Electric double layer capacitor

An electric double layer capacitor employs a paste electrode between collector electrodes. The paste electrode is compressed under small load because the collector electrodes are pressed by compression plates in only an area corresponding to the past electrode. With the paste electrode thus compressed, the internal resistance of the capacitance is reduced and the capacitance of the capacitor is stable. The electric double layer capacitor comprises a stack of basic cells of capacitor elements. The basic cells are held closely together by a bolt which extends through central holes in the basic cells and interconnects the compression plates, so that the basic cells are compressed under a uniform and appropriate pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electric double layer capacitor which 
includes a basic cell or a stack of basic cells each having a carbon paste 
polarized electrode sealed between a pair of collector electrodes and 
divided into two layers, the polarized electrode being compressed between 
the collector electrodes from outside thereof. 
2. Description of the Prior Art 
Motor vehicles powered by internal combustion engines, such as automobiles, 
have a starter motor for starting the engine, an alternator drivable by 
the engine for generating electric energy, and a lead storage battery for 
temporarily storing the electric energy generated by the alternator and 
for supplying the stored electric energy to the starter motor when the 
engine is to be started, or to other electric devices. 
There has recently been developed a large-capacitance capacitor of the 
electric double layer type. Some such large-capacitance capacitors can be 
used as batteries. Japanese Patent Application No. 63(1988)-329846 
discloses a motor vehicle power supply device in which such a 
large-capacitance capacitor is used to start an internal combustion 
engine. 
FIG. 9 of the accompanying drawings shows an electric double layer 
capacitor having paste electrodes. The illustrated electric double layer 
capacitor is disclosed in U.S. Pat. No. 3,536,936. The electric double 
layer capacitor comprises a single basic cell composed of a pair of 
current collectors 1 made of an electron conductor and which serves as a 
pair of collector electrodes, a pair of carbon electrodes 2 made of active 
carbon particles, a pair of nonconductive gaskets 3, and an isolating 
plate 4 for preventing electrons from moving between the electrodes 2. 
The carbon electrodes 2 are made from a concentrated slurry which is a 
mixture of powdery or particulate active carbon and an electrolyte. The 
electrolyte has three functions to perform. It serves as an accelerator 
for ion conduction, an ion source, and a binder for carbon particles. 
In order for an electric double layer capacitor to be used as a motor 
vehicle power supply, it has to have a large capacitance ranging from 100 
to 150 F (farads), for example. If the necessary capacitance is to be 
achieved with an increased number of basic cells, then the capacitor 
becomes too heavy and bulky to be carried on the motor vehicle. It is 
necessary to increase the size of a basic cell in order to increase the 
energy density, i.e., the capacitance per unit volume or the capacitance 
per unit weight. 
If the size of a basic cell is increased, the surface areas of the 
collector electrodes are also increased. Should different pressures be 
applied to the central and peripheral regions of the basic cell of 
increased size, a differential pressure between these different regions is 
developed. For an electric double layer capacitor to have stable 
performance as a power supply, it is necessary that the collector 
electrodes of the stacked basic cells be held in contact under uniform and 
sufficient adhesive forces. 
The internal resistance of an electric double layer capacitor used as a 
motor vehicle power supply should be as low as possible. The internal 
resistance of an electric double layer capacitor is greatly affected by 
the contact resistance of active carbon of the polarized electrodes and 
the contact resistance between the collector electrodes and the polarized 
electrodes. Therefore, each basic cell should be kept under vertical 
pressure in order to reduce the internal resistance of the electric double 
layer capacitor. Conventional electric double layer capacitors require 
each cell to be kept under a pressure of about 100 kg/cm.sup.2, though it 
depends on the size of the electrodes, the size of the particles of the 
carbon material, or the kind of the electrolyte used. 
In ordinary electric double layer capacitors, the cells are kept under 
pressure by staking in order to reduce the internal resistance thereof. A 
self-supporting capacitor disclosed in Japanese Laid-Open Patent 
Publication No. 56(1981)-2621, for example, has an encased structure as 
shown in FIG. 10 of the accompanying drawings. More specifically, the 
self-supporting capacitor has an outer case 5 housing a first electrode 
plate 7 from which a first electrode terminal 6 extends vertically 
upwardly and a second electrode plate 10 from which a second electrode 
terminal 9 extends vertically upwardly, the second electrode plate 10 
lying below the first electrode plate 7 with an insulating plate 8 
interposed therebetween. With a certain number of basic cells 12 
compressed by a reinforcing plate 11 and accommodated in the outer case 5, 
the upper peripheral edge of the outer case 5 is bent inwardly down 
against the first electrode plate 7 by staking near the electrode 
terminals 6, 9. 
The electric double layer capacitor shown in FIG. 10 requires each cell to 
be kept under a pressure of ranging from 10 to 30 kg/cm.sup.2, though it 
depends on the size of the electrodes, the size of the particles of the 
carbon material, or the kind of the electrolyte used. If the size of an 
electric double layer capacitor is so large that its surface area is 100 
cm.sup.2 or more, then it may be kept under a pressure of several tons or 
higher. 
When an electric double layer capacitor is compressed by staking, the 
pressure is also imposed on the gasket of each of the cells of the 
capacitor. Therefore, if the pressure applied to a electric double layer 
capacitor is to be increased, the thickness of the outer case has to be 
increased or the rigidity of the capacitor has to be increased by other 
methods. As a result, the prior electric double layer capacitors cannot be 
large because there is certain limitation on the overall weight and the 
cost of materials to be used. Furthermore, there are required a device for 
compressing the cells and also a device for retaining the cells by 
staking, and the process of applying a suitable pressure to the cells is 
complex. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electric double 
layer capacitor which includes a plurality of basic cells or capacitor 
elements that can easily be compressed. 
Another object of the present invention is to provide an electric double 
layer capacitor in which a stack of basic cells or capacitor elements are 
compressed uniformly under a pressure that can easily be regulated, and 
the internal resistance can be reduced with a relatively small force that 
is effectively utilized. 
Still another object of the present invention is to provide an electric 
double layer capacitor element or cell that may easily be increased in 
size, and a method of manufacturing such an electric double layer cell. 
According to the present invention, there is provided an electric double 
layer capacitor comprising at least one basic cell which comprises a pair 
of collector electrodes, the collector electrodes having respective 
through holes, a polarized electrode having a carbon paste sealed between 
the collector electrodes, and a separator by which the polarized electrode 
is divided into two layers, a pair of plates disposed on opposite surfaces 
of the collector electrodes, respectively, and fastening means 
interconnecting the plates through the through holes, for thereby 
compressing the basic cell. 
According to one aspect of the invention, the electric double layer 
comprises a plurality of basic cells stacked together, the plates being 
disposed on opposite surfaces of the stacked basic cells and 
interconnected by the fastening means, thereby compressing the stacked 
basic cells. 
Even if the basic cells are large in size, it is possible to reduce the 
internal resistance thereof by compressing the cells under high pressure. 
The basic cells are held together by the fastening means at the center 
thereof. The collector electrodes of the basic cells are therefore held 
closely against each other, with the result the electric double layer 
capacitor has a low contact resistance. 
According to another aspect of the invention, the plates are held in 
contact with the opposite surfaces of the collector electrodes in only an 
area corresponding to the polarized electrode. 
Since the basic cells are compressed by the plates which are held in 
contact with the opposite surfaces of the collector electrodes in only an 
area corresponding to the polarized electrode, the internal resistance of 
the cells can be reduced under a small load even if the cells are large in 
size. 
Any desired capacitance can be obtained by a desired number of basic cells 
stacked together. The fastening means, typically a bolt and a nut, and a 
collar for use therewith can be varied in length depending on the number 
of basic cells used. Therefore, the electric double layer capacitor of 
desired capacitance can be manufactured with ease. 
Even if the stacked basic cells have different internal resistances, the 
total internal resistance of the electric double layer capacitor can 
easily be adjusted while measuring the internal resistance when the basic 
cells are tightened together by the fastening means. 
Since the maximum compressive distortion of the capacitor is set by the 
collar, the basic cells will not be broken when the force to tighten the 
fastening means happens to be excessive. 
The capacitor is relatively light in weight because the basic cells, even 
if they are large, have central through holes. 
The electric double layer capacitor of the present invention can be 
manufactured to have a necessary capacitance when it is used as a power 
supply in a motor vehicle. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which preferred 
embodiments of the present invention are shown by way of illustrative 
example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIGS. 1 and 2, an electric double layer capacitor 20 according 
to an embodiment of the present invention comprises a stack of basic cells 
or capacitor elements 30 each in the shape of a circular flat body and 
having a circular through hole 31 at its center. 
Compression plates 21 are disposed on the upper and lower surfaces, 
respectively, of the cell stack, the compression plates 21 doubling as 
electrode plates of the capacitor. The stack of basic cells 30 and the 
compression plates 21 are housed in a cylindrical outer case 22 which are 
held against the outer peripheral edges of the basic cells 30 and the 
compression plates 21. Each of the compression plates 21, being a plate 
for fastening the basic cells 30 in place, is made of a highly rigid 
metallic material such as stainless steel, and has a circular shape 
identical to the shape of the basic cells 30. The compression plates 21 
have respective through holes corresponding to the through holes 31 in the 
basic cells 30. A bolt 23 extends through the holes 31 in the basic cells 
30 and also the corresponding holes in the compression plates 21. The head 
of the bolt 23 is held against one of the compression plates 21, whereas a 
nut 24 is threaded over the tip end of the bolt 23 and held against the 
other compression plate 21, so that the compression plates 21 are pressed 
toward each other, holding the basic cells 30 in position therebetween. A 
collar composed of a pair of collar members 25 made of resin or the like 
is inserted into the holes 31 from the opposite sides of the capacitor. 
The collar members 25 electrically isolate the outer peripheral surface of 
the bolt 23 from the inner peripheral surfaces of the basic cells 30 and 
the compression plates 21. 
Each of the basic cells 30 comprises a polarized electrode 33 made of 
carbon paste which is separated into two layers by a separator 32, and a 
pair of collector electrodes 34 between which the polarized electrode 33 
is axially sealed. The polarized electrode 33 is also axially sealed by 
annular gaskets 35, 36 disposed immediately around the hole 31 and along 
the outer peripheral edges of the collector electrodes 34. The annular 
gasket 35 which seals the polarized electrode 33 serves to define the 
circular hole 31 therein, a feature which is different from the 
conventional electric double layer capacitor cell or element shown in FIG. 
9. 
The stacked basic cells 30 can easily be compressed when the compression 
plates 21 and the basic cells 30 are tightened together by the nut 24 on 
the bolt 23 which extends through the compression plates 21 and the basic 
cells 30. 
FIG. 3 shows a junction between the collar members 25 with a gap or 
clearance .DELTA.l left across the junction. More specifically, the collar 
members 25 are separate from each other in the longitudinal direction of 
the bolt 23 at an intermediate portion of the bolt 23. The clearance 
.DELTA.l between the confronting ends of the collar members 25 corresponds 
to the maximum compressive distortion or deformation to which the stacked 
basic cells 30 can be subjected. When the capacitor is assembled, the nut 
24 is tightened on the bolt 23 to compress the basic cells 30 to the 
extent that is allowed by the clearance .DELTA.l without destruction under 
compression. In this manner, the internal resistance of the capacitor can 
easily be adjusted. 
FIG. 4 shows an electric double layer capacitor according to another 
embodiment of the present invention. 
According to the embodiment shown in FIG. 4, a stack of basic cells 30 is 
housed in an outer case 42 of resin, and the outer case 42 has a central 
partition 42a which is positioned between two groups of the basic cells 
30. The basic cells 30 are compressed against the partition 42a by upper 
and lower compression plates 41 disposed on the opposite sides of the 
stack of basic cells 30. Conductive plates 43 from which leads extends 
outwardly are disposed between the compression plates 41 and the basic 
cells 30. The compression plates 41 are interconnected by a bolt 44 
extending through the basic cells 30, with a nut 45 tightened over the 
bolt 44. The bolt 44 and the nut 45 are insulated when they are encased in 
molded resin or applied tar, after the capacitor has been assembled. 
The leads extending from the respective conductive plates 41 are of the 
same polarity. The bolt 44 is electrically connected to the basic cells 30 
in an intermediate region of the outer case 42, and serves as an electrode 
lead of the opposite polarity. In use, a plurality of electric double 
layer capacitors may be connected in series with or parallel to each other 
by outer cables. 
The basic cells 30 which are compressed from their opposite sides toward 
the central partition 42a of the outer case 42 are held more closely 
together in their stack than they are in the stack shown in FIG. 1. If the 
basic cells 30 are larger in size and their collector electrodes have 
larger surface areas, the capacitance per unit weight can be increased by 
enlarging the central holes 31 of the basic cells 30. The contact 
resistance between the collector electrodes is small because any 
differential pressure thereon is minimized when the nut 45 is tightened on 
the bolt 44. 
FIGS. 5(a) and 5(b) show a modification of the compression plates of the 
electric double layer capacitor shown in FIG. 4. A modified compression 
plate 51 has a central area thicker than its peripheral area. The 
compression plate 51 with such a raised central area allows the basic 
cells to be compressed more uniformly. 
FIGS. 6(a) through 6(e) illustrate successive steps of manufacturing the 
basic cell or capacitor element 30 shown in FIG. 2. 
First, gaskets 36, 35 are bonded to outer peripheral and central portions 
of a disc which will serve as a collector electrode 34, as shown in FIG. 
6(a). The gaskets 36, 35 should preferably be made of hard rubber. Then, a 
carbon paste which will serve as a polarized electrode 33 is filled in the 
groove or space between the gaskets 35, 36, as shown in FIG. 6(b). The 
polarized electrode 33 is sealed by an ion-permeable separator 32, as 
shown in FIG. 6(c). The assembly, and another identical assembly which is 
prepared in the same manner as described above, except that no separator 
is provided, are joined together with the separator 32 interposed 
therebetween, thereby producing a cell blank, as shown in FIG. 6(d). 
Thereafter, a hole 31 is punched centrally through the collector 
electrodes 34 and the gasket 35, as shown in FIG. 6(e), whereupon a basic 
cell 30 is completed. 
FIG. 8 shows how the compressive displacement and the resistance of an 
electric double layer capacitor having paste electrodes, as shown in FIG. 
9, vary with the load that is applied to the collector electrodes all over 
their surface. 
Each basic cell of the capacitor to which the load F is applied has a 
thickness of 3.5 mm. The collector electrodes of the basic cell have a 
surface area of 300 cm.sup.2, i.e., are of a rectangular shape which is 20 
cm long and 15 cm wide. Values shown in FIG. 8 were obtained when the load 
F is applied to the capacitor which comprises a stack of fifteen basic 
cells of such dimensions. The compressive displacement of the capacitor, 
which is indicated by the solid-line curve, linearly increased as the load 
F increased up to about 2,400 kg at a point P. After the applied load F 
increased beyond 2,400 kg, the compressive displacement increased at a 
lower rate. The resistance of the capacitor, which is indicated by the 
broken-line curve, was at minimum when the load F was 3,000 kg, which is 
about 600 kg greater than at the point P, and remained substantially 
constant after the load further increased. 
It should be noted that the experimental results shown in FIG. 8 were 
obtained when the load was applied also to the gaskets of the basic cells. 
Generally, the gaskets are made of a nonconductive material such as 
elastic hard rubber, and the paste electrodes which are sealed by the 
gaskets are in the form of a mixture of particulate or fibrous active 
carbon and a solvent such as sulfuric acid. The gaskets and the paste 
electrodes have different moduli of elasticity. Therefore, the load F is 
first borne by the gaskets, and then applied to the paste electrodes, 
which are porous and have a smaller bulk density, essentially after the 
point P. 
The results shown FIG. 8 indicate that the load of 3,000 kg or more is 
required to compress the entire basic cell including the gaskets, as is 
the case with the conventional electric double layer capacitor, and only 
the load of 600 kg, which is one fifth of 3,000 kg, is required to 
compress only the paste electrodes. 
FIG. 7 shows an electric double layer capacitor according to still another 
embodiment of the present invention. Those parts shown in FIG. 7 which are 
identical to those shown in FIG. 1 are designated by identical reference 
numerals. 
As with the electric double layer capacitor 20 shown in FIG. 1, an electric 
double layer capacitor 20 shown in FIG. 7 comprises a stack of basic cells 
or capacitor elements 30 each in the shape of a circular flat body and 
having a circular through hole 31 at its center. Each of the basic cells 
30 comprises a polarized electrode 33 of carbon paste which is separated 
into two layers by a separator 32, and a pair of collector electrodes 34 
between which the polarized electrode 33 is axially sealed. The polarized 
electrode 33 is also axially sealed by annular gaskets 35, 36 disposed 
immediately around the hole 31 and along the outer peripheral edges of the 
collector electrodes 34. 
Compression plates 21' are disposed on the upper and lower surfaces, 
respectively, of the cell stack, the compression plates 21' doubling as 
electrode plates of the capacitor. Each of the compression plates 21', 
being a plate for fastening the basic cells 30 in place, is made of a 
highly rigid metallic material such as stainless steel, and has a circular 
shape large enough to compress the polarized electrode 33 of each basic 
cell 30. Each compression plate 21' has a raised central area and is held 
in contact with one of the collector electrodes 34 of one of the basic 
cells 30. The raised central areas of the compression plates 21' have 
respective through holes corresponding to the through holes 31 in the 
basic cells 30. Collars 22 are fitted in the holes in the compression 
plates 21' and a collar 25 is fitted in the holes 31 in the basic cells 
30. A bolt 23 extends through the collars 22, 25, and has a head held 
against one of the compression plates 21', whereas a nut 24 is threaded 
over the tip end of the bolt 23 and held against the other compression 
plate 21', so that the compression plates 21' are pressed toward each 
other, holding the basic cells 30 in position therebetween. 
The stacked basic cells 30 can easily be compressed when the compression 
plates 21' and the basic cells 30 are tightened together by the nut 24 on 
the bolt 23 which extends through the compression plates 21' and the basic 
cells 30. The compression plates 21' do not apply a load to the gaskets 
35, 36 of the basic cells 30, but compress only the polarized electrodes 
33 of the basic cells 30. Therefore, the internal resistance of the 
capacitor is reduced under a small load, while the basic cells 30 are held 
together stably. 
Although certain preferred embodiments have been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.