Abstract:
A electrical connector for connection to an electrochemical cell is described. The connector comprises a conductive lid; a ferrule disposed within an opening through the conductive lid; a conductive center pin disposed within the ferrule; and a hermetic seal formed between the pin and an interior surface of the ferrule. A mating terminal connector adapted to be connected to the ferrule and the conductive center pin is further provided. The terminal connector is a device for easily and quickly connecting the cell to a circuit board of the kind found in an implantable medical device, such as a cardiac pacemaker, defibrillator, neuro-stimulator, drug pump, and the like.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a terminal connector or connector assembly for connection to an electrochemical cell. More particularly, the present invention relates in one embodiment to a terminal connector for attaining a simple, reliable connection of a device to a high capacity, high energy density cell used in implantable medical devices. The terminal connector connects to the cell&#39;s terminal assembly disposed on the exterior of the cell. That way, the terminal assembly occupies very little of the internal volume of the cell. 
     2. Description of Related Art 
     A case-grounded implantable electrochemical cell most commonly provides two terminals of opposed polarity for connection of the cell to a device to be powered. One terminal extends from a glass-to-metal seal, and the other terminal extends from the case. Prior art terminals are disadvantageous in that one of the terminals typically protrudes inwardly into the cell casing, thereby reducing the casing volume available for containing the active cell materials. The use of prior art terminals thus results in a reduction of the overall cell volumetric efficiency. Additionally, the connection of the cell terminals to the device to be powered is typically accomplished by soldering, welding, or wire-bonding the cell terminals to the device. 
     For both the central terminal pin extending from the glass-to-metal seal, and the case grounding terminal that is welded or otherwise joined to the cell casing, each is comprised of a fine wire extending outwardly from the casing. These wires are fragile and are thus susceptible to displacement and fracture by bending during cell handling. Additionally, it is difficult to position the terminal wires precisely and to maintain such positioning in the event they are to be matched with specific corresponding terminals of the device to be powered by the cell when making connection with such a device. The terminal wires are easily bent and misaligned during handling of the cell prior to connection to the device. 
     Heretofore, a number of patents and publications have disclosed cells including terminal connections to powered devices. For example, U.S. Pat. No. 5,250,373 to Muffoletto et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes an electrochemical cell including a central lead that is fixed relative to a lid by a generally cylindrical ferrule element, which is fitted at one end to an opening in the lid and secured to the lid by welding. The space between the lead and the ferrule is filled by a glass seal that extends along the major portion of the length of the ferrule. This glass-to-metal seal insulates the lead electrically from the lid and the cell casing, and seals the lead so that no egress or ingress of gases or liquids into or out of the casing can occur. The entire glass-to-metal seal is disposed beneath the lid on the interior of the cell casing. This assembly thus occupies space within the cell casing that could be more advantageously used for active materials, which would increase the cell&#39;s volumetric efficiency and capacity. 
     Additionally, U.S. Pat. No. 6,004,692 to Muffoletto et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes an electrochemical cell including an electrode assembly constructed of a serpentine electrode with interleaved counter electrode plates. The electrochemical cell disclosed therein also includes a glass-to-metal seal assembly similar to that disclosed in the aforementioned U.S. Pat. No. 5,250,373 to Muffoletto et al., wherein the lead is connected to the interleaved cathode plates by a cathode bridge. These are only two examples of many conventionally constructed electrochemical cells having the glass-to-metal seal assembly disposed beneath the lid on the interior of the casing, thus using space within the cell that could be more advantageously used for active materials. 
     SUMMARY OF THE INVENTION 
     There is therefore a need for an electrical terminal connector for attaining a simple, reliable connection to the terminal assembly of a high capacity, high energy density cell used in implantable medical devices, wherein the terminal assembly is exterior of the cell, and occupies very little, if any, of the valuable internal volume of the cell that is better used to contain active materials. 
     Accordingly, embodiments of the present invention are provided that meet at least one or more of the following objects of the present invention. 
     It is an object of this invention to provide a cell for use in an implantable medical device that includes a terminal assembly that is external to the cell casing. 
     It is a further object of this invention to provide a cell that includes a terminal assembly that is simple to connect to and disconnect from a terminal connector connectable to an implantable medical device. 
     It is a further object of this invention to provide a cell for connection to a device that includes a terminal connector that is simple to connect to a circuit board of the device. 
     It is a further object of this invention to provide a cell for use in an implantable medical device that includes a terminal connector that is connectable to a corresponding mating fitting of the device, wherein the mating fitting is a standardized, mass-produced fitting. 
     According to the present invention, therefore, a terminal connector is provided for connection to a cell. The cell includes a conductive lid comprised of a wall having an inside surface, an outside surface, and an opening therethrough from the inside surface to the outside surface. The cell comprises a ferrule of a conductive hollow cylindrical body having a central axis, an interior surface, an exterior surface, an outer end, and an inner end. The inner end is electrically joined and sealed within the opening through the wall of the conductive lid and is substantially coplanar with the inside surface of the wall of the conductive lid; the cell further includes a conductive center pin or terminal pin comprising an elongated surface, an inner end, and an outer end. The terminal pin is disposed within the hollow cylindrical body of the ferrule and aligned along the central axis thereof, thereby forming an annular cavity between the elongated surface of the terminal pin and the interior surface of the ferrule. A hermetic glass-to-metal seal is formed in the annular cavity. 
     The inner end of the ferrule is preferably joined to the lid by a seam weld that is flush with the inside surface of the lid. The exterior surface of the ferrule may be threaded to enable connection with a correspondingly threaded terminal connector of the present invention. Alternatively, the exterior surface of the ferrule may be provided with small protruding pins to enable connection with a BNC terminal connector of the present invention. The conductive lid, the ferrule, and the center pin may be made of titanium, or of stainless steel. In the latter instance, the terminal pin may be made of molybdenum. 
     Also according to the present invention, a terminal connector is provided for connection to the ferrule electrically joined to the casing and for connection to the insulated conductive terminal pin. The terminal connector comprises a non-conductive cap including a hollow cylindrical body covered at an outer end by a flange, a center terminal embedded in the flange, and a perimeter terminal disposed within the hollow cylindrical body of the cap. When the terminal connector is connected to the ferrule, the exterior surface of the ferrule is in contact with the perimeter terminal of the terminal connector, and the cell&#39;s terminal pin is in contact with the center terminal of the terminal connector. This provides opposite polarity electrical conduction through the terminal pin and through the body of the ferrule. 
     If the exterior surface of the ferrule is threaded, the perimeter terminal of the terminal connector is correspondingly threaded to enable a threaded connection between the terminal connector and the ferrule. If the exterior surface of the ferrule is provided with small protruding pins, the terminal connector is formed as a BNC connector adapted to engage with the protruding pins of the ferrule. If the exterior surface of the ferrule is provided with a smooth cylindrical surface, the perimeter terminal of the terminal connector is provided with a smooth inner surface dimensioned such that the terminal connector is press-fittingly engageable with the exterior surface of the ferrule. If a permanent, rather than detachable electrical connection between the ferrule and the terminal connector is desired, the perimeter terminal of the terminal connector may be joined to the exterior surface of the ferrule by a weld. In this embodiment, a cap of the terminal connector is preferably made of a ceramic material rather than a plastic, in order to prevent damage to the cap during the welding process. 
     To facilitate connection of the terminal connector to an external device to be powered by the cell, the terminal connector preferably further comprises a first conductor extending from the center terminal through a flange of the cap, and a second conductor extending from the perimeter terminal through the cap flange. In one preferred embodiment, the first and second conductors are comprised of elongated tabs adapted for connection into an electrical receptacle. The electrical receptacle typically is comprised of a pair of slots in a circuit board dimensioned to permit the elongated tabs of the conductors to be “plugged” into them. The center contact, the perimeter contact, and the electrical conductor of the terminal connector may be made of stainless steel or titanium. 
     Also according to the present invention, an electrochemical cell is provided comprising a conductive casing closed at one end and having an opening at the opposite end thereof closed by a conductive lid, the conductive lid is comprised of a wall having an inside surface, an outside surface, and an opening therethrough from the inside surface to the outside surface; an electrode assembly housed inside the casing and comprised of a first electrode of a first polarity and including a current collector; a second electrode of a second, opposite polarity; an electrolyte provided in the casing to activate the electrode assembly; a ferrule comprised of a conductive hollow cylindrical body having a central axis, an interior surface, an exterior surface, an outer end, and an inner end, the inner end electrically joined and sealed within the opening through the wall of the conductive lid and being substantially coplanar with the inside surface of the wall of the conductive lid; a conductive center terminal pin comprising an elongated surface, an inner end electrically connected to the current collector, and an outer end, the terminal pin disposed within the hollow cylindrical body of the ferrule and aligned along the central axis thereof, thereby forming an annular cavity between the elongated surface of the terminal pin and the interior surface of the ferrule; and a hermetic seal formed in the annular cavity. The electrochemical cell may further include the aforementioned terminal connector. 
     The electrochemical cell may be configured with a case-negative or case-positive ground configuration, the former being preferred. The case-negative cell may be comprised of a simple rectilinear electrode structure, or the electrodes may be configured in a serpentine or jellyroll arrangement. In the serpentine arrangement, the cell may further comprise a cathode bridge to facilitate connection of a current collector to the multiple cathode plates interleaved between the folds of the serpentine anode. 
     The terminal connectors of the present invention are advantageous because they provide a simple, reliable connection to high capacity, high energy density electrochemical cells, such as e.g., those used in implantable medical devices. The terminal connectors of the present invention are connectable to terminal assemblies disposed on the exterior of the cell. Electrochemical cells using the terminal connectors of the present invention can therefore contain a greater amount of active materials, thereby increasing the incapacity and energy density. 
     The foregoing and additional objects, advantages, and characterizing features of the present invention will become increasingly more apparent upon a reading of the following detailed description together with the included drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be described by reference to the following drawings, in which like numerals refer to like elements, and in which: 
         FIG. 1  is a perspective view of one electrochemical cell including a cell terminal assembly and a mating terminal connector of the present invention separated from the cell&#39;s terminals; 
         FIG. 2  is a perspective view of the cell mated to the terminal connector of  FIG. 1 ; 
         FIG. 3  is a side cross-sectional view of the cell&#39;s terminal assembly taken along line  3 - 3  of  FIG. 1 ; 
         FIG. 4  is a side cross-sectional view taken along line  4 - 4  of  FIG. 2  showing the cell&#39;s terminal assembly mated to the terminal connector; 
         FIG. 4A  is a side cross-sectional view of a cell that is an alternative to the cell of  FIG. 4 , the cell including a terminal assembly mated to the terminal connector depicted also in  FIG. 4 ; 
         FIG. 5  is a top view of the terminal connector of  FIGS. 1 ,  2 ,  4 , and  4 A, depicting conductors provided for connection to a device to be powered by an electrochemical cell; 
         FIG. 6  is a side cross-sectional view of the terminal connector of  FIG. 2  and  FIG. 5 , taken along line  6 - 6  of  FIG. 5 ; 
         FIG. 7  is a side cross-sectional view of a first alternative embodiment of the terminal connector of the present invention connected by a press fit to a cell&#39;s terminal assembly; 
         FIG. 8  is a side cross-sectional view of a second alternative embodiment of a terminal connector of the present invention connected by welding to a cell&#39;s terminal assembly; 
         FIG. 9  is a side cross-sectional view of a third alternative embodiment of a terminal connector of the present invention using a BNC configuration connected to a cell&#39;s terminal assembly; and 
         FIGS. 10A-10C  are detailed side elevation views of the engagement means for securing the mating terminal connector of  FIG. 9  to the cell&#39;s terminal assembly. 
     
    
    
     The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to the embodiment described. On the contrary, the intent is to cover all alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For a general understanding of the present invention, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. In describing the present invention, a variety of terms are used in the description. 
     As used herein, the term “active material” is meant to indicate a cathode or anode material used in an electrochemical cell, which participates in the oxidation-reduction reaction(s) therein, thereby resulting in the discharge of energy from the electrochemical cell. 
     The present invention provides a direct and simple means of connecting a cell to a device. The present invention adapts the terminal feed-through system that is used in case-grounded implantable grade cells. That system includes a glass-to-metal seal comprising a cylindrical metal ferrule, a non-conductive hermetic sealing material, and a centralized terminal pin. The pin and ferrule are of opposed polarity and are electrically insulated from one another with the hermetic sealing material. The seal also holds the terminal pin concentric with the ferrule and hermetically separates the internal cell components from their outside environment. Typically, the ferrule is contained within the cell body, and cannot be used as a terminal circuit connector. 
     In that respect, the cell&#39;s terminal assembly is characterized by a cylindrical metal ferrule that protrudes from the outer surface of the casing, and the cell lid in particular. The ferrule is made of the same conductive material as the casing and lid to which it is joined. The terminal assembly is configured to accept a mating terminal connector connected to the circuitry of the device to be powered by the cell. The ferrule of the cell&#39;s terminal assembly thus eliminates the need to provide a second terminal pin extending from the cell lid or casing. 
     The outer and inner geometry of the ferrule may be threaded or straight walled, but must correspond with the configuration of the mating terminal connector connectable to the device to be powered. The ferrule may be configured for a mechanical connection (e.g., a press-fit, a threaded, of a BNC-type connection), or a soldered, brazed, or welded connection. 
       FIG. 1  is a perspective view of one electrochemical cell including a cell terminal assembly and a mating terminal connector.  FIG. 2  is a perspective view of the cell and mating terminal connector of  FIG. 1 , wherein the mating terminal connector is shown connected to the cell. Cell  10  is comprised of a conductive casing  12  closed at lower end  14 , and having walls  16  and  17 , and an opening  19  at the upper end  18  thereof. The casing  12  is closed at opening  19  by a conductive lid  20 , which is sealed to casing  12  by suitable means such as e.g. seam weld  15  (see  FIG. 3 ). Cell  10  further comprises an electrode assembly housed inside casing  12  and comprised of a first electrode of a first polarity including a current collector  40 , a second electrode of a second, opposite polarity (not shown), and an electrolyte (not shown) provided in the casing  12  to activate the electrode assembly. 
       FIG. 3  is a side cross-sectional view of the cell terminal assembly taken along line  3 - 3  of  FIG. 1 . The terminal assembly  100  comprises a ferrule  110  comprised of a conductive hollow cylindrical body  112  having a central axis  199 , an interior surface  114 , an exterior surface  116 , an outer end  118 , and an inner end  119 , the inner end  119  electrically joined and sealed within an opening  22  through the wall  24  of the conductive lid  20  and substantially coplanar with the inside surface  26  of the lid wall  24 . The coaxial connector assembly  100  further comprises a conductive terminal pin  130  comprising an elongated surface  132 , an inner end  134 , and an outer end  136 . The pin  130  is disposed within the hollow cylindrical body  112  of the ferrule  110  and aligned along the central axis  199  thereof, thereby forming an annular cavity  129  between the elongated surface  132  of the terminal pin  130  and the interior surface  114  of the ferrule  110 . Ferrule  110  and center pin  130  are thus coaxial, sharing a common central axis  199 . 
     Coaxial connector assembly  100  further comprises a hermetic sealing material  140  disposed in the annular cavity  129  to form a glass-to-metal seal (GMTS) with the ferrule  110  and terminal pin  130 . As used herein, a GMTS is to be considered as being completely impervious to the passage of liquid or gas either into or out of the cell, and completely resistant to corrosion or other degradation by materials within the cell, or in the external environment. The sealing material  140  is formed of electrically insulative material, so that terminal pin  130  is electrically insulated from ferrule  110 . In one preferred embodiment, the sealing material  140  is a glass seal, such as is described and shown in the aforementioned U.S. Pat. No. 5,250,373 to Muffoletto et al. 
     The inner end  119  of ferrule  110  is preferably joined to lid  20  by a peripheral seam weld  29  that is flush with the inside surface  26  of the lid  20 . Ferrule  110  is thus electrically connected and sealed to lid  20 . 
     Terminal pin  130  is electrically connected to one of the electrodes of cell  10 . In the embodiment depicted in  FIG. 3 , terminal pin  130  extends downwardly into the cell  10 , and terminal pin  130  is joined to a coupling element  42  extending from the current collector  40  by suitable means such as e.g., welding. In other embodiments, the coupling element  42  of current collector  40  may be formed as a hollow sleeve, and the inner end  134  of terminal pin  130  may be roughened and fitted within a sleeve, as is shown and described in the aforementioned U.S. Pat. No. 6,929,881 to Wutz et al. 
     It is preferable that the conductive materials of connector assembly  100  are made of the same material as the lid  20 . In one embodiment in which lid  20  is made of titanium, ferrule  110  and center pin  130  are also made of titanium. In another embodiment in which lid  20  is made of stainless steel, ferrule  110  is made of stainless steel. In this embodiment terminal pin  130  may also made of stainless steel, or molybdenum. It is to be understood that these materials are to be considered exemplary, and not limiting, and that other suitable metallic materials may be used that are formable to precise dimensions and resistant to degradation by materials within the cell and/or materials in the external environment. 
       FIG. 4  is a side cross-sectional view of the cell&#39;s terminal assembly  100  connected to the mating terminal connector  200 . The mating terminal connector  200  is adapted to be connected to coaxial connector assembly  100 , and more specifically, the ferrule  110  and the conductive center pin  130  thereof. Terminal connector  200  is comprised of a non-conductive cap  210  including a hollow cylindrical body  212  covered at an outer end  214  by a flange  216 , a center terminal  220  embedded in the flange  216 , and a perimeter terminal  230  disposed within the hollow cylindrical body  212  of the cap  210 . When the mating terminal connector  200  is connected to the ferrule  110 , the exterior surface  116  of the ferrule  110  is brought into contact with the perimeter terminal  230  and the terminal pin  130  is brought into contact with the center terminal  220 , thereby enabling electrical conduction from the cell  10  through the center terminal pin  130  and through the body  112  of the ferrule  110 . 
     In one embodiment depicted in  FIGS. 3 ,  4 , and  4 A, the exterior surface  116  of the ferrule  110  is provided with threads  122  and the inside of the hollow cylindrical body  212  of the mating terminal connector is correspondingly provided with matching threads  232  to enable a threaded connection between the terminal connector  200  and the ferrule  110 . 
     In another embodiment, the ferrule is formed as the male fitting of a Bayonet Neill Concelman connector. Such a connector, also known as a BNC connector, is a standard industrial electrical connector often used in the transmission of high fidelity electrical signals, such as radio frequency signals. The BNC connector is a type of bayonet connector, named after the resemblance to the standard twist-on attachment for a bayonet. 
       FIG. 9  is a side cross-sectional view of a BNC connector embodiment of the terminal connector of the present invention connected to a corresponding alternative BNC mating connector.  FIGS. 10A-10C  are detailed side elevation views of the engagement of means for securing the terminal connector  201  of  FIG. 9  to the cell&#39;s terminal assembly  101 . The terminal assembly  101  is similar to the terminal assembly  100  previously described herein and shown in  FIGS. 3-4A , and is comprised of ferrule  111 , terminal pin  130 , and hermetic sealing material  140 . Ferrule  111  includes an exterior surface  116 , an interior surface  114 , an outer end  118 , and an inner end  119 . Mating terminal connector  201  is similar to terminal connector  200  previously described herein and shown in  FIG. 4 , and is comprised of non-conductive cap  211  including a hollow cylindrical body  212  covered at an outer end  214  by a flange  216 , a center terminal  220  embedded in the flange  216  and a perimeter terminal  231  disposed within the hollow cylindrical body  212  of the cap  211 . 
     In lieu of threads  122  as shown on ferrule  110  of the terminal assembly  100  of  FIG. 3 , ferrule  111  is provided with a smooth exterior surface  116 , from which protrudes a first pin  124  and a second pin  126 , spaced at about 180 degrees opposite each other. When the terminal connector  201  is first brought into contact with the cell&#39;s terminal assembly  101 , pin  124  is separated slightly from an engagement latch  233  formed on the inner surface  234  of perimeter terminal  231 , as shown in  FIG. 10A . A twisting motion of the terminal connector  201  relative to terminal connector  101  is performed, as indicated by arrows  99  and  98 , causing pin  124  and engagement latch  233  to approach each other as indicated by arrows  97  and  96 . When pin  124  makes contact with engagement latch  233  as indicated in  FIG. 10B , the tang  235  is displaced diagonally upwardly relative to pin  124  as indicated by arrows  95  and  94 . When pin  124  reaches slot  236 , pin  124  pops into slot  236  as indicated by arrow  93  in  FIG. 10C , and pin  124  is engaged with tang  235  of engagement latch  233 . In a like manner, a similar engagement latch (not shown) is provided on the inner surface  234  of perimeter terminal  231  about 180 degrees opposite latch  233  for engagement with pin  126  of ferrule  111  of the terminal connector  101 . 
     Referring again to  FIG. 10C , in an alternative embodiment of the BNC connector configuration, perimeter terminal  231  is provided with a pair of slots such as slot  237  with which pins  124  and  126  are engaged. Such a slotted configuration is commonly used in the female BNC connectors such as mating terminal connector  201 . 
     Referring again to  FIG. 9 , for either configuration of the BNC connector assembly, the mating terminal connector  201  is preferably provided with an elastomeric flat washer  219 , which provides an axial force that helps to secure connector  201  to the terminal assembly  101 . When mating terminal connector  201  is brought into contact with the cell&#39;s terminal assembly  101  and twisted in the previously described joining motion, elastomeric washer  219  is compressed. When the mating terminal connector  201  is fully engaged with the cell&#39;s terminal assembly  101  by the action of engagement latches or slots with pins  124  and  126 , elastomeric washer  219  remains compressed, thereby providing axial biasing forces against both the mating terminal connector  201  and terminal assembly  101 . The biasing force applied by the elastomeric washer  219  to the terminal connector  201  is indicated by arrow  92 , while the force applied by the washer  219  against the cell&#39;s terminal assembly  101  is indicated by arrow  91 . These forces serve to retain pins  124  and  126  securely in the engagement latches  235 , or in the slots  237  provided in the perimeter terminal  231  of the mating connector. It will be apparent that the mating terminal connector  201  is separable from the terminal assembly  101  by the application of a compressive axial force upon washer  219 , and a twisting motion applied to mating terminal connector  201  that is opposite to the twisting motion previously described. 
     In another embodiment of the present invention, the ferrule is provided with a smooth cylindrical surface, and the perimeter terminal of the terminal connector is provided with a smooth inner surface dimensioned such that the mating connector is press-fittingly engageable with the exterior surface of the ferrule.  FIG. 7  is a side cross-sectional view of this alternative embodiment of the cell&#39;s terminal assembly connected by a press fit to a corresponding alternative mating terminal connector. The cell&#39;s terminal assembly  102  is similar to the terminal assembly  100  previously described herein and shown in  FIGS. 3-4A , and is comprised of ferrule  113 , terminal pin  130 , and hermetic sealing material  140 . Ferrule  113  includes an exterior surface  116 , an interior surface  114 , an outer end  118 , and an inner end  119 . The terminal connector  202  is similar to the terminal connector  200  previously described herein and shown in  FIG. 4 , and is comprised of non-conductive cap  210  including a hollow cylindrical body  212  covered at an outer end  214  by a flange  216 , a center terminal  220  embedded in the flange  216  and a perimeter terminal  238  disposed within the hollow cylindrical body  212  of the cap  210 . 
     In lieu of threads  122  as shown on ferrule  110  of the terminal connector  100  of  FIG. 3 , ferrule  113  of terminal assembly  102  of  FIG. 7  is provided with a smooth exterior surface  116 . Mating terminal connector  202  is provided with a corresponding smooth cylindrical interior surface  239  that is slightly smaller in diameter than the diameter of surface  116  of ferrule  113 , such that the terminal connector  202  may be fitted to the cell&#39;s terminal assembly  102  by a mild press fit of ferrule  113  within perimeter terminal  238 . As used herein, a mild press fit is a fit between two parts that is achievable by the application of forces easily delivered by a person&#39;s thumb and fingers. It will be apparent that the terminal connector  202  is separable from ferrule  113  by the application of similar forces applied in the opposite direction to those used in the connection with the cell&#39;s terminal assembly. 
     If a permanent, rather than detachable electrical connection between the ferrule and the terminal connector is desired, the perimeter terminal of the mating terminal connector may be joined to the exterior surface of the ferrule by welding, soldering, or brazing.  FIG. 8  is a side cross-sectional view of this alternative embodiment of the cell&#39;s terminal assembly  103  connected by welding to a corresponding alternative mating terminal connector. The cell&#39;s terminal assembly  103  is similar to the terminal assembly  102  previously described herein and shown in  FIG. 7 , and is comprised of ferrule  113 , terminal pin  130 , and hermetic sealing material  140 . Ferrule  113  includes an exterior surface  116 , an interior surface  114 , an outer end  118 , and an inner end  119 . Mating terminal connector  203  is similar to the terminal connector  202  previously described herein and shown in  FIG. 7 , and is comprised of non-conductive cap  213  including a hollow cylindrical body  215  covered at an outer end  217  by a flange  218 , a center terminal  220  embedded in the flange  218  and a perimeter terminal  240  disposed within the hollow cylindrical body  215  of the cap  213 . 
     The mating terminal connector  203  is fitted to the cell&#39;s terminal assembly  103  by a mild press fit or a sliding fit between the interior surface  242  of perimeter terminal  240  and the exterior surface  116  of ferrule  113 . To effect the permanent attachment of the terminal connector  203  to the cell&#39;s terminal assembly  103 , a weld  243  is made which joins perimeter terminal  240  to ferrule  113 . Weld  243  is preferably a plug weld made within a hole  244  that is provided in perimeter terminal  240  prior to the assembly operation. In the preferred embodiment, an additional plug weld  246  is made at a location 180 degrees around perimeter terminal  240 ; and additional plug welds (not shown) are made at equally spaced locations, such a e.g., four welds spaced at 90 degree intervals. 
     In the embodiment depicted in  FIG. 8 , the cap  213  of the terminal connector  203  is preferably made of a ceramic material rather than a plastic, in order to prevent melting, decomposition or other thermal damage to cap  213  during the welding process. Suitable ceramic materials include, but are not limited to alumina, titania, zirconia, silicon nitride, and silicon carbide. 
     Each of the terminal assemblies and mating terminal connectors depicted in  FIGS. 4 ,  7 ,  8 , and  9  are dimensioned to ensure that an effective electrical contact (i.e. one essentially of zero resistance) is made between the cell&#39;s terminal pin and the center terminal of the mating terminal connector, and between the cell&#39;s ferrule and the perimeter terminal of the terminal connector. For the contact between the cell ferrule and the perimeter terminal of the mating terminal connector, for each of the embodiments depicted in  FIGS. 4 ,  7 ,  8 , and  9 , such effective electrical contact is easily ensured, by virtue of the extensive engagement of the respective surfaces with each other. 
     In the embodiment  150  of  FIG. 4 , threads  232  of the mating terminal connector  200  make firm and extensive contact with threads  122  of ferrule  110 . In the embodiment of  FIG. 7 , interior surface  239  of perimeter terminal  238  of the mating terminal connector  202  makes firm and extensive contact with surface  116  of the cell&#39;s ferrule  113 , particularly due to the press fit between ferrule  110  and perimeter terminal  238 . In the embodiment of  FIG. 8 , interior surface  242  of perimeter terminal  240  of the terminal connector  203  makes firm and extensive contact with surface  116  of the cell&#39;s ferrule  113 , particularly due to the welds  243  and  246  between ferrule  113  and perimeter terminal  240 . In the embodiment of  FIG. 9 , interior surface  234  of perimeter terminal  231 of the mating terminal connector  201 makes firm and extensive contact with surface  116  of the cell&#39;s ferrule  111 , particularly due to engagement of pins  124  and  126  of ferrule  113  and the engagement latches  235  or slots  237  of the perimeter terminal  231 . 
     For the contact between the terminal pin  130  of the cell&#39;s terminal assembly and the center terminal  220  of the mating terminal connector, for each of the embodiments depicted in  FIGS. 4 ,  7 ,  8 , and  9 , such effective electrical contact must be attained by proper dimensioning of the respective terminal assemblies and mating terminal connectors. This point will be illustrated with reference to the embodiment of  FIG. 4 , but is equally applicable to those of  FIGS. 7 ,  8 , and  9 . 
     Referring to  FIG. 4 , when the mating terminal connector  200  is fitted to the cell&#39;s terminal assembly  100 , center electrical contact  220  must be brought into forced contact with outer end  136  of terminal pin  130 , in order to achieve effective electrical contact between these two conductors. This is most simply accomplished by having outer end  136  of terminal pin  130  function as a stop against center electrical contact  220  of the mating terminal connector  200 . In other words, when the mating terminal connector  200  is fitted to the cell&#39;s terminal assembly  100 , it is the outer end  136  of the terminal pin  130  coming into contact with center electrical contact  220  that stops the axial movement of the terminal connector  200  toward the cell&#39;s terminal assembly  100 . 
     In one embodiment, center electrical contact  220  is made flush (i.e. coplanar) with inner surface  209  of flange  210  of the mating terminal connector  200 . In order for the outer end  136  of the terminal pin  130  to act as a stop against center electrical contact  220  of the mating terminal connector  200 , outer end  136  of terminal pin  130  protrudes slightly beyond the plane defined by the end  118  of ferrule  100 . Accordingly, when the terminal connector  200  is threadedly engaged with ferrule  200 , the rotation of mating connector  200  on ferrule  110  will be stopped because of contact between the outer end  136  of the terminal pin  130  and center electrical contact  220  of the mating terminal connector  200 . A small additional tightening torque applied to the terminal connector  200  will firmly force center electrical contact  220  against the end  136  of the terminal pin  130 , thereby ensuring effective electrical contact there between. In this embodiment, it is preferred that the outer end  136  of the terminal pin  130  protrudes at least about  0 . 020  inches beyond the plane defined by the end  118  of ferrule  100  in order to provide effective electrical contact. 
     In another embodiment (not shown), the outer end  136  of the terminal pin  130  may be recessed beneath the plane defined by the outer end  118  of ferrule  110 . In this embodiment, center electrical contact  220  of the mating terminal connector  200  protrudes beyond inner surface  209  of flange  210  a corresponding distance such that when the terminal connector  200  is threadedly engaged with ferrule  200 , the rotation of the connector  200  on ferrule  110  is stopped by contact between the outer end  136  of the terminal pin  130  and center electrical contact  220  of the terminal connector  200 . The distance which center electrical contact  220  protrudes beyond inner surface  209  of flange  210  is preferably at least about 0.020 inches greater than the distance that outer end  136  is recessed beneath the plane defined by the outer end  118  of ferrule  110 . In this manner, substantially the same effective electrical contact is provided between the end  136  of the terminal pin  130  and the center electrical contact  220  as is provided in the previously described embodiment. 
     It will be apparent that although the attainment of effective electrical contact between the terminal pin  130  of the cell&#39;s terminal assembly  100  and the center electrical contact  220  of the mating terminal connector  200  has been illustrated with reference to the embodiment of  FIG. 4 , these principles are equally applicable to the embodiments of  FIGS. 7 ,  8 , and  9 . 
     To facilitate connection of the cell&#39;s terminal assembly to an external device to be powered by the cell, the mating terminal connector preferably further comprises a first conductor extending from the center terminal through the flange of the cap, and a second conductor extending from the perimeter terminal through the flange of the cap.  FIG. 5  is a top view of the mating terminal connector  200  of  FIGS. 1 ,  2 ,  4 , and  4 A, depicting such conductors provided for connection to a device to be powered by a cell.  FIG. 6  is a side cross-sectional view of the terminal connector  200  of  FIG. 2  and  FIG. 5 , taken along line  6 - 6  of  FIG. 5 . 
     Referring in particular to  FIGS. 4 ,  5 , and  6 , center contact  220  is joined to first conductor  250 , which comprises an elongated horizontal bar  252  that extends from an upper edge  222  of center contact  220 , an elbow  253 , and an elongated vertical tab  254 . The majority of elongated horizontal bar  252  is embedded within flange  216  of cap  210 , with a short section protruding from flange  216  proximate to elbow  253 . In the preferred embodiment, center contact  220  and first conductor  250  are integrally formed as a single piece, and insert molded into the desired position when cap  210  is formed. Cap  210  is preferably made of an injection moldable or curable polymer, such as polysulfone, polyetheretherketone, or polyurethane. 
     Perimeter contact  230  is joined to second conductor  260 , which comprises a short vertical section  261  that extends from an upper edge  248  of perimeter contact  230 , an elongated horizontal bar  262 , an elbow  263  (see also  FIG. 1 ), and an elongated vertical tab  264 . The majority of elongated horizontal bar  262  is embedded within flange  216  of cap  210 , with a short section protruding from flange  216  proximate to elbow  263 . In the preferred embodiment, perimeter contact  230  and second conductor  260  are also integrally formed as a single piece, and insert molded into the desired position when cap  210  is formed. 
     The elongated tabs  254  and  264  of the first and second conductors  250  and  260  are adapted for connection into an electrical receptacle (not shown). The electrical receptacle typically is comprised of a pair of slots in a circuit board dimensioned so as to permit the elongated tabs of the conductors to be “plugged” into them. The center contact  220 , the perimeter contact  230 , and the electrical conductors  250  and  260  of the mating terminal connector may be made of stainless steel or titanium, or other suitable metals are described previously herein for the ferrule  110  and the terminal pin  130 . 
     The embodiments described herein may be used with a variety of cells. In general, such a cell is comprised of a conductive casing closed at one end and having an opening at the opposite end thereof closed by a conductive lid. The conductive lid is comprised of a wall having an inside surface, an outside surface, and an opening therethrough from the inside surface to the outside surface to which the cell&#39;s terminal assembly is joined and sealed; an electrode assembly housed inside the casing and comprised of a first electrode of a first polarity including a currant collector and a second electrode of a second, opposite polarity. The current collector includes a coupling element for connection to the terminal pin. The open volume within the conductive casing is filled with an electrolyte to activate the electrode assembly. 
     The cell may be configured with a case-negative or case-positive ground configuration. The cell may be comprised of a simple sandwich-like rectilinear electrode structure, such as is disclosed in the aforementioned U.S. Pat. No. 5,250,373 to Muffoletto et al. Alternatively, the electrodes may be configured in a jellyroll arrangement, such as is disclosed in U.S. Pat. No. 6,951,576 to Takeuchi, which is assigned to the assignee of the present invention and incorporated herein by reference. In another embodiment, the electrodes may be configured in a serpentine arrangement such as is disclosed in the aforementioned U.S. Pat. No. 6,929,881 to Wutz et al. In a cell in which the electrodes are configured in a serpentine arrangement, the cell may further comprise a cathode bridge to facilitate connection of a current collector to the multiple cathode plates of the cell, as is disclosed in the aforementioned U.S. Pat. No. 6,004,692 to Muffoletto et al. It is to be understood that these suitable cell configurations are to be considered as exemplary and not limiting, and that the terminal connectors may be adapted to many other cell configurations. 
       FIG. 4A  is a side cross-sectional view of a cell that is an alternative to the cell of  FIG. 4 . The cell  10  includes a terminal assembly connected to the mating terminal connector  200  also depicted in  FIG. 4 . Terminal pin  131  differs from terminal pin  130  of the cell of  FIG. 4 , in that it includes an elbow  133  proximate to the inner end  135  thereof. This feature aligns the inner section  137  of the terminal pin  131  with the coupling element  43  of current collector  40 , thereby providing a greater length of contact between the terminal pin  131  and current collector  40  at which a bond may be formed. An example of such a terminal pin and current collector configuration and bond is provided in the aforementioned U.S. Pat. No. 5,250,373 to Muffoletto et al. 
     It is, therefore, apparent that there has been provided, in accordance with the present invention, a coaxial electrical connector fitting for connection to a cell used in implantable medical devices. While this invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims.