Abstract:
Contacts  3, 6 , which are opened/closed by the displacement of a heat responsive element  7 , are interposed between first and second terminals  2, 9 , and a heat generating resistor  8  is interposed between the first terminal  9  and a third terminal  10 , whereby the heat responsive element  7  is displaced by heat generated when a current is caused to flow in the heat generating resistor  8  via the third terminal  10.

Description:
FIELD OF THE INVENTION AND RELATED ART 
     The present invention relates to a thermal shut-off device used suitably for a secondary battery pack and a battery pack using the thermal shut-off device. 
     FIG. 9 shows a battery pack proposed by Japanese Patent Provisional Publication No. 7-153499. This battery pack incorporates a safety device a having a configuration in which a bimetallic switch b, a resistor c connected in parallel to the bimetallic switch b, and a resistor d connected in series to the bimetallic switch b are incorporated in a casing, not shown. 
     In this battery pack, if an excessive charging current flows in through a charging terminal e, Joule heat is generated in the resistor d, so that the bimetallic switch b is turned off to stop the charging to an incorporated battery f. If the bimetallic switch b is turned off, the off state of the bimetallic switch b is maintained by the heat generated in the resistors c and d. 
     FIGS. 10 and 11 show safety shut-off devices proposed by Japanese Patent Provisional Publication No. 63-503020. The shut-off device shown in FIG. 10 has a configuration in which a resistor i is connected in parallel to a bimetallic switch h connected in series to a load g, and a resistor k is interposed between a common connection point between the load g and the bimetallic switch h and the switch j. 
     In this device, if the switch j is closed, Joule heat is generated in the resistor k to turn off the bimetallic switch h, so that the current carried to the load g is interrupted. If the bimetallic switch h is turned off, the off state of the bimetallic switch h is maintained by the heat generated in the resistors i and k. 
     The shut-off device shown in FIG. 11 has a configuration in which resistors m and n connected in series to one another are connected in parallel to the bimetallic switch h, and the switch j is connected to a common connection point of the resistors m and n. 
     In this device, if the switch j is closed, Joule heat is generated in the resistors m and n to turn off the bimetallic switch h, with the result that the current carried to the load g is interrupted. If the bimetallic switch h is turned off, the off state of the bimetallic switch h is maintained by the heat generated in the resistor m. 
     The safety device shown in FIG. 9 has following problems. 
     (1) In order to maintain the off state of the bimetallic switch b by means of the heat generated in the resistor c, the impedance of the battery f, which is a load, must be low. This means that when the battery f is short-circuited externally, the on state is maintained. In the recent condition in which a battery pack is used, a secondary battery pack is scarcely removed from equipment such as a computer, so that the external short circuit of the battery occurs in fewer cases. Therefore, the function as a safety device is greatly limited. 
     (2) In recent years, a battery that requires exact charging/discharging control, such as a lithium battery, has been used frequently for a second battery pack. In the case of such a battery, overvoltage and overheating due to overcharging create a danger, so that multiple safety measures are taken against these phenomena. 
     As one example, both a safety device using a bimetallic switch and a temperature fuse are used for a protective control circuit. However, since the function of the safety device is greatly limited as described in the above item (1), the temperature fuse finally performs the shutting-off operation. 
     However, the temperature fuse cannot be reused once it performs the shutting-off operation. Therefore, even if the temperature fuse is operated by misuse, the expensive battery pack becomes incapable of being used. 
     (3) Since the resistor c is connected in parallel to the bimetallic switch b, complete electrical shutoff is not effected even if the bimetallic switch b is turned off. Therefore, after the bimetallic switch b is turned off, a phenomenon takes place in which a current flows into the battery f from a charger, not shown, connected to the charging terminal e or inversely a current leaks from the battery f to the equipment. This phenomenon is undesirable in terms of safety. 
     On the other hand, the shut-off device shown in FIG. 10 is configured so that the resistor i is connected in parallel to the bimetallic switch h, and the shut-off device shown in FIG. 11 is configured so that a series composite resistor consisting of the resistors m and n is connected in parallel to the bimetallic switch h. 
     Therefore, in these shut-off devices, complete electrical shutoff is not effected even if the bimetallic switch h is turned off, so that a current flows to the load g even after the bimetallic switch h has been turned off. That is to say, these shut-off devices pose the same problem as those described in the above item (3). 
     OBJECTS AND SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above situation. Accordingly, an object of the present invention is to provide a thermal shut-off device capable of shutting-off operation under various abnormal conditions such as overcurrent, overvoltage, etc., maintenance of a shut-off state, reuse after an abnormality is eliminated, and complete electrical shutoff. 
     Another object of the present invention is to provide a battery pack using the above-described thermal shut-off device. 
     To achieve the above object, the present invention provides a thermal shut-off device comprising contacts interposed between first and second terminals, the contacts being opened/closed by the displacement of a heat responsive element, and a heat generating resistor interposed between the first terminal and a third terminal, wherein the heat responsive element is displaced by heat generated when a current is caused to flow in the heat generating resistor via the third terminal. 
     In the thermal shut-off device, a PTC element can be used as the heat generating resistor. 
     Also, the heat generating resistor can be formed into a film form, and the film-form resistor can be disposed so as to be in contact with the heat responsive element. 
     This thermal shut-off device achieves the following effects. 
     (1) Since a shutting-off operation can be performed based on various pieces of information and the shut-off state can be maintained by using one heat generating resistor, the device can be made small in size and low in cost, and the application range thereof can be broadened. 
     (2) Since no resistor is connected in parallel to the contacts to be opened/closed, a reliable electrical shut-off property can be provided. Therefore, the reliability as a protector is improved. 
     Also, the present invention provides a battery pack incorporating a thermal shut-off device comprising contacts interposed between first and second terminals, the contacts being opened/closed by the displacement of a heat responsive element, and a heat generating resistor interposed between the first terminal and a third terminal; and a chargeable battery, wherein the first terminal of the thermal shut-off device is connected to a first terminal for external connection, the second terminal thereof is connected to a second terminal for external connection via the battery, and the third terminal thereof is connected to a third terminal for external connection; and a current is caused to flow in the heat generating resistor via the third terminal for external connection, whereby the contacts are opened by the displacement of the heat responsive element caused by the heat generated by the heat generating resistor. 
     This battery pack achieves the following effects. 
     (3) An opening/closing operation of the thermal shut-off device can be performed and the open state can be maintained by an operation from a charger or external equipment such as a computer. Therefore, not only a protective operation against short circuit etc. but also a variety of protective operations based on various pieces of information can be performed. 
     (4) Since electrical shutoff can be effected reliably, the inflow and outflow of a current into and out of the battery during the time when the shutoff is continued is prevented. As a result, safety is improved. 
     (5) The battery pack has an economical advantage that it can be reused by disconnecting a heat generating power source. 
     Further, the present invention provides a battery pack incorporating a thermal shut-off device comprising contacts interposed between first and second terminals, the contacts being opened/closed by the displacement of a heat responsive element, and a heat generating resistor interposed between the first terminal and a third terminal; a chargeable battery; and abnormality detecting means for detecting the condition of the battery to close a switch element if the condition is abnormal, wherein the second terminal of the thermal shut-off device is connected to a first terminal for external connection, the first terminal thereof is connected to a second terminal for external connection via the battery, and the third terminal thereof is connected to the switch element; and a current from the battery is caused to flow to the heat generating resistor by closing the switch element, whereby the contacts are opened by the displacement of the heat responsive element caused by the heat generated by the heat generating resistor. 
     In this battery pack, abnormality detecting means can detect the condition of the battery based on the current of the battery. 
     Also, the abnormality detecting means can detect the condition of the battery based on the voltage of the battery. 
     Further, the abnormality detecting means can detect the condition of the battery based on the temperature of the battery. 
     This battery pack achieves the following effects in addition to the effect described in the above item (4). 
     (6) Since the shut-off state of the thermal shut-off device is maintained by the heat generation of the resistor caused by the discharge current of the battery, the thermal shut-off device automatically returns to the closed state when the discharge current of the battery decreases down to a predetermined value. Therefore, a returning operation of the thermal shut-off device is needed. 
     (7) A variety of protective operations can be performed, and also the battery pack can be reused. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view taken along the line B—B of FIG. 2, showing one embodiment of a thermal shut-off device in accordance with the present invention; 
     FIG. 2 is a sectional view taken along the line A—A of FIG. 1; 
     FIG. 3 is a sectional view showing another embodiment of a thermal shut-off device in accordance with the present invention; 
     FIG. 4 is a longitudinal sectional view of the thermal shut-off device shown in FIG. 3; 
     FIG. 5 is an exploded perspective view of a film-form resistor; 
     FIG. 6 is a circuit diagram for the thermal shut-off devices shown in FIGS. 1 and 3; 
     FIG. 7 is a circuit diagram showing one embodiment of a battery pack in accordance with the present invention; 
     FIG. 8 is a circuit diagram showing another embodiment of a battery pack in accordance with the present invention; 
     FIG. 9 is a circuit diagram showing a configuration of a conventional battery pack; 
     FIG. 10 is a circuit diagram showing one example of a conventional thermal shut-off device; and 
     FIG. 11 is a circuit diagram showing another example of a conventional thermal shut-off device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIGS. 1 and 2, a fixed plate  1  formed of a conductive material is formed with a terminal  2  at the rear end portion thereof and is formed with a fixed contact  3  at the front end portion thereof. On the fixed plate  1  is fixed an insulating plate  4 . The insulating plate  4  has two columnar protrusions  4   a  and  4   b  at the rear part thereof, and the columnar protrusions  4   a  and  4   b  penetrate holes formed in a movable plate  5 , a resistor  8 , a terminal  9 , etc., described later. 
     The movable plate  5  formed of a metal plate having elasticity is disposed on the insulating plate  4 . The movable plate  5  is supported by fitting the rear part thereof onto the columnar protrusions  4   a  and  4   b  of the insulating plate  4 . At the front and rear end portions of the movable plate  5  are formed with a movable contact  6  opposed to the fixed contact  3  and a terminal  9 , respectively. 
     A bimetal  7 , which is a heat responsive element, is placed on the upper face of the movable plate  5 . This bimetal  7  is supported on the movable plate  5  by being fitted with play into claws  5   c  and  5   d  provided at both ends of the movable plate  5 . 
     The bimetal  7  performs reversing operation to deform the movable plate  5  when the temperature reaches a predetermined value. This deformation of the movable plate  5  separates the movable contact  6  from the fixed contact  2 . To make the deformation of the movable plate  5  caused by the reversing operation of the bimetal  7  as large as possible, a circular protrusion  5   f  is provided at the central portion of the movable plate  5 . 
     A resistor  8  fitted onto the columnar protrusions  4   a  and  4   b  is disposed above the rear part of the movable plate  5 , and the terminal  10  fitted onto the columnar protrusions  4   a  and  4   b  is disposed on the upper face of the resistor  8 . Therefore, one end of the resistor  8  is electrically connected to the upper face of the rear part of the movable plate  5 , and the other end thereof is electrically connected to the lower face of the terminal  10 . 
     An insulating plate  11  having electrical insulation properties is disposed on the upper face of the terminal  10 . The columnar protrusions  4   a  and  4   b  are fused or bonded to the upper face of the insulating plate  11  after penetrating the insulating plate  11 . 
     The components thus assembled are inserted in a housing  12 , and the opening of the housing  12  is sealed by resin  13  or the like. 
     FIGS. 3,  4  and  5  show another embodiment of a thermal shut-off device in accordance with the present invention. In these figures, to the elements corresponding to the elements shown in FIGS. 1 and 2 are applied reference numerals that are formed by adding 0 to the reference numerals applied to the elements shown in FIGS. 1 and 2. 
     In this embodiment, a protrusion  41  is provided on an insulating plate  40  instead of the protrusion  5   f  provided on a movable plate  50  as shown in FIG.  2 . Also, a bimetal  70  has a shape such that the rear end of the bimetal  7  shown in FIG. 2 is extended. 
     Between the movable plate  50  and the bimetal  70  is disposed a film-form resistor  80  shown in FIGS. 3 and 5. This film-form resistor  80  is constructed so that one face and the other face of a metallic foil (for example, a stainless steel foil with a thickness of 50 μm)  80   b  having a relatively high resistance are covered by flexible thin films (for example, polyimide resin)  80   a  and  80   c  having electrical insulation properties, respectively. 
     The right and left rear end portions  80   d  and  80   e  of the metallic foil  80   b  are exposed from the rear ends of the thin films  80   a  and  80   c . The end portion  80   d  is fixed to the rear end portion of the movable plate  50  by means such as welding or fusing, and the end portion  80   e  is formed with a terminal  100  for connection to an external circuit. 
     The rear end portions of the movable plate  50 , the bimetal  70 , and the film-form resistor  80  are penetrated by a prismatic member  42  provided on the insulating plate  40 , and are tightened together and fixed by an insulating member  110 . 
     The film-form resistor  80  has a main heat generating region, which generates 50% or more of the calorific value, located on the outside of the fixed portion in this state. Specifically, the main heat generating region extends forward so as to be located in a space between the movable plate  50  and the bimetal  70 , and the front end portion thereof is in contact with the lower face of the bimetal  70 . 
     Since the resistor  80  is flexible, it does not hinder the reversing operation of the bimetal  70 . Also, since the main heat generating region of the resistor  80  is formed by zigzagging the metallic foil  80   b , the electrical resistance can be regulated by changing the thickness, width, and zigzag shape of the metallic foil  80   b.    
     FIG. 6 is a circuit diagram for a thermal shut-off device S in accordance with the above-described embodiments. According to this thermal shut-off device S, when an abnormal load current flows between the terminals  2  ( 20 ) and  9  ( 90 ), the reversing operation of the bimetal  7  ( 70 ) caused by the heat generated in the movable plate  5  separates the movable contact  6  ( 60 ) from the fixed contact  3  ( 30 ). If the current for generating heat is supplied continuously, the open state of the contacts  6  ( 60 ) and  3  ( 30 ) is maintained. 
     In this thermal shut-off device S, the resistor  8  ( 80 ) is not connected in parallel to the bimetallic switch element. When the switch element becomes in an open state, therefore, the load current is completely interrupted. That is to say, this thermal shut-off device S provides a very good shut-off quality. 
     If what is called a PTC (Positive Temperature Coefficient) element such as a positive thermistor is used as the heat generating resistor  8  ( 80 ), the response of the thermal shut-off device can be improved. As is well known, the PCT element has a property that the electrical resistance rises suddenly as the temperature increases. 
     FIG. 7 shows an embodiment of a battery pack in accordance with the present invention in which the above-described thermal shut-off device S is incorporated. 
     A battery pack  200 - 1  is configured so that the terminal  9  ( 90 ) of the thermal shut-off device S, the terminal  2  ( 20 ) thereof, and the terminal  10  ( 100 ) thereof are connected to a terminal  201  for external connection, a chargeable battery (for example, lithium battery)  202 , and a terminal  204  for external connection, respectively. 
     The terminals  201  and  203  for external connection are connected to a positive output terminal and a negative output terminal of a charger  300 , respectively. Also, the terminals  203  and  204  for external connection are connected to a normally-open switch element  302  (including a semiconductor switch element) in an abnormality detecting circuit  301  incorporated in the charger  300  or equipment such as a computer, not shown. 
     In this battery pack  200 - 1 , if an abnormal charging current flows in the thermal shut-off device S, the abnormality detecting circuit  301  detects the abnormal charging current to close the switch element  302 . Accordingly, a current flows in the resistor  8  ( 80 ) of the thermal shut-off device S so that the resistor generates heat, by which the thermal shut-off device S is opened. As a result, the charging current is interrupted. 
     After detecting the abnormal charging current, the abnormality detecting circuit  301  maintains the closed state of the switch element  302 . During this time, the heat generating state of the heat generating element  80  is maintained by the current supplied from the charger  300 . 
     When the battery pack  200 - 1  is removed from the charger  300 , or when the power source of equipment such as a computer incorporating the charger  300  is turned off, no current flows in the resistor  8  ( 80 ), so that the temperature of the resistor  8  ( 80 ) lowers. When the temperature lowers to a predetermined value, the bimetal  7  ( 70 ) of the thermal shutoff device S performs a returning operation, becoming in a reusable state. 
     Although the operation in the case where an abnormal charging current (overcurrent) flows has been explained in the above description, the abnormality detecting circuit  301  also has a function of detecting an abnormal charging voltage (overvoltage) to close the switch element  302 . 
     It is a matter of course that a temperature sensor such as a thermistor for detecting the temperature of the battery  202  can be included in the abnormality detecting circuit  301  so that the switch element  302  is closed when the temperature of the battery  202  becomes abnormal (excessive temperature). 
     FIG. 8 shows another embodiment of a battery pack in accordance with the present invention in which the thermal shut-off device S is incorporated. 
     A battery pack  200 - 2  incorporates an abnormality detecting circuit  205  corresponding to the abnormality detecting circuit shown in FIG.  7 . The terminal  2  ( 20 ) of the thermal shut-off device S is connected to a first terminal  201  for external connection, the terminal  9  ( 90 ) thereof is connected to a second terminal  203  for external connection via a battery  202  and the abnormality detecting circuit  205 , and the terminal  10  ( 100 ) thereof is connected to a negative pole of the battery  202  via a normally-open switch element  206  of the abnormality detecting circuit  205 . 
     In this battery pack  200 - 2 , if an abnormal current flows in the thermal shut-off device S, the abnormality detecting circuit  205  detects the abnormal current to close the switch element  206 . Accordingly, a current flows in the resistor  8  ( 80 ) of the thermal shut-off device S so that the resistor generates heat, by which the thermal shut-off device S is opened. As a result, the abnormal current is interrupted. 
     When the battery  202  is charged, the abnormality detecting circuit  205  detects an abnormal charging current as the aforementioned abnormal current. In this case, after detecting the abnormal charging current, the abnormality detecting circuit  205  maintains the closed state of the switch element  206 . During this time, the resistor  8  ( 80 ) is supplied with a heat generating current from the battery  202 . The heat generating current decreases with decreasing voltage of the battery  202  caused by the power consumption of the resistor  8  ( 80 ). When the heat generating current decreases to a predetermined value, the heat generating temperature of the resistor  8  ( 80 ) lowers, so that the thermal shut-off device S returns to the closed state. 
     On the other hand, when the battery  202  is discharged, the abnormality detecting circuit  205  detects an abnormal discharge current caused by overload, external short circuit, etc. as the aforementioned abnormal current. In this case as well, the abnormality detecting circuit  205  closes the switch element  206  and maintains the closed state, so that the abnormal current is interrupted. 
     The abnormality detecting circuit  205  also has a function of detecting an abnormal charging voltage of the battery  202  to close the switch element  302 . As described above, a temperature sensor such as a thermistor for detecting the temperature of the battery  202  can be included in the abnormality detecting circuit  205  so that the switch element  206  is closed when the temperature of the battery  202  becomes abnormal.