Patent Application
20050147873
This invention relates in general to battery cans with rupturable safety vents and methods of forming battery cans with such vents. More particularly, this invention relates to a battery can having such a vent that is formed on an interior surface of the can.
Disposable and rechargeable battery cells are utilized in a wide variety of applications to provide either a main source of power or backup power to a wide variety of devices. These cells may contain acidic or alkaline electrolytic pastes in metal cans. The cans are fabricated from thin metal sheeting, such as nickel-plated steel, aluminum and stainless steel, and have zinc and carbon, or zinc and manganese oxide or other type of electrodes. The cans are typically manufactured in standard sizes such as A, AA, AAA, C, D, etc. for applications such as toys, flashlights, portable radios, and the like. They may also be manufactured in specialty sizes or in prismatic shapes for other applications such as cameras, watches, calculators, personal digital assistants, hand-held games and the like.
The cans oftentimes incorporate a joint that is designed to fail in the event that excessive pressure is built up within the electrolyte, such pressure typically being established when a recharging operation is attempted. The incorporated failure joint provides a weakness that ruptures and compromises the structural integrity of the can when the can is subjected to excessive internal pressure.
One type of failure joint is a groove that is formed on the exterior surface of the can. Such a groove is generally stamped into the exterior surface. Stamping a groove into the can, however, because of the impact forces required and the disparate fragility of the can, oftentimes results in a vent that is not uniform in depth over its length, which may provide a vent that will rupture at a pressure that is different from the pressure at which the vent is calculated to rupture. Furthermore, because the stamping operation typically requires a high impact force, significant wear of the stamping tool may be realized over relatively short production periods, thereby resulting in substantial inconsistencies in the vents in subsequently stamped cans. Moreover, stamping a groove into the exterior surface of the can mars the outer surface. If the can is made of nickel plated steel, this causes a “nickel-break,” which may provide a foothold for corrosion of the underlying metal sheeting from which the can is fabricated. The stamped groove may cause other detrimental metallurgical defects in the case of other can materials such as aluminum or stainless steel. Inconsistency in raw material physical properties may cause variation in rupture pressures.
What is needed is a battery can in which a failure joint can be incorporated that does not allow conditions to occur that detract from the appearance of the can. What is also needed is a method for providing a failure joint in a battery can such that failure joints provided in a plurality of cans produced in a high volume manufacturing process are uniform so as to allow consistent failure characteristics to be realized across production quantities.
According to one aspect of the present invention, a battery can comprises a casing and a roll-formed vent formed preferably on an interior surface of the casing, the vent being rupturable by a condition internal to a battery cell in which the battery can is employed. In another aspect of the present invention, a method of forming a rupturable vent in a casing of a battery cell comprises positioning the casing adjacent to a tool such that a first surface of the casing faces the tool. The casing is then biased against the tool to form a groove in the first surface, the groove being configured to provide a weakness in the casing. In yet another aspect of the present invention, a method of forming a battery can comprises forming a casing and roll-forming a rupturable vent in an interior surface of the casing.
One advantage of a battery can as described above is the uniformity of the exterior surface of the can. Uniformity of the exterior surface (i.e., the lack of sharp interruptions in the surface) improves the external appearance and uniformity of the battery can, also reducing the chances that a nickel-break will form and allow corrosion to gain a foothold on the outer surface of a nickel-plated steel casing.
One advantage of the use of the roll-forming process to incorporate the vents into the battery cans is substantial uniformity of the physical attributes of the vents across production quantities. Because the physical attributes across production quantities of cans are substantially uniform, each individual battery of a particular type can be predicted to fail at substantially the same point (pressure).
Another advantage of the use of the roll-forming process is the realization of lower costs due to increased tooling life. In particular, because the roll-forming process of forming the vents is substantially less violent than the stamping operations used in prior art processes, better control of the tooling can be attained, which in turn results in extended tooling life.
Another advantage of the use of the roll-forming process is the reduction of variation in burst pressure normally caused by inconsistency in raw material physical properties. Roll-formed vents are less susceptible than stamped vents to minor fluctuations in tensile strength, elongation, hardness, and temper.
Referring to
Referring now to
The vent 14, as stated above, is preferably linear in configuration and extends along a portion of the interior surface 18 of the wall 16. The vent 14 is preferably formed by two opposingly-angled facing surfaces 20 that meet along a common edge 22 to define a V-shaped groove. The edge 22 of the groove defines a weakness in the wall 16 that, upon a buildup of a predetermined amount of pressure at the interior surface 18 of the casing, causes the wall 16 to rupture along the edge 22, thereby relieving the pressure at the interior surface 18.
The wall 16 is preferably dimensioned to be of a total thickness T. The force that causes the vent 14 to rupture along the edge 22 is determined to be a function of the maximum desired pressure at the interior surface 18 and the remaining wall thickness after subtracting the depth d of the vent 14 from the thickness T of the wall 16. The angle of the facing surfaces 20 may also be a factor in determining the amount of force that causes the vent 14 to rupture.
The preferred process by which a vent may be formed in a casing is a roll-forming process that is used after the casing is formed. In a roll-forming process, a casing (or at least a portion of a wall of a casing) is mounted such that one surface of the casing is positioned against or a short distance from a coin or similar tooling. The surface opposing the surface of the casing at which the coin or similar tooling is positioned is then engaged by a roller or similar device to bias the casing against the coin. Translatory pressure applied by the roller is sufficient to cause a deformation of the surface engaged by the coin. The opposing surface (engaged by the roller) is either minimally deformed or (preferably) not deformed at all. In another exemplary process by which a vent may be formed in a casing, the sheet material that comprises the casing may be roll-formed to include the vent prior to the working of the sheet material to form the casing.
Referring now to
In
Upon removal of the roll-formed casing 12 from the coin punch, the vent 14 is disposed in the interior surface thereof, as can be seen in
Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of the appended claims.
