Sealed lead-acid battery

Abstract: A sealed lead-acid battery is improved in manufacturability by providing a container (1) with a major side left open, and being sealed by a laminated sheet (3) heat-bonded to side edges of the opening after the battery components have been inserted. The laminated sheet (3) may include a metal layer to improve rigidity.

Description: The invention relates to an improvement in a sealed lead-acid battery suitable for use as a power supply for portable devices. A prior art lead storage battery (JP-A-61 32 347) is formed by enclosing a positive plate, a negative plate, a separator and an electrolytic solution in a battery housing made of synthetic resin. The housing is formed by joining the battery inner room to a cover portion by means of heat welding. The side and bottom surfaces of the housing is subjected to electrolytic copper plating and next nickel plating and further to chrome plating, thus a flat metal surface having a plated layer is formed. A similar lead storage battery is disclosed in EP-A-0 160 491 in which a plate assembly is enveloped in a jacket. The jacket is made of a film-formed or sheet-formed synthetic resin surrounding the plate assembly and electrolyte. The jacket is of a multi-layer laminate structure made by disposing a polyolefin film on the inside contacting the plate assembly and laminating a film of thermoplastic synthetic resin in two or more layers on the outside thereof. A further prior art storage battery is disclosed in FR-A-2 156 104 comprising a group of various positive plates and negative plates within a container which is open on one major side parallel to the plates. This open side is closed by a sealing sheet. A lithium battery is disclosed in JP-A-61 61 363 which is sealed with a multiple layer moisture prove form in order to shut outside moisture during storage. A typical example of a sealed lead-acid battery fabricated according to the prior art is described hereinafter with reference to Fig. 4. An assembled element composed of a positive plate 8, a negative plate 9 and a separator 10 is accommodated in a container 11 in box form molded from ABS or other resins. The container 11 is sealed by bonding a cover 12 having a vent hole 6, terminal channels 15, etc. to the open end of the container. The battery shown in Fig. 4 is also furnished with a safety valve holder 14. In order to fabricate a sealed lead-acid battery having the construction described above, the assembled element has to be pressed into the opening in the container, but this causes great difficulty when the element is elongated or flat. Furthermore, the friction between each of the electrode plates and the inner surface of the container will cause shedding or cracking of the active materials (the powders of lead oxide and spongy lead) or other potential risks that lead to shorting or shorter life of the battery. The conventional practice of bonding the cover to the battery container with adhesives typically based on urethane or epoxy resins causes another problem in that batteries in process have to stay on the production line until the adhesive cures to a satisfactory degree. It is the object of the invention to provide a sealed lead-acid battery having a container eliminating or simplifying the conventionally required outer reinforcing case. This object is solved by the features as claimed in claims 1 or 2. The present invention offers the following advantages over the prior art in terms of battery performance and manufacturability. (1) An assembled element can be readily accommodated in the container and this reduces not only the number of steps involved in assembly but also the chance of the active materials (the powders of lead oxide and spongy lead) shedding or cracking. (2) The battery is sealed by fusing a sealing sheet to the container. In comparison with the prior art in which a cover is bonded to an ABS resin molded container with an adhesive, both the processing time and the residence time of product on the production line is shortened. (3) Part of the container is replaced by a thin-walled member (i.e., sheet) and the area conventionally required for bonding the container and the cover with an adhesive is eliminated. As a result, the proportion of the battery's inner space occupied by the volume of the assembled element is increased, leading to higher performance of the battery. (4) By laminating two or more layers selected from among films having low moisture and oxygen gas permeabilitites, films having high mechanical strength and films having good heat bondability, a sheet having the advantages of each of the respective films can be obtained. Compared with the conventional container made of a single resin, this laminated sheet is thin, has higher performance and yet contributes to an improvement in the production rate of batteries. (5) The sealing sheet is thin-walled and lacks rigidity but on the other hand, it has good sealing properties and ensures good protection against potential leakage of sulfuric acid electrolyte. Therefore, it can be used in combination with an outer reinforcing case made of steel or other metals that exhibit sufficient strength and rigidity even if they are not thick. (6) Two or more layered synthetic resin films may be bonded to a sheet of a metal such as iron, stainless steel or aluminium so as to make laminated metal sheet, one side of which has good bondability to the container. This laminated metal sheet exhibits not only the sulfuric acid resistance and moisture and oxygen permeabilities of the synthetic resin film but also the strength and rigidity of the metal sheet. Therefore, by using such a laminated metal sheet as a container sealing member, the conventionally required outer reinforcing case can be eliminated or simplified. In a preferred embodiment, the heat-bonded sheet applied to close the open side of the container is a laminated film of two or more synthetic resin layers. In another preferred embodiment, the sealing sheet is a combination of a metal sheet bonded to a laminated film of two or more synthetic resin layers. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a perspective view of the container of a sealed lead-acid battery according to one embodiment of the present invention; Fig. 2 is a cross section of the battery before the container is sealed; Fig. 3 is a cross section of the battery with its container sealed by fusing a sealing sheet; Fig. 4 is a perspective view showing the construction of a prior art sealed lead-acid battery; and Fig. 5 is a graph comparing the performance of the prior art sealed lead-acid battery with that of two prototypes of the battery of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention is described hereinafter with reference to Figs. 1-3. Shown by numeral 1 in these figures is a battery container molded from a thermoplastic synthetic resin such as polypropylene resin and which is open on one major side (usually a surface parallel to the electrode plates). This container is provided with terminal channels 4 and 5 and a vent hole 6 at one end. An assembled element consisting of a positive plate 8, a negative plate 9 and a separator 10 is accommodated in the recess 2 in the container 1. The positive and negative plate lugs are passed through a through hole 7 and soldered to current pickup terminals (not shown) in the form of a bar (or strip) or lead wire. Thereafter, the terminal channels 4 and 5 are filled with an epoxy resin or other suitable sealing resins so as to fix the electrode plates and prevent leakage of the electrolyte from the container. The vent hole 6 is furnished with a safety valve. Fig. 2 is a cross section of the container having the assembled element accommodated therein, and Fig. 3 is a cross section of the same container having its open side closed by heat bonding of a sealing sheet. Fig. 5 shows the results of a performance test conducted on two prototypes of the sealed lead-acid battery of the present invention and a prior art battery having essentially the same dimensions and which has the geometry shown in Fig. 4. In Fig. 5, curve A refers to the data for the prior art battery employing an ABS resin container having a wall thickness of 2.5 mm. Curves B and C refer to the two prototypes of the battery of the present invention, which employed a polypropylene resin molded container having a wall thickness of about 1 mm. The container of battery B was sealed with a laminated film consisting of polyethylene terephthalate (PET, 12µm), polyvinylidene chloride (PVDC, 2µm) and polypropylene (CPP, 40µm). The container of battery C was sealed with a laminated film consisting of polyethylene terephthalate (PET, 12µm) and polypropylene (CPP, 40µm). The first laminated film had a water vapor permeability of 7 g/m² per day and an oxygen gas permeability of 10 cc/m² per day-atmosphere 10 cc/m² per day-bar, whereas the second laminated film had a water vapor permeability of 20 g/m² per day and an oxygen gas permeability of 130 cc/m² per day-atmosphere. 130 cc/m² per day-bar. Judging from the data shown in Fig. 4, the prototype referred to by curve C was not capable of maintaining the intended performance until the end of its life. It was therefore concluded that the sealing sheet to be used in the present invention should desirably have a water vapor permeability of not more than 10 g/m² per day and an O₂ gas permeability of not more than 20 cc/m² per day atmosphere (20 cc/m² per day bar). Such a sealing sheet is thin-walled and lacks rigidity and strength, and can be easily torn or damaged with a sharp-edged metal tool or the like. In addition, it is vulnerable to external shocks such as drops that will damage the assembled element in the container. Therefore, the sealed lead-acid batteries usually require outer protection such as metal cases when they are put to normal use. However, if the combination of a synthetic resin layer laminated to a metal sheet is used as a container sealing member, such outer protecting cases can be eliminated or simplified since such a laminated metal sheet exhibits both the sulfuric acid resistance and moisture and oxygen barrier properties of the synthetic resin layer and the strength and rigidity of the metal. The synthetic resin layer in the laminated metal sheet is required to have heat bondability to the container. However, in the presence of a metal layer, it is difficult to control the state of fusion between the container and the synthetic resin layer, and sulfuric acid used as an electrolyte can penetrate through the resin layer to reach the metal sheet if it is insufficiently fused to the container. It is therefore necessary to provide between the fused layer and the metal layer an intermediate layer that is not only resistant to sulfuric acid but also stable when subjected the heat applied during the fusion. To meet this requirement, the synthetic resin layer must consist of at least two layers. The metal sheet may be made of a material such as iron, stainless steel or aluminum. The sealing member may be fused to the container by a variety of methods including heat sealing using a heated plate, impulse sealing, and ultrasonic sealing.

Claims: A sealed lead-acid battery, comprising: an assembled battery element comprised of a positive plate (8), a negative plate (9) and a separator (10); a container (1) molded from a thermoplastic synthetic resin and open on one major side parallel to said plates (8,9); for accommodating said assembled element; and means for sealing said container (1) by closing its open side comprising a heat-bondable sealing sheet (3), wherein said sealing sheet (3) is a laminated metal sheet having at least two synthetic resin films being bonded to an inner surface of a sheet of metal, with the innermost film of said laminated sheet being heat bondable to the container (1). A sealed lead-acid battery comprising: An assembled battery element comprised of a positive plate (8), a negative plate (9) and a separator (10); a container (1) molded from a thermoplastic synthetic resin and open on one major side parallel to said plates (8,9) for accommodating said assembled element; and means for sealing said container (1) by closing its open side comprising a heat-bondable sealing sheet (3), wherein said sealing sheet (3) is a laminated film of at least two synthetic resin layers which exhibits a moisture permeability of no more than 10 g/m²-day and an oxygen gas permeability of no more than 20 cc/m²-day-bar, with one side of said film being heat-bondable to the container (1). A sealed lead-acid battery according to claim 2, wherein said laminated film is comprised of layers of polyethylene terephthalate, polyvinylidene chloride and polypropylene. A sealed lead-acid battery according to claim 1, wherein said metal sheet (3) is comprised of one of aluminium, iron and stainless steel. A sealed lead-acid battery according to claim 1, wherein said laminated metal sheet (3) comprises at least a first synthetic resin film which is heat-bondable to the container, and a second synthetic resin film which is resistant to sulfuric acid and stable when subjected to the heat of heat bonding and is disposed intermediate said first synthetic resin film and said sheet of metal. A sealed lead-acid battery according to claim 1, wherein said container (1) includes a vent (6) and terminal channels (4,5) located at an end thereof. A sealed lead-acid battery according to claim 6, wherein said sealing sheet extends over the assembled element accommodated in said container (1), but not over said terminal channels (4,5) located at said end.

Inventors: FUJIMOTO NAOKI C O JAPAN STORA, KANO SEIZO C O JAPAN STORAGE B, FUJIMOTO, NAOKI C/O JAPAN STORAGE BATTERY COMP., KANO, SEIZO C/O JAPAN STORAGE BATTERY COMP.
Applicants: JAPAN STORAGE BATTERY CO LTD, JAPAN STORAGE BATTERY COMPANY LIMITED
Application year: 1988
Main classification: H01M2
IPC codes: H01M2, H01M10