Support assembly for cells of a secondary battery

A secondary battery, such as a nickel-hydrogen (NiH.sub.2) cell, has a first and a second cell stack, each cantilevered from opposite sides of a weld ring. The weld ring is located at the approximate center or middle of a pressure vessel composed of two half sections which are welded to opposite sides of the weld ring. The cell stacks are essentially unsupported at their outer ends. Electrical current carriers are received in longitudinal channels formed on opposite sides of an axially located cell stack support member, on which the cell elements are captured. Cantilever mounting of two cell stacks from a central weld ring is found to improve the integrating and durability of the cell. It also increases the eneregy capacity of the battery through the ability to add more cells through the use of axially longer pressure vessels, increasing the capacity of the battery. Location of the weld ring between the pair of cell stacks improves heat transfer between the cells and the pressure vessel during discharge.

FIELD OF THE INVENTION 
This invention relates generally to metalgas secondary batteries or cells, 
such as NiH.sub.2 cells, and more particularly relates to an improved 
support assembly and arrangement of the cell stack. 
BACKGROUND OF THE INVENTION 
Metal-gas cells or batteries, such as nickel-hydrogen cells, are contained 
in sealed metal vessels or casings (often referred to as cans) which 
contain hydrogen gas under high pressure. This gas pressure may, for 
example, vary between about 0 psi and 1000 psi during each cycle 
charge-discharge operation. Each such cell has at least one 
nickel-containing positive electrode which is spaced from a 
hydrogen-forming negative electrode. A pair of positive and negative 
electrodes makes up the individual cell, with a plurality of such cells 
forming the battery. A plurality of such electrode pairs are generally 
organized in the form of plates which are stacked to form a plate stack. 
The stack also includes gas diffusion plate separators which prevent short 
circuiting contact between the positive and negative electrodes, and which 
also hold a sufficient quantity of electrolyte for cell operation. 
The electrolyte is typically an alkaline medium such as an aqueous solution 
of alkali metal hydroxide, generally approximately a 30% potassium 
hydroxide solution. The negative (hydrogen-forming) electrode or anode is 
typically a plastic bonded, metal powder plate. The metal powder is 
usually platinum or palladium which will catalyze a hydrogen dissociation 
reaction in the aqueous electrolyte. The plastic bonding material can be a 
tetrafluoroethylene, for example. The active material of the positive 
plate or cathode is generally nickeloxyhydroxide. 
Hydrogen in the vessel diffuses through a diffusion mesh of 
tetrafluoroethylene or the like to reach the catalytic anode. The anode 
causes molecular H.sub.2 to dissociate into atomic hydrogen, which in turn 
reacts with free hydroxyl groups to form water plus free electrons. The 
water and the free electrons react with the nickeloxyhydroxide cathode to 
form nickel hydroxide plus free hydroxyl groups. Reverse reactions occur 
during charging. 
The components of the cell stack (i.e., anodes, cathodes, separators) are 
conventionally made in a disc-shape and are arranged along a common axis. 
A single plate stack has been used, which is mounted PG,4 in a cylindrical 
shaped pressure vessel having hemispherical ends. The pressure vessel is 
ordinarily formed from two casing portions joined at an equatorial weld 
ring. 
Nickel-hydrogen cells are relatively long lived, have a wide operating 
temperature range and a high energy density. They have been widely adopted 
as a preferred electrical storage system for earth-orbiting satellites. 
Due to the great expense of these satellites, the chance of cell failure 
must be absolutely minimized. The cells must as well be designed to endure 
the forces encountered when the satellite is launched, for example. It is 
also most critical that the mass and volume of these cells be as low as 
possible, while their energy storage and generation capabilities are 
maximized. For these reasons, considerable attention has been directed to 
the structure which supports the cell stack in the pressure vessel. 
One known way to support the plate stack is by mounting it on a retaining 
rod which extends axially through a central aperture formed in the 
components of the stack. The retaining rod is supported at its ends by 
terminals which extend axially outwardly from the centers of the domed 
ends of the pressure vessel. The plate stack is fixed and may be 
compressed on the rod by stops or retaining elements. 
Mounting the plate stack on a central rod in this manner leads to a 
relatively large mass and volume for the nickel-hydrogen cell, and further 
places an undesirable stress on the terminals themselves. This makes the 
cell more prone to failure. It also makes the cell relatively long due to 
the axially extending terminals. 
One previously known alternative to mounting the plate stack on a central 
rod is to mount or support the stack in a cantilever fashion from a 
support surface formed inside the pressure vessel, such as from one side 
of the weld ring used in interconnecting the two domed casing portions. In 
this arrangement, the weld ring has been located considerably to one axial 
side of the center or middle of the pressure vessel. The plate stack is 
cantilevered (as viewed horizontally) from one side of the weld ring, as 
on an elongated support rod fixed to the weld ring, and extends through 
much of the remaining volume of the pressure vessel. 
One difficulty with this type of mount for the plate stack is the 
relatively long lever arm represented by the plate stack on the support 
rod. Jolting of the cell from acceleration and deceleration or other 
movements causes the relatively heavy plate stack to vibrate off-axis, 
particularly at its free end, imposing a torque on the weld ring. Such 
movements can also cause the stack to bang against the inside of the 
pressure vessel. Failure of the cell can result, such as from damage to 
the components of the cell stack from banging into the pressure vessel 
walls, shorting of the electrodes through contact with the metal pressure 
vessel, or rupture of the weld ring seal from fatigue. 
One possible solution to this problem is to reduce the number of individual 
cells making up the cell stack, to thereby shorten the lever arm. This 
reduces the capacity of the battery, however. 
Another drawback to cantilever mounting of the cell stack in this fashion 
is inherent in the fabrication of the pressure vessel itself. The draw 
process which is typically used for making the pressure vessel limits the 
maximum size of a can portion to approximately 5-7" in axial length for a 
31/2" diameter cell. (A 31/2" diameter cell is at present standard in the 
industry for satellite applications.) A pressure vessel is ordinarily 
constructed with one can portion of about this maximum axial length and 
within which substantially all of the cell stack is received, and a 
shorter can portion (or cap) completing the vessel. The maximum length of 
the plate stack is thus limited by the maximum length of the larger can 
portion. This construction further reduces the volume available for 
mounting the plate stack within the pressure vessel, again limiting the 
capacity of the cell. 
Another problem with mounting the plate stack from one side of a weld ring 
in the foregoing fashion is that heat transfer between the plate stack and 
the pressure vessel is not efficient. The point of highest heat generation 
(i.e., during discharge) in a plate stack is at the longitudinal center of 
the stack. In the cantilever arrangement described above, the center of 
the stack is considerably offset from the weld ring which transfers heat 
to the pressure vessel for dissipation. 
Also, the larger can portion containing the plate stack has a tendency to 
balloon or bulge somewhat from the internal pressure, particularly if this 
larger portion is made to maximum length. This causes undesirable flexing 
of the pressure vessel during internal pressure changes associated with 
cyclic charge-discharge operation. It also increases the gap between the 
plate stack and the pressure vessel wall within which the plate stack can 
vibrate or shift, again causing greater torque on the weld ring. 
SUMMARY OF THE INVENTION 
The shortcomings of the prior art have been overcome by the present 
invention, wherein two cell stack assemblies are cantilevered from 
opposite sides of a central weld ring. The weld ring is located at the 
approximate center or middle of a pressure vessel having two half 
sections. Each half section of the pressure vessel is fixed to a 
respective side of the weld ring. The two cell stack assemblies are 
preferably of about equal size and weight and are therefore substantially 
balanced on the weld ring. This also locates the center of gravity 
approximately in the middle of the battery. 
The single cell stack of the prior art has effectively been split into two 
halves in this invention, with one half mounted on each of the two sides 
of the central weld ring. The lever arm presented by each of the two cell 
stacks is thus effectively one-half of that presented by the prior art 
single stack structure of the same size (i.e., component weight). The 
torques imposed on the weld ring by the two cell stacks also tend to 
offset each other. The integrity and durability of the cell is thus 
greatly improved. 
The capacity of the battery can thereby be considerably increased, since 
the size of the two cell stacks can each be increased, although this does 
result in an increase in their respective lever arms. Larger cell stacks 
can still be provided with an acceptable increase in the length of each 
lever arm. 
This increase in cell capacity goes hand in hand with the feasibility of 
now increasing the length of the pressure vessel in its axial dimension. 
This advantage is gained from locating the weld ring in the center or 
middle of the pressure vessel. That is, two pressure vessel halves can now 
be drawn to the maximum length possible in the drawing process. This is to 
be compared with the "one long and one short" pressure vessel portions 
used in the prior art. It is estimated that use of the longer cans can 
increase the capacity of a nickel-hydrogen battery from a typical 70 to 90 
ampH to 140 to 180 ampH. 
Location of the weld ring in the center of the pressure vessel also places 
it nearer the area of maximum heat generation in the plate stacks, at 
least for a pair of plate stacks having the same component volume as the 
prior art single stack. A shorter heat transfer path from the source of 
heat generation in the stacks to the weld ring and then to the pressure 
vessel is thereby provided. This improves the life and operation of the 
battery. Location of the weld ring in the longitudinal center of the can 
also reduces the "ballooning" effect previously noted. This is because two 
pressure vessel halves can now be used which are each shorter in length 
than the one long pressure vessel portion previously used (in combination 
with a shorter vessel end portion). The shorter length pressure vessel 
halves are less subject to ballooning. 
A preferred embodiment of the invention uses a sectional core piece 
assembly on which the cell stacks are oppositely mounted. The core piece 
assembly is comprised of a back-to-back pair of one piece molded 
polysulfone core halves having a generally planar disc shaped base 
portion, and a stack support member extending generally perpendicularly 
(axially) from the base portions. The two core pieces are mounted in back 
to back relationship on either side of a weld ring/core piece mount and 
are fixed together and to the weld ring/core piece mount. 
A pair of parallel internal channels are formed in each of the 
perpendicular stack support members. These channels provide routing 
channels for electrical leads, and give a sturdy I-beam type construction 
to the core assembly. The cell stacks are mounted on the stack support 
members of respective core pieces, and are held in place by a polysulfone 
end cap secured at the free end of the stack support member. 
The sectional core piece assembly further provides ease in assembly of the 
cell. It also facilitates "sizing" the core assembly to fit particular 
configurations, as by simple substitution of a core piece having a larger 
support member to support a larger stack for a higher capacity 
configuration. 
The weld ring/core piece support particularly adapted for this invention 
has a scalloped internal ring design which provides great rigidity for the 
core piece attachment points, yet restricts heat diffusion to the cell 
stacks during the weld closure operation. Heat transfer from the cell 
stacks to the pressure vessel is not substantially affected, however. 
The features and advantages of the present invention will be more readily 
understood upon consideration of the following detailed description of the 
invention taken in conjunction with the accompanying drawings, in which:

DETAILED DESCRIPTION OF THE INVENTION 
The specific embodiment of the invention hereinafter described is related 
to nickel-hydrogen cell technology. It should be recognized that the 
invention is not so limited to nickel-hydrogen cell technology, however, 
and those skilled in the art of designing electrochemical energy storage 
cells can readily adapt the principles of this invention to other high 
pressure cell designs. 
With reference to FIG. 1, a nickel-hydrogen cell is illustrated having a 
casing or pressure vessel 10 (often referred to as the can) and a pair of 
axially aligned cell stacks 11 and 12. The can 10 is cylindrical with 
hemispherical ends, and is formed from two halves 10a10b. As previously 
noted, these halves or portions of the can 10 are typically fabricated by 
a drawing process which is limited as to maximum draw length, at least in 
so far as these cans are adapted for satellite applications. For instance, 
the maximum length for a draw portion of a can having a radial diameter of 
31/2" (which is a standard diameter for satellite applications) is about 5 
to 7" in axial length. It may be noted that while the diameter of the can 
may be limited for use in satellites, the axial length of the can is not 
as restricted. 
Each of the cell stacks 11, 12 is comprised of a stack of essentially 
disc-shaped components arranged along a common axis. These components 
ordinarily include a plurality of anodes which are connected electrically 
to an off-axis negative terminal 15 through leads or connectors 16, and a 
plurality of cathodes connected electrically by leads or connectors 17 to 
a positive terminal 18. Separators, gas screens, and the like, of 
conventional function are disposed between the anodes and the cathodes. 
Each of the cell stacks is partly saturated with an aqueous solution of 
potassium-hydroxide electrolyte, and the pressure vessel is filled with 
hydrogen gas, as through a fill tube 19, to a pressure which may be about 
20-30 atmospheres. It will be noted that none of the details concerning 
the function of the components of the cell stack assemblies forms any part 
of this invention, and the technology for constructing the same is well 
known in the art. Reference can be made to U.S. Pat. No. 4,115,630, for 
example, for the particular arrangement and components of such a metal 
oxide-hydrogen cell. 
It may be noted that the two cell stacks assemblies 11, 12 may 
advantageously be connected either in series or in parallel, as desired. 
With particular reference to FIG. 2, a sectional core assembly is provided 
upon which the cell stacks 11, 12 are mounted. More specifically, this 
core assembly comprises a pair of similar but oppositely oriented core 
pieces 22, 23 upon which the cell stacks 11, 12 are respectively 
supported, and a central core piece mount 24. The core piece mount 24 is 
fixed within a weld ring 25, but alternatively can be formed integrally 
with the weld ring 25. 
The two core pieces 22, 23 are made of a non-conducting material such as 
polysulfone, which is resistant to the high alkaline environment within 
the cell. Each core piece 22, 23 has a circular base 26 having a shoulder 
26a formed on the face which is toward the weld ring. A cylindrical stack 
support portion 27 is axially centered on the outboard side of the base 26 
and extends generally perpendicularly or axially therefrom. A pair of 
channels 28a, 28b are formed along diametrically opposite sides of each 
support 27 and extend along the entire length of the support and through 
base 26. Channels 28a, 28b define an I-beam configuration for supports 27, 
27. The electrical connectors 16, 17 to the respective anodes and cathodes 
are received in the channels 28a, 28b. Each of the core pieces 22, 23 is 
desirably molded as a single unit. 
The two core pieces 22, 23 are secured to opposite sides of the core piece 
mount 24. Core piece mount 24 is made of Inconel and is stamped or 
otherwise fabricated from two portions 24a, 24b which are arranged in back 
to back fashion and spot welded together. Each of the core piece mount 
portions 24a, 24b have an outwardly turned edge or rim 21a, 21b and an 
open, sprocket-shaped planar portion 29a, 29b, respectively (see FIG. 1A). 
An open center is defined in the core piece mount 24, and spaced scallops 
are formed between the planar portions and the rims, thus defining a 
sprocket-shape (see FIG. 2). 
The core piece mount 24 is fixed within weld ring 25 as by spot welding. 
Weld ring 25 has a bead 32 formed around its outside perimeter. As 
particularly shown in FIG. 1A, the edges of the respective can halves 10a, 
10b fit against the lateral side edges of the bead 32 and are fixed to the 
weld ring 25 as by tungsten inert gas or e-beam welding. The can 10 is 
thereby sealed. It will be noted that the scalloped configuration of the 
core piece mount 24 advantageously minimizes heat transfer to the 
components of the stack assemblies 11, 12 during the weld closure 
operation. 
The core pieces 22, 23 are assembled to the combined core piece mount/weld 
ring 24, 25 assembly by non-electrically conductive rivets 34 (FIG. 1) 
which extend through holes 35a provided in the bases 26 of the core 
pieces, and holes 36 provided in the core piece mount 24. In assembly, the 
core pieces 22, 23 are arranged with their respective channels 28a, 28b 
aligned with each other, with each of the core pieces then riveted to a 
respective side of the core piece mount 24. The core pieces 22, 23 are 
also directly attached to each other through rivets passing through holes 
35b provided in their respective bases. 
The stack assemblies 11, 12 are received on respective support members 27, 
and held under compression thereon by polysulfone end caps 37, 38, 
respectively. These end caps 37, 38 are threaded onto the free ends of the 
support members 27 of the core pieces 22, 23. The entire core piece 
mount/weld ring assembly with the cell stacks 11, 12 thereon is then fixed 
in the can 10 by assembly of the can halves 10a, 10b to the weld ring 25 
in the manner previously described. 
It will be noted that the cell stack assemblies 11, 12 are slightly spaced 
away from the interior can wall to prevent electrical contact between the 
electrodes and the wall and to enable hydrogen gas to permeate the spaces 
within the cell stack assemblies. The sides of the polysulfone end caps 
37, 38 also do not contact the interior can wall. The two cell stack 
assemblies are thus mounted in a cantilever fashion on either side of the 
core piece mount/weld ring assembly. The electrical connectors or leads 
16, 17 provide essentially no support for the cell stacks, and in fact 
each lead 16, 17 has a strain relief loop formed therein. 
This design for a nickel-hydrogen cell provides for the components of the 
cell stack assemblies, and the end plates, to be held firmly in position 
and adjacent to each other, while still providing the necessary insulation 
between the electrodes of the plate stack assemblies and the pressure 
vessel walls. The sectional core design promotes ready assembly of the 
cell, and also permits desirable sizing options for cell design 
variations. That is, various size core pieces 22, 23 with longer or 
shorter support members 27 can be assembled and used as desired. 
The use of two cell stack assemblies supported in a cantilever fashion on 
opposite sides of the core piece mount/weld ring greatly reduces the 
length of the lever arm represented by each separate stack assembly on a 
support member 27, as compared to the use of a single stack of the same 
weight and capacity supported by an equivalent single support member. 
Problems associated with the cell stack assemblies wiggling on the support 
members from vibration or shock to the cell is thus substantially reduced, 
improving the overall integrity and durability of the cell. 
This design further permits an increase in the energy capacity of the cell, 
since each of the cell stack assemblies 11, 12 can be increased in terms 
of the number of individual cells contained therein. This, of course, also 
increases the lever arm represented by the support member 27 used to 
support each of the individual stack assemblies, but it is very 
conceivable that a significant gain in capacity achieved would offset any 
potential loss of integrity in certain applications. 
In conjunction with the ability to increase the size of the cell stack 
assemblies, the axial length of the can 10 can be increased to the limits 
of the ability to draw each of the can halves 10a, 10b. A much longer can 
can thus be fabricated using existing technology than could be used before 
with the single stack, end supported arrangement. That is, two maximum 
length can halves can now be joined together, compared to the one short 
and one long can portions used in the past. The larger cell stack 
assemblies can therefore be used through this increase in available can 
volume. 
An improvement in heat transfer from within the cell stacks is also 
achieved by locating the weld ring between the two cell stack assemblies. 
That is, given two cell stack assemblies 11, 12 which together are the 
equivalent of a single cell stack assembly of the prior art, location of 
the weld ring between the two cell stack assemblies places the weld ring 
much closer to the center of each of the cell stack assemblies, which is 
the point of highest heat generation. The path of heat transfer is thus 
significantly reduced, improving the efficiency and longevity of the cell. 
Thus, while the invention has been described in connection with a certain 
presently preferred embodiment, those skilled in the art will recognize 
modifications of structure, arrangement, portions, elements, materials and 
components which can be used in the practice of the invention without 
departing from the principles of this invention.