Abstract:
A battery assembly includes a sealed battery cell container holds a positive and negative electrode and an electrolyte. Deflection-responsive means engages at least one outer face of the container and is responsive to deflection thereof for operating a cut-off switch to break the electrical continuity of the battery assembly. Flat battery cell containers may be stacked in different arrangements within a coupling structure, the arrangements defining either a cavity between the coupling structure and an adjacent container face, or a cavity between faces of adjacent cell containers. A membrane switch received in the cavity is actuated by bulging of a cell container wall. A control circuit operates the cut-off switch in response to actuation of the membrane switch.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a battery assembly in which the electrical continuity of the assembly is broken in response to an increase in internal pressure in the cells. 
     BACKGROUND OF THE INVENTION 
     Rechargeable batteries are widely used for powering small portable electronic devices. Lithium ion batteries offer a high energy density and high operating voltage, together with a reasonable life. Other rechargeable batteries commonly used are lead batteries and nickel-metal-hydride batteries. 
     Sealed rechargeable battery cells can pose an explosion hazard when internal gas pressure rises uncontrollably, through over-charge or over-discharge. U.S. Pat. No. 4,025,696 describes a cell with an internal overpressure safety switch responsive to deflection of the walls of the cell for breaking an electrical contact. The cell has a casing with cylindrical side walls and incorporates a Belleville spring element adjacent the bottom of the casing which is moved from a first to a second stable position by outward bulging of the bottom of the casing. However these switches take up space within each cell in a multi-cell battery, and are expensive. 
     Common over-pressure relief devices include vents or seals which are opened or ruptured under excess pressure. The venting or rupture are irreversible and once the seal of the cell is broken the corrosive and potentially harmful electrolyte can escape. 
     As an alternative, or further safeguard, rechargeable batteries are often provided with an integrated protection circuit which stops the charging of the battery due to avoid overcharging. The circuit senses battery temperature and/or pressure and cuts off current flow through the cell. These circuits are also expensive and suffer from a lack of positive action. It is an object of the present invention to overcome or substantially ameliorate the above disadvantages or more generally to provide an improved rechargeable battery. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a battery assembly comprising: 
     a sealed battery cell container holding a positive and negative electrode and an electrolyte, the container having outer faces; 
     a cut-off switch electrically connected between one of the electrodes and an output of the device, the switch being operable to break the electrical continuity of the battery assembly, and 
     deflection-responsive means engaging at least a first outer face and responsive to deflection thereof for operating the cut-off switch. 
     Preferably the deflection-responsive means comprises coupling structure fixed to the cell container and forming a cavity between the first outer face and the coupling structure, the cut-off switch is received in the cavity and is pressure-sensitive, whereby outward deflection of the first outer face tends to compress the cut-off switch. 
     Alternatively the deflection-responsive means comprises a member having first and second ends with complementary mechanical couplings fixed thereto, the couplings being connected such that the member forms a band extending about the cell container and tensioned to hold the member against the outer faces, the cut-off switch comprising a respective electrical contact fixed to each of the couplings, whereby the contacts are engaged when the couplings are connected and deflection of the outer faces tensions the member and tends to separate the couplings and contacts to break the electrical continuity of the battery assembly. In another alternative embodiment the cell container includes a plurality of sealed battery cell containers, the cell containers being layered to form a stack with outer stack faces; the deflection-responsive means comprises a member having first and second ends with complementary mechanical couplings fixed thereto, the couplings being connected such that the member forms a band extending about the stack and tensioned to hold the member against the outer stack faces, the cut-off switch comprising a respective electrical contact fixed to each of the couplings, whereby the contacts are engaged when the couplings are connected and deflection of the outer faces tensions the member and tends to separate the couplings and contacts to break the electrical continuity of the battery assembly. 
     The cell container preferably includes a plurality of sealed battery cell containers, each with opposing major outer faces; the deflection-responsive means comprising: 
     coupling structure joining the cell containers and forming a cavity between two adjacent outer faces, 
     the cut-off switch comprising a pressure-sensitive switch received in the cavity. 
     Alternatively a pressure-sensitive switch is received in the cavity, the deflection-responsive means further including a control circuit operating the cut-off switch in response to a change of state of the pressure-sensitive switch. 
     The coupling structure preferably comprises a ring extending around the cavity and held between the two adjacent outer faces. 
     Preferably the pressure-sensitive switch comprises a membrane switch. Preferably the membrane switch comprises an array of switch elements. The array is preferably a linear array aligned with a central axis of both adjacent outer faces, with one switch element lying between central parts of the adjacent outer faces. Most preferably the switch elements are normally-open switch elements electrically connected in parallel. 
     Preferably the coupling structure comprises a tubular member and the opposing major outer faces are substantially planar and parallel, the control circuit including a PCB lying in a plane substantially orthogonal to a plane of the major outer faces. 
     Preferably the battery assembly further includes a cable connected to the PCB that terminates in an output jack for making an electrical connection to the battery, the cell containers, coupling structure and PCB being enclosed between shell-type casing halves from which the cable and output jack protrude. Preferably the battery cell containers are lithium ion battery cell containers. 
     This invention provides a battery assembly which is effective and efficient in operational use, and which has an overall simple design which minimizes manufacturing costs. Therefore it may be used, not only as a primary over-pressure protection, but can be used as further safeguard in addition to over-pressure protection sensors or features provided in the battery cells themselves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred forms of the present invention will now be described by way of example with reference to the accompanying drawings, wherein: 
         FIG. 1  is a pictorial view of a first embodiment of the battery of the invention; 
         FIG. 2  is an exploded pictorial view of the battery of  FIG. 1 ; 
         FIG. 3  is an exploded pictorial view of the cell and circuit assembly of the battery of  FIG. 1 ; 
         FIG. 4  is a schematic longitudinal cross-section of the cell and circuit assembly of the battery of  FIG. 1 ; 
         FIG. 5  is a schematic circuit diagram of the circuit of the battery of  FIG. 1 ; 
         FIG. 6  is a large-scale schematic cross-section through a switch element of the switch element of  FIG. 4 ; 
         FIG. 7  is a plan view of the cell and circuit assembly of a second embodiment of the battery of the invention; 
         FIGS. 8 to 21  are schematic longitudinal cross-sections of the cell and circuit assembly of second through fifteenth embodiments respectively of the battery of the invention; 
         FIG. 22  is a pictorial view of the cell and circuit assembly of a sixteenth embodiment of the invention, and 
         FIG. 23  is an exploded pictorial view of the cell and circuit assembly of  FIG. 22 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to the drawings, particularly  FIGS. 1-6 , a battery assembly  1  includes shell-type casing halves  2 ,  3  molded from polymeric material and fixed together to enclose a cell and circuit assembly  4 . The assembly  4  includes two sealed lithium ion battery cell containers  5   a ,  5   b  electrically connected by couplings  9  to a printed circuit board (PCB)  6 . A cable  8  connected to the PCB  6  terminates in an output jack  7  for making an electrical connection to the battery. 
     The cell containers  5   a ,  5   b  are generally in the form of a flat rectangular prism, having planar major surfaces  10   a ,  10   b ,  11   a ,  11   b  on respective opposing sides. A deflection-responsive assembly includes cover  18  and ring  15  forming a coupling structure and the membrane switch assembly  12 . The switch assembly  12  is received in a cavity  14  defined between the innermost faces  11   a ,  10   b  and the ring  15 . The ring  15  has rectangular shape complementary to the outer edge of the switch assembly  12  and opposing planar faces  17   a ,  17   b . The ring  15  is formed of a substantially rigid polymer. A neck  16  of the switch assembly  12  includes electrical conductors (not shown) and extends through a channel (not shown) in the ring  15  to connect to the PCB  6 . 
     The rigid tubular outer cover  18  is elongated parallel to a longitudinal axis of the cell containers  5   a ,  5   b  and conforms tightly to the outer surfaces  10   a ,  11   b  to clamp the ring  15  between the surfaces  10   b  and  11   a . The PCB  6  lies generally perpendicular to the parallel planes of the cell containers  5   a ,  5   b  and adjacent to a transverse edge  19 , to provide a compact package that nests within the casing halves  2 ,  3 . 
     Referring to  FIGS. 5 and 6 , the sealed battery cell containers  5   a ,  5   b  each hold a positive and negative electrode  30 ,  31  and an electrolyte  32 . The membrane switch assembly  12  is of a type comprising three normally-open switch elements  13   a ,  13   b ,  13   c  electrically connected in parallel. As used herein the term “membrane switch” refers to a pressure-sensitive momentary switch in which at least one electrical contact is on, or made of, a thin flexible substrate. Each of the switch elements  13   a ,  13   b ,  13   c  includes thin flexible substrates  20 ,  21  each carrying a respective pole  22 ,  23  held apart by resilient action of the dome-shaped substrate  20 . The substrate  21  abuts the surface  11   a  of cell container  5   a  and substrate  20  is spaced apart from surface  10   b  of cell container  5   b  such that bulging of either of the cell containers  5   a ,  5   b  beyond a pre-determined value will close the switch. 
     In an alternative arrangement not shown, instead of being normally open, the switch elements  13   a ,  13   b ,  13   c  may be normally closed with both poled carried on one substrate and may, for example, be connected in the circuit in series such that each one provides a cut-off switch for breaking the electrical continuity of the assembly. 
     When the battery cell containers  5   a ,  5   b  are pressurized internally, the relatively large major surfaces  10   a ,  10   b ,  11   a ,  11   b  tend to bulge outwardly. In use, the outer cover  18  not only holds the cell containers  5   a ,  5   b  together, it also serves to stiffen outer surfaces  10   a ,  11   b  against which it is held. Any internal pressure increase in one or both of the cell containers  5   a ,  5   b  preferentially deflects one or both of the inner surfaces  10   b ,  11   a , compressing the switch elements  13   a - 13   b . The PCB  6  includes a controller that detects a current flow when at least one of the switch elements  13   a ,  13   b ,  13   c  has closed and then acts to open a cut-off switch  34  electrically connected in series in the circuit  33 , thereby preventing current flow to or from the cell containers  5   a ,  5   b.    
     The invention lends itself to a large number is variations in the stacking of the containers  5   a ,  5   b , switch assembly  12  and ring  15 , some of which are schematically shown in  FIGS. 7-21 . In all of these variants a switch assembly  12  with three longitudinally aligned switch elements  13   a ,  13   b ,  13   c  is used as shown in  FIG. 7 , although it will be understood that any number and arrangement of switches may be used. 
       FIGS. 8 and 9  show second and third embodiments respectively, with each employing a single cell container  5 . In the second embodiment one switch assembly  12  may abut the major surface  10   a  ( FIG. 8 ) with the outer cover (not shown) conforming tightly to the outer surface  10   b  and face  17   a . In the third embodiment ( FIG. 9 ) switch assemblies  12   a ,  12   b  abut the major surfaces  10   a ,  10   b  respectively with the outer cover (not shown) conforming tightly to the outer faces  17   a ,  17   b . The switch elements  13   a ,  13   b ,  13   c  etc of both of the switch assemblies  12   a ,  12   b  are connected in parallel, such that the controller PCB  6  can operate the cut-off switch  34  in response to closure of any one of the switch elements. 
     The fourth, fifth and sixth embodiments of  FIGS. 10-12  show the stacking of two cell container  5   a ,  5   b  with one, two or three switch assemblies  12   a ,  12   b ,  12   c  respectively. The seventh to fifteenth embodiments of  FIGS. 13-21  show the stacking of three cell container  5   a ,  5   b ,  5   c  with one, two, three or four switch assemblies  12   a ,  12   b ,  12   c ,  12   d  respectively. In each of these embodiments the size of the outer cover (not shown) varies to firmly fit about the components and hold them together. It will be apparent from these embodiments that any switch assembly  12  should abut at least one major surface of a cell container  5 . 
     A sixteenth embodiment of the invention is shown in  FIGS. 22 and 23  and the illustrated cell and circuit assembly  104  is enclosed in shell-type casing halves  2 ,  3  to provide a battery assembly. In this embodiment of the deflection-responsive assembly the coupling structure is in the form of band assembly  20 , with integrated contacts forming a cut-off switch. 
     The band assembly  20  includes a band  21  which extends transversely about the stack  30  of cell containers  5   a ,  5   b ,  5   c  and has male and female couplers  22 ,  23  at opposing ends. A connecting strip  24  electrically connects the band  21  to the PCB  6 . Connecting the couplers  22 ,  23  mechanically holds the band  21  firmly in contact with the outer faces of the stack of cell containers  5   a ,  5   b ,  5   c  (as seen in  FIG. 23 ) and makes an electrical contact (not shown). Deflection of the walls of one of the cell containers  5   a ,  5   b ,  5   c  acts to tension the band  21  tending to separate the couplers  22 ,  23  (as seen in  FIG. 24 ) and a pre-determined deflection is sufficient to separate the couplers  22 ,  23  and thereby break the electrical contact. The electrical contacts (not shown) may form a cut-off switch which breaks the electrical continuity of the assembly, or the controller may sense a current flow through the closed contacts to operate a separate cut-off switch. This band assembly  20  may be used in like manner with a single cell container, or any number of stacked cell containers. 
     Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof.