Abstract:
A ventilation system for a metal-air battery is disclosed. The metal-air battery has one or more air cathodes, one or more air pathways from the air cathode to a reactive gas source, a closure member associated with each of the air pathways to selectively prevent the flow of gas through the air pathways and one or more volume-changeable plenums in fluid communication with the air pathway and the air cathode. When the battery is not in use, the closure member is closed and the door is closed to save the battery life time. When the battery is activated, the air pathway is opened and the volume of the plenum is increased to introduce fresh air from the outside to the plenum. A method of producing electricity using a metal-air battery is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The following patent applications, all of which are commonly assigned and are incorporated herein by reference, contain related subject matter and are being filed concurrently with the present application. 
     “CYLINDRICAL METAL-AIR BATTERY WITH A CYLINDRICAL PERIPHERAL AIR CATHODE”, Ser. No. 09/215,820, now U.S. Pat. No. 6,274,261; 
     “AIR MANAGER SYSTEMS FOR METAL-AIR BATTERIES UTILIZING A DIAPHRAGM OR BELLOWS”, Ser. No. 09/216026; 
     “AIR MOVER FOR A METAL-AIR BATTERY UTILIZING A VARIABLE VOLUME ENCLOSURE”, Ser. No. 09/216118; 
     “DIFFUSION CONTROLLED AIR VENT WITH AN INTERIOR FAN”, Ser. No. 09/215,879; 
     “UNIFORM SHELL FOR A METAL-AIR BATTERY”, Ser. No. 09/216,114, now U.S. Pat. No. 6,235,418; 
     “LOAD RESPONSIVE AIR DOOR FOR A METAL-AIR CELL”, Ser. No. 09/216,115; 
     “GEOMETRY CHANGE DIFFUSION TUBE FOR METAL-AIR BATTERIES”, Ser. No.  09 / 216 , 273 ; and 
     “AIR-MANAGING SYSTEM FOR METAL-AIR BATTERY USING RESEALABLE SEPTUM”, Ser. No. 09/216,343, now U.S. Pat. No. 6,168,877. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to batteries, and more particularly relates to an air delivery system with a volume-changeable plenum for a metal-air battery. 
     DESCRIPTION OF THE RELATED ART 
     Metal-air battery cells include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. During discharge of a metal-air battery, such as a zinc-air battery, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air batteries have a relatively high energy density because the cathode utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material, such as a metal or metallic composition. Metal-air battery cells are often arranged in multiple cell packs within a common housing to provide a sufficient amount of power output. The result is a relatively light-weight battery. 
     To operate a metal-air battery cell, it is necessary therefore to provide a supply of oxygen to the air cathodes of the cells. An air pathway and a plenum are typically used to supply the air cathodes with the oxygen. The air pathway serves as a conduit of the oxygen and the plenum diffuses the oxygen evenly over the air cathodes so as to increase efficiency of the electrodes. The plenum, therefore, has to have a certain volume to work properly while the battery is operating. The air pathway is generally sealed or closed during non-use by sealing tapes, plugs, mechanical doors, etc., because water vapor and oxygen in the ambient air may cause the cell to flood, dry out, or discharge in certain circumstances, thereby leading to a reduction in cell efficiency and life. Furthermore, after a period of use, residual water vapor and oxygen in the plenum may cause similar problems, at least immediately after the air pathway is sealed or closed. 
     Thus, a need exists for an improved metal-air battery cell that overcomes or minimizes the above-referenced disadvantages of prior art metal-air battery cells. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies the above-described needs by providing a novel metal-air battery. The metal-air battery of the present invention comprises one or more air cathodes; one or more air pathways from the air cathode to a reactive gas source; a closure member associated with each of the air pathways to selectively prevent the flow of gas through the air pathways; and one or more volume-changeable plenums in fluid communication with the air pathway and the air cathode. 
     The present invention also comprises a method of producing electricity using a metal-air battery comprising one or more air cathodes, an enclosure member at least partially defining a volume-changeable plenum in fluid communication with the air cathode, and at least one closable air pathway in fluid communication with the plenum and ambient air surrounding the enclosure. The method comprising the steps of opening the air pathway so that the ambient air can flow to said plenum through the pathway; and increasing the volume of the plenum so that air is drawn through the pathway to the plenum. 
     In another disclosed embodiment, the present invention comprises an electric device using the metal-air battery mentioned above. 
     Accordingly, it is an object of the present invention to provide an improved metal-air battery. 
     Another object of the present invention is to provide a metal-air battery that is relatively simple and inexpensive to manufacture. 
     A further object of the present invention is to provide a ventilation system for a metal-air battery that does not require any electric mechanisms to operate it. 
     Yet another object of the present invention is to provide a metal-air battery that reduces the amount of air in communication with the air cathode when the battery is not in use and increases the amount of air in communication with the air cathode when the battery is in use. 
     Another object of the present invention is to provide a metal-air battery with an initial charge of fresh reactive gas for reaction with an air cathode when the battery is activated. 
     These and other objects, features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended drawing and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a disclosed embodiment of the battery of the present invention. 
     FIG. 2 is a vertical cross-sectional view taken along line  2 — 2  of the battery shown in FIG.  1 . 
     FIG. 3 is an alternate vertical cross-sectional view taken along line  2 — 2  of the battery shown in FIG. 1 when the battery is in a closed condition. 
     FIG. 4 is a perspective view of another disclosed embodiment of the battery of the present invention. 
     FIG. 5 is a vertical cross-sectional view taken along line  5 — 5  of the battery shown in FIG.  4 . 
     FIG. 6 is an alternate vertical cross-sectional view taken along line  5 — 5  of the battery shown in FIG. 4 when the battery is in a closed condition. 
     FIG. 7A is a partial vertical cross-sectional view of a pop-open mechanism of the battery shown in FIG.  4 . 
     FIG. 7B is partial detail view of the pop-open mechanism shown in FIG.  7 A. 
     FIG. 7C is an enlarged perspective view of a portion of a pivoting mechanism of the pop-open mechanism shown in FIG.  7 . 
     FIG. 8 is a vertical cross-sectional view of another disclosed embodiment of the battery of the present invention. 
     FIG. 9 is an alternate vertical cross-sectional view of the battery shown in FIG. 8 when the battery is in a closed condition. 
     FIG. 10A is a partial perspective view of a portion of the closure mechanism of the battery shown in FIG.  8 . 
     FIG. 10B is partial detail cross-sectional view of the closure mechanism shown in FIG.  10 A. 
     FIG. 10C is partial detail cross-sectional view of the closure mechanism shown in FIG.  10 A. 
     FIG. 11 is a vertical cross-sectional view of another disclosed embodiment of the battery of the present invention. 
     FIG. 12 is a vertical cross-sectional view of the battery shown in FIG. 11 showing the battery in a closed condition. 
     FIG. 13 is a schematic view of a disclosed embodiment of an electric device employing a battery of the present invention showing the electric device in an operational condition. 
     FIG. 14 is a schematic view of the electric device shown in FIG. 13 showing the electric device in non-operational condition. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in more detail to the drawing, in which like numerals refer to like elements throughout the several views, FIGS. 1,  2 ,  3 A and  3 B show a disclosed embodiment of the metal-air battery  10  of the present invention. The metal-air battery  10  includes a plurality of metal-air cells  12 , such as zinc-air cells, enclosed within an upper cell case  14  or a lower cell case  16 . The present invention is useful with both primary and secondary metal-air cells. Primary and secondary metal-air cells are known in the art and suitable components for primary metal-air cells are described in U.S. Pat. No. 5,721,065 (the disclosure of which is incorporated herein by reference). Secondary cells also can be used in the present invention, such as that described in U.S. Pat. No. 5,569,551 (the disclosure of which is incorporated herein by reference). Although the use of the present invention is disclosed as useful with a zinc-air battery, it should be understood that the present invention is applicable to other types of metal-air battery cells. 
     The upper case  14 , which is a rectangular box shape with the bottom face open, contains the cells  12  inside and optionally has two openings  18  on the top face  20  for injecting electrolyte into the cells and venting or exhausting evolved gas. The upper case  14  also has two projections  22  on its opposite side faces  24 . The lower case  16  has a tray  26  with two leg portions  28  projecting upwardly at the side edges  30  and a four-sided wall  32  sitting on the tray  26  and extending upwardly. Each leg portion  28  has a projection  34  extending inwardly. The wall  32  has several openings, such as a circular opening  36 , and slits or vents  38 ,  40 ,  42 ,  44 . The vents  38 ,  40 ,  42 ,  44  have a variety of widths and each has a round end at its bottom portion. The vents  38 ,  40 ,  42 ,  44  may be open at the top edge  46 . On the top face of the tray  16 , a circumferential elastomeric gasket  47 , which may be made of a rectangular sheet of rubber with a large rectangular hole at the center, is placed to surround the wall  32  and pressed by a circumferential bottom edge  48  of a circumferential wall  49  of the upper case  14  and the top surface of the tray  16  to seal the case. 
     Within the upper case  14 , a plurality of metal-air cells  12  are secured to the underside of the top face  20  of the upper case  14 . Two identical upper pillars  52  of a telescopic mechanism are also secured to the underside of the top face  20  of the upper case  14 . At the other end of each pillar  52  is formed a salient portion  54 . A lower hollow pillar  56  of the telescopic mechanism surrounds each the upper pillar  52  and has a smaller opening at the top end  58 . The salient portion  54  of each upper pillar  52  is smaller than the hollow tube of each lower pillar  56  but larger than the smaller opening  58  at the top end so as to prevent the upper pillar  52  from being removed completely from the lower pillar  56 . The upper and lower pillars  52 ,  56  are surrounded by a coil compression spring  60 . Opposite ends of the springs  60  contact the underside of the top face  20  of the upper case  14  and the top side  62  of the bottom face  63  of the lower case  16 . The springs  60  apply a force to the upper case  14  and lower case  16  tending to bias the two case halves apart. 
     The upper case  14  and lower case  16  define a volume-changeable plenum  64 . The plenum  64  is disposed under and around the cells  12  so as to diffuse a reactive gas from the plenum  64  to the cells  12 . 
     The outside dimension of the four-sided wall  32  of the lower case  16  is slightly smaller than the inside dimension of the box-shaped upper case  14  so that the upper and lower cases fit together telescopically, and, thus, permit the wall  32  to slide on the inside of the box-shaped upper case. This telescopic arrangement permits the upper case  14  to move upwardly and downwardly with respect to the lower case  16 . When the upper case  14  is in the position shown in FIG. 2, the vents  36 ,  38 ,  40 ,  42 ,  44  permit the ambient air surrounding the battery  10  to enter the plenum  64  and also permit air inside the plenum  64  to escape to the outside. 
     When the battery  10  is stored, inactive or otherwise not operative, the upper and lower cases  14 ,  16  are pressed together so that the battery case may be in a collapsed or closed position, as shown in FIG. 3, and the projections  22  of the upper case  14  engage the projections  34  of the portion  28  of the lower case  16 . The projections  22  have a horizontally flat portion  65  at their upper portion and a wedge shape  66  at their lower portion. The projections  34 , on the other hand, have a horizontally flat portion  67  at their lower part and a wedge shape  68  at their upper part. Thus, the two wedgeshaped portions  66 ,  68  slide on their surfaces to push outwardly when the upper and lower cases  14 ,  16  are pressed together. When the battery case is in a collapsed position, the projection  34  of the lower case  16  moves back or springs back inwardly to engage with the projection  22  on its flat portion  67  like a ratchet so that the battery case may be retained in the collapsed position. In the collapsed or closed position (FIG.  3 ), the box-shaped upper case  14  covers the openings  36 ,  38 ,  40 ,  42 ,  44  thereby closing them. The circumferential bottom edge  48  of the circumferential wall  49  is pressed on the circumferential elastomeric gasket  47  placed on the top face of the tray  16  such that the battery case may be airtight., and, thus, preventing the ambient air surrounding the battery  10  from entering the plenum  64  and also preventing air inside the plenum  64  from escaping to the outside. 
     When the battery is ready to use, the projections  34  are moved outwardly against the elastic force of the leg portions  28 . Thus, the engaged projections  22 ,  34  are released and the upper case  14  is popped open by the elastic force of the coil spring  60 . At the same time, the volume-changeable plenum  64  is expanded and a negative gauge pressure is produced inside of the case, which causes ambient air surrounding the battery  10  to be drawn through the vents  36 ,  38 ,  40 ,  42 ,  44  during the opening movement. The fresh air, including a reactive gas; i.e., oxygen, is therefore, introduced into the expanded plenum  64  and supplies the air cathodes (not shown) with oxygen for immediate use. The inflow of air through the vents  36 ,  38 ,  40 ,  42 ,  44  during the opening movement also causes turbulence inside of the case which aids in the diffusion of oxygen evenly over the air cathodes (not shown). After a while, when the battery is operating at steady state, the oxygen makes it way to the air cathodes (not shown) by diffusion. Thus, no expensive air moving device is needed. 
     When the battery is not in use, the upper and lower cases  14 ,  16  are pressed together against the elastic force of the coil spring  60  to the collapsed position (FIG.  3 ). As the upper and lower cases  14 ,  16  move toward each other, the plenum  64  becomes smaller which thereby cause air inside the plenum  64  to be exhausted through the vents  36 ,  38 ,  40 ,  42 ,  44 . When completely closed, the upper case  14  covers the vents  36 ,  38 ,  40 ,  42 ,  44 , as described above, and prevents air from both entering and exiting the plenum  64 . 
     The four-sided wall  32  of the lower case  16  can have any number or any shape of vents  36 ,  38 ,  40 ,  42 ,  44 . It is within the skill of the art to design the vents to provide sufficient ventilation for optimum battery performance during steady state, as well as transition state, use. The distance that the upper and lower cases  14 ,  16  move in the opening and closing movement directly affects a volume change ratio of the plenum  64  and can also be adjusted for optimum battery performance. If that distance is relatively short, the volume change ratio is relatively small and produces relatively small pressure differences; if that distance is relatively long, the volume change ratio is relatively large and produces relatively large pressure differences. The distance may also be varied by the planar area of the cell  10 . In general a cell with a large planar area needs a longer distance to have the same diffusion air flow from the peripheral vents, that is, the same supplying current. If the cell with circumferential gap, which may be referred to the distance, communicating with the plenum at the cell&#39;s periphery is tested, the experimental data suggests a somewhat linear relationship between plenum thickness, which is equal to the gap height, and a limiting current. Useful closing distances are typically from 0.06 to 0.5 inches if the wall  32  has many vents. 
     Since a relatively small positive pressure remains in the plenum  64  after all the openings  36 ,  38 ,  40 ,  42 ,  44  are closed, it is desirable to provide the plenum  64  with a vent hole with a check valve as described further hereinbelow. 
     The top view of the upper case  14  is rectangular in this embodiment, but it will be understood that the upper case can have any desired shape, such as polygonal or circular, as described later. Any number or any shape of the engaging projections  22 ,  34  may also be used in accordance with the present invention as long as the engaging mechanism retains the upper and lower cases  14 ,  16  in the collapsed or closed position and can be relatively easily released so that the upper and lower cases can assume the open position. The upper and lower cases  14 ,  16  can be made from any suitable material, however, plastic is preferred because it is gas-impermeable, relatively easy and inexpensive to form into a desired shape, and because it can be self-lubricating so as to facilitate the sliding mechanism. Metal or other materials may also be used. 
     The coil spring  60  is of a suitable size so as to provide sufficient force for opening the upper and lower cases  12 ,  14 . The elastic force may be adjusted to achieve the optimal pop-opening rate and to give suitable resistance for closing by hand. However, the spring  60  can also be designed so as to provide more space for the cells  12 . 
     Another disclosed embodiment of the present invention is shown in FIGS. 4,  5 ,  6 ,  7 A,  7 B and  7 C. The metal-air battery  10 ′ includes a cylindrical cell  70  enclosed within a circular upper cell case  72  and a circular lower cell case  74 . 
     The upper case  72 , which is shaped like an upside down cup, has an optional opening  76  on the top face  78  for injecting electrolyte into the cell  70  and ventilating evolved gas. The lower case  74  has a base  80  and a circular wall  82  extending upwardly therefrom. The wall  82  has several slit openings or vents  84  each having the same size and shape for symmetry. Each opening  84  is round at the bottom ending and may be open at the top edge  88 . The base  80  has an opening  90  and a reducing valve  92  to leak pressured air from a volume-changeable plenum  94  during battery collapse or closing. In a similar manner as described in the first embodiment, a circular elastomeric gasket  50  may be disposed on the top face of the base  80  of the lowercase  74 . The gasket is pressed between the bottom edge of the peripheral vertical wall  51  and the top face of the base  80  to seal off the battery when the case is in the closed position. 
     Inside the upper and lower cases  72 ,  74  (collectively “the battery case”), a cell  70  is secured to the underside of the top face  78  of the upper case  72 . An upper pillar  96  of a telescopic mechanism is also secured to the underside at the top face  78  of the upper case  72 . At the distal end of the pillar  96  a snap ring  98  is secured to retain an opening mechanism described later. A lower pillar  100  of the telescopic mechanism is secured to the center of the top face  102  of the base  80 . The hollow pillar  100  surrounds a lower portion of the upper pillar  96  and has a smaller opening  104  at the top end of the lower pillar  100 . A coil spring  106  surrounds the upper pillar  96  and is disposed between the topside of the top face  78  of the upper case  72  and a circular plate  108  at the top end of the lower pillar  100  so as to push the upper case  72  and the lower case  74  apart. Therefore, the opening, which is defined by the circular plate  108  at the top end of the lower pillar  100  is smaller than the diameter of the coil spring  106 . 
     In FIGS. 7A,  7 B, and  7 C, the opening mechanism is illustrated. The upper pillar  96  holds a rotatable cylindrical member  110  between the snap ring  98  and another snap ring  112  secured to the upper pillar  96 . The cylindrical member  110  has a clearance hole  114  for the upper pillar  96 , rectangular teeth  116  on the top face  118 , and saw teeth  126  on the bottom face  122 . Inside the hollow pillar  100 , a stationary cylinder  124  with rectangle teeth  126  on the bottom face  128  is secured to the circular plate  108 , and a stationary cylinder  130  with saw teeth  132  on the top face  134  is secured to the base plate  80 . Each rectangular tooth  136  of the stationary cylinder  124  has a plateau  138  and a triangle portion  140  on the right corner with neighboring concavities  142 , as shown in FIG.  7 B. On the top face  118  of the rotatable cylindrical member  110 , each rectangle tooth  143  has a plateau  144  and a chamfered right corner  146  with neighboring concavities  148 . Each cavity  148  has a triangular cavity  150  on the right. The triangular portion  140  fits the chamfered corner  146  and the cavity  150 . On the bottom face  122  of the rotatable cylindrical member  110 , each saw tooth  152  has a peak  154  and a wedge-like slope  156  at the right hand side. On the top face  134  of the stationary cylinder  130 , each saw tooth  158  has a peak  160  and a wedge-like slope  162  on the left-hand side. Thus, both saw teeth  126 ,  132  engage with each other. A typical bottom face  122  of the rotatable cylindrical member  110  or a typical bottom face  134  of the stationary cylinder  130  is shown in FIG.  7 C. 
     When the battery is in a collapsed or closed position, the plateau  138  of the tooth  136  of the stationary cylinder  124  coincides with the plateau  144  of the tooth  116  of the rotatable cylinder  110  such that the rotatable cylindrical member  110  is kept in a lower position in the hollow pillar  100  (FIG.  6 ). That means the upper case  72  is in the collapsed position. When the upper case  72  and the lower case  74  are pressed together to move the upper shaft  96  downwardly, the peak  154  of each saw tooth of the rotatable cylinder  110  hits on the wedge-like slope  162  of the stationary cylinder  130  and the peak  160  of each saw tooth  158  of the stationary cylinder  130  hits on the wedge-like slope  156  of the rotatable cylinder  110 . Because of the slope, the peak  154  moves left in FIG. 7B while the upper case  72  and the lower case  74  are pressed together; i.e., while the rotatable cylinder  110  and the stationary cylinder  130  are pressed together. Here, the term “moves left” means “rotates clockwise” if the rotatable cylinder  110  is viewed from the top. After the peak  154  of the saw tooth  152  reaches a notch  164  of the counter saw tooth of the stationary cylinder  130 , the clockwise movement of the rotatable cylinder  110  is blocked by each vertical wall of the saw teeth of both the rotatable cylinder  110  and the stationary cylinder  130 . 
     When the pressure is released, the coil spring  106  pushes the upper case  72  back to the open position in a manner described hereafter. The rotatable cylinder  110  is lifted to bring the rectangle tooth  136  into contact with the corresponding rectangle tooth  116  when the spring  106  pushes up on the upper case  72  and the rotatable cylinder  110 . However, since the rotatable cylinder  110  has been moved left about half of the breadth of the rectangle tooth  136  as described before, the left corner of the tooth  136  hits on the chamfered corner  146  and each tooth  143  of the rotatable cylinder  110  rotates to fall into each corresponding concavity  142 . Thus, the rotatable cylinder  110  is retained in the open position (FIG.  5 ). 
     When the upper case  72  and the lower case  74  are pressed together, the rotatable cylinder  110  is pushed downward to bring the saw teeth  120  at the bottom face  122  into contact with those at the top face  134  of the stationary cylinder  130 . The peak  154  of each saw tooth  152  of the rotatable cylinder  110 , thereby, hits on the wedge-like slope  162  of the stationary cylinder  130  to move clockwise as described above. Then, the pressure is released so that the rotatable cylinder  110  is pushed upwardly by the spring  106  to bring the rectangular teeth  116  into contact with the rectangular teeth  136  again. The triangular portion  140  slides on the chamfered corner  146  to move the rotatable cylinder  110  slightly clockwise such that the plateau portion  144  of the rotatable cylinder  110  sits on the plateau portion  138  of the stationary cylinder  124 . Thus, the battery case is kept in the closed position again and the rotatable cylinder  110  is kept in the lower position in the hollow cylinder  100 . 
     When the battery  10 ′ is stored or is otherwise inactive, the upper and lower cases  72 ,  74  are pressed together so that the battery case is in the collapsed or closed position (FIG.  6 ). Thus, the openings  84  are closed by the side wall of the upper case  72  and the volume-changeable plenum  94  is also collapsed (FIG.  6 ). 
     When the battery is ready to be used, the battery is opened by the elastic force of the coil spring  106  with the opening mechanism described above (FIG.  5 ). At the same time, the volume-changeable plenum  94  is expanded and a negative gauge pressure is produced inside of the case, which causes air outside of the openings  84  to be drawn into the plenum  94  during the pop-open movement. The fresh air, including a reactive gas; i.e., oxygen, is introduced into the expanded plenum  94  and supplies the air cathode (not shown) with oxygen for immediate use. The inflow of air through the openings  84  also causes turbulence within the plenum  94  which causes the oxygen to diffuse evenly over the air cathode (not shown). 
     When the battery is not in use, the upper and lower cases  72 ,  74  are pressed together against the elastic force of the coil spring  106  to the collapsed position (FIG.  6 ). As the upper and lower cases  72 ,  74  move toward each other, the plenum  94  becomes smaller which causes air inside the plenum  94  to be exhausted through the openings  84  just before they are closed completely by the upper case  72  and through the opening  90  after the openings  84  are closed. The reducing valve  92  is composed of a plate like a petal and a hinge (not shown) securing the valve  92  to the bottom face of the base  80 . The valve  92  closes the opening  90  with a weak closing force provided by a spring at the hinge (not shown) unless there is a positive gauge pressure in the plenum  94 . 
     The cylindrical wall  82  of the lower case  74  may have any number or any shape of vent openings  84 . It is within the skill of the art to design the vents to provide sufficient ventilation for optimum battery performance during steady state, as well as transition state, use. The distance that the upper and lower cases  72 ,  74  move in the opening and closing movement directly affects the volume change ratio of the plenum  94  and can also be adjusted for optimum battery performance as previously described. 
     The top view of upper case  72  is circular in this embodiment, but it can be any desired shape, such as polygonal, as mentioned above. Any suitable mechanism for opening and closing the upper and lower cases  72 ,  74  can be employed in accordance with the present invention. 
     Another embodiment of the present invention is illustrated in FIGS. 8,  9 ,  10 A,  10 B and  10 C. Although most of the parts of this battery  10 ″ are commonly used in the previous embodiment (FIGS.  4 - 7 C), the battery has a mechanical door located beneath an air cathode (not shown) of a metal-air cell  70 , a plurality of pillars  170  for a door opening and closing mechanism, and a plurality of openings  172  for air communication. The mechanical door comprises two circular disks  174 ,  176  with a plurality of holes  178 ,  180 ,  182 ,  184  formed therein and a spring (not shown) to push it back to an open position. Each pillar  170  for the door opening and closing mechanism has a wedge-like portion  186  at the top end (FIGS. 10A,  10 B and  10 C). The base  80  has a plurality of openings  172  so that the air can enter the plenum  94  from the outside and can also exit the plenum  94  through the openings  172 . 
     The circular disks  174 ,  176  are disposed just beneath the cell  70 . Therefore, the volume-changeable plenum  94  is separated from the air cathode (not shown) by the circular disks  174 ,  176 . Air freely enters and exits the plenum  94  through the openings  172 . The openings  172  can be diffusion tubes that restrict the flow of excess air and water vapor or larger tubes for continuous operation. The circular disks  174 ,  176  may have many openings  182 ,  184  formed therein because only a small space is available for diffusion between the openings and the cathode. However, the openings  172  may prevent solid particles and other harmful materials from entering the battery cells  70 . 
     The opening and closing of the mechanical door coincides with a volume change of the plenum  94  as described hereafter. The mechanical door is open; i.e., the openings  182 ,  184  on both disks  174 ,  176  align with each other, by the action of a spring (not shown) when the battery case is in an open position (FIG.  10 B). However, the mechanical door is closed; i.e., the openings  182 ,  184  do not align with each other (FIG.  10 C), because the lower disk  176  is moved by the pillars  170 . Each pillar  170  extends through a corresponding opening  180  for alignment and the slant cut end  186  of each pillar  170  moves the lower disk  176  when the case is collapsed or closed. The use of the mechanical door to isolate the metal-air cell from ambient air provides a longer shelf life than for conventional metal-air battery cells. 
     Another embodiment of the present invention is illustrated in FIGS. 11 and 12. The battery  10 ″′ includes a metal-air cell  190  secured within an upper case  192 , which may be shaped like an upside down cup. Disposed under the cell  190  is a volume-changeable plenum  194 . The plenum  194  is also defined by a lower case  196  including a bellows portion  198  with openings  200  formed therein. The bellows can be made of rubber, plastics, and other suitable flexible materials, so that the openings  200  can be closed by the adjacent rib  202  of the bellows facing the openings  200  when the battery case is in a collapsed or closed position (FIG.  12 ). 
     When the battery is stored or is otherwise inactive, the upper and lower cases  192 ,  196  are pressed together thereby compressing the bellows so that the battery case is in a collapsed position (FIG. 12) and so that the openings  200  are closed as described above. 
     When the battery  10 ″′ is ready for use, the battery case is popped-open by the elastic force of bellows or is stretched by hand (FIG.  11 ). At the same time, the volume-changeable plenum  194  is expanded and the openings  200  accordingly become open. A negative gauge pressure is produced in the plenum  194  and causes ambient air surrounding the battery  10 ″′ to flow into the plenum through the openings or vents  200 . Fresh air, including a reactive gas; i.e., oxygen, therefore, is introduced into the expanded plenum  194  to supply the air cathode with oxygen for immediate use. 
     When the battery  10 ″′is not in use, the upper and lower cases  192 ,  196  are pressed together against the elastic force of the bellows to the collapsed position (FIG.  12 ). At the same time, the plenum  194  becomes smaller so as to exhaust air inside the plenum through the openings  200 . The lower case  196  optionally includes a reducing valve, of the type described above, such that a positive air pressure in the plenum  194  is avoided. 
     Further the opening and closing motion facilitates air movement and fresh air introduction into the plenum  186  and the air cathode (not shown). Thus, the volume-changeable plenum works as a fresh air pump or an air manager of the metal-air battery which is disclosed, for example, in U.S. Pat. No. 5,356,729. 
     Another embodiment of the present invention is illustrated in FIGS. 13 and 14. The battery of the present invention is illustrated as being installed in a camcorder  210 , which has a foldable handle  212  to hold the body of the camcorder while shooting. Any type of batteries described theretofore can be used in the camcorder  210 . When the handle  212  is unfolded (FIG.  13 ), the battery case  214  expands thereby expanding the volume-changeable plenum so that fresh air enters the plenum. When the handle  212  is folded (FIG.  14 ); i.e., the camcorder is not in use, the expanded battery case is pushed back by the handle  212  to the closed position and volume-changeable plenum is compressed expelling air therefrom and the openings are closed to save battery life. 
     It should be understood, of course, that the foregoing relates only to certain disclosed embodiments of the present invention and that numerous modifications or alterations may be made therein without departing from the spirit and scope of the invention as set forth in the appended claims.