Abstract:
A fault tolerant battery system includes an electrical storage cell having a positive terminal and a negative terminal. The electrical storage cell is provided with a normally open bypass circuit path that is closed in the event of an overdischarged, or open-circuit failure of, the electrical storage cell. The bypass circuit path includes a first electrical conductor connected to the negative terminal of the electrical storage cell, a second electrical conductor connected to the positive terminal of the electrical storage cell, and a shorting gap between the first electrical conductor and the second electrical conductor. The first and second electrical conductors and a non-conductive barrier define an enclosed chamber that encompasses a mass of a fusible material, a spring that is positioned to force the mass of the fusible material into the shorting gap when the mass of the fusible material is at least partially molten, and a heat source is activatable upon the occurrence of the failed electrical storage cell. Two diodes connected in electrical series serve as heat sources, one of which is operable to melt at least a portion of the mass of the fusible material and one of which is operable to heat the conductive web. The chamber entraps the partially molten fusible material such that the spring forces the molten material into the shorting gap, where it closes the shorting gap so that the first electrical conductor is in electrical communication with the second electrical conductor.

Description:
TECHNOLOGICAL FIELD 
       [0001]    The present disclosure relates generally to electrical storage batteries, and, in particular, to a fault tolerant battery cell bypass device and system. 
       BACKGROUND 
       [0002]    Rechargeable cells or batteries are electrochemical energy storage devices for storing and retaining an electrical charge and later delivering that charge as useful power. Familiar examples of rechargeable electrical storage cells are lithium-ion (Li-on) cells and nickel-cadmium cells used in various portable devices and lead-acid cells used in automobiles. Another type of electrical storage cell is the nickel oxide/pressurized hydrogen electrical storage cell, commonly called the nickel-hydrogen electrical storage cell, which is used in spacecraft applications. 
         [0003]    Although electrical storage cells are designed for excellent reliability, there is always the chance of a failure. One common failure is when a cell, in a battery comprised of an array of series-connected cells, is defective and, therefore, has diminished storage capacity. When the battery discharges, the defective cell fully discharges prior to the other non-defective cells in the array. As the battery remains in discharge mode, the defective cell becomes overdischarged resulting in a reversal of the defective cell&#39;s voltage. This adversely affects the battery&#39;s operation because the defective cell is no longer able to carry the same current load as the other cells in the array. Another common failure is when there is an open-circuit failure, in which there is no longer a conducting path through the cell. In the event of such open-circuit failure of a single cell in a series-connected array of cells, all of the storage capacity of the array is lost. In a spacecraft battery, for example, a loss of the battery&#39;s storage capacity can lead to failure of the mission. 
         [0004]    A bypass around a potentially defective or failed cell is required to prevent loss of the storage capacity of the entire array. The bypass must not conduct when the electrical storage cell is functioning properly, but it must activate to provide an electrically conductive bypass when the electrical storage cell fails. An example circuit diagram for a prior art bypass device  10  is shown in  FIG. 1 , in which a diode  22  is connected across the cell  20  such that the cathode of the diode  22  is connected to the positive terminal of the cell  20 , and the anode of the diode  22  is connected to the negative terminal of the cell  20 . If the voltage across the diode  22  is negative at the anode and positive at the cathode, as in normal operation of the cell  20 , no significant current flows through the diode  22 . If the cell  20  fails to an open-cell condition, for example, the voltage across the diode  22  reverses, and current flows through the diode  22  in the forward direction. Current flowing through the diode  22  causes the diode  22  to heat substantially, to at least 183 degrees C. A mass of a fusible material  24  is positioned at an initial mass location such that it is not within the shorting gap  30 , but such that it is heated and melted by the heat produced by the diode  22 . The melted fusible material  24  is driven into the shorting gap  30  and serves to cause the shorting gap  30  to be closed, which closure is indicated schematically by a switch  32  in  FIG. 1 . 
         [0005]    The prior art bypass device circuitry shown in  FIG. 1  has a significant shortcoming because if the single diode  22  suffers a short-circuit failure, the bypass device  10  could be inadvertently activated even when the cell  20  is not defective or in an open-cell condition. This could cause the cell  20  to overheat, vent, and cause a fire. Even apart from the use of a single diode  22 , the prior art bypass circuitry is activated only when the heat from the diode  22  creates an effective thermal path to the fusible material  24 , such that the fusible material  24  melts. If the heat dissipates as it travels along the thermal path from the diode  22  to the fusible material  24  such that it is then insufficient for heating the fusible material  24 , the bypass device  10  fails. Finally, in the prior art bypass device  10 , the fusible material  24  is unconstrained, which means that the molten flow is not guaranteed to close the shorting gap  30  for activation of the device, thereby resulting in device unreliability. 
         [0006]    Thus, there is a need in the art for an improved technique for achieving an electrical bypass of electrical storage cells. The present invention fulfills that need, and further provides related advantages. 
       BRIEF SUMMARY 
       [0007]    In view of the foregoing background, example implementations of the present disclosure provide a battery system including an electrical storage cell having a positive terminal and a negative terminal and a normally open bypass circuit path. The normally open bypass circuit path includes a first electrical conductor connected to the negative terminal of the electrical storage cell, a second electrical conductor connected to the positive terminal of the electrical storage cell, and a shorting gap between the first electrical conductor and the second electrical conductor. The battery system further includes a mass of electrically conductive fusible material, and first and second diodes electrically connected in series. The fusible material may be a metal, such as a metal alloy. The fusible material has a melting point of no more than about 183 degrees C. 
         [0008]    Each of the first and second diodes has a cathode and an anode, the anode of the first diode being electrically connected to the negative terminal of the electrical storage cell and the cathode of the second diode being electrically connected to the positive terminal of the electrical storage cell. Each of the first and second diodes is heat source activatable upon the occurrence of a voltage reversal of the electrical storage cell. The first diode has a sufficient heat output to melt the mass of the fusible metallic alloy. A biasing mechanism, such as a spring, is positioned to force the mass of the fusible material into the shorting gap, when the mass of the fusible material is at least partially molten, thereby closing the shorting gap so that the first electrical conductor is in electrical communication with the second electrical conductor. 
         [0009]    The battery system further includes a nonconductive barrier disposed between the first electrical conductor and the second electrical conductor to define a bypass chamber that encloses the mass of fusible material, first and second diodes, gap, biasing mechanism, and shorting gap. The bypass chamber entraps the mass of the fusible material when it is at least partially molten. 
         [0010]    The normally open bypass circuit path may also include an electrically conductive web that it is electrically connected to the second conductor, the web being disposed within the chamber. The shorting gap is disposed between the web and the first conductor, and the biasing mechanism forces the mass of the fusible material into the shorting gap, when the mass of the fusible material is at least partially molten, thereby closing the shorting gap so that the first electrical conductor is in electrical communication with the second electrical conductor via the web. The second diode has a sufficient heat output to heat the web. 
         [0011]    In example implementations, the chamber further encloses an electrically conductive spreader that is disposed between the fusible material and the first diode. Also, the battery system may include a second electrical storage cell in an electrical series relationship. 
         [0012]    Example implementations of the present disclosure further provide a battery system including an electrical storage cell having a positive terminal and a negative terminal and a normally open bypass circuit path. The normally open bypass circuit path includes a first electrical conductor connected to the negative terminal of the electrical storage cell, a second electrical conductor connected to the positive terminal of the electrical storage cell, and a shorting gap between the first electrical conductor and the second electrical conductor. The battery system further includes a mass of fusible material, a heat source activatable upon the occurrence of a voltage reversal of the electrical storage cell, the heat source being operable to melt at least a portion of the mass of the fusible material, and a biasing mechanism positioned to force the mass of the fusible material into the shorting gap when the mass of the fusible material is at least partially molten, thereby closing the shorting gap so that the first electrical conductor is in electrical communication with the second electrical conductor. 
         [0013]    The mass of fusible material, heat source, and biasing mechanism are disposed within an enclosed bypass chamber defined by the first electrical conductor, the second electrical conductor, and a nonconductive barrier disposed between the first electrical conductor and the second electrical conductor. The bypass chamber entraps the mass of the fusible material when it is at least partially molten. 
         [0014]    The normally open bypass circuit path further includes an electrically conductive web that it is electrically connected to the second conductor within the chamber such that the shorting gap is disposed between the web and the first conductor, and the biasing mechanism forces the mass of the fusible material into the shorting gap, when the mass of the fusible material is at least partially molten, thereby closing the shorting gap so that the first electrical conductor is in electrical communication with the second electrical conductor via the web. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING(S) 
         [0015]    Having thus described example implementations of the disclosure in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
           [0016]      FIG. 1  is a schematic circuit diagram of a prior art battery bypass device. 
           [0017]      FIG. 2  is a schematic diagram of a battery system according to an example implementation of the present disclosure; 
           [0018]      FIGS. 3  is a schematic diagram of a battery bypass device prior to activation according to an example implementation of the present disclosure; 
           [0019]      FIG. 4  is a schematic diagram of a battery bypass device during activation according to an example implementation of the present disclosure; 
           [0020]      FIG. 5  is a schematic diagram of a battery bypass device after activation according to an example implementation of the present disclosure; 
           [0021]      FIG. 6  is a schematic diagram of a battery bypass prior to activation according to an additional implementation of the present disclosure; 
           [0022]      FIG. 7A  is a schematic diagram of a battery system prior to activation of a battery bypass device according to the example implementation of  FIG. 3 ; 
           [0023]      FIG. 7B  is a schematic diagram of a battery system during activation of a battery bypass device according to the example implementation of  FIG. 4 ; and 
           [0024]      FIG. 7C  is a schematic diagram of a battery system after activation of a battery bypass device according to the example implementation of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Some implementations of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all implementations of the disclosure are shown. Indeed, various implementations of the disclosure may be embodied in many different forms and should not be construed as limited to the implementations set forth herein; rather, these example implementations are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. For example, unless otherwise indicated, reference something as being a first, second or the like should not be construed to imply a particular order. Also, something may be described as being above something else (unless otherwise indicated) may instead be below, and vice versa; and similarly, something described as being to the left of something else may instead be to the right, and vice versa. Like reference numerals refer to like elements throughout. 
         [0026]    Example implementations of the present disclosure relate generally to an apparatus and system for sensing a failed cell in a battery and bypassing such failed cell to restore near-normal operation of the battery. 
         [0027]      FIG. 2  illustrates a battery  40  having three electrical storage cells  50   a ,  50   b,  and  50   c  in an electrical series relationship in accordance with an implementation of the present disclosure, although in practice the number of electrical storage cells  50  in series is usually greater. In this example implementation, via operation through a controller  52 , the electrical storage cells  50  are charged by a solar panel power supply  54  and discharge to provide power to consuming components  56 . More specifically, the battery  40  goes into discharge mode when the demand for power from the consuming components  56  can&#39;t be satisfied by the solar panel supply  54 , such as during an eclipse. 
         [0028]    As noted above, one common failure mode of electrical storage cells  50 , such as lithium ion cells, is when a cell  50  in an array of series-connected cells  50  is defective and, therefore, has diminished storage capacity. When the battery  40  discharges, the defective cell  50  fully discharges prior to the other non-defective cells  50 , which ultimately results in the defective cell  50  being overdischarged and having a negative voltage. Another example of a failure mode is an open-circuit failure, in which there is no longer a conducting path through the cell  50 . In both of these examples of cell  50  failures, the consequences of such failures are compounded because the storage cell  50  is in electrical series with other cells  50  in the battery  40 . Specifically, the other electrical cells  50  in the battery  40  are rendered useless by the failure of the defective cell  50  unless a suitable bypass is provided to remove the failed cell  50  from the series arrangement. Thus, in the example implementation of the present disclosure shown in  FIG. 2 , a bypass  58  is provided for each storage cell  50  in the array. Particularly, there is a bypass  58   a  for cell  50   a,  a bypass  58   b  for cell  50   b,  and a bypass  58   c  for cell  50   c.  When the cells  50  functions normally, the corresponding bypasses  58  are inactive and carry no current. Upon failure of a cell  50 , the corresponding bypass  58  becomes active and carries current around the failed cell  50 . The remaining properly functioning cells  50  continue to store and deliver power, so the battery as a whole continues to store and deliver power, but at a diminished voltage because one of the cells is at zero volts. 
         [0029]      FIG. 3  illustrates the bypass device  58  according to example implementations of the present disclosure, in which the bypass device  58  is an enclosed chamber with borders defined by: a first electrical conductor  66 ; a second electrical conductor  68 ; and an electrically non-conductive barrier  72  that is disposed between the first conductor  66  and the second conductor  68 . In the implementation shown in  FIG. 3 , the non-conductive barrier  72  is cylindrically disposed between the first conductor  66  and the second conductor  68 . It should be understood, however, that the present disclosure is not limited to the non-conductive barrier  72  being cylindrically shaped. Rather, the barrier  72  could be a different shape with walls that still nonetheless form the chamber. The non-conductive barrier  72  may be formed from material such as plastic with a high melting temperature or ceramic. 
         [0030]    Disposed within the enclosed chamber of the bypass device  58  are: a mass of electrically conductive fusible material  64 ; a first diode  60   a;  an electrically conductive biasing mechanism such as a spring  62 ; a second diode  60   b;  and an electrically conductive web  70  that is electrically connected to the second conductor  68 . Even though spring  62  physically separates the first diode  60   a  and the second diode  60   b,  the pair of diodes  60  are connected in electrical series. A shorting gap  74  prevents the web  70  from electrically connecting the first conductor  66  to the second conductor  68 , thereby electrically isolating the battery cell&#39;s  50  positive and negative terminals. 
         [0031]    The bypass device  58  is connected to the battery cell  50  such that the anode of the first diode  60   a  is electrically connected to the negative terminal of the cell  50  by means of the first electrical conductor  66  and the fusible material  64 . The cathode of the second diode  60   b  is electrically connected to the positive terminal of the cell  50  by means of the second conductor  68 . Thus, the first conductor  66 , fusible material  64 , pair of diodes  60  and second conductor  68  are electrically connected through physical contact and compression provided by the conductive spring  62 . 
         [0032]    In example implementations of the present disclosure, in order to minimize the voltage difference between the second conductor  68  and the first conductor  66 , the diodes  60  are selected such that the sum of the forward voltage drops is minimized. For example, it is optimal for the voltage drop across the bypass device  58  to be less than the voltage at which copper plating occurs in a lithium ion cell because the copper plating internally shorts the cell. One example of suitable diode is a Schottky diode. By minimizing the sum of the voltage forward drops of the diodes  60 , the bypass device  58  is activated at a lower voltage than the voltage level at which copper plating begins. 
         [0033]    Referring again to  FIG. 3 , during normal operation of the cell  50 , the voltage across the pair of series connected diodes  60  is negative at the anodes and positive at the cathodes, respectively, so no significant current flows through the diodes  60 . However, if the voltage of the cell  50  is reversed during a discharge operation, such as because the cell  50  has been overdischarged or because the cell  50  has failed to an open-cell condition, the current flows from the cell&#39;s  50  negative terminal through the first conductor  66 , the fusible material  64 , the first diode  60   a,  the spring  62 , and the second diode  60   b  to the second conductor  68 , as illustrated in  FIG. 4 . The diodes  60  are sized so that this forward flow of current through the diodes  60  causes them to heat, such as to at least about 183 degrees C. Unlike the prior art bypass device  10  wherein the fusible material  24  is positioned a distance away from the diode  22 , in example implementations of the present disclosure, the mass of a fusible material  64  is positioned directly next to diode  60   a  and in close proximity to the second diode  60   b,  thereby eliminating the thermal path associated with the prior art bypass device  10 . More specifically, as the diodes  60  conduct heat, the fusible material  64 , which is closely coupled and in physical contact with the diodes  60 , begins to melt. In example implementations of the present disclosure, the fusible material  64  is eutectic solder, which is an alloy commonly used in electronic assembly that melts at about 183 degrees C. An example ratio of lead to tin in eutectic solder is Sn 63 Pb 37 . 
         [0034]    Referring now to  FIG. 5 , as the fusible material  64  melts from the heat of the diodes  60  and becomes at least partially molten, such molten material  64  remains trapped within the chamber and, therefore, moves towards the shorting gap  74  under the influence of the spring&#39;s  62  biasing force against the diodes  60  and the fusible material  64 . Eventually, the molten fusible material  64  fills the shorting gap  74  to provide an electrically conductive, low resistance, bridge between the first conductor  66  and the second conductor  68  via the conductive web  70 , which conductive bridge is retained even after the fusible material  63  re-solidifes. The newly created electrical path is bidirectional, in that it allows current to flow in either direction, thereby allowing for battery cell  50  charging and discharging. For example, during battery cell  50  discharging, current flows from the first conductor  66  through the fusible material  64  filled gap, the conductive web  70 , and the second conductor  68 . 
         [0035]    In yet a further implementation of the present disclosure shown in  FIG. 6 , the bypass device  58  further includes an electrically conductive, but non-melting spreader  76  comprise of a material such as copper. The spreader  76  is disposed within the chamber between the fusible material  64  and the first diode  60   a.  In this implementation, because of the spreader  76 , the collective mass of the first diode  60   a,  spreader  76 , fusible material  64 , and first conductor  66  (collectively, the “first diode  60   a  mass”) is more than the collective mass of the second diode  60   b,  web  70 , and second conductor  68  (collectively, the “second diode mass”). Because the second diode mass is less than the first diode mass, the second diode mass gets hotter during bypass  58  activation than the first diode mass. Thus, the web  70  gets hotter than the fusible material  64  which is located in the first diode mass. When the web  70  is hotter than the fusible material  64 , even in its molten state, the fusible material  64  forms a stronger bond with the web  70 . Conversely, if the web  70  is cooler than the fusible material  64  when the bond is formed (commonly referred to as a cold solder joint), the bond is more likely to breakdown over time. Thus, in the implementation shown in  FIG. 6  wherein the web  70  is hotter than the fusible material  64  because of the second diode mass being less than the first diode mass, the bond formed between the fusible material  64  and web  70  is much stronger and less likely to breakdown over time. 
         [0036]    Example implementations of the bypass device  58  in operation are shown in  FIGS. 7A-7C .  FIG. 7A  depicts normal operation the battery  40 , meaning that none of the cells  50  have diminished capacity or are in an open-cell condition. In this scenario, during the battery&#39;s  40  discharge operation, the battery&#39;s  40  current flows equally through each cell  50  and no current flows through the bypass devices  58 .  FIG. 7B  depicts operation of the battery  40  during activation of a bypass device  58   b  because of a failure in the corresponding cell  50   b.  In this scenario, as the battery  40  discharges, part of the current flows through the defective cell  50   b  and the balance of the current flows through the bypass device  58   b.  As the battery  40  continues to discharge and the defective cell  50   b  becomes overdischarged to a higher degree, an increasingly higher proportion of the current flows through the bypass device  58   b  which causes the diodes  60  in the bypass device  58   b  to heat to activate the device  58   b,  as described above with respect to  FIG. 4 .  FIG. 7C  depicts operation of the battery  40  after the bypass device  58   b  has been activated. In this scenario, all of the current flows through the bypass device  58   b  and no current flows through the defective cell  50   b,  as described above with respect to  FIG. 5 . 
         [0037]    According to the example implementations of the present disclosure, the improved bypass device  58  is fault tolerant unlike the prior art device  10  because the bypass device  58  utilizes two diodes  60 , operating in electrical series, for activation of the device  58 . Thus, if one of the diodes  60  fails when the cell  50  is not defective, the other diode  60  will prevent the device  58  from activating prematurely. Additionally, by closely coupling the diodes  60  with the fusible material  64 , such as by physical contact, the bypass device  58  of the present disclosure provides for better heat transfer between the pair of diodes and the fusible material  64 . Because the heat doesn&#39;t have to travel a thermal path, as in the prior art device  10 , the bypass device  58  of the present disclosure can be activated at lower currents, thereby making the device  58  more likely to short the lithium ion cell before the cell can form copper plating and short internally. Additionally, because of the fusible material&#39;s  64  close coupling with the diodes  60 , the bypass device  58  may utilize a lower volume of fusible material  64  than in the prior art device  10 . In yet a further improvement over the prior art device  10 , the bypass device  58  of the present disclosure provides an enclosed chamber for entrapping the molten fusible material  64  when the device  58  is activated. This results in a significantly higher likelihood that the molten fusible material  64  will fill the shorting gap  70 , unlike the prior art device  10  wherein the molten fusible material is unconstrained so directional flow of the molten is not definite or predictable. 
         [0038]    It should be understood that variations on the general principals of the invention are possible. For example, in some implementations of the present disclosure, the enclosed chamber of the bypass device  58  may include only one diode as a heating device. Also, a number of practical aspects have been omitted from the description that should be obvious to a practitioner skilled in the art. 
         [0039]    Different examples of the apparatus(es) and method(s) disclosed herein include a variety of components, features, and functionalities. It should be understood that the various examples of the apparatus(es) and method(s) disclosed herein may include any of the components, features, and functionalities of any of the other examples of the apparatus(es) and method(s) disclosed herein in any combination, and all of such possibilities are intended to be within the spirit and scope of the present disclosure. 
         [0040]    Many modifications and other implementations of the disclosure set forth herein will come to mind to one skilled in the art to which this disclosure pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the disclosure is not to be limited to the specific implementations disclosed and that modifications and other implementations are intended to be included within the scope of the appended claims. Moreover, although the foregoing descriptions and the associated drawings describe example implementations in the context of certain example combinations of elements and/or functions, it should be appreciated that different combinations of elements and/or functions may be provided by alternative implementations without departing from the scope of the appended claims. In this regard, for example, different combinations of elements and/or functions than those explicitly described above are also contemplated as may be set forth in some of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.