Abstract:
A battery separator for use in enveloping the plates of a flooded cell type lead acid battery. The separator has improved puncture resistance in the shoulder areas provided by a plurality of improved mini-ribs located therein. The mini-ribs have substantially flat upper surfaces, and the space between adjacent mini-ribs is less than about 0.0225 inch. The separator may also have a plurality of micro-ribs located on the backside.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a battery separator for use in flooded cell type lead acid batteries. The separator has a shoulder design that provides improved resistance to puncture.  
           [0002]    Separators are used in lead acid batteries to separate the positive and negative plates. Such separators are formed of materials that have sufficient porosity to permit the battery&#39;s electrolyte to reside in the pores of the material, thereby permitting ionic current to be established between adjacent positive and negative plates, but not so porous as to allow physical contact between the plates or “treeing” of lead between adjacent plates.  
           [0003]    The most commonly used material in flooded cell type lead acid batteries is microporous polyethylene. Such separators and their method of manufacture are, essentially, described in U.S. Pat. No. 3,351,495.  
           [0004]    Typically such separators have multiple “major” ribs formed on at least that planar face of the “backweb” which is to face the positive plate. Such major ribs are typically formed parallel to the longitudinal edges of the backweb and typically have a height above the backweb that is greater than the thickness of the backweb.  
           [0005]    Although at one time separators were manufactured as sheets which were placed between the plates, the overwhelming configuration currently used is to wrap the separator material around either the negative or positive plates and seal the separators, which has a width greater than the wrapped plates, along both edges to thereby form an “envelope” around each wrapped plate. The separator edges adjacent the plate edges, where sealing of the separator is effected, is known as the “shoulder” area of the separator.  
           [0006]    U.S. Pat. No. 4,407,063 discloses the most commonly used method and apparatus for enveloping and sealing separators around plates for lead acid batteries.  
           [0007]    One method of making plates for lead acid batteries is to cut, slit, and expand a lead sheet. Such “expanded metal” plates often have sharp metal protrusions, called “bent wires”, which can abrade and puncture the adjacent separator shoulder thereby causing a short circuit and premature failure of the battery. Such punctures can occur during enveloping, during battery assembly, or after manufacture of the battery.  
           [0008]    One approach to preventing such punctures is to make the shoulder area thicker than the thickness of the adjacent backweb. This approach is disclosed in U.S. Pat. No. 4,788,113.  
           [0009]    Another approach has been to form “mini-ribs” in the shoulder area. Such mini-ribs have a cross-section that is the arc of a circle. The height of such mini-ribs above the backweb is less than that of the major ribs, typically 0.007 inch. Such prior art mini-ribs are also spaced closer together than the major ribs, a typical such mini-rib spacing being 0.0626 inch. The use of mini-ribs is preferable over using a thicker shoulder area because of cost considerations. However, it has been found that many punctures still occur in the shoulder area with separators using current mini-rib configurations.  
           [0010]    It is an object of the present invention to provide a separator having an improved shoulder design that greatly reduces puncture by bent wires.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is a battery separator having a shoulder design that has improved puncture resistance.  
           [0012]    The separator of the invention has a plurality of mini-ribs extending from one planar face of the separator in the shoulder area, the mini-ribs having a flattened upper surface rather than the rounded upper surfaces of prior art mini-ribs. The maximum spacing between the mini-ribs of the invention is 0.0225 inch.  
           [0013]    In another embodiment of the invention, a plurality of micro-ribs extend from the planar surface of the separator opposite the planar surface from which the mini-ribs extend. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a partial end view of one embodiment of the battery separator of the invention;  
         [0015]    [0015]FIG. 2 is an enlarged end view of one shoulder portion of the battery separator illustrated in FIG. 1;  
         [0016]    [0016]FIG. 3 is an enlarged end view of two adjacent mini-ribs of the invention; and  
         [0017]    [0017]FIG. 4 is a partial end view of another embodiment of the battery separator of the invention using micro-ribs. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]    The battery separator  10  of this invention is comprised of a backweb  12  having a first (upper) planar surface  13 , a second (lower) planar surface  14 , a first edge  16 , and a second edge  18 . Lower planar surface  14  will also be referred to as the “backside”.  
         [0019]    A plurality of major ribs  20  are located across the width of separator  10 , only the two major ribs  20   a  and  20   b  closest to edges  16  and  18 , respectively, being shown in FIG. 1. Major ribs  20  are typically disposed longitudinally along the length of separator  10 , parallel to each other and to edges  16  and  18 . However, major ribs  20  may be located at an angle to edges  16  and  18  (i.e., be diagonally disposed), or may be sinusoidal rather than straight.  
         [0020]    Major ribs  20  may have any cross-sectional configuration used in the art, including those configurations disclosed in U.S. Pat. No. 5,679,479. Additional ribs located in that portion of the separator where the major ribs are located, such as disclosed in U.S. Pat. No. 5,789,103, may also be used.  
         [0021]    Located between the end ribs  20   a  and  20   b  and the edges  16  and  18 , respectively, are the shoulder areas of separator  10 . A plurality of mini-ribs  30  are located in the shoulder areas. Mini-ribs  30  are longitudinally disposed along the length of separator  10 , and are substantially evenly spaced from each other. Although it is preferred that mini-ribs  30  extend from only one planar face of separator  10 , they may extend from both planar faces in the shoulder area, and may be staggered so that a mini-rib on one planar surface is located between two mini-ribs located on the other planar surface.  
         [0022]    Mini-ribs  30  are preferably disposed substantially parallel to each other and to edges  16  and  18 . However, mini-ribs  30  may have other configurations, such as being at an angle to the edges  16  and  18  of separator  10 , or be sinusoidal rather than straight.  
         [0023]    [0023]FIG. 3 is an enlarged end view of two adjacent mini-ribs  30   a  and  30   b . As can be seen, mini-rib  30   a  has a flat apex or upper surface  31   a . Flat upper surface  31   a  is bounded by two upper edges  32   a  and  33   a . The base of mini-rib  30   a  is bounded by two lower edges  34   a  and  35   a , located at the juncture of mini-rib  30   a  and backweb  12 . Mini-rib  30   a  has sloping side walls  36   a  and  37   a  which extend between upper edges  32   a  and  33   a  and lower edges  34   a  and  35   a,  respectively. Upper edges  32   a  and  33   a  and lower edges  34   a  and  35   a  are, preferably, rounded as shown in FIG. 3.  
         [0024]    Likewise, mini-rib  30   b , which is identical to mini-rib  30   a,  has a flat apex or upper surface  31   b  bounded by upper edges  32   b  and  33   b , lower edges  34   b  and  35   b , and sloping side walls  36   b  and  37   b.    
         [0025]    Currently used mini-ribs have rounded apexes, i.e., the cross-section of such prior art mini-ribs is solely the arc of a circle, typically having a radius of 0.007 inch.  
         [0026]    The height of the flat upper surface  31  of mini-ribs  30  above the upper planar surface  13  of the backweb  12  of separator  10  is between about 0.003 and about 0.006 inch, preferably between about 0.003 and about 0.004 inch. Currently used mini-ribs typically have a height of about 0.007 inch.  
         [0027]    The width of the flat upper surface  31  is between about 0.008 and about 0.018 inch, preferably between about 0.010 and about 0.012 inch.  
         [0028]    The spacing between adjacent upper edges of adjacent mini-ribs  30 , i.e., the distance between edges  33   a  and  32   b  as seen in FIG. 3, is critical to minimizing puncture resistance in the shoulder area. The present inventors been found that if that spacing is less than about 0.0225 inch, and preferably between about 0.018 and about 0.020 inch, the vast majority of grid wires will not be able to contact the thinner backweb  12  where puncture resistance is low, but will contact the thicker area occupied by a mini-rib  30 , thereby minimizing the ability of the wire to penetrate through the separator  10  in the shoulder area.  
         [0029]    Currently used mini-ribs typically are spaced apart a distance of 0.0626 inch, which the current inventors have found is much greater than the cross-sectional size of many grid wires, thereby more easily permitting the grid wire to come into contact with the backweb where penetration is more easily achieved.  
         [0030]    Another preferred embodiment of the invention is one wherein a plurality of “micro-ribs”  40  extend from the backside  14  of the separator.  
         [0031]    Micro-ribs  40  are preferably substantially evenly spaced apart across the entire width of the backside  14  of separator  10 , and run longitudinally along the length of the separator  10 , substantially parallel to each other and to edges  16  and  18 . However, the micro-ribs  40  may be disposed at an angle to edges  16  and  18 , i.e., be diagonally disposed, or be sinusoidal rather than straight.  
         [0032]    The cross-section of micro-ribs  40  is preferably the arc of a circle, as can be seen in FIG. 4 which illustrates four adjacent micro-ribs  40   a,    40   b ,  40   c , and  40   d . The height of micro-ribs  40  above the backside is between about 0.003 and about 0.006 inch, preferably between about 0.003 and about 0.004 inch.  
         [0033]    The distance between micro-ribs  40 , measured center-to-center, is less than about 0.0225 inch, preferably between about 0.018 and about 0.020 inch.  
         [0034]    Example 1 below describes a specific configuration for a battery separator having the mini-rib configuration of this invention.  
       EXAMPLE 1  
       [0035]    A microporous polyethylene battery separator was made having the following characteristics, all dimensions being in inches:  
                                                       Separator width:   6.4           Backweb thickness:   0.006           Major ribs:           Number:   20           Height:   0.019           Width:   0.015           Spacing:   0.263           Mini-Ribs:           Number:   18 (each shoulder)           Height:   0.004           Width:   0.010           Spacing:           Upper edges:   0.0225           Centers:   0.0325           Lower edges:   0.0066           Other:   0.131 (distance between centers               of adjacent major rib and mini-               rib                      
 
       EXAMPLE 2  
       [0036]    Separators having the configuration described in Example 1 were tested for puncture resistance. The method used for testing for puncture resistance was a modified BCI Test Method 3.214. The modification consisted of substituting a pin having cross-sectional dimensions of 0.0225 inch×0.0325 inch for the pin size specified in the original test method. The reason for the substitution of pins was to use a pin which more closely approximated smaller grid wires. The puncture resistance of a large number of Example 1 separators averaged 0.75 pound.  
       Comparative Example 1  
       [0037]    Prior art separators were also tested for puncture resistance using the same modified test procedure used in Example 2. These prior art separators had the same dimensions of the Example 1 separators with the exception that the mini-ribs of such prior art separators were spaced apart a distance of 0.0626 inch (center-to-center) and the cross-section of the mini-ribs was the arc of a circle having a radius of 0.007 inch. The average puncture resistance of a large number of such prior art separators was 0.62 pound.  
         [0038]    While specific embodiments have been described, it is not intended that they restrict the scope of the invention beyond that set forth in the appended claims.