Alkaline dry cell containing a can of nickel-plated steel having an inner surface of graphite

An object of the present invention is to improve the alkaline dry cell performance by modifying a positive cell case. An alkaline dry cell comprises a can as both of a terminal and a cell case 1 for accommodating a power generating element, prepared by drawing processing of a cold rolled steel material preliminarily applied with nickel-plating on both sides so as to form a can, having a conductive film formed in an inner surface portion contacting with a positive electrode compound, wherein the area of the portion with the conductive film is in the range of 75% to 90% with respect to the area of the whole portion contacting with the positive electrode compound in the inner surface. Since the conductive film is provided, the increase of the contact resistance can be prevented so as to improve the short-circuit current and the storage property. With less than 75% conductive film area, the above-mentioned effect cannot be achieved sufficiently. On the other hand, with more than 90% area, the filler compound can be separated in the dry cell production. Therefore, the above range is appropriate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an alkaline dry cell having an excellent 
performance, suitable for the recent super heavy load or a heavy load 
application. 
2. Description of the Related Art 
Recently, super heavy load discharge or a heavy load application, such as 
portable AV devices including personal digital assistants (PDA), CD 
players, MD players and liquid crystal television sets, and portable 
telephones has been required for an alkaline dry cell. However, 
conventional positive electrode cell cases of alkaline dry cells are 
produced by drawing a steel sheet preliminarily applied with 
nickel-plating on both sides, or by drawing a steel sheet and then 
applying nickel-plating on it. 
Since the former process comprising of drawing a steel sheet preliminarily 
applied with nickel-plate generates cracks on the nickel-plating surface 
to expose the iron base, the contact resistance rises therefor The latter 
process comprising of drawing a steel plate and then applying 
nickel-plating involves a problem in that the inner surface of the cell 
case is poorly applied with nickel-plating although the outer surface is 
well applied with the nickel-plating. For example, when nickel-plating is 
applied on the outer surface by a 3 .mu.m thickness, only about 0.5 .mu.m 
thickness of nickel-plating is applied on the inner surface. Therefore, 
due to the poor contact of the inner surface with a positive electrode 
compound, the contact resistance is raised, as a result the short-circuit 
current decreases and the performance of an alkaline dry cell falls after 
a long term storage, furthermore generation of hydrogen gas increases. 
SUMMARY OF THE INVENTION 
Accordingly, one object of the present invention is to provide an alkaline 
dry cell suitable for the above-mentioned recent super heavy load or a 
heavy load application by improving the positive electrode cell case of an 
alkaline dry cell to obtain the high performance of a dry cell. 
That is, the present invention relates to an alkaline dry cell having a 
cell case (acting also as a terminal) for accommodating a power generating 
element which is prepared by drawing a cold rolled steel material 
preliminarily applied with nickel-plating on both sides so as to form a 
can and have a conductive film in its inner surface portion contacting 
with a positive electrode compound, wherein the portion with the 
conductive film is formed in the 75% to 90% area with respect to the area 
of the whole portion contacting with the positive electrode compound in 
the inner surface portion. 
As mentioned above, since the conductive film is applied in the inner 
surface of a can to be used as a terminal and cell case, a layer with an 
excellent conductivity can be obtained. Further, since the exposed iron 
base is covered by the conductive film even if cracks are generated on the 
surface at the time of drawing processing, a bad influence of cracks can 
be solved. Therefore, in an alkaline dry cell of the present invention, 
increase of the contact resistance and decrease of the short-circuit 
current can be prevented. If the above-mentioned conductive film formation 
area is less than 75%, the above-mentioned object cannot be achieved 
sufficiently, on the other hand, in the case of more than 90%, filler 
compounds rise up during the dry cell production process. Therefore the 
range of 75% to 90% is favorable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Examples 
A JIS standard LR 6 type alkaline dry cell shown in FIG. 1 was assembled. 
In this figure, numeral 1 represents a cylindrical metal can with a bottom 
face, which also serves as a positive electrode terminal. A positive 
electrode compound 2 cylindrically shaped with pressure was filled in the 
metal can 1. The positive electrode compound 2 was prepared by mixing 
manganese dioxide powders and carbon powders, accommodating the same in 
the metal can 1 to have a hollow cylindrical shape by applying a certain 
pressure. A gel negative electrode 4 was filled in the hollow portion of 
the positive electrode compound 2 via a cylindrical separator 3 having a 
bottom face comprising a non-woven fabric of acetalized polyvinyl alcohol 
fibers. 
A brass negative electrode collecting bar 5 was inserted in the gel 
negative electrode 4 such that the upper end part of the collecting bar 5 
projects from the gel negative electrode 4. An insulating gasket 6 
comprising a double ring shape polyamide resin was provided on the 
projecting outer periphery surface of the negative electrode collecting 
bar 5 and the upper part inner periphery surface of the metal can 1. A 
ring shape metal plate 7 was provided between the double ring shape part 
of the gasket 6. A hat-like metal sealing plate 8, which serves also as a 
negative electrode terminal was provided to the metal plate 7 such that it 
contacts with the head part of the collecting bar 5. By bending the 
opening rim of the metal can 1 inward, the inside of the metal can 1 was 
tightly sealed with the gasket 6 and the metal sealing plate 8. 
In the alkaline dry cell, the cylindrical metal can 1 having a bottom face, 
which serves also as a positive electrode terminal was produced as 
mentioned below. 
A steel plate applied with nickel-plating on both sides by 2 to 3 .mu.m was 
applied with drawing processing into a cylindrical shape with a bottom 
face. A conductive film mainly containing graphite was formed on the inner 
side of the metal can 1 except the portion contacting with the gasket. 
FIG. 2 shows a cross-sectional configuration of the metal can. In the 
figure, numeral 11 represents a steel plate, 10 and 12 a plating layer, 
and 13 a conductive film mainly containing graphite. FIG. 3 is a developed 
view of the metal can inner surface. The conductive film was applied as 
mentioned below. A conductive coating material mainly containing graphite, 
diluted with a low boiling point organic solvent such as methyl ethyl 
ketone (MEK), was sprayed to the inner surface of the metal can with a 
spray gun. At the time, the portion which is contacted with the gasket at 
the metal can opening portion was not applied with the coating. After the 
application with the conductive coating material with the spray gun, the 
solvent was evaporated with a drier. About 1 to 10 .mu.m thickness is 
preferable as the remained conductive film. The area applied with the 
conductive film was 90% with respect to the area of the whole inner 
surface of the metal can contacting with the positive electrode compound. 
Comparative Example 1 
A can was prepared by drawing a cool rolled steel plate applied with 
nickel-plating on both inner and outer surfaces but without the conductive 
film. Using this can as a positive cell case (also as a terminal), a JIS 
standard LR 6 type alkaline dry cell was assembled by the same process as 
the above-mentioned example. 
Comparative Example 2 
A can was prepared by drawing a cool rolled steel plate without 
nickel-plating and then applying nickel-plating on surfaces but without 
the conductive film. Using this can as a positive cell case (also as a 
terminal), a JIS standard LR 6 type alkaline dry cell was assembled by the 
same process as the above-mentioned example. 
The open circuit voltage (average of n=100 pieces), the short-circuit 
current (average of n=100 pieces) and the hydrogen gas amount [the gas 
inside the dry cell was collected by disassembling in water] (average of 
n=100 pieces) of the each above alkaline dry cell were examined after 
storing at 60.degree. C. for 10 days and 60 days. Results are shown in 
Table 1. 
TABLE 1 
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Open circuit 
Short-circuit 
Hydrogen gas 
voltage current amount 
60.degree. C. 
after after after after after after 
storage 10 days 60 days 10 days 60 days 10 days 60 days 
______________________________________ 
Example 1.611 1.597 14.0 13.1 0.35 0.53 
comparative 1.610 1.586 10.1 8.0 0.37 0.82 
Example 1 
comparative 1.610 1.584 9.9 7.2 0.39 1.23 
Example 2 
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Compared with Example, deterioration of the short-circuit current was more 
conspicuous in Comparative Example 1 already after 10 days storage at 
60.degree. C. Also the difference in hydrogen gas amount was conspicuous 
after 60 days storage at 60.degree. C. In the comparison of Example and 
Comparative Example 2, the difference was more conspicuous than the case 
of Comparative Example 1, so that it was confirmed the nickel-plating 
after drawing is disadvantageous. 
The difference in the dry cell performance caused by the conductive film 
area was examined. As shown in Table 2, the open circuit voltage (average 
of n=100 pieces) and the short-circuit current (average of n=100 pieces) 
at the initial stage and after storing at 60.degree. C. for 60 days, and 
the hydrogen gas amount [the gas inside the dry cell was collected by 
disassembling in water] (average of n=100 pieces) at 60.degree. C. for 10 
days and 60 days of the alkaline dry cell of the above-mentioned example 
(conductive film application area 90%) and the alkaline dry cells with the 
conductive film application area of 85%, 80%, 75%, 70%, 50%, and 30% were 
examined after storing. Results are shown in Table 2. 
TABLE 2 
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Open circuit 
Short-circuit 
Hydrogen gas 
voltage current amount 
application 
initial after initial 
after initial 
after 
area stage 60 days stage 60 days stage 60 days 
______________________________________ 
90% 1.615 1.597 14.6 13.1 0.35 0.53 
85% 1.615 1.595 14.6 12.7 0.35 0.55 
80% 1.615 1.594 14.6 12.3 0.35 0.56 
75% 1.615 1.592 14.5 11.2 0.36 0.58 
70% 1.614 1.589 14.5 10.1 0.36 0.63 
50% 1.614 1.587 14.5 9.7 0.36 0.71 
30% 1.614 1.585 14.4 8.8 0.38 0.78 
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From the results shown in table 2, it was confirmed that deterioration 
after storage becomes larger with a smaller application area of less than 
70%. Furthermore, a 70% or less application area is not appropriate since 
the open circuit voltage becomes less than 1.590 V, the short-circuit 
current less than 10 A, and the gas amount of more than 0.60 cc. 
Therefore, at least 75% application area is necessary. On the other hand, 
with a 100% application amount, the filler compound becomes separated in 
the dry cell production process. Therefore, a slight metal surface area is 
necessary and thus the application amount up to 90% is appropriate. 
As heretofore mentioned, according to an alkaline dry cell of the present 
invention, by the improvement of the positive electrode cell case, the 
increase of the contact resistance can be prevented so as to improve the 
short-circuit current. Therefore, the present invention can provide an 
alkaline dry cell having an excellent performance appropriate for the 
recent super heavy load or a heavy load application.