Releasable pressure vents

A resealable pressure vent for a container such as a battery container having a wall containing an aperture connecting the interior of the container with the atmosphere comprising a cover member in the form of a metal plate which is spot welded at its center to the center of the wall of the container. The aperture is located in the wall away from the center thereof and an elastic O-ring is interposed between the metal plate and container wall such that the O-ring seals the aperture from the exterior of the container. In the event of excess pressure within the container, the O-ring is displaced by the said excess pressure to vent the container and the O-ring returns to its original sealing position when the excess pressure is relieved.

BACKGROUND OF THE INVENTION 
This invention relates to a resealable pressure vent for a container and 
more particularly for venting a battery container. 
In the manufacture of rechargeable batteries, especially nickel cadmium 
cells, a safety vent is generally provided to release any abnormal 
pressure which might develop in the cell during charging or abuse of the 
battery. 
Vents for battery cells are known and fall into two main categories, 
"one-time" or "resealable". The "one-time" vent is of simple design and 
comprises weakened areas in the battery casing, when the internal gas 
pressure in the battery reaches a predetermined venting pressure, the 
weakened area is punctured thereby protecting the battery from exploding. 
The main disadvantage of the "one-time" design is that once the vent is 
punctured, the interior of the battery becomes exposed to the atmosphere 
and the battery quickly dries up or fails because of carbonation. Of the 
two, the preferred safety vent is the "resealable" design. 
The "resealable" vent is generally provided with an elastic element such as 
a spring or a rubber ball which will release the excess pressure inside 
the battery. Furthermore, after the excess pressure is released, the 
elastic element recovers its original state or form and reseals the 
battery. The battery continues to function although possibly at a slightly 
deteriorated rating. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a resealable pressure 
vent for containers which overcomes or mitigates the above-mentioned 
disadvantages of "one-time" vents. 
According to the present invention, there is provided a resealable pressure 
vent for a pressurized container having a wall containing an aperture 
connecting the interior of the container with the atmosphere, said 
releasable pressure vent comprising a cover member adapted to close the 
aperture and comprising a metal plate spot welded at its center to said 
wall of the casing and an elastic O-ring located between the wall of the 
container and the cover member such that the O-ring encircles the spot 
welded center of the metal plate and the aperture, the O-ring serving to 
seal the aperture from the exterior of the container and being 
displaceable by an excess of pressure in the container to vent the 
interior before returning to its sealing position when the excess pressure 
is relieved. 
According to one embodiment of the present invention the resealable 
pressure vent is incorporated in an electric battery container. 
According to a further preferred embodiment of the present invention the 
resealable pressure vent is incorporated in the container of a nickel 
cadmium battery cell. 
Preferably, the metal cover plate is located in a recess in the container 
wall such that it lies flush with the container wall exterior. 
In a preferred construction the resilient O-ring is made from Butyl rubber 
with a hardness of 60 to 75 shore A and has a cross-sectional diameter in 
the uncompressed condition of 1 mm. 
The cover member for the vent is preferably made of 0.30 mm nickel plated 
steel with a gap of 0.5 mm between the container wall and the cover 
member. 
The venting pressure is ideally from 15 to 16 Kg/cm.sup.2.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The nickel cadmium battery cell shown in FIG. 1 comprises a cylindrical 
casing 1 having an end wall 2 with a central vent hole 3 and a cathode 
cell cap 4. The peripheral edges of the end wall and cap are crimped by 
the end of the cylindrical casing with the interposition of an annular 
insulating washer 5. The vent hole 3 is sealed by an elastic rubber member 
6. 
FIG. 2 shows another prior art construction similar to FIG. 1 where like 
parts have the same reference numerals. The cathode cell cap 4 is welded 
to the end wall 2 instead of being crimped by the casing 1 and the annular 
insulating washer 5. 
The prior art sealable pressure vent cell construction shown in FIG. 3 has 
a similar construction to that shown in FIG. 1 where like parts have the 
same reference numerals. The end wall 2 is dished to receive a 
displaceable plate 7 with a locating member 8 for a coil spring 9 covered 
with a plastic coating. 
The above three embodiments all share one common factor in that the vent 
hole is directly covered by an elastic element which affects the vent 
pressure and is dependent on the size of the vent hole. This type of vent 
arrangement generally requires a quite sizeable volume and a high profile 
so as to allow for the compression of the elastic element. The size and 
surface finish of the vent hole is very critical for reproducible results 
in mass production. 
The prior art construction shown in FIG. 4 illustrates a typical nickel 
cadmium button cell. These are electrochemical cells which have a large 
diameter to height ratio. Their external shape has the appearance of coins 
and they are sometimes referred to as coin cells. The button cell shown in 
FIG. 4 comprises a metal cup 10 which serves as the anode and a metal cap 
11 which serves as the cathode separated by an insulator 12 of plastic 
material which is compressed to seal the battery against leakage. The cell 
contains a screen wrapped positive electrode 13 and a screen wrapped 
negative electrode 14 separated by an absorber layer 15 and surrounded by 
a gasket 16. The cathode metal cap 11 has a central depression 17 which 
locates a contact spring 18 which makes an electrical connection with the 
negative cell electrode. 
Although simple in construction and easy to mass produce, hermetically 
sealed cells such as these tend to burst open explosively when they are 
charged at an excessively high rate, overcharged at excessively high rate, 
prolonged reverse-charge at a high rate or disposed of in a fire. Such 
disadvantages limit the popularity of these button cells. 
To overcome these disadvantages a "one-time" seal is also currently used as 
illustrated in FIGS. 5a to 5b. The anode plate 19 is formed with a heavily 
indented "+" depression 20 at the base of the cell cup. This heavily 
indented area reduces the metal thickness to about 20% of the nominal 
thickness of the cup. As the internal pressure builds up, the bottom of 
the cell cup swells and cracks open at the weakened area, thereby 
releasing the excess pressure. 
This "one-time" vent, although simple in concept is very difficult to 
implement, for example, if the indent is made slightly too deep, the vent 
could prematurely operate during normal usage, rendering a new battery 
unserviceable. If the indent is too shallow because of tool wear, the vent 
might not operate properly resulting in a dangerous condition. 
One embodiment of an improved resealable pressure vent according to the 
present invention is illustrated in FIG. 6 which shows a cross-section of 
a cathode cell cap 21 formed with a central depression 22 and a vent hole 
23 off-set from the center of the depression. Spot welded to the center of 
the depression 22 is a cover plate 24 covering the vent. To seal the vent 
an elastic O-ring 25 is compressed between the cover plate 24 and the 
depression 22 in the cathode cell cap 21. The O-ring encompasses the vent 
hole 23 to seal the vent from the atmosphere. 
The size of the vent hole 23 is not very significant as it does not affect 
the vent pressure. 
The dimension of the O-ring is chosen so as to be in a state of compression 
when the vent cover plate 24 is welded in place. A reduction of 30 to 60% 
in cross-sectional diameter of the O-ring is considered sufficient and the 
diameter is chosen to allow for easy welding of the vent cover plate and 
to accommodate the placement of the vent hole. 
In operation when the pressure in the cell increases, this increase is 
transmitted to the O-ring 25 through the vent hole 23. The diameter of the 
O-ring increases in response to increasing pressure, which has the effect 
of reducing the cross-sectional diameter of the rubber O-ring. When the 
cross-sectional diameter of the O-ring becomes reduced to the point where 
it can no longer accommodate the cell pressure the excess pressure is 
vented, whereupon the O-ring seal snaps back to its original dimension 
resealing the vent. By choosing different hardness of rubber, different 
cross-sectional diameter, different sizes of O-ring as well as different 
dimensions of the vent cover plate, the cell venting pressure can be 
varied as desired. 
In the releasable pressure vent for the nickel cadmium button cell the 
O-ring is ideally made of Butyl rubber with a hardness of 60 to 75 shore A 
and a cross-sectional diameter of 1 mm. The vent cover plate is suitably 
made of 0.30 mm nickel plated steel with the gap between the cell cap and 
the cover plate of 0.5 mm to compress the O-ring. The resulting pressure 
vents at a relief pressure of 15 to 16 Kg/cm.sup.2. 
In the alternative embodiment shown in FIG. 7, the resealable pressure vent 
according to the invention is similar to that shown in FIG. 6 and like 
parts have the same reference numerals. This construction differs in that 
the cover plate 24 is a flat disc flush with the upper wall of the cell 
cap 21. The cell cap depression 22 has a central upwardly extending 
depression 26 which is spot welded to the cover plate 24. As in the 
embodiment of FIG. 6 the O-ring 25 encompasses the vent hole 23 to seal 
the vent from the atmosphere. 
Although the resealable pressure vent according to the invention has been 
described for use in the container of a battery cell, it is to be 
understood that it can be incorporated into any convenient flat surface of 
any pressurized container. 
Further, the invention is not restricted to the above described embodiment 
but variations and modifications may be made without departing from the 
spirit and scope of the invention as defined by the appended claims.