the motorized vehicle 10. However, it is within the scope of the present disclosure that traction battery pack 18 may be located elsewhere on electric vehicle 10.

Fig. 2 and 3 show further details associated with the traction battery pack 18 of the motorized vehicle 10. The traction battery pack 18 may include a plurality of cell stacks 22 housed within an interior region 30 of the housing assembly 24. The housing assembly 24 of the traction battery pack 18 may include a housing cover 26 and a housing tray 28. The housing cover 26 may be secured (e.g., bolted, welded, adhered, etc.) to the housing tray 28 to provide an interior region 30 for receiving the cell stack 22 and other battery internal components of the traction battery pack 18.

Each cell stack 22 may include a plurality of battery cells 32. The battery cells 32 of each cell stack 22 may be stacked side-by-side with respect to one another along a stack axis a. The battery cells 32 store and supply electrical power for powering the various components of the motorized vehicle 10. Although a particular number of cell stacks 22 and battery cells 32 are shown in the various figures of the present disclosure, the traction battery pack 18 may include any number of cell stacks 22, with each cell stack 22 having any number of individual battery cells 32.

In one embodiment, the battery cells 32 are lithium ion pouch cells. However, battery cells having other geometries (cylindrical, prismatic, etc.) and/or chemistries (nickel-metal hydride, lead acid, etc.) may alternatively be utilized within the scope of the present disclosure.

One or more structural thermal barriers 34 may be disposed along a respective cell stack axis a of each cell stack 22. The structural thermal barrier 34 may divide each cell stack 22 into two or more groups or compartments 36 of battery cells 32. Each compartment 36 may hold one or more of the battery cells 32 within one of the cell stacks 22. In one embodiment, the battery cells 32 of each cell stack 22 are held within one of the four compartments 36. However, other configurations are possible within the scope of the present disclosure, including configurations utilizing a greater or lesser number of compartments 36.

The battery cells 32 of each cell stack 22 may be disposed between a pair of cross-member beams 38. The cross-member beam 38 may be configured to retain the battery cells 32 and at least partially identify the cell stack 22.

The cross member beams 38 may be adhesively secured to the housing cover 26 and to the housing tray 28 or to heat exchanger plates 44 located between the housing tray 28 and the one or more cell stacks 22. The adhesive may seal these interfaces to inhibit escape of battery cell exhaust byproducts through these areas.

A cross member assembly 40 may be established immediately adjacent the cross member beam 38, which is disposed between adjacent cell stacks 22 of the traction battery pack 18. For example, the cross member assembly 40 may be configured to transfer loads applied to the sides of the motorized vehicle 10. Each cross member beam 38 of the cross member assembly 40 may be a structural beam that may help accommodate tensile and compressive loads from expansion of the battery cells 32. Thus, the cross member assembly 40 is configured to increase the structural integrity of the traction battery pack 18.

The cross member assembly 40 may also establish a battery pack venting system for conveying battery cell venting byproducts from the traction battery pack 18 during a battery thermal event. For example, the cross member assembly 40 may establish a passageway 42 (best shown in fig. 3) that conveys the cell exhaust byproducts from the cell stack 22 toward a location where the cell exhaust byproducts may be exhausted from the traction battery pack 18.

In the exemplary embodiment shown in fig. 3, adjacent first and second cross member beams 38 may establish first and second sides, respectively, of a passageway 42 of a cross member assembly 40. Further, a vertically upper side of the passageway 42 may be established by the housing cover 26, and a vertically lower side of the passageway 42 may be established by the heat exchanger plates 44 positioned against the housing tray 28. In another embodiment, the heat exchanger plates 44 may be omitted and the vertically underside of the passages 42 may be established by the housing tray 28. For purposes of this disclosure, vertical and horizontal are the general orientations of the traction battery pack 18 with reference to the ground and when installed in the motorized vehicle 10 of fig. 1.

In one embodiment, the cell stack 22, the cross member assembly 40, and the corresponding channels 42 extend longitudinally in the vehicle transverse direction. However, other configurations are also contemplated within the scope of the present disclosure.

Fig. 4 and 5 (with continued reference to fig. 2 and 3) illustrate exemplary designs of the cell stack 22 of the traction battery pack 18. The additional cell stack 22 of the traction battery pack 18 may comprise the same design as the cell stack 22 shown in fig. 4-5 or a similar design, as its electrical connection to an adjacent cell stack may be varied to complete the necessary electrical circuit.

The cell stack 22 may include a plurality of cell packets 46 stacked horizontally between a pair of cross-member beams 38 and longitudinally (e.g., side-by-side along the cell stack axis a) between a pair of compression plates 50. The total number of cell packets 46 disposed within the cell stack 22 may vary and, thus, is not intended to limit the present disclosure.

Each compression plate 50 may be made of a plastic material. The compression plate 50 may be configured to accommodate and maintain compression of the cell stack 22 along the cell stack axis a. Compression plates 50 may be attached to cross member beams 38. In one embodiment, compression plate 50 includes tabs 54 that are received by cross member beam 38.

Each cell pouch 46 of the cell stack 22 may include a combination of battery cells 32, one or more structural thermal barriers 34, and one or more cell expansion pads 48 stacked together along a cell stack axis a. An exemplary stacked configuration of each cell pouch 46 may include the following arrangement of subcomponents: battery cell 32-cell expansion pad 48-structural thermal barrier 34-cell expansion pad 48-battery cell 32-cell expansion pad 48. However, it is within the scope of the present disclosure that the battery cell pouch 46 may embody various other stacked arrangements/configurations.

For example, the various subcomponents of each cell pouch 46 may be secured together using an adhesive (such as a strip of double-sided tape 52). Strips of double-sided tape 52 may be interspersed between each pair of adjacent sub-components of the cell pouch 46.

The structural thermal barriers 34 may each comprise a single-piece structure or a multi-layer sandwich structure configured to slow or even prevent thermal propagation across the cell-to-cell of the cell stack 22. In one embodiment, the structural thermal barrier 34 may be made of, for example, a metallic material (such as stainless steel or aluminum) or a thermoplastic material. In another embodiment, the structural thermal barrier 34 comprises an insulating material, such as an aerogel material or a foam material. However, other materials or combinations of materials may be utilized to provide insulating properties to the structural thermal barrier 34 within the scope of the present disclosure.

The cell expansion pad 48 may include a compliant material for accommodating battery cell swelling. The compliant material may include, for example, polyurethane foam or silicone foam. However, other materials or combinations of materials may be utilized to provide the cell expansion pad 48 with compliance properties within the scope of the present disclosure.

Each cross member beam 38 may include a beam body 74 and one or more reinforcing sections. In the illustrated embodiment, the cross member beam 38 includes an upper or first reinforcing section 76 and a lower or second reinforcing section 78. However, other configurations are also contemplated within the scope of the present disclosure.

The beam body 74 may be a unitary structure that includes an upper portion 83, a lower portion 82, and an intermediate portion 84 extending between the upper portion 83 and the lower portion 82 and connecting the upper portion 83 and the lower portion 82. The upper portion 83 may establish an upper platform 86 of the cross member beam 38 and the lower portion 82 may establish a lower base 88 of the cross member beam 38. When positioned within the housing assembly 24 of the traction battery pack 18 in the manner shown in fig. 3, the upper platform 86 may interface with the housing cover 26 and the lower base 88 may interface with the heat exchanger plates 44 or the housing tray 28.

The beam body 74 of each cross member beam 38 may be made of any suitable thermoplastic material. In one embodiment, the beam body 74 is overmolded around each of the first and second reinforcing sections 76, 78. Thus, the first reinforcing section 76 may extend inside the upper portion 83 of the beam body 74, and the second reinforcing section 78 may extend inside the lower portion 82 of the beam body 74. Accordingly, the first and second reinforcing sections 76, 78 may be positioned to structurally reinforce selected portions (e.g., stress areas) of the beam body 74.

In one embodiment, the beam body 74, the first reinforcing section 76, and the second reinforcing section 78 each comprise substantially equal lengths. In other embodiments, the length of the beam body 74 may be greater than the respective lengths of the first and second reinforcing sections 76, 78.

In one embodiment, the first and second reinforcing sections 76, 78 are pultrusions, which implies a structure of these beam-like sections. Those skilled in the art having the benefit of this disclosure will appreciate how to structurally distinguish a pultruded beam structure from another type of structure, such as an extruded beam, for example.

The first and second reinforcing sections 76, 78 may be manufactured as part of a pultrusion process utilizing glass or carbon fibers (unidirectional or multidirectional mats) and a thermosetting resin. As part of the pultrusion process used to fabricate the first and second reinforcing sections 76, 78, a plurality of glass fiber bundles or carbon fiber bundles may be pulled through the thermosetting resin. The first and second reinforcing sections 76, 78 may then be overmolded by the beam body 74 to provide the desired cross-section of the cross-member beam 38. The beam body 74 may be made of any suitable thermoplastic material.

Each cross-member beam 38 of the cell stack 22 may include a plurality of vent openings 56 for conveying cell vent byproducts through the beam and into one of the passageways 42 (note that the passageways 42 are best shown in fig. 3). Thus, the vent openings 56 provide a path for battery cell vent byproducts to move through the cross member beam 38 and into the passageway 42 as needed during a vent event.

The vent opening 56 may be formed through a beam body 74 of the cross member beam 38. In one embodiment, the vent opening 56 is formed through the middle portion 84 of the beam body 74.

The vent opening 56 may be covered by a segmented membrane 58 when the cell cells 32 of the cell stack 22 are not vented. The increase in pressure differential associated with the venting of one or more of the battery cells 32 may rupture a localized section of the segmented membrane 58, allowing battery cell venting byproducts to enter the passageway 42 through the vent opening 56 of the individual cell packet 46 that experienced the thermal event. When a single cell packet 46 experiences a thermal event to release battery cell exhaust byproducts into the passageway 42, a localized section of the segmented membrane 58 may locally disengage. The cell exhaust byproducts may exit from one cell pouch 46 on both sides of the cell stack 22.

Each cross-member beam 38 may additionally include a plurality of cell tab openings 60 disposed vertically below the vent opening 56. The cell tab openings 60 may be formed through the beam body 74. In one embodiment, the cell tab openings 60 are formed through the middle portion 84 of the beam body 74.

Each cell tab opening 60 may be configured to receive a cell tab terminal 62 of a battery cell 32. Cell tab terminals 62 extend from the battery cell housing. For example, an aluminum film may provide the battery cell housing.

In one embodiment, each cell tab opening 60 may receive one cell tab terminal 62. In another embodiment, each cell tab opening 60 may be sized to receive a cell tab terminal 62 from a plurality of adjacent battery cells 32 (see, e.g., fig. 6). During a thermal event, in addition to the vent openings 56, battery vent byproducts may also be at least partially vented through each cell tab opening 60.

Referring now primarily to fig. 7 (with continued reference to fig. 2-5), at least a portion of adjacent cell tab openings 60 may be separated by a backing tab 64 of the cross-member beam 38. The cross member beams 38 may each include a plurality of backing tabs 64. Each backing tab 64 may provide a suitable backing surface to join (e.g., weld) the cell tab terminals 62 together to electrically connect the battery cells 32 of the battery pouch 46. To electrically connect the cell tab terminals 62, the cell tab terminals 62 may extend through their respective cell tab openings 60 and then be folded over the backing tab 64 such that the cell tab terminals 62 overlap one another. When folded, the cell tab terminals 62 are located on the side of the cross member beam 38 opposite the housing of the battery cell 32. A bead 66, such as formed during a laser welding process, for example, may then be applied to the overlapping cell tab terminals 62 to electrically connect the cell tab terminals 62.

The backing tab 64 may additionally provide a sense lead that may be used to collect data. For example, the voltage of the cell tab terminal 62 of the battery cell 32 may be monitored and collected by the backing tab 64.

The inner surface 68 of the backing tab 64 (e.g., the surface facing the cell packet 46) or the cross-member beam 38 to which the backing tab 64 may be attached may interface with the structural thermal barrier 34 of the cell packet 46. For example, the inner surface 68 may include a C-shaped channel 70 sized to receive a protruding barb 72 of the structural thermal barrier 34. Together, the protruding barbs 72 and the C-shaped channels 70 establish a tongue and groove connection between the structural thermal barrier 34 and the cross member beam 38.

Each cross-member beam 38 may also include features for supporting one or more bus bars that may be used to electrically connect the cell packets 46 of the cell stack 22. Referring to fig. 8, for example, each cross-member beam 38 may include a slot 92 configured to support a bus bar 90. The slot 92 may extend outwardly from both the upper and lower sections of the cell tab opening 60. Thus, the slot 92 is configured to support both the upper and lower sections of the bus bar 90.

The bus bar 90 may be inserted into the slot 92 prior to or during welding of the cell tab terminals 62. The bus bar 90 may be secured to the cell tab terminal 62 by additional weld beads 94.

The multi-functional cross-member beam of the present disclosure is capable of supporting various subcomponents of the battery cell stack. For example, the cross-member beams may provide features for supporting battery cells, cell tab terminals, bus bars, thermal barriers, etc. of the cell stack. The cross-member beam may also include features for facilitating the venting of battery cell exhaust byproducts during a battery thermal event.

Although various non-limiting embodiments are shown with specific components or steps, embodiments of the present disclosure are not limited to these specific combinations. It is possible to use some of the features or components from any one of the non-limiting embodiments in combination with features or components from any one of the other non-limiting embodiments.

It should be understood that the same reference numerals indicate corresponding or similar elements throughout the several views. It should be understood that while particular component arrangements are disclosed and illustrated in the exemplary embodiments, other arrangements may benefit from the teachings of this disclosure.

The above description should be construed as illustrative and not in any limiting sense. A worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this disclosure. For these reasons, the following claims should be studied to determine the true scope and content of this disclosure.

Claims (15)

1. A traction battery pack, comprising:

a cell stack, the cell stack comprising:

a first cross member beam;

a first battery cell including a first cell tab terminal extending through a first cell tab opening of the first cross member beam; and

a second battery cell including a second cell tab terminal extending through a second cell tab opening of the first cross member beam,

wherein the first cell tab terminal overlaps the second cell tab terminal.

2. The traction battery pack of claim 1, wherein the first battery cell and the second battery cell are part of a cell pouch supported between the first cross-member beam and a second cross-member beam.

3. The traction battery pack of claim 2, wherein the cell packet further comprises a structural thermal barrier between the first battery cell and the second battery cell, and optionally wherein the first cross-member beam further comprises a channel sized to receive a protruding barb of the structural thermal barrier.

4. The traction battery pack of claim 3, comprising a third cross-member beam adjacent to the first cross-member beam, wherein the first and third cross-member beams establish a cross-member assembly disposed between the cell stack and a second cell stack of the traction battery pack, and optionally the traction battery pack comprises a vent passage disposed between the first and third cross-member beams.

5. The traction battery pack of claim 4, wherein a housing cover provides a vertically upper side of the exhaust passage and a housing tray or heat exchanger plate provides a vertically lower side of the exhaust passage.

6. The traction battery pack of claim 5, wherein a vent opening formed through the first cross-member beam establishes a vent path between the cell stack and the vent passageway.

7. The traction battery pack of any preceding claim, wherein a backing tab of the first cross-member beam separates the first cell tab opening from the second cell tab opening, and optionally wherein the backing tab establishes a backing surface for joining the first cell tab terminal and the second cell tab terminal together.

8. The traction battery pack of claim 7, wherein the first cell tab terminal overlaps the second cell tab terminal on a side of the first cross member beam opposite the first battery cell and the second battery cell.

9. The traction battery pack of claim 8, comprising a bead joining the first cell tab terminal to the second cell tab terminal.

10. The traction battery pack of any preceding claim, wherein the first cross-member beam comprises a beam body, a first reinforcing section establishing a first pultrusion, and a second reinforcing section establishing a second pultrusion, and optionally wherein the first pultrusion is disposed within an upper portion of the beam body and the second pultrusion is disposed within a lower portion of the beam body.

11. The traction battery pack of any preceding claim, wherein the first cross member beam further comprises a slot sized to receive a bus bar.

12. A traction battery pack, comprising:

a first cell stack;

a second cell stack; and

a cross member assembly disposed between the first cell stack and the second cell stack,

wherein the cross member assembly includes a first cross member beam supporting the first cell stack and a second cross member beam supporting the second cell stack.

13. The traction battery pack of claim 12, comprising an exhaust passage disposed between the first and second cross member beams, and optionally wherein a housing cover provides a vertically upper side of the exhaust passage and a housing tray or heat exchanger plate provides a vertically lower side of the exhaust passage.

14. The traction battery pack of claim 13, wherein the first and second cross-member beams each include a vent opening fluidly connectable to the vent passage.

15. The traction battery pack of any one of claims 12-14, wherein the first cross-member beam includes a cell tab opening for receiving a cell tab terminal of a cell of the first cell stack.