Note: Descriptions are shown in the official language in which they were submitted.
60BA-102
687
ELECTROCHEMICAL CELL HAVING INTERNAL SHORT INHIBITOR
Background of the Invention
The present invention relates to electrochemical cells,
and specifically to a new sealed lead acid cell having
improved performance and less susceptibility to shorting
due to severe discharge and charge reversal.
A significant improvement in the well-known lead acid
cell is the fully sealed lead acid cell which makes use of
a cylindrical spiral-wound plate design for high energy
density and low internal impedance, and can be used, i.e.,
charged and discharged, in any position. Such cells
typically include spaced-apart positive and negative lead
plates having a grid-like construction. The grid structure
is filled with the active materials to form either positive
(lead dioxide) or negative (sponge lead) electrodes.
Sandwiched between the plates is a thin porous separator,
the plate-separator assembly being wound into a compact,
rugged cylindrical form. The separator electrically
isolates the plates, and also functions as an effective wick
to retain the cell's electrolyte (an aqueous solution of
sulfuric acid) and keep it evenly distributed in the
working area. The thin, highly porous separator keeps the
ionic path between the plates short and permits rapid
diffusion of electrolyte, these factors all contributing to
the cell's ability to be discharged at high rates. The
typical cell being described also generally includes means
such as excess negative plate material, for minimizing
the formation of gases in the cell, and a resealable vent for
releasing internal pressure in the cell should unwanted
gases be generated.
While the above-described cell represents a significant
improvement over prior lead acid cells, it is desirable to
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further improve its performance and lengthen its life cycle,
especially when the cell is subjected to extreme conditions.
For example, when a lead acid cell is allowed to stand on
open circuit, a slow electrochemical discharge occurs, the
rate of selfdischarge depending on the cell temperature
and its state of charge. If a cell is allowed to self-
discharge completely, i.e., until substantially all of the
sulfate ion in the electrolyte has reacted with the plate
materials, the lead sulfate becomes slightly soluble in
the very dilute electrolyte and is free to diffuse into
the separator between the plates. Attempting to recharge
the cell in this condition may result in the formation
of lead dendrites in the separator between the plates,
eventually shorting the cell and ending its useful life.
Similarly, where a discharged lead-acid cell is for one
reason or another connected to a charge in reverse, the
cell will accept a charge (the positive plate becomes a
negative plate and vice versa), but is vulnerable to being
shorted out by deposits of metallic lead and lead sulfate
in the separator.
In addition, it has been found that there are
certain problems associated with winding the plate-
separator assembly of known cells. More particularly,
in the manufacture of electrolytic cells or batteries, the
positive and the negative plate materials are formed as
strips which are then cut into cell strip lengths and wound,
with separator material therebetween, into a coil. The
coil is then inserted into a preformed cylindrical container.
Electrolyte is added to the container and the container is
closed and sealed. The sealed cell is then charged. The
coilad positive plate is connected to the positive terminal
and the coiled negative plate is connected to the negative
~i66~87 6OBA-102
terminal. Typically, the plate strips and separators are
wound into the composite coiled assembly about a mandrel
with a kiss roller or the inner surface of a cylindrical
winding nest engaging one side of the coil as it is being
wound. One of the difficulties with this arrangement,
however, is that the forces required for winding are
applied through the mandrel directly to the strips being
wound. Thus, tensile forces are applied to the separators
as the separators and plates are pulled by the arbor into
the composite, coiled assembly. Because the separators
have a low tensile strength, these winding forces can
cause damage to and/or breakage of the separators, and
can result in a shorted coil, which, of course, adversely
affects overall cell quality.
Accordingly, it is an object of the present invention
to provide a new and improved structure and method of
manufacturing a sealed lead-acid cell which has an
extended useful life, and which is less vulnerable to
electrical shorts as a result of the cell being subjected
to extreme conditions such as over-discharge or charge
reversal.
It is another object of the present invention to
provide a sealed lead-acid cell having an electrode
assembly which may be wound automatically into a cylindrical
coil without damage to the separators.
SUMMARY OF THE INVENTION
In accordance with the above-recited objectives, the
present invention provides a new and improved rechargeable
sealed lead-acid cell which includes an inner assembly
comprising spaced-apart positive and negative plate members,
a thin porous separator member disposed between the positive
and negative plate members, and a short-inhibitor member
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1~66~37
sandwiched between the negative plate member and the
separator member. In the preferred embodiment of the
present invention, the plate-separator assembly is spiral-
wound into a coil such that a short-inhibitor member is
adjacent each of the major surfaces of the negative plate
member; a separator member is adjacent each of the short-
inhibitor members; and the positive plate member is
adjacnet one of the separator members. In accordance with
the present invention each short-inhibitor member is
formed from a fine mesh fabric, either woven or non-woven,
comprising an acid and oxidation resistant polymer fiber
such as polyester, polypropylene, etc., which may or may
not be combined with a sintered polymer filler. The
short-inhibitor member may have a thickness in the range of
approximately .0009-010 inches, preferably about ~0012
inches; a basic weight (gms/yd2) in the range of approxi-
mately 18-30, preferably, about 20; a bulk density
(gm/cm3) in the range of approximately .64-78; preferably
about .78; and a porosity in the range of approximately
40-50%. It will be noted that while the preferred location
of the short-inhibitor member, as described above, is on
the negative plate member of the cell, said short-
inhibitor member may be located anywhere in the plate-
separator sandwich. Each separator member, which may
include one or more layers, may be typically formed from
a non-woven glass micro fiber mat. It will be understood,
however, that other types of separators in common use
may also be used.
The subject plate/separator/short-inhibitor assembly
is contained in a housing which also contains the electrolyte,
typ~cally an equeous solution of sulfuric acid. The
housing of course includes a pair of external terminals
116~687 6OBA-102
each of which being connected to a positive or negative
plate member.
It will be noted that while the subject assembly has
been described above with respect to a lead acid cell, the
assembly may also be used in cells having zinc electrodes.
Brief Description of the Drawings
Figure 1 is a perspective, cross-sectional view of a
battery cell formed in accordance with the present invention.
Figure 2 is a plan view, broken away in part, of the
subject battery cell of ~igure 1.
Figure 3 is a schematic diagram illustrating the
plate-separator-short inhibitor assembly of the subject
invention prior to its being wound into a cylindrical coil.
Detailed Description of the Preferred Embodiments
-
Referring to Figures 1 and 2, the electrochemical
cell of the present invention 10 includes an inner
assembly 20 which comprises a negative plate electrode
23; first and second short-inhibitor members 22, each of
which being disposed adjacent one of the major surfaces
of negative plate electrode 23; first and second separator
members 26, each of which being disposed adjacent one of
the short-inhibitor members 22; and a positive plate electrode
25 disposed adjacent one of the separator members 26. As
illustrated in the drawings, the preferred embodiment
comprises a spirally wound assembly 20, but it will be
understood that a flat plate-separator-short inhibitor
assembly may also be employed in cells which incorporate a
starved electrolyte system. Where the subject cell is of
the lead acid type, negative plate electrode 23 and positive
plate electrode 25 are each constructed from lead metal grids
which are cut into strips. These grids are filled with the
active materials to form a negative (sponge lead) electrode
60BA-102
23 and a positive (lead dioxide) electrode 25. As indicated
above, the present invention may also be employed with zinc
electrode cells, and thus, in such an embodiment, the
specific construction of the negative and positive electrode
plates of the cell will be changed pursuant to known
practice.
In accordance with the present invention, each short-
inhibitor member 22 is formed from an inert, fine mesh
fabric, either ~oven or non-woven, comprising an acid and
oxidation resistant polymer fiber such as polyester, poly-
propylene, etc..., which may or may not be combined with a
sintered polymer filler. In the preferred embodiment of
the invention, short-inhibitor members 22 are formed from
polyester, and have a thickness in the range of approxi-
mately .0009 - C10 inches, preferably about .0012 inches,
a basic weight (gms/yd2) in the range of approximately 18
to 30, preferably, about 20; a bulk density (gm/cm ) in
the range of approximately .64 to .78, preferably about
.78; and a porosity in the range of approximately 40 to
20, 50%, preferably about 45%. In addition, it is preferable
that short-inhibitor members 22 have a length greater than
plates 23 and ~5 and separator members 26 such that
extension portions 27 on members 22 are provided (See Fig.3),
the function of these extension portions being described
below.
Separator members 26 are each formed from a thin,
highly porous insulating material which, along with short-
inhibitor members 22, completely electrically isolate
positive and negative electrode plates 25 and 23. As
mentioned above, separators 26 function as an effective
wick to retain the cell's electrolyte and keep it evenly
distributed in the working area. The porous separator 26
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and inhibitor members 22 provide a short ionic path
between plates 23 and 25 and permit rapid diffusion of the
electrolyte. Preferably, separator members 26 are formed
from a non-woven glass micro fiber mat, but other types of
separators in common use may also be employed. In
addition, as illustrated in Figures 1 and 2, each separator
26 may include one or more layers, such as, for example,
four layers.
In accordance with the invention, the electrode
assembly 20 may be spirally wound using automatic belt
winders, manual belt winders or an arbor winding technique
such as that schematically illustrated in Figure 3. More
particularly, as shown in Figure 3 (and referred to above),
it is preferable that short inhibitor members 22 have a
length greater than that of electrode plates 23 and 25,
and separator members 26 such that extension portions 27
are provided on short inhibitors 22. In winding assembly 20,
the arbor 24 pulls on inhibitor extensions 27, which in
turn, also effects the winding of the electrode plates and
separators. Because inhibitor members 22 have sufficient
tensile strength to withstand the pull forces exerted by
the arbor in winding the electrode/separator/short-
inhibitor strips, the above-described problems associated
with the tensile forces being directly applied to the
fragile separator members,which generally have a low tensile
strength, are obviated.
Referring again to Figures 1 and 2, the coiled
electrode assembly 20 of the invention is enclosed in a
cylindrical housing 15, which preferably includes an inner
case 12 formed from an electrically non-conductive material,
e~g., a chemically stable polypropylene. Housing 15 also
includes an outer metallic case 11, encasing inner case
60sA-102
12, which provides for mechanical rigidity and strength.
Typically, outer case 11 may be formed from aluminum or
steel. By providing housing 15 with inner and outer cases
12 and 11, respectively, the rate of gas diffusion through
the housing wall is minimized, and loss of water from the
electrolyte is virtually eliminated.
Housing 15 further includes an outer cover member
13 and a pair of terminals 14 and 16. Terminals 14 and 16
protrude through outer cover 13, with each terminal being
electrically connected to either positive plate 25 or
negative plate 23 within housing 15. Housing 15 may also
include a resealable safety vent mechanism 17 which
provides for the harmless venting of gasses that can be
generated under extreme operating conditions such as an
excessive overcharge rate. The electrolyte, typically an
aqueous solution of sulfuric acid, is preferably added
to the housing under pressure. The acid concentration within
a cell varies with the state of charge, the concentration
being highest when the cell is fully charged and lowest
when the cell is discharged. The amount of electrolyte used
in the cell is selected to permit efficient utilization
of the active plate materials while still preventing the
accumulation of any free electrolyre, i.e., unretained by
the electrode plates 23 and 25, separators 26 and short-
inhibitor layers 22. As indicated above, under normal use
conditions, virtually no water is lost from the electrolyte.
However, a small portion of the water may be temporarily
involved in generating gas during overcharging.
The following examples are presented to compare results
obtained in testing conventional cells against cells
containing short-inhibiting layers in accordance with the
invention.
,
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Example l
Test cells were built in accordance with the preferred
embodiment of the prPsent invention, i.e., incorporating
polyester short-inhibitor layers. Some of the test cells
were filled with electrolyte in the conventional manner,
the other test cells having the electrolyte forced into
the cell under pressure to increase its rate of entry.
After filling the cell, the pressure was relieved. Each
short-inhibitor member had a thickness in the range of
approximate]y .0009 - .010 inches, a basic weight in the
range of approximately 18 - 30 grams per square yard, a
density in the range of approximately .64 - .78 grams per
cubic centimeter, and a porosity in the range of about 40 to
50%. In addition, control cells, i.e., cells not incorporat-
ing the subject polyester short-inhibitor members were
prepared, some of said control cells being filled with
electrolyte in the conventional manner, the other control
cells having the electrolyte forced into the cell as
described above with respect to some of the test cells.
All of the test cells and control cells were then
subjected to a reversal test which comprises discharging
the freshly formed cells at a rate of 250 mA for 24 hours.
This completely discharged the cells, tending to drive into
polarity reversal. The control cells, regardless of how
the electrolyte was added, tended to develop shorts by
deposists of metallic lead and lead sulfate in the
separator with ninety percent (90%) of the control cells
developing shorts between the electrodes. None of the
test cells developed shorts. Moreover, the electrolyte was
added under pressure revealed no traces of lead sulfate in
the separators or short-inhibitor members.
Example 2
Test cells incorporating the short-inhibitor members of
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1~6~87 60BA-102
the present invention were prepared having the electrolyte
forced into the cells under pressure, the pressure being
relieved after the cells were filled. The short-inhibitor
members in each cell had the same thickness, weight,
density and porosity as the test cells of Example 1. In
addition, control cells, i.e., cells which did not in-
corporate the short-inhibitor members of the present
invention, were also prepared. The cells were cycled at
48C with each cycle comprising a charge period of 18
hours on a constant voltage of 2.45 volts, and a discharge
of 1.8 amperes to a cell cutoff voltage of 1.4 volts. With
"failure" being defined as a drop in capacity to one-half
the rated capacity of 1.8 ampere-hours at the above
discharge rate, the test cells lived, i.e., did not undergo
"failure" for a period about twice as long as the control
cells.
In summary, the present invention provides an im-
proved electrochemical cell employing short-inhibitor
members within the electrode plate/separator sandwich.
This cell extends the time for the incidence of permanent
cell failure as a result of shorting caused by deposits
of lead dentrites or tracks in the plate insulation (sep-
arators) when the cell is subjected to abusive conditions
such as overdischarge and cell reversal. In addition,
the subject cell has a significantly improved cycle life
over known cells. Further, the subject cell is very
resistant to damage to the cell separator as a result of
winding the plate-separator assembly, the relatively
strong polyester short-inhibitor being the portion of the
assembly that carries the largest portion of the tensile
forces during winding.
While there have been described herein what are at
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present considered preferred embodiments of the invention,
it will be obvious to those skilled in the art that many
modifications and changes may be made therein without
departing from the essence of the invention. It is therefore
to be understood that the exemplary embodiments are
illustrative and not restrictive of the invention, the
scope of which is defined in the appended claims, and that
all modifications that come within the meaning and range
of equivalency of the claims are intended to be included
therein.
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