Abstract:
A secondary electrochemical cell with at least one positive electrode includes an active material and one negative electrode, which are held directly in a hermetically tight housing which has at least one detector element or which is dynamically connected to one such detector element. The detector element is designed or can be set to acquire a predetermined unallowable operating state of the secondary electrochemical cell, and in an unallowable operating state of the secondary electrochemical cell, to actuate at least one switching element which prevents recharging and/or discharging of the secondary electrochemical cell. By making the housing as a hermetically tight protective housing, which is part of an implantable medical device, contamination of surrounding tissue with toxic substances and hazard to the implant wearer by the malfunction of the secondary electrochemical cell can be precluded under all operating conditions with simultaneously great reduction of all dimensions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a divisional application of U.S. patent application Ser. No. 10/101,193 entitled “Secondary Electrochemical Cell” and filed on Mar. 20, 2002, which is a continuation application of U.S. patent application Ser. 09/627,449, filed Jul. 27, 2000, now abandoned. The entire disclosure and contents of the above applications are hereby incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The invention relates to an secondary electrochemical cell with at least one positive electrode which comprises an active material and one negative electrode, which are held directly in a housing which has at least one detector element or which is dynamically connected to one such detector element. The latter being designed or can be set to acquire a predetermined unallowable operating state of the secondary electrochemical cell and in an unallowable operating state of the secondary electrochemical cell to actuate at least one switching element which prevents recharging and/or discharging of the secondary electrochemical cell.  
         [0004]     2. Description of the Related Art  
         [0005]     In secondary electrochemical cells a pressure rise in the interior of the housing of the secondary electrochemical cell can occur, for example, upon excess charging or discharging with an unallowably high current and it leads to deformation of the housing which can become so large that chemicals, especially in gaseous or liquid form, emerge from the housing in large amounts. Various protective mechanisms have been proposed to increase the operating safety of secondary electrochemical cells.  
         [0006]     Thus, publication EP-A-0 470 726 discloses a secondary electrochemical cell which has a cylindrical housing with a pressure membrane as the detector element. The housing directly accommodates the electrodes of the secondary electrochemical cell, the pressure membrane as part of the housing being integrated on the face side in the housing and bulging when the pressure rises in the interior of the housing, so that a plate-shaped switching element which is centrally connected to the pressure membrane reversibly or irreversibly interrupts the electrical contact between the electrode and a terminal contact which is provided on the outside of the housing as soon as the pressure within the housing reaches a predetermined value.  
         [0007]     EP-A-0 322 112, EP-A-0 360 395 and EP-A-0 370 634 disclose providing electrochemical cells with a switching element which, when a certain still tolerable boundary deformation of the housing is exceeded, preferably irreversibly separates an electrical terminal contact from a pertinent electrochemically active electrode in order to prevent further deformation of the housing. The housing directly accommodates the electrodes and surrounds an electrically conductive cylindrical housing section which is closed on the face side and which makes contact with an electrode, on the end face the plate-shaped switching element being attached centrally by means of an electrically insulating cement from the outside. The electrically, likewise, conductive switching element in its base position, via its outside edge, forms an electrical connection between the housing section and the electrical terminal contact which projects to the outside and which is located in the center of the switching element. When the pressure rises within the housing the face side of the housing section acting as the detector element bulges to the outside, by which the contact between the housing section and the outside edge of the switching element and, thus, between the one electrode and the terminal contact is interrupted. One typical application of these switching elements is standard D cells. The bulging of the face side, starting from which the switching element interrupts electrical contact, in this case is 0.76 mm to 1.8 mm. For bulging of more than 1.8 mm emergence of chemicals from within the housing can ordinarily be expected.  
         [0008]     EP-A-0 674 351 discloses a secondary electrochemical cell with a housing which comprises a cutting device which can be actuated by a pressure membrane and which, when a boundary pressure is exceeded within the housing, irreversibly, severs an electrical conductor which connects the terminal contact of the secondary electrochemical cell to an electrochemically active electrode.  
         [0009]     Also, when using a switching element which interrupts the electrical connection between the terminal contact and the pertinent electrochemically active electrode when a certain pressure within the housing is exceeded, it is possible for the pressure to continue to rise and ultimately for chemicals to emerge from the housing of the electrochemical cell or for the latter even to explode. For this reason, for example in EP-A-0 364 995, EP-A-0 573 998 or EP-A-0 739 047 it is proposed that a pressure membrane which actuates the switching element and which is integrated into the housing be provided with a bursting area via which after activation of the switching element and a further pressure rise chemicals can emerge from the interior of the housing.  
       SUMMARY OF THE INVENTION  
       [0010]     The mechanisms cited in the aforementioned prior art for increasing the operating safety of secondary electrochemical cells are not suited for those cells which are used as part of implantable medical devices, since especially high demands must be met for this purpose, especially with regard to safety and reliability, with a simultaneously very extensive reduction of all dimensions. Thus, for example, contamination of surrounding tissue with toxic substances and hazard to the implant wearer by the malfunction of the secondary electrochemical cell must be precluded under all operating conditions. In electrochemical cells with a structure according to the described prior art this cannot be done since it happens that especially toxic gases emerge in intolerably large amounts from the housing of the electrochemical cells even during proper, normal operation of the electrochemical cell.  
         [0011]     U.S. Pat. No. 6,143,440 discloses a protective device for a repeatedly rechargeable electrochemical battery with a battery housing is disclosed, the protective device having at least one switching element which can be actuated by a detector element and which is designed or can be set to prevent recharging and/or discharging of the battery in a predetermined unallowable operating state thereof. Here the protective device comprises a hermetically tight protective housing which holds the battery housing, impresses on the detector element a change of shape in an unallowable operating state of the battery, and is part of an implantable device. Especially in a predetermined unallowably large expansion in the volume of the battery housing or emergence of gas from the interior of the battery housing which leads to an unallowably large pressure rise in the interior of the protective housing, a change of shape is impressed on the detector element by the protective housing, which change actuates the switching element. The latter can be made as a make contact which electrically short circuits a recharging circuit which can be supplied by means of a charging device.  
         [0012]     The invention is a secondary electrochemical cell which meets the specific requirements for parts of implantable medical devices, and at the same time the construction effort is to be minimized.  
         [0013]     The invention is a secondary electrochemical cell with the housing being made as a hermetically tight protective housing which is part of an implantable medical device.  
         [0014]     All chemical substances of the secondary electrochemical cell are securely retained in the protective housing by the housing which directly accommodates at least one positive and at least one negative electrode of the secondary electrochemical cell being made as a hermetically tight protective housing.  
         [0015]     Here hermetic tightness is preferably defined as hermetic gas tightness as per Mil-Std 883 D. This ensures that when using the secondary electrochemical cell in an electronic implant which is itself in turn accommodated in a hermetically tight and furthermore biocompatible housing, besides liquid toxic substances, no gases can emerge from the protective housing of the secondary electrochemical cell either. The hermetic gas tightness of the protective housing reliably prevents danger to the surrounding electronics. This means that the electronic circuits, especially integrated circuits, can remain unprotected since contamination even by extremely small amounts of emerging gases is not possible. If the secondary electrochemical cell is not accommodated in the housing of an electronic implant, but is implanted directly in its protective housing, the hermetic gas tightness as per Mil-Std 883 D precludes corresponding contamination of the surrounding tissue. In this case, at least the outside of the protective housing as a whole is furthermore made biocompatible.  
         [0016]     The concept of direct accommodation of at least one positive electrode which comprises an active material and a negative electrode in the housing of the secondary electrochemical cell which has been made as a protective housing is defined here in that between the hermetically tight protective housing and the electrodes there is no further housing which holds the electrodes, especially no housing which is used essentially to accommodate mechanical loads such as for example to accommodate compressive forces by gas evolution or the like of the secondary electrochemical cell and/or which has an electrically conductive housing section with an inside which makes contact with an electrode. This does not preclude electrical insulation or the like between at least one electrode and the protective housing, or electrical insulation and/or flexibly soft jacketing surrounding the electrodes. Furthermore, in the protective housing there can be mechanical internals and/or it can be divided into several interior volumes, and one interior volume throughout can directly accommodate only one of the electrodes.  
         [0017]     It goes without saying that the protective housing, besides the electrodes, accommodates other components of the secondary electrochemical cell which are necessary for the sequence of electrochemical reactions on the electrodes. This applies especially to an electrolyte and optionally to a separator (diaphragm), which latter can be omitted when its function as an electrical insulator which allows ion migration is assumed by the electrolyte, as is the case for example in polymer electrolytes.  
         [0018]     This invention is not limited to certain electrodes/electrolyte systems, but rather fundamentally any common systems can be used. Examples include the following: nickel-cadmium systems (sinter electrodes: ground electrodes; liquid, pasty or solid electrolyte systems; separator); nickel-metal hydride systems (sinter electrodes: ground electrodes; liquid, pasty or solid electrolyte systems; separator); lithium-based systems ((a) lithium metal or alloy anode, inorganic or organic dispersion or redox or other cathode; liquid, gel, pasty or solid electrolyte systems; optionally separator; or (b): lithium intercalation anode, inorganic or organic dispersion or redox or other cathode; liquid, gel, pasty or solid electrolyte systems; optionally separator); nickel-iron systems; nickel-zinc systems; zinc-silver oxide systems; cadmium-silver oxide systems; zinc-manganese dioxide systems; redox systems such as, for example, quinone/hydroquinone systems; or zinc-mercury oxide systems and silver-metal hydride systems.  
         [0019]     Within the framework of this invention, more than only one positive and one negative electrode can also be used, the electrodes being accommodated directly by the protective housing and in the conventional manner they can be series-connected or arranged in bipolar configuration for voltage multiplication or can be connected in parallel for capacitance multiplication. A combination of these types of connection is likewise possible.  
         [0020]     In the implantable medical device among others it can be an active electronic hearing implant, a cardiac pacemaker, a drug dispenser, a neurostimulator or the like.  
         [0021]     Recharging of the secondary electrochemical cell is preferably prevented by making at least one switching element as a break contact which is designed or which can be set to electrically interrupt a recharging circuit which can be supplied by means of a charging device in an unallowable operating state of the secondary electrochemical cell.  
         [0022]     At least one switching element can furthermore be made as a break contact which is designed or which can be set to electrically interrupt a consumer circuit which is connected to the secondary electrochemical cell in an unallowable operating state of the secondary electrochemical cell. Thus, it is possible, for example, to prevent in case of a short circuit in the consumer circuit an overly high electrical power from being withdrawn from the secondary electrochemical cell. This, likewise, can cause an unallowable operating state of the secondary electrochemical cell, for example, in the form of exceeding a stipulated boundary temperature or boundary pressure within the hermetically tight protective housing. It is also conceivable for the break contact to be arranged such that it simultaneously interrupts the recharging circuit with the consumer circuit.  
         [0023]     In another preferred embodiment of the invention at least one switching element is made as a make contact which is designed or which can be set to electrically short circuit a recharging circuit which can be supplied by means of a charging device in an unallowable operating state of the secondary electrochemical cell, by which further energy supply to the secondary electrochemical cell is interrupted.  
         [0024]     Furthermore, there can be at least one switching element which is made as a make contact which is designed or which can be set to electrically short circuit the secondary electrochemical cell in an unallowable operating state thereof. This make contact is especially feasible in serial coupling to an overcurrent fuse which can irreversibly interrupt a current in a recharging circuit and/or consumer circuit of the secondary electrochemical cell. The overcurrent fuse can be made as a fusible link which is burned through when the residual energy stored in the secondary electrochemical cell is short circuited. If the energy is not sufficient to do this, the secondary electrochemical cell can be completely discharged. It is likewise possible for at least one overcurrent fuse to simply limit the current in the recharging circuit and/or consumer circuit, advantageously the overcurrent fuse being made as a cold conductor (PTC resistor) with a resistor which increases as the temperature increases.  
         [0025]     The switching elements can fundamentally, reversibly or irreversibly, interrupt a circuit into which they have been incorporated. Reversibly operating switching elements offer the advantage that their operation can be tested nondestructively, for example in the assembled state in interaction with other components of the secondary electrochemical cell or the implantable medical device.  
         [0026]     If the stipulated unallowable operating state of the secondary electrochemical cell is a boundary temperature as the stipulated still allowable temperature being exceeded at a defined position of the secondary electrochemical cell, at least one detector element is designed or can be set to acquire the temperature of the secondary electrochemical cell, especially within the protective housing, and when an unallowable temperature is reached to actuate at least one switching element.  
         [0027]     Evolution of gas and/or swelling of the electrodes and/or a temperature rise during operation of the secondary electrochemical cell can lead to an increase of the pressure within the protective housing. If a boundary temperature as the stipulated still allowable temperature within the protective housing being exceeded is chosen as the criterion for the stipulated unallowable operating state of the secondary electrochemical cell, at least one detector element is designed or can be set to acquire the pressure in the interior of the protective housing which houses at least one electrode and when an unallowable pressure is reached to actuate at least one switching element. If the protective housing, which is hermetically tight to the outside, is divided within into several sections, which are sealed relative to one another without, however, the need to ensure hermetic tightness among one another, the pressure can preferably be acquired in one of the sections. Swelling of the electrodes, which lead to expansion of their volume and the resulting pressure rise within the protective housing, can be acquired via a detector element, also by direct interaction of the detector element with at least one electrode by, preferably, positioning the detector element such that at least one electrode in an unallowable operating state of the secondary electrochemical cell adjoins the detector element, possibly with the interposition of electrical insulation, and impresses on it at least one change of shape which actuates the switching element.  
         [0028]     At least one detector element is preferably designed or can be set such that in an unallowable operating state of the secondary electrochemical cell a change in shape is impressed on the element, which change of shape actuates at least one switching element. In doing so, the detector element can be arranged and/or made such that an elastic and/or plastic shape change is impressed on it, depending on the internal pressure and/or the temperature in the protective housing and/or swelling of the electrodes. Feasibly the shape change which is impressed on the detector element in an unallowable operating state of the secondary electrochemical cell mechanically actuates at least one switching element. Direct mechanical actuation of the switching element by the change in shape of the detector element is especially reliable since it works necessarily without being dependent on nonmechanical transmission elements.  
         [0029]     This does not preclude the fact that especially when two or more switching elements are redundantly present, at least one switching element can be actuated by means of evaluation electronics which acquire the change in shape of the detector element. Advantageously, an electrical extensometer is used which acquires the change in shape of the detector element and responds with a change in an electrical quantity which is monitored by the evaluation electronics. If the electrical extensometer is a passive system, it can convert the change in shape of the detector element into a change in its electrical resistance (strain gauge), its inductance or its capacitance. Alternatively, an active extensometer can be used which, like a piezoelement, reacts with a change in charge to the change in shape which is applied to the extensometer by the detector element.  
         [0030]     In another embodiment of the invention at least one detector element is part of the protective housing and is made especially as a bulging membrane which preferably forms the outside or separating wall of the hermetically tight protective housing. At least one detector element can be accommodated within the protective housing. This allows space-saving construction and an easily predictable change in shape of the detector element when the pressure in the protective housing rises.  
         [0031]     In particular, the thickness of the protective housing, which is especially critical for an implantable medical device, can be minimized when at least one membrane is integrated into a side wall of the hermetically tight protective housing, the direction of bulging of the membrane running essentially perpendicular to the smallest extension of the protective housing. The cover and bottom of the protective housing which is provided in the direction of the smallest extent of the protective housing above or below the latter then feasiblely remain free of detector elements and switching elements so that the thickness of the protective housing exceeds only slightly that of the electrode/electrolyte arrangement. Furthermore, it is possible to arrange at least one detector element outside the section of the protective housing which houses the electrodes and to bring it into fluid connection with the interior of the protective housing such that there is greater freedom in adaptation to the circumstances of the implantation site. Redundant protection against an unallowable operating state of the secondary electrochemical cell arises when, for example, there are two membranes, of which one is made as part of a make contact, and by means of this membrane, in an unallowable operating state of the secondary electrochemical cell one contact pair of the make contact can be electrically short circuited. The second membrane can be made as part of a break contact and, by means of this second membrane, in an unallowable operating state of the secondary electrochemical cell one contact pair of the break contact can be electrically disengaged.  
         [0032]     The protective housing under all operating conditions must form a hermetically tight barrier with respect to the space located outside the protective housing. In particular, neither energy supply nor energy removal, nor the energy stored in the secondary electrochemical cell and the chemical processes which take place during operation may lead to destruction of the hermetic tightness. Factors which should be considered, in particular, in the design of the protective housing and the choice of materials are: chemical effects of the electrolyte and electrodes on the protective housing at the storage and operating temperature; electrochemical processes within the protective housing as a result of all conceivably possible charging and discharging processes; mechanical loads of the protective housing as a result of chemical, electrochemical or physical (especially thermal) processes such as gas evolution within the protective housing; swelling of the electrodes; changes in the ambient pressure; heating by energy supply or energy removal; and a change of the storage temperature.  
         [0033]     A suitable protective housing can be attained especially as a hermetically tight material interconnection between at least one electrically conductive metallic housing section and at least one electrically insulating inorganic-nonmetallic housing section. The housing sections are preferably welded to one another, but can also be joined to one another hermetically tight by means of a soldering process.  
         [0034]     Materials which are especially chemically inert to the electrodes/electrolyte systems ordinarily used and which are resistant to the electrochemical processes which take place include metals and precious metals which form a passivation layer against chemical decomposition. They include among others: platinum chromium nickel steel, nickel alloys, titanium tantalum and niobium. Preferably, ceramics which can be materially joined hermetically tight to at least one metallic housing section are used as the inorganic-nonmetallic insulator materials.  
         [0035]     In at least one electrically insulating inorganic-nonmetallic housing section at least one single-pole electrically conductive penetration can be accommodated hermetically tight and electrically insulated. In doing so, the insulator material can be used for electrical insulation of the poles of the penetration both among one another and also with regard to adjoining electrically conductive metallic housing sections.  
         [0036]     Preferably, at least one pole of the hermetically tight penetration has a metallic contact pin which penetrates, hermetically tightly, the electrically insulating inorganic-nonmetallic housing section. The contact pins are feasiblely held in the insulating material, for example in a ceramic substrate, and this for its part in the outside wall of the protective housing hermetically tightly by means of a material connection, preferably a solder connection, especially a gold solder connection. Especially suitable materials for the contact pin can be platinum-iridium alloys.  
         [0037]     It can be advantageous for the contact pins to be held hermetically tight by means of a material connection in the electrically insulating inorganic-nonmetallic housing section and the latter in an metallic receiver, the metallic receiver likewise being materially connected to the outside wall of the protective housing. To connect the metallic receiver to the outside wall of the protective housing a weld joint can be used.  
         [0038]     The penetration can fundamentally be made with one or more poles. Advantageously, at least one pole of the hermetically tight penetration having made contact with the electrode of the secondary electrochemical cell. But, the hermetically tight penetration enables not only transmission of energy but also, in principle, transmission of signals through one wall of the protective housing. For example, the potential of a potential probe can be routed through the wall of the protective housing by means of one pole of the penetration. The potential probe being located especially in the electrolyte between one positive and one negative electrode of the secondary electrochemical cell, as is described in EP-A-0 981 262 (corresponding to commonly owned U.S. Pat. No. 6,192,272).  
         [0039]     The dimensions of the electrically insulating section, which is provided in the protective housing and which is formed preferably by a ceramic material, can be minimized and limited to the section which accommodates the penetration when the number of poles of the penetration corresponds to the number of energy and signal taps, via which the secondary electrochemical cell is connected to other components of the implantable medical device. In this case, the remaining wall of the protective housing can be made in a metallic material with an inside which is electrically insulated with reference to the electrodes.  
         [0040]     Also at least one electrically conductive metallic housing section can make contact with an energy or signal tap which is accommodated within the protective housing. In particular, at least one electrically conductive metallic housing section makes electrical contact with one electrode of the secondary electrochemical cell. One especially simple structure of the protective housing arises when the positive and the negative electrode of the secondary electrochemical cell are each connected to an electrically conductive metallic housing section, the two metallic housing sections being electrically insulated against one another, and a penetration can be omitted.  
         [0041]     Preferably, the electrodes conventionally have an electrical tap which for its part makes contact either with one pole of the hermetically tight penetration or an electrically conductive metallic housing section from the inside of the protective housing. Contact of the taps on the electrically conductive housing section or on the pole of the penetration can be accomplished preferably directly via weld or solder connections, or indirectly via electron-conductive intermediate elements which adjoin by force-fit, such as for example springs, pins, metal foams or the like.  
         [0042]     In another embodiment of the invention, to increase the operating safety and to create redundancy, the protective housing is dimensioned such that its hermetic tightness is preserved even if in an unallowable operating state of the secondary electrochemical cell it can continue to be recharged or discharged. This means that even in a malfunction for example of the detector element and/or the switching element the protective housing up to certain upper boundaries withstands the physical loads which occur when the supply or removal of energy is not interrupted even if the secondary electrochemical cell enters an unallowable state.  
         [0043]     If the outside of the protective housing as a whole is made biocompatible, it can be directly implanted and connected to a consumer via electrical lines which are permanently connected or which are detachable. Thus, the implantation site is irrelevant, as is described also in EP-A-0 982 784 (corresponding to commonly owned U.S. Pat. No. 6,269,266). Materials for the biocompatible protective housing can be preferably titanium, titanium alloys, niobium, tantalum, implantable steels or a composite of them or other implantable metallic with ceramic materials such as aluminum oxide ceramics, and jacketing of the entire protective housing with a biocompatible polymer, for example silicone, polytetrafluorethylene (PTFE), polymethane, parylene or the like.  
     
    
       [0044]     These and further objects, features and advantages of the present invention will become apparent from the following description when taken in connection with the accompanying drawings which, for purposes of illustration only, show several embodiments in accordance with the present invention.  
         [0045]      FIG. 1  is a schematic circuit diagram with a secondary electrochemical cell which is incorporated into a consumer circuit and a recharging circuit;  
         [0046]      FIG. 2  is a cross-sectional view of a first embodiment of a secondary electrochemical cell with a hermetically tight protective housing and a reversibly operating switching element;  
         [0047]      FIG. 3  is a perspective view of the embodiment of  FIG. 2 ;  
         [0048]      FIG. 4  is a partial extract of the view of  FIG. 2 ;  
         [0049]      FIG. 5  is a cross-sectional view of a second embodiment of another secondary electrochemical cell;  
         [0050]      FIG. 6  is a cross-sectional view of a third embodiment of a secondary electrochemical cell with a switching element in the closed state;  
         [0051]      FIG. 7  is a cross-sectional view of the embodiment of  FIG. 6  with a switching element in the opened state;  
         [0052]      FIG. 8  is a cross-sectional view of another embodiment of a secondary electrochemical cell with a switching element in the closed state;  
         [0053]      FIG. 9  is a cross-sectional view of the embodiment of  FIG. 8  with a switching element in the opened state;  
         [0054]      FIG. 10  is a cross-sectional view of another embodiment of a secondary electrochemical cell with an irreversibly operating switching element;  
         [0055]      FIG. 11  is a cross-sectional view of another modified embodiment of a secondary electrochemical cell;  
         [0056]      FIG. 12  is a cross-sectional view of a secondary electrochemical cell with a hermetically tight single-pole penetration through the outside wall of a protective housing to form a current path to or from an electrode of the secondary electrochemical cell;  
         [0057]      FIG. 13  is a perspective view of another embodiment of a secondary electrochemical cell with a short circuit switch and a bursting strip as an irreversible break-contact;  
         [0058]      FIG. 14  is a cross-sectional view of the embodiment of  FIG. 13 ;  
         [0059]      FIG. 15  is a perspective view of another embodiment of a secondary electrochemical cell;  
         [0060]      FIG. 16  is a cross-sectional view of a secondary electrochemical cell with a hermetically tight double-pole penetration as a current path to or from one positive and one negative electrode of the secondary electrochemical cell at a time; and  
         [0061]      FIG. 17  is a cross-sectional view of a secondary electrochemical cell with a double-pole penetration, the penetration and two switching and detector elements at a time being integrated in one side wall of the protective housing. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0062]     According to  FIG. 1 , the secondary electrochemical cell as part of an implantable medical device has a protective housing  14  which hermetically tightly accommodates a positive electrode  2 , a negative electrode  4 , an electrolyte  12  and a separator  6  which is used to prevent an electrical short circuit between the electrodes  2 ,  4 . The separator  6  which is positioned between the two electrodes  2  and  4  is used as an electrical insulator which, however, allows ion migration between the electrodes  2  and  4 . The positive electrode  2  and the negative electrode  4  each have one electron-conductive tap  8  and  10 , the tap  8  making contact with an electrically conductive housing section  13  and the tap  10  making contact with an electrically conductive housing section  11  of the protective housing  14 . The two housing sections  11  and  13  are electrically insulated against one another by means of an inorganic-nonmetallic housing section  15  which is preferably made of ceramic material. Another electrically insulating inorganic-nonmetallic housing section  19  holds an electrically conductive penetration which is used to pass a signal from a detector element  21  which is made as a temperature sensor and which records the temperature of the secondary electrochemical cell at a stipulated site within the protective housing  14  and acquires an unallowable operating state of the secondary electrochemical cell in the form of an unduly high temperature.  
         [0063]     The housing sections  11  and  13  on their outsides each have one terminal  18  and  16  via which the negative electrode  4  and the positive electrode  2  are connected to a recharging circuit  22  and a consumer circuit  24 . The protective housing  14  is dynamically connected to another detector element  20  such that, in an unallowable operating state of the secondary electrochemical cell in the form of an unduly high internal pressure within the protective housing  14 , a change in shape is impressed on the detector element  20 . In doing so, gas evolution and/or swelling of the electrodes  2 ,  4  and/or a temperature rise during operation of the secondary electrochemical cell can cause a pressure rise within the protective housing  14 .  
         [0064]     The secondary electrochemical cell in the consumer circuit  24  supplies, via a break contact  50 , a consumer  26  of the implantable medical device which can be, for example, an implantable active hearing aid. The recharging circuit  22  has a receiver coil  28  which, together with a capacitor  30 , forms a serial resonant circuit which is excited by a second serial resonant circuit (not shown) of an external sending part with the same resonance range as is detailed in U.S. Pat. No. 5,279,292. Depending on the phase, the recharging circuit  22  is connected via diodes  32 ,  38  and  34 ,  36 , a break contact  48 , an overcurrent fuse  42  and the electrodes  2 ,  4  which are accommodated in the protective housing  14 . A Zener diode  40  protects the secondary electrochemical cell from overly high voltage of the recharging circuit. Parallel to the receiving coil  28  and the capacitor  30  there is a make contact  44 . Another make contact  46  is located parallel to the secondary electrochemical cell and the overcurrent fuse  42 .  
         [0065]     As shown by the dot-dash line in  FIG. 1 , the detector elements  20  and  21  actuate the switching elements  44 ,  46 ,  48  and  50  which are part of the implantable medical device. This can take place in the detector element  20  either directly, for example by mechanical coupling of the detector element  20  and one or more switching elements, or indirectly via optional evaluation electronics  52  which acquires the change in shape of the detector element  20  and actuates one or more switching elements electrically or electromechanically. The evaluation electronics  52  is, furthermore, used to acquire the signal of the detector element  21  which is made as a temperature sensor and to actuate one or more of the switching elements  44 ,  46 ,  48 , and  50 . It goes without saying that not all switching elements  44 ,  46 ,  48 , and  50  need be present and that the classification, which of the switching elements are actuated directly or via the optional evaluation electronics  52 , can be varied depending on the application and the desired redundancy. It can furthermore be provided that one switching element or several switching elements be actuated only when the two detector elements  20  and  21  signal an unallowable operating state of the secondary electrochemical cell.  
         [0066]     The position of the switching elements  44 ,  46 ,  48  and  50  which is shown in  FIG. 1  corresponds to a base position in normal operation. In the case of an unallowable operating state of the secondary electrochemical cell at least one of these switching elements is actuated.  
         [0067]     A first embodiment of the secondary electrochemical cell is shown in FIGS.  2  to  4  and includes a hermetically tight protective housing  54  with a cup-shaped cover  56  and a likewise cup-shaped bottom  58 . A hollow cylindrical side wall  60  of the cover  56  is welded to a cover plate  62 , which, as a bulging membrane, forms the detector element of the secondary electrochemical cell. The bottom  58  is formed by a hollow cylindrical side wall  64  and a bottom plate  66  which is welded to it. The electrodes  2  and  4  of the secondary electrochemical cell are accommodated within the protective housing  54 , the positive electrode  2  having a tap which is a contact clip  68  and which makes contact with the inside of the side wall  60 . The negative electrode  4  of the secondary electrochemical cell is brought into electrically conductive contact by means of a contact clip  70  as a tap from the inside with the side wall  64 , a recess  74  in the edge area of the bottom plate  66  facilitating the installation of the contact clip  70 . The contact clips  68  and  70  are each soldered or welded electrically conductively to the side wall  60  and  64  and both can be provided with an insulating layer  73  which surrounds them, for example in the form of an insulating hose which has been slipped on. It goes without saying that instead of the solder or weld connection it can also be provided that the contact clips  68  and/or  70  are allowed to adjoin the corresponding housing parts merely elastically pretensioned, for which purpose either contact pressure elements such as metal foams or springs can be used or the contact clips  68 ,  70  themselves provide for the necessary contact pressure. In principle, any other form of electrical contact-making can also be used instead of the contact clip  69  and/or  70 , thus for example a metal foam which has been inserted with pretensioning between the bottom of the cover plate  62  and one end face of the electrode  2  facing it, advantageously another metal foam being interposed between the top of the bottom plate  66  and one end face of the electrode  4  facing it.  
         [0068]     The interior of the protective housing  54  is filled with an electrolyte  12 , the positive and the negative electrode  2  and  4  being separated from one another by a separator  6  which is drawn schematically and which prevents direct electrical contact between the electrodes  2  and  4 , but allows ion migration. The cover  56  and the bottom  58  each have an electrical terminal  76  and  78  which correspond to the terminals  16  and  18  as shown in  FIG. 1 , and both are made of an electrically conductive metallic material (for example, titanium) which is chemically inert to the electrolytes  12  and the electrodes  2 ,  4  and is resistant to the electrochemical processes which take place.  
         [0069]     The side walls  60  and  64  are welded or soldered hermetically tight to one another via their end faces towards one another, with the interposition of an insulating ring  80 , the insulating ring  80  having a smaller inside diameter than the side walls  60  and  64 . For example, oxide ceramic can be used as the material for the insulating ring  80 .  
         [0070]     On the cover plate  62  a switching element labeled  86  throughout is attached; it corresponds to the make contact  44  of  FIG. 1 . As an important component the make contact  86  has a flexible contact carrier  92 , for example of polyimide, which has the shape of a roughly rectangular thin-walled plate with one short side which has a U-shaped notch so that two spring arms  102 ,  104  are formed. Along the two longer sides on the top of the contact carrier  92  metal coatings  94  are attached which extend into the spring arms  102  and  104 , where on the metal coatings  94  one section of a platinum wire at a time is soldered as a contact  106  or  108  via a solder connection  109 . In the vicinity of the second short side of the contact carrier  94  a contact plate  96  and  98  is connected by means of a solder layer  100  to one of the two metal coatings  94 . In doing so, the contact plates  96  and  98  each bear one terminal  112  and  114 , so that the terminal  112  is electrically connected to the contact  106  and the terminal  114  is electrically connected to the contact  108 . At a short distance over the contacts  106  and  108  there is a contact bridge  110  with which the contacts  106 ,  108  can be brought into contact in order to electrically short circuit them. The flexible contact carrier  92  is connected via a multilayer structure to the cover plate  62  such that the contacts  106 ,  108  are located in the middle over the cover plate  62  and in an overhead view run along the axis of symmetry of the round cover plate  62 . As is shown in  FIG. 4 , the multilayer structure between the contact carrier  92  and the cover plate  62  includes, proceeding from the bottom of the contact carrier  92  facing the cover plate  62 , an adhesive layer  88 , a spacer plate  90  and a second adhesive layer  88 . The multilayer structure extends roughly from the short side of the contact carrier  92 , which has the contact plates  96 ,  98 , as far as the base of the U-shaped notch which separates the two spring arms  102 ,  104  so that they stand freely at an exactly stipulated distance over the cover plate  62 .  
         [0071]     The contact clip  68  can be a fusible link which interrupts the electrical connection between the positive electrode  2  and its terminal  76  when the current flowing over it exceeds a stipulated boundary value. In this case, there can be insulation between one end face of the positive electrode  2  facing the cover plate  62  and the cover plate  62  itself. Accordingly the contact clip  70  can also be designed as a fusible link, and between the negative electrode  4  on the one side and the side wall  64  and/or the bottom plate  66  on the other side there can be insulation which prevents electrical contact when this fusible link triggers by means of the negative electrode  4  directly adjoining the housing parts  64  and/or  66 .  
         [0072]     If during operation of the secondary electrochemical cell the internal pressure within the protective housing  54  rises due to swelling (increase of volume) of the electrodes  2 ,  4  and/or by gas evolution and/or by a temperature increase, a bulge is specifically impressed on the detector element (cover plate  62 ) which is designed as a membrane by the other outside walls of the protective housing  54  being designed to be stiffer than the cover plate  62  and deforming only little. When the electrodes  2 ,  4  swell they can also touch the inner surfaces of the cover plate  62  and the bottom plate  66  directly or indirectly via insulating layers and/or the electrolyte  12  which can be a solid electrolyte, and in this way cause a change in shape of the cover plate  62 .  
         [0073]     The bulging of the cover plate  62  causes the contacts  106  and  108  of the contact bridge  110  to approach one another in order to touch the contact bridge  110  and to electrically short circuit the terminals  112  and  114  and via then the recharging circuit  22  when a boundary value of the internal pressure in the protective housing  54  which signals a still allowable operating state of the secondary electrochemical cell is exceeded, further power supply to the electrodes  2 ,  4  of the secondary electrochemical cell being suppressed. The flexible configuration of the spring arms  102 ,  104  prevents them from being damaged when the switching element  86  closes so that the break contact  86  in principle works reversibly. As the internal pressure in the protective housing  54  decreases, the switching element  86  again assumes its base position which is reproduced in  FIG. 2 .  
         [0074]     The bulging of the cover plate  62  which leads to actuation of the switching element  86  is generally less than 300 microns, the protective housing  54  having, for example, an outside diameter of roughly 18 mm at a height of less than 5.5 mm measured from the bottom plate  66  to the cover plate  62 .  
         [0075]      FIG. 5  shows a second embodiment of a secondary electrochemical cell which has a protective housing  116  with a cup-shaped, one-part bottom  118  of electrically conductive material. The protective housing  116  is sealed by a likewise electrically conductive cover  120 , between the cover  120  and the bottom  118 , an insulating ring  122  of oxide ceramic being soldered in. The insulating ring  122  having an inside diameter which is less than that of the side wall of the bottom  118  bears on its bottom a membrane  138  and on its top a contact membrane  134  electrically insulated against one another. Both membranes  134  and  138  are made of electrically conductive material, the contact membrane  134  with its top being electrically insulated against the adjacent cover  120  by means of an insulating layer  148 , and via a metal coating  132 , through plating  130  and a solder layer  128  it being connected, in an electrically conductive manner, to the bottom  118 . The electrodes  2  and  4 , the separator  6  and the electrolyte  12  are surrounded hermetically tight by the bottom  118 , the lower side of the insulating ring  122  and the membrane  138 . The positive electrode  2  is centered in the protective housing  116  by an electrically insulating annular receiver  124  which is positioned between the side wall of the bottom  118  and the positive electrode  2  in the area between the separator and the lower side of the insulating ring  122  such that electrical contact between the side wall of the bottom  118  and the positive electrode  2  is not possible. A recess  140  in the receiver  124  facilitates installation of the contact clip  68  by means of which the positive electrode  2  makes electrical contact with the bottom of the membrane  138 . A metal coating  142  on the lower side of the insulating ring  122 , through plating  144  through the insulating ring  122 , and a solder layer  146  close the electrical connection between the membrane  138  and the cover  120 , with which, for its part, contact is made with an electrical terminal  152 . An electrical terminal  150  on the outer side wall of the bottom  118  is electrically connected to the negative electrode  4  via the bottom  118  and the contact clip  70  which is connected to the inner side wall of the bottom  118 .  
         [0076]     While, therefore, the membrane  138  is connected to the positive electrode  2 , the contact membrane  134 , which is located at a distance which corresponds to the thickness of the insulating ring  122  from the membrane  138 , makes contact with the negative electrode  4 . This distance is such that in an unallowable operating state of the secondary electrochemical cell a bulge is impressed on the membrane  138  which acts as the detector element, which bulge is enough to establish electrically conductive contact with the contact membrane  134  so that the secondary electrochemical cell is electrically short circuited. Furthermore, a section of the solder connection  146  can be a fusible link which irreversibly burns through if a recharging or a discharging current exceeds a stipulated boundary value. Further energy supply and removal via the terminals  150 ,  152  is, thus, suppressed.  
         [0077]     In the embodiment of the secondary electrochemical cell which is shown in  FIG. 5 , the combination of the membrane  138  and the contact membrane  134  is used as the reversibly operating switching element which is a make contact and which is mechanically actuated by the detector element  138 . Since the two terminals  150 ,  152  are surrounded by a biocompatible insulating jacket  149  and a biocompatible polymer  153 , for example, silicone jackets the protective housing  116  and the housing-side ends of the terminals  150 ,  152 , the protective housing  116  can be directly implanted. The entire unit which is shown in  FIG. 5  can be used as an energy supply module with terminals  150 ,  152  which are electrically connected to other components of the implantable device, preferably detachably via a coupling element, as is described in EP-A-0 982 784 (corresponding to coassigned U.S. patent application Ser. No. 09/359,050) of the applicant which has not been published.  
         [0078]     According to  FIGS. 6 and 7 , a third embodiment of a secondary electrochemical cell has a protective housing  154  with an electrically conductive bottom which has a side wall  156  in the form of a tube section which is sealed on the face side by a bottom plate  158 . A peripherally closed weld  160  joins the bottom plate  158  to the side wall  156 . On the second face side of the side wall  156 , a solder layer  164  fixes a ceramic insulating ring  162  which has a round opening with an inside diameter which is smaller than the inside diameter of the side wall  156 , on the bottom of the insulating ring  162  which points in the direction to the bottom plate  158  a membrane  166  of electrically conductive material, which spans the opening, being attached and a hermetically tight internal space being formed. This internal space holds the positive electrode via the receiver  124  such that electrical contact between the positive electrode  2  and the side wall  156  is precluded and the former is centered at the same time in the protective housing  154 . While the positive electrode  2  makes electrical contact with the membrane  166  from underneath by means of the contact clip  68 , the contact clip  70  forms an electrical contact between the negative electrode  4  and the side wall  156 . An electrically conductive contact membrane  168  with a center contact point  170  which points downward is connected on the top of the insulating ring  162  to the latter and spans its opening. The contact membrane  168  has an elastic preliminary bulge to the inside in the direction to the membrane  166  and is in electrical contact with the membrane  166  by means of the contact point  170 . In this position of the contact membrane  168  energy can be supplied and removed to and from the secondary electrochemical cell via a terminal  174  which is electrically connected to the contact membrane  168  and a terminal  172  which has made contact with the outside of the side wall  156 . If an unallowable operating state of the secondary electrochemical cell occurs, the membrane  166  and with it the contact membrane  168  are caused to bulge so far to the outside that the contact membrane  168  springs to the outside via an unstable equilibrium position and electrical contact between the two membranes  166 ,  168  remains interrupted even if the membrane  166  should again return to its original position as shown in  FIG. 6 .  
         [0079]     A fourth embodiment of the secondary electrochemical cell which is illustrated in  FIGS. 8 and 9  has a protective housing which differs from the protective housing  154  essentially only by the configuration of the membranes  166 ,  168  and their electrical contact-making. A membrane  176  which corresponds to the membrane  166  centrally bears on its top a contact spring which is labeled  180  throughout, with a pin  182  and a spring plate  184 . The cylindrical pin  182  is fixed with one face side on the membrane  182  on which it stands vertically with its longitudinal axis, and with its second face side is connected to the spring plate  184 . In doing so, the pin  182  penetrates an opening in a contact membrane  178  which corresponds to the contact membrane  168  of the protective housing  154 , the spring plate  184  being in electrically conductive contact with the contact membrane  178  in the base position which is shown in  FIG. 8  with a contact surface  185  which faces the top of the contact membrane  178  and which is located in the vicinity of its outside edge. In the base position the membranes  176 ,  178  run roughly parallel and the contact surface  185  lies elastically pretensioned on the contact membrane  178 . In an unallowable operating state of the secondary electrochemical cell a bulge to the outside in the direction to the contact membrane  178  is impressed on the membrane  176  and the contact membrane  178  essentially does not deform and retains its location. The bulging of the membrane  176  is enough to lift the contact surface  185  from the contact membrane  178  and to reversibly interrupt electrical contact, but is not so large that the top of the membrane  176  comes into contact with the bottom of the contact membrane  178 . To guarantee the latter, even when the membrane  176  bulges more dramatically, the bottom of the contact membrane  178  is provided with an insulating layer  186 .  
         [0080]     A fifth embodiment of a secondary electrochemical cell as shown in  FIG. 10  includes a protective housing  190  with a one-piece, cup-shaped bottom  192 , a likewise cup-shaped cover  194  and a membrane  196  which is connected by a weld  198  on the facing end sides of the bottom  192  and the cover  194  to them the bottom  192 , the cover  194  and the membrane  196  preferably bring of the same electrically conductive material. The membrane  196  divides the protective housing  190  horizontally into a top cover area and a bottom, hermetically tight space which holds the positive and the negative electrode  2  and  4 , the separator  6  and the electrolyte  12 . The negative electrode  4  makes contact by means of the contact clip  70  with the bottom  192 , with a side wall which holds a single-pole penetration  214  via a ceramic substrate  212  hermetically tight and electrically insulated with reference to the side wall of the bottom  192 . The contact clip  68  establishes electrical contact between the positive electrode  2  and one end of the penetration  214  which projects into the hermetically tight space of the protective housing  190 ; its other end projects over the side wall of the bottom  192  to the outside and is electrically connected to a terminal  216 .  
         [0081]     On the bottom of the membrane  196  facing the positive electrode  2  and on the inner surface of the side wall of the bottom  192  adjacent to this electrode, an insulating layer  218  is applied to prevent electrical contact between the membrane  196  and the side wall of the bottom  192  and the positive electrode  2 . On the top of the membrane  196  in the center is a plunger  222  which projects into an opening in the cover  194 . At a short distance above the upper edge of the plunger  222 , there is a bursting element  204  which spans the opening. This element includes, on its side facing the top of the cover  194 , a substrate  206  with a conductive layer  208 . The substrate  206  can be ceramic, for example, oxide ceramic, glass or the like. The conductive layer  208  on one side of the opening in the cover  194  makes contact with the latter via a contact clip  202  and on the opposite side of the opening is provided with a terminal  210  which in this way is electrically connected to the negative electrode  4 .  
         [0082]     In an unallowable operating state of the secondary electrochemical cell a bulge is impressed on the membrane  196 , whether by an increase in the volume of the electrodes  2 ,  4  and/or a temperature rise and/or gas evolution in the operation of the secondary electrochemical cell and it is enough for the plunger  222  to destroy the bursting element  204  so that the conductive layer  208  between the contact clip  202  and the terminal  210  is irreversibly interrupted.  
         [0083]     As is shown in  FIG. 11 , a sixth embodiment of a secondary electrochemical cell has a protective housing  224  with a flat, electrically conductive, shell-shaped bottom  226  which is connected via its end face which points upward with the interposition of a ceramic insulating ring  229  to a side wall  228  which has the shape of a tube section. The side wall  228  is sealed hermetically tight on its upper face side with an electrically conductive membrane  230 . On the outside of the membrane  230 , which points upward, an insulating layer  232  is applied and on the latter an electrically conductive, brittle bursting layer  234  is applied. The bursting layer  234  in the area of the side wall  228  is connected electrically conductively to a membrane  230  at a first site via contact-making  236  and at a second diametrically opposite site to a terminal  238 . The bursting layer  234  extending in the manner of a strip between these two sites. Via the terminal  238 , the positive electrode  2  is tapped from outside of the protective housing  224 , for which purpose the positive electrode  2  makes contact by means of a contact clip  242 , which latter forms via a weld connection  244  electrical contact with the inside of the side wall  228 . The negative electrode  4  is connected via the contact clip  70  to the inside of the bottom  226  and can be tapped from outside the protective housing  224  by means of the terminal  240 . It goes without saying that alternatively to the weld connection  244 , for example, there can also be a solder connection. In an unallowable operating state of the secondary electrochemical cell a bulge is impressed on the membrane  230  and it irreversibly destroys the bursting layer  234  and, thus, interrupts the electrical connection between the positive electrode  2  and the terminal  238 .  
         [0084]     A seventh embodiment of a secondary electrochemical cell is shown in  FIG. 12  and includes a protective housing  252  which differs from the protective housing  224  essentially only by the configuration of the bottom  226  and that of the current path to and from the positive electrode  2 . The protective housing  252  has a bottom  254 , with side walls which are routed as far as the membrane  230  and are welded to it in a hermetically tight and electrically conductive manner, so that the insulating ring  229  of the protective housing  224  is eliminated. The positive electrode  2  is held in a cup-shaped, electrically insulating receiver  256  which electrically insulates the positive electrode  2  with reference to the side wall of the bottom  254  and the membrane  230  and keeps it in a predetermined position. The contact clip  68  is placed in the area of a recess  258  of the receiver  256  and is used for contact-making of the positive electrode  2  to the first end of a single-pole penetration  260  which is held hermetically tight and electrically insulated in the side wall of the bottom  254 . The second end of the penetration  260  projects over the outside of the side wall of the bottom  254  and is brought via a contact clip  262  into electrical connection with the electrically conductive bursting layer  234  which is insulated completely by an insulating layer  255  relative to the membrane  230  and the bottom  254 .  
         [0085]     According to  FIGS. 13 and 14 , an eighth embodiment of a secondary electrochemical cell uses a modified protective housing  264  which corresponds for the most part to the protective housing  54  of FIGS.  2  to  4 , but does not use its switching element  86  and in which the current path from and to the positive electrode  2  is accomplished in a modified form. A rectangular bursting strip  266  is coupled from the outside via adhesive connections  268  and  270  each roughly in the outer third of its longer side to the cover plate  62  of the protective housing  264  at a stipulated distance so that a bridging zone  272  is formed between the adhesive connections  268  and  270 . The line of symmetry in the direction of the longer side of the bursting strip  266  runs essentially parallel to the center line of the round cover plate  62 , and the center of the bridging zone  272  is located above the center of the cover plate  62 . On the top of the bursting strip  266 , a conductive layer  276  is applied which extends over almost the entire top of the bursting strip  266  and which makes contact with the cover plate  62  in the area of one short side of the bursting strip by a contact clip  284  and with a terminal  286  in the area of the other short side. Thus, an electrical connection is established between the terminal  286  and the positive electrode  2 .  
         [0086]     In the area of the bridging zone  272  which is free of the adhesive layer, near one side bisector of the longer side of the bursting strip  266 , in the latter there is scoring in the form of a perforation  274 . The scoring can of course also be produced in some other suitable way, for example by notching, scratching or narrowing of the bursting strip  266 . Adjacent to the perforation  274 , by means of a solder connection  280  a wire section is fixed as a short circuit contact  278  via which there is an opposite contact  282  at a certain distance. The opposite contact  282  is an electrical contact via a contact clip  288  with the side wall  64 , i.e with the negative electrode  4 . In an unallowable operating state of the secondary electrochemical cell, a bulge is impressed on the cover plate  62  as a bulging membrane and it is transferred via the adhesive connections  268 ,  270  to the bursting strip  266  which breaks along the perforation  274  according to  FIG. 14 . The short circuit contact  278  is pressed against the opposite contact  282  and the short circuit current between the positive and the negative electrode  2  and  4  is enough to burn through a possibly residual fragment  290  of the conductive layer which at least partially spans the fracture site. In this way, the current path between the positive electrode  2  and the terminal  286  is irreversibly interrupted.  
         [0087]     Since it is necessary to predict the bulging necessary to break the bursting strip  266  in the range of a few hundred microns, preferably in the range of less than 200 microns, the predetermined geometry of the bursting strip  266  must be maintained very precisely. The material for the bursting strip  266  can be, for example, glass or ceramic, and it can be advantageous to impress mechanical pretension on the bursting strip which increases the distance of the fracture sides after bursting of the scoring. To do this, the bursting strip  266  can be made as a composite element of at least one glass or ceramic material and at least one metal which are cemented or soldered to one another. Pretensioning of the bursting strip  266  can likewise be induced by using a metal with shape memory (memory effect).  
         [0088]     It is furthermore possible to produce the bursting strip  266  from a piezoelectric material or at least provide it with one such layer. In deformations of the bursting strip  266  which do not lead to its rupture, therefore which do not signal an unallowable operating state of the secondary electrochemical cell, electrical information delivered by the piezoelectric converter can be tapped in the form of charge changes by the optional evaluation electronics  52  in order for example to actuate a switching element which corresponds to the break contact  48  or  50  of  FIG. 1  and to interrupt recharging or discharging of the electrodes  2 ,  4  of the secondary electrochemical cell before the secondary electrochemical cell reaches an unallowable operating state.  
         [0089]     In the embodiment as shown in  FIGS. 13 and 14 , it is conceivable that the position of the perforation  274  and the short circuit contact  278  could be exchanged for one another with reference to the side bisector of the longer side of the bursting strip  266  so that the short circuit contact  278  comes to rest on the side of the rupture edge which is the right one in  FIG. 14 . In this case a short circuit current cannot burn through a possible remaining fragment  290  of the conductive layer, but the secondary electrochemical cells are forced to completely discharge, and then the contact clips  68  and/or  70  could not be designed as fusible links, as is fundamentally possible (compare the corresponding statements on the protective housing  54  of FIGS.  2  to  4 ). If repeatedly redundant protection is unnecessary, it is furthermore conceivable to completely abandon the short circuit contact  278  and the opposite contact  282 , to use only the irreversible break contact function of the bursting strip  266  and to design it such that in any case it is ensured that when the bursting strip  266  ruptures a fragment  290  of the conductive layer does not remain.  
         [0090]     A ninth embodiment of the secondary electrochemical cell is shown in  FIG. 15  and differs from the embodiment of FIGS.  2  to  4  essentially only in that, to increase the redundancy, a strain gauge  292  is applied in a meander configuration to the top of the cover plate  62 . When the cover plate  62  bulges, the strain gauge  292  experiences a change in shape which leads to a change of its electrical resistance which is acquired via the terminals  292  and  296  by the evaluation electronics  52  and is used for example to actuate other switching elements, for example, for an electrically actuatated make contact which can be placed away from the protective housing and in function corresponds to the make contact  46  or the break contact  50  of  FIG. 1 . The evaluation electronics  52  can furthermore activate a warning means which is not shown and which notifies a user about the malfunction of the secondary electrochemical cell.  
         [0091]     A protective housing  298  which is shown in  FIG. 16  encompasses the switching element  86  which has already been described in conjunction with the protective housing  54  of FIGS.  2  to  4  and differs from the protective housing  54  essentially only by the following configuration features: A metallic, preferably titanium bottom is made cup-shaped, a hollow cylindrical side wall  300  on its bottom face side being sealed in one piece with a bottom plate  302 . On the top face side of the side wall  300  a cover plate  304 , which is a bulging membrane, is welded hermetically tight and it is likewise made of titanium. A cup-shaped receiver  306  is held within the protective housing  298  such that its cylindrical side wall has an outside diameter which corresponds to the inside diameter of the side wall  300  and its bottom plate rests on the bottom plate  302 . The inside contour of the receiver  306  is matched to the outside contour of the positive electrode  2 , the latter being fixed and centered in the radial direction within the protective housing  298 . The receiver  306  being, like the already described receivers  124  (FIGS.  5  to  9 ) and  256  ( FIG. 12 ), of an electrically insulating material, preferably of a plastic material such as polytetrafluorethylene (PTFE), and insulates the positive electrode  2  with reference to the metallic outside walls of the protective housing  298 . The receiver  306  is provided on the outside of its side wall with a recess  308  and is, thus, inserted into the protective housing  298  such that the recess  308  is facing an opening  310  which is made in the side wall  300  in the radial direction.  
         [0092]     The cylindrical opening  310  in the side wall  300  is surrounded on the outside of the side wall  300  by a flat cylindrical surface  312  into which a shoulder  314 , which points to the outside, of a sleeve-shaped, metallic, preferably titanium receiver  316  is fitted and welded hermetically tight. In the receiver  316  an electrically insulating, inorganic-nonmetallic housing section of the protective housing  298  in the form of a ceramic substrate  318 , for example, of AL 2 O 3 , is held such that the substrate  318  with its outside diameter is matched to the inside diameter of the receiver  316  and up to the collar on the inside wall of the receiver  316  is inserted into the latter and is located in the axial direction to the side of the receiver  316  which faces the shoulder  314 . On the opposite side the substrate  318  projects above the receiver  316  and is connected hermetically tight to it via a gold solder connection  320 . The substrate  318  for its part accommodates a twin-pole penetration  315  which includes two metallic contact pins  322  and  324 , which preferably is a platinum-iridium compound and form one pole each of the twin-pole penetration  315 . The contact pins  322  and  324  which are shown in  FIG. 16  in a position which has been turned by 90° around the axis of the receiver  316  penetrate the substrate  318  in the axial direction and are fixed hermetically tight in it in the same manner by a gold solder connection  326 . They have a length which is sufficient to axially project above the face side of the shoulder  314  and that of the substrate  318 , the contact pins  322  and  324  projecting into the recess  308  on the side facing the interior of the protective housing  298 . The pin  322  makes contact, via a contact clip  332 , with the positive electrode  2  and the contact clip  330  forms a current path between the negative electrode  4  and the contact pin  324 . The two contact clips  330  and  332  are each surrounded by an insulating hose  334 . To place the metallic housing sections (i.e. the side wall  300 , the bottom plate  302 , the cover plate  304  and the receiver  316 ) of the protective housing  298  specifically at the potential of the negative electrode  4 , the penetration  315  has a solder bridge  328  which extends between the contact pin  324  and the receiver  316 . Outside of the protective housing  298  the positive electrode  2  and the negative electrode  4  are each tapped via a terminal  338  or  336 .  
         [0093]     The protective housing  298  compared to the protective housing  54  can be produced with lower cost since the bottom of the protective housing  298  is made in one piece and no ceramic insulating ring  80  is used. The especially critical metal ceramic transition is minimized and limited to a part which is to be produced and tested separated and which includes the components  316 ,  318 ,  322  and  324 . This contributes to increased compressive strength of the protective housing  298 .  
         [0094]     As follows from  FIG. 17 , the protective housing  340  viewed in the direction of its smallest extension, instead of a cylindrical cross section, can also have a different, for example roughly droplet-shaped cross section. In the embodiment of the protective housing  340  as shown in  FIG. 17 , the side wall is formed by one segment  342  in the form of a three quarters circle and two linear segments  344  and  346  which tangentially join one another to both sides of the three quarters circle, the segments adjoining one another via a rounded area which lies between them. The side wall is metallic, preferably of titanium and together with a one-piece bottom plate forms a cup-shaped bottom. A cover plate, which is not shown and which likewise is preferably titanium, is welded hermetically tight to the upper face of the side wall. For the sake of clarity the wall thickness of the segments  342 ,  344  ad  346  is shown enlarged. The electrodes  2  and  4  are fixed within the protective housing  340  by a centering arrangement which is not shown, at least one of the electrodes  2  and  4  being electrically insulated with reference to the cup-shaped bottom and the cover plate preferably by the centering arrangement. In doing so, the centering arrangement can be a cup-shaped receiver similar to receiver  256  (compare  FIG. 12 ) or the receiver  306  ( FIG. 16 ) which in addition to the centering function also satisfies the object of electrical insulation. Furthermore, the inside of the cup-shaped bottom which faces the electrodes  2 ,  4  and/or of the cover plate can be provided with an insulating layer.  
         [0095]     A penetration is made with two poles, in contrast to the embodiment as shown in  FIG. 16  the receiver  316  is omitted, so that a ceramic substrate  350  is soldered, not in the receiver, but directly into the side wall of the protective housing  340 , hermetically tight, for which preferably gold solder is used. The ceramic substrate  350  is soldered into the linear segment  344 , but it is also possible to house the former in the circular segment  342 . Metallic contact pins  354  and  356  which each form one pole of the twin-pole penetration  348  are soldered hermetically tight in the ceramic substrate  352  and are used for separate routing of the current path from or to the two electrodes  2  and  4  through the wall of the segment  344 . With respect to the preferred selection of materials for the contact pins  354 ,  356  and the substrate  350  reference is made to the statements on the penetration  315 . In the penetration  348  there can also be an electrical connection which is similar to the solder bridge  328  (see  FIG. 16  between the contact pin  354  or  356  and the segment  344  in order to place the housing specifically at a negative or positive potential. Advantageously, the housing is connected electrically conductively to the negative electrode  4  and the positive electrode  2  is insulated with respect to the housing.  
         [0096]     A cylindrical opening in the segment  346  by means of a solder connection accommodates a ceramic substrate  358  which is penetrated by two contact pins  360  and  362  which are held in the substrate  358  via a solder connection and project above it axially to either side. The opening in the segment  346  is sealed hermetically tight on the outside by a metallic membrane  364 , the membrane  364  in the base state, i.e. at a not unduly elevated pressure within the protective housing  340  with its inside which points toward the interior of the protective housing  340 , adjoining the contact pins  360  and  362  with a defined prestress and electrically connecting them. The membrane  364  which acts as the detector element is located outside of a section of the protective housing  340  which holds the electrodes  2  and  4 , an opening  366  which run essentially in the direction of the lengthwise axis of the contact pins  360 ,  362  in the substrate  358  ensuring a fluid connection between the membrane  364  and the section of the protective housing  340  which holds the electrodes  2  and  4 .  
         [0097]     While one contact clip  372  connects a tap  384 , provided on the positive electrode  2 , directly to the contact pin  356  of the penetration  348 , the second contact pin  354  of the penetration  348  makes contact with one tap  382  of the negative electrode  4  with the interposition of a break contact. The break contact includes a contact pair which is formed from the contact pins  360 ,  362 , and the membrane  364 . In doing so, there is one contact clip  368  between the tap  382  and the contact pin  360  and there is one contact clip  370  between the contact pin  362  and the contact pin  354 .  
         [0098]     The membrane  364  does allow detection of swelling of the electrodes  2 ,  4  by the electrodes  2 ,  4  directly or indirectly adjoining the membrane  364 , but swelling of the electrodes  2  and  4  leads, in the same way as gas evolution and/or a temperature rise during operation of the secondary electrochemical cell to an increase in the internal pressure in the protective housing  340 . If within the hermetically tight protective housing  340  a predetermined pressure boundary value which signals a still allowable operating state of the secondary electrochemical cell is exceeded, a bulge is impressed on the membrane  364  which is in fluid communication with the interior of the protective housing  340  via the opening  366  and the bulge is enough to lift the membrane  364  off the contact pins  360  and  362 . The electrical connection between the two contact pins  360 ,  362  and thus between the negative electrode  4  and the contact pin  354  of the penetration  348  is then reversibly interrupted.  
         [0099]     The protective housing  340  has another switching element which is integrated in the circular segment  342  and is made as a reversible make contact. This switching element can be present additionally to or instead of the above described break contact. A membrane  380  is at the same time the detector element and part of the make contact. It is provided on the inside of the segment  342  which faces the electrodes  2 ,  4  and closes an opening in the segment  342 . In the cylindrical opening a ceramic substrate  374  is soldered such that two contact pins  376  and  378 , which axially penetrate the substrate  374  in the base position of the membrane  380 , are at a predetermined distance to their outside. The membrane  380  is made electrically conductive at least in the area of its outside, which area is adjacent to the contact pins  376 ,  378 . But, preferably, the entire membrane  380  is made electrically conductive, especially of a material which agrees with the material of the metallic bottom and the cover plate of the protective housing  340  and it is welded in the segment  342 . In this case it is entirely at the electrical potential of the indicated metallic housing parts.  
         [0100]     The membrane  380  need not necessarily seal the opening in the segment  342  hermetically tight. It is enough if it is tight enough to experience a bulge for an unallowably high pressure rise within the protective housing  340  (in  FIG. 17  shown by a broken line) which brings at least its conductive area into electrical contact with the contact pins  376  and  378  and electrically short circuits them. The hermetic tightness must then be ensured by the substrate  374  which would have to be soldered hermetically tight to the segment  342  and likewise to the contact pins  376 ,  378 . Furthermore, a compression space formed between the substrate  374  and the membrane  380  would have to be considered in the design of the membrane  380 . Conversely, if the membrane  380  seals the opening in the segment  374  hermetically tight, which is preferred, a hermetically tight configuration of the substrate  374  and the corresponding solder connections between the substrate  374  and the contact pins  376 ,  378  and the segment  342  is not absolutely necessary, but makes sense under certain circumstances.  
         [0101]     It goes without saying that the protective housing  340  can be designed such that swelling or expansion of the volume of the electrodes  2  and/or  4  in the direction of bulging of the membrane  380 , i.e. essentially perpendicular to the smallest extension of the protective housing  340 , actuates the membrane  380  by the electrodes  2  and/or  4  adjoining the membrane  380  directly or indirectly for example via an insulating layer. Preferred materials for contact pins  360 ,  362 ,  376 ,  378  and the substrates  358  and  374  of the break contact or the make contact correspond to those of the contact pins  354  and  356  of the penetration  348  and the substrate  350 .  
         [0102]     The make contact of the protective housing  340 , which includes the membrane  380  and a contact pair which is formed by the contact pins  376 ,  378 , can be used for example according to the make contact  44  (see  FIG. 1 ) in order to directly short circuit the recharging circuit  22 . It is likewise possible to monitor the make contact by means of the evaluation electronics  52  which, for its part, actuates other switching elements or the already mentioned warning device.  
         [0103]     One major advantage of the protective housing  340  is that it fits very flatly into the side wall of the protective housing  340  by integration of all detector and switching elements and the penetration and in the direction of its smallest extension has a thickness which is essentially larger than that of the electrodes/electrolyte system by the wall thicknesses of the bottom and the cover plate.  
         [0104]     While various embodiments in accordance with the present invention have been shown and described, it is understood that the invention is not limited thereto, and is susceptible to numerous changes and modifications as known to those skilled in the art. Therefore, this invention is not limited toe the details shown and described herein, and includes all such changes and modifications.