Abstract:
A passive battery charging control system for charging a battery is devoid of active electrical components. The passive battery charging control system includes one or more passive electrical control elements configured to limit the charging state of the battery.

Description:
BACKGROUND 
       [0001]    This invention relates generally to sealed batteries, and more particularly to a system and method for safely charging sealed batteries such as sodium/metal chloride batteries. 
         [0002]    Sodium chloride (NaCl) in the cathode of a sodium/metal chloride battery is converted into sodium (Na) ions and M-Cl complex through a series of chemical and transport steps during charging of a sodium/metal chloride battery. The Na ions are transported out of the cathode through a solid ion-conducting electrolyte into the anode compartment. The corresponding anode volume must be large enough to accommodate complete charging of the cathode. The total anode volume is typically about 0.42 times the total cathode volume for typical cathode configurations. If the anode volume is less, the liquid Na will pressurize the anode and could cause failure of either the solid electrolyte or anode compartment. 
         [0003]    Although techniques are known for avoiding overcharging in the anode compartment of a sodium/metal chloride battery, these known techniques generally rely on the use of active circuit elements that add cost and reduce the reliability of a corresponding charging control system. 
         [0004]    In view of the foregoing, it would be advantageous to provide a system and method for preventing overcharging (too much Na) in the anode compartment of a sodium/metal chloride battery in a manner that is more cost effective and achieve higher reliability than techniques that employ active circuit elements. It would be beneficial if the system and method could be successfully applied to any sealed battery having a variable fluid level anode compartment. 
       BRIEF DESCRIPTION 
       [0005]    According to one embodiment, a passive battery charging control system devoid of active electrical control elements for charging a battery comprises: 
         [0006]    an anode compartment comprising an anode fluid sealed therein; 
         [0007]    a first anode current collector configured to physically remain in contact with the anode fluid during operation of the battery; 
         [0008]    a second anode current collector configured to physically contact the anode fluid only when the anode fluid reaches a desired maximum level within the anode compartment during the operation of the battery; and 
         [0009]    a passive control element configured to limit the charging state of the battery when the second anode current collector makes physical contact with the anode fluid during operation of the battery, wherein the passive battery charging control system is free of active electrical components. 
         [0010]    According to another embodiment, a passive battery charging control system devoid of active electrical control elements for charging a battery comprises one or more passive electrical control elements configured to limit the charging state of the battery. 
         [0011]    According to yet another embodiment, a passive battery charging control system devoid of active electrical control elements for charging a battery comprises: 
         [0012]    an anode compartment comprising an anode fluid sealed therein; 
         [0013]    a plurality of current collectors; and 
         [0014]    a passive control element, the plurality of current collectors and the passive control element together configured to limit the charging state of the battery when the anode fluid reaches a desired maximum level within the anode compartment, wherein the passive battery charging control system is free of active electrical components. 
     
    
     
       DRAWINGS 
         [0015]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawing, wherein: 
           [0016]      FIG. 1  is a simplified schematic diagram illustrating a control system for charging a battery cell according to one embodiment; 
           [0017]      FIG. 2  is a schematic diagram illustrating a control system for charging a plurality of battery cells according to one embodiment; and 
           [0018]      FIG. 3  is a cross-sectional view of an anode tube assembly according to one embodiment. 
       
    
    
       [0019]    While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
       DETAILED DESCRIPTION 
       [0020]      FIG. 1  illustrates a control system  10  for charging a battery cell  12  according to one embodiment. Battery cell  12  comprises an anode compartment  14 . Anodic material  16  in fluidized form is sealed within anode compartment  14 . 
         [0021]    A first anode current collector  18  is configured to physically remain in contact with the anodic material  16  during operation of the battery cell  12 , such as during charging and discharging of the battery cell  12 . A second anode current collector  20  is configured to physically contact the anodic material  16  only when the anodic material reaches a desired maximum level  22  within the anode compartment  14  during the operation of the battery cell. 
         [0022]    A passive control element  24 , such as without limitation, a high temperature resistor, is configured to limit the charging state of the battery cell  12  when the second anode current collector  20  makes physical contact with the anodic material  16  during operation of the battery cell  12 . During charging of some batteries such as, for example, a sodium/metal chloride battery, sodium chloride (NaCl) in the cathode  26  through a series of chemical and transport steps, is converted into sodium (Na) ions and metal-chloride (M-Cl) complex. The Na ions are transported out of the cathode  26  through a solid ion-conducting electrolyte  28  into the anode compartment  14 . The anode compartment volume must be large enough to accommodate the complete charging of the cathode  26 . The anode compartment volume is typically about 0.42 times the cathode volume for typical sodium/metal chloride battery cathode configurations. If the anode compartment volume is less, then the anodic material  16  such as Na will pressurize the anode compartment  14  beyond its physical limitations, causing failure of either the solid electrolyte  28  or anode compartment  16 . 
         [0023]    With continued reference to  FIG. 1 , the first anode current collector  18  extends downward through the height of the anode compartment  14  to maintain physical contact with the varying anodic material  16 , e.g. liquid Na, level during operation of the battery cell  12 . The second anode current collector  20 , e.g. wire, is also extended into the anode compartment  14 . Second anode current collector  20  however extends only down to approximately a desired maximum level  22  of anodic material, e.g. Na. The opposite end of the second anode current collector  20  is connected to a passive control element  24 , such as without limitation, a shorting resistor Rs. The passive element  24  is also electrically connected to a corresponding cathode current collector  30  such as depicted in  FIG. 1 . 
         [0024]    The final state of charge of the cell  12 , and any cells in parallel with the cell  12  can be controlled in response to the physical extension of the second anode current collector  20 , the value of the passive element  24 , e.g. resistance, and the resistance of the battery cell  12 , which depends upon the state of charge of the battery cell  12 . 
         [0025]    Although the first and second anode current collectors  18 ,  20  may be implemented using a pair of distinct and separate current conductors, e.g. wires, another embodiment may use a structure such as a coaxial cable to implement the current collector pair  18 ,  20 . In this embodiment, the inner coaxial cable conductor can be used to form one of the current collectors such as current collector  18 , while the outer electrical shield can be used to form the other current collector such as current collector  20 . Another embodiment employs a U-shaped anode tube  40  such as depicted in  FIG. 3 . In this embodiment, the anodic material  16  naturally equalizes to the same level in each side of the U-shaped anode tube  40  during operation of the battery cell. The first anode current collector  18  is inserted into one side of the U-shaped anode tube  40 , while the second, shorter anode current collector  20  that connects with the passive element  24  shown in  FIG. 1 , is inserted into the other side of the U-shaped anode tube  40  such as depicted in  FIG. 3 . The foregoing coaxial cable and U-shaped anode tube embodiments are particularly useful when the corresponding anode tube structure cross-sectional area when viewed in the axial direction of the anode tube is too small to insert two workable separate and distinct anode current collectors such as two separate and distinct wire elements. 
         [0026]    According to one embodiment, the passive element  24  may be a typical high temperature resistor and may have a resistance value that ranges between zero or no resistance and up to an upper limit that is based upon and depends upon the value of battery cell resistance between the cathode current collector  30  and the first anode current collector  18  such as depicted in  FIG. 1 . 
         [0027]      FIG. 2  is a schematic diagram illustrating a control system  100  for charging a plurality of battery cells  102 ,  104  according to one embodiment. Each battery cell  102 ,  104  may be implemented as shown in  FIG. 1  described herein. 
         [0028]    A workable range of resistance values can be defined for passive element  24  using the simplified model shown in  FIG. 2 . A battery comprises a collection of one or more (Ns) cells, e.g.  102 ,  104 , in series. Each cell Ns has a common cathode and one or more (Np) anodes. The workable range of resistance value for passive element  24  can be defined as follows: 
         [0000]    
       
      
       Voc/I&lt;Rs&lt;Voc*Np/I,  
      
     
         [0000]    where Rs is the resistance of passive element  24 , I=maximum recharge current used for charging, and Voc is the open circuit voltage, which depends on the chemistry of the cell, e.g.  102 ,  104 . Resistance values smaller than the lower bound Voc/I allows other (parallel) anodes in the cell to discharge through the shorted anode(s). This condition also provides a workable solution though less charge is stored in the corresponding cell than the cell is capable of storing. Resistance values larger than the upper bound Voc*Np/I results in a condition that all anode cells will continue to charge (and potentially completely fill) even after all anodes are shorted. In between these two limits, any anode cell that is shorted will continue to charge until all anode cells become shorted, at which time the cell will be charged and no anode will be further filled. The expected variation of the resistance for each anode determines the distance of the second electrode  20  into the anode chamber and the value of Rs so that no anode can completely fill. A completely filled anode may lead to failure of that anode (and therefore the failure of that cell). If a cell, e.g.  102 ,  104 , fails, the corresponding battery itself may still work; but now the battery comprises Ns−1 working cells in series. 
         [0029]    The maximum recharge current I, is determined by chemical aspects (e.g. too much current can be harmful to the battery), or is based upon the allowable current density through the solid electrolyte  28  (i.e. I&lt;Jmax*Aanode, where Jmax is the maximum allowable current density in the solid electrolyte  28  and Aanode is the total working surface area of all the anodes in a cell. A subcell resistance  106  can be defined by Voc*Np/I described herein; the resistance of the entire cell, e.g.  102  or  104 , can be defined by Voc/I described herein. 
         [0030]    Although particular embodiments described with reference to  FIG. 2  depict a plurality of passive elements  24 , other embodiments can just as easily employ a single passive element  24  to achieve the desired results using the principles described herein. In one such embodiment, a single passive control element  24  such as a typical high temperature resistor can be electrically connected at one end to a common battery cathode, while the other end of the resistor can be connected to a plurality of anode current collectors  20 . Generally the use of a high temperature resistor is only needed for high temperature applications, sometimes greater than about 125° C., such as when using Na/M-Cl batteries, since they operate at high temperatures. 
         [0031]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.