Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Separator – retainer – spacer or materials for use therewith
Reexamination Certificate
2001-09-07
2004-03-16
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Separator, retainer, spacer or materials for use therewith
C429S252000, C429S255000, C429S247000, C429S248000, C429S249000, C429S129000
Reexamination Certificate
active
06706450
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a separator for a valve-regulated lead acid battery. More particularly, it relates to a separator for a valve-regulated lead acid battery, which is mainly made of fine glass fibers and also includes inorganic powder and natural pulp in order to improve its property of suppressing short circuits between positive and negative electrode plates of the battery.
BACKGROUND OF THE INVENTION
A conventional separator for a valve-regulated lead acid battery has a shape of a sheet, and contains glass fibers as the main component. A conventional separator is occasionally penetrated or ripped by local pressure given from a salience on an electrode plate or by crystal growth of lead in the separator during charging thereof, which causes a short circuit between positive and negative electrode plates and interrupts electrical charge or discharge of the battery. This is because the conventional separator does not have a sufficient mechanical strength, so that it is easily penetrated or ripped by the salience on the electrode plate, and because the conventional separator has a relatively low density and contains relatively large pores to easily allow crystal growth. With a thinner conventional separator, a short circuit occurs more often because the decrease of the thickness makes the mechanical strength of the separator weaker and the positive and negative electrode plates closer.
Increasing density and decreasing pore size and/or pore volume in a separator may prevent crystal growth of lead and also prevent an electrical short circuit. It is one possible way to increase the density and decrease the pore size and/or pore volume of the separator that the separator includes particles of inorganic powder such as silica powder between the glass fibers. However, when the separator includes the inorganic powder, the relative amount of the glass fibers becomes less and interlacing of the glass fibers is reduced. As a result, the mechanical strength of the separator is decreased, so that a short circuit caused by pressure from salience of the electrode plates becomes easy to occur.
Increasing the mechanical strength of a separator can prevent another cause, penetration or ripping of the separator. Japanese patent publications S54-22531A, S56-99968A, and S58-663B describe to mix synthetic fibers with glass fibers for strengthening the separator. However, since the synthetic fiber is less hydrophilic than the glass fiber, the separator including the synthetic fibers has a drawback of lower liquid absorbency and liquid retention for sulfuric acid solution.
Japanese patent publication S64-52375A discloses a separator including beaten cellulose to improve its mechanical strength without lowering its liquid absorbency or liquid retention. However, beaten cellulose brings only a slight change in the density of the separator so that it does not prevent short circuits caused by crystal growth of lead.
OBJECT AND SUMMARY OF THE INVENTION
It is an object of the present invention to overcome disadvantages described above and to provide a separator for a valve-regulated lead acid battery which is mainly made of fine glass fibers and also includes inorganic powder and natural pulp in order to improve the property of suppressing the short circuits between the positive and negative electrode plates of the battery, and, further, which has an increased density to make the separator thinner and applicable to flat electrode plates.
A separator for a valve-regulated lead acid battery of the present invention is mainly made of fine glass fibers and includes inorganic powder and beaten natural pulp. The separator contains 5 to 30% by weight of the inorganic powder and 3 to 20% by weight of the natural pulp. The density of the separator is equal to or more than 0.165 g/cm
3
.
An electrical short circuit between positive and negative electrode plates in a valve-regulated lead acid battery occurs mostly due to the following two causes;
[1] Mechanical Short Circuit
A salience on an electrode plate (unevenness of a grid, granule of active material, and the like) gives local pressure or shearing force to a separator. When the separator is not strong enough, the salience penetrates or rips the separator and reaches the opposite plate to cause a short circuit.
[2] Electrochemical Short Circuit
At the end of electrical discharging of the battery, sulfate ions in electrolyte are consumed and the electrolyte becomes almost pure water, whereby the electrolyte is increased in its solubility of the lead ion. Consequently, a part of lead sulfate formed on the positive and negative electrode dissolves into the electrolyte. When the battery is charged after that, lead ions in the electrolyte are reduced on the negative plate and lead is deposited thereon and crystallized. The lead crystal grows into the separator until it reaches the opposite plate to cause a short circuit.
In the present invention, in order to suppress the occurrence of short circuits due to the above cause [2], inorganic powder such as silica powder is mixed into the separator to decrease the size of pores in the separator and increase the density of the separator. Furthermore, beaten natural pulp is mixed into the separator to improve the tensile strength and the durability to penetration so that short circuits due to the above cause [1] is suppressed. Since both inorganic powder and natural pulp are highly hydrophilic, the performance of the separator is not reduced.
The separator of the present invention suppresses effectively the short circuit due to the above two causes. The separator can be produced in one process, thereby lowering the production cost.
Furthermore, since the separator of the present invention has a relatively high density, it can be thinner and successfully applicable to flat electrode plates placed in a small distance from 0.3 to 0.7 mm.
In the present invention, acid resistant glass fibers having a mean fiber diameter of not greater than 1 &mgr;m are suitable for the fine glass fibers and the silica powder having a specific surface area of not less than 100 m
2
/g is suitable for the inorganic powder. Natural pulp to be used is preferably beaten to have the Canadian freeness of 250 mL or less.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, preferred embodiments of the present invention will be described in detail.
The separator for a valve-regulated lead acid battery of the present invention is mainly made of fine glass fibers, includes inorganic powder in an amount of 5 to 30% by weight and beaten natural pulp in an amount of 3 to 20% by weight, and has a density of 0.165 g/cm
3
or higher.
The fine glass fibers are preferably acid resistant glass fibers, more preferably alkali-contained glass fibers with good acid resistance and having a mean fiber diameter equal to or smaller than 1 &mgr;m. The separator includes the fine glass fibers preferably in a range from 50 to 92% by weight. Fine glass fibers with a mean fiber diameter larger than 1 &mgr;m have lower liquid retention and sheet-formability. When the separator includes the fine glass fibers in an amount of less than 50% by weight, the separator does not have a good liquid retention. More than 92% by weight of the fine glass fibers results in relatively small amounts of inorganic powder and natural pulp, whereby the separator is not fully prevented from short circuits.
When the amount of the inorganic powder is less than 5% by weight, the occurrence of short circuits is not suppressed successfully. More than 30% by weight of the inorganic powder means relatively small amounts of the fine glass fiber and the natural pulp, whereby the separator does not have enough high mechanical strength. Therefore, the amount of inorganic powder is preferably in a range from 5 to 30% by weight.
Silica, titanium dioxide and diatomaceous earth can be used as the inorganic powder. Silica powder having a specific surface area equal to 100 m
2
/g or larger is most suitable because of its high hydroph
Kanesaka & Takeuchi
Martin Angela J
Nippon Sheet Glass Co. Ltd.
Ryan Patrick
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