Abstract:
A cell grading test fixture includes an array of cell sockets electrically interconnected in series so that current through the entire array of cells flows through each of the cells once inserted into the fixture. Associated with each socket is an indicator which indicates to the operator when a cell voltage has fallen to a predetermined level. Each socket of the test fixture includes a spring-loaded switch contact such that upon removal of a cell which has dropped below a threshold voltage, the movable contact engages the contact of an adjacent socket, shorting out the cell location such that the series circuit of cells continues to provide a current path for the remaining cells.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a test unit for grading battery cells. 
     In order to assemble battery packs employing multiple cells made of, for example, a series and/or series parallel connection of individual secondary cells, the cells should have substantially the same capacity. However, NiCd or metal hydride batteries can display different discharge characteristics from one another upon manufacturing. Thus, it is desirable to grade the cells prior to battery pack assembly to assure that the cells have substantially the same capacity, particularly when in a series interconnection, so that one cell, which may have less capacity than the other, does not disable or otherwise reduce the capacity of the entire battery pack. 
     In the past, in order to grade the cells, a test fixture has been employed for holding cells which have been fully charged, coupling them in series, and loading and discharging them through a resistive load. Such fixtures hold, for example, an array of one hundred cells with the operator inserting the cells into the test fixture to begin their discharge. Periodically, and typically on a two-minute basis, an operator utilizes a voltage test probe to test each cell. The probe provides an audible or visible indication that a cell has fallen below a predetermined reference voltage, such as 1.3 volts for a 1.5 volt cell. Thus, the operator must periodically manually test each of the hundred cells and, if a cell has fallen below the threshold voltage, the test probe will provide the operator with an indication that the cell has reached a lowered voltage. When a cell falls below the threshold voltage, it is then removed, which interrupts the series circuit of the test fixture, and the last most cell is repositioned in the removed cell&#39;s location to continue the series discharge circuit. As can be appreciated as more and more cells reach a diminished capacity, the shuffling and reshuffling of batteries becomes a labor intensive and inefficient process by which to grade cells. 
     SUMMARY OF THE PRESENT INVENTION 
     In order to overcome the deficiencies of the prior art cell grading process and test equipment, the cell grading test fixture of the present invention includes a plurality of cell sockets for receiving individual cells, with the sockets being electrically interconnected in series so that current through the entire array of cells flows through each of the cells once inserted into the fixture. Associated with each socket is an indicator which indicates to the operator when a cell voltage has fallen to a predetermined level. Each socket of the test fixture includes an electrical contact which is spring-loaded and movable such that upon removal of a cell which has dropped below a threshold voltage, the movable contact engages the contact of an adjacent socket, in effect shorting out the cell location such that the series circuit of cells continues to provide a current path for the remaining cells. A plurality of voltage comparators are coupled to each of the cells and compares the voltage of the cell to a reference voltage. When the cell voltage drops below the predetermined voltage, the indicator, such as an LED mounted to the test fixture immediately adjacent the cell, is illuminated to indicate to the operator that the cell has reached a reduced voltage and should be removed. 
     Thus, with the cell grading fixture of the present invention, the operator can insert an array of cells for testing and watch the test fixture until such time as cells begin to drop below the threshold voltage, which is indicated by an LED immediately adjacent the individual cell. At such time, the cell is removed and, knowing the time duration it took from the initiation of the testing procedure until the cell reached its lower threshold voltage, the cells can be automatically graded without reshuffling them in the test fixture. Such a test fixture and the method of testing cells greatly improves the efficiency of grading cells and allows a single operator to test multiple cell banks at a given time as opposed to being occupied with the shuffling of cells in a single test fixture. 
     These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a top plan view of a test fixture embodying the present invention; 
     FIG. 2 is an enlarged, fragmentary, vertical, cross-sectional view of a pair of individual sockets in the test fixture shown in FIG. 1; 
     FIG. 3 is an exploded perspective view of one of the cell sockets; 
     FIG. 4 is a partial electrical circuit diagram for the electrical test circuit employed in connection with the test fixture of FIGS. 1-3; 
     FIG. 5 is a top plan view of a preferred second embodiment of the test fixture; 
     FIG. 6 is an enlarged, fragmentary cross-sectional view taken along plane VI—VI of a few of the individual socket inserts in the test fixture shown in FIG. 5; 
     FIG. 7 is a top plan view of the lower plate of the test fixture shown in FIG. 5; 
     FIG. 8 is a bottom plan view of the upper plate of the test fixture shown in FIG. 5; and 
     FIG. 9 is a perspective view of a socket insert of the test fixture shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, there is shown a test fixture for grading cells embodying the present invention. The test fixture  10  comprises a horizontally extending panel  12  supported on a framework sufficient to elevate the panel  12  above a work surface onto which the test fixture is placed. Alternatively, the test fixture could be rack mounted vertically to a suitable framework if desired. The panel  12 , in the embodiment illustrated in FIG. 1, includes an array of cell-receiving sockets which have the flexibility of interchanging their shape for use in connection with different cells. For such purpose, the panel  12  includes an array of generally rectangular openings  13  comprising ten rows and columns of such openings for receiving test sockets  20  therein. Thus, the test fixture  10  of the present invention can receive up to one hundred test sockets  20  and be capable of testing up to one hundred cells. With the system of the present invention, however, the array can be decreased or increased for testing a fewer or greater number of cells. 
     Mounted to the panel  12  adjacent and below each of the sockets  20  is a light emitting diode (LED)  15  which is coupled to an electrical circuit, shown in FIG.  4 . An LED  15  is coupled to each socket and cell being tested for indicating when the cell voltage has fallen below a predetermined reference level. A wiring harness conventionally couples each of the sockets to the test circuit and LED as well as to a suitable resistive load, as shown in FIG. 4, for drawing current through the series coupled cells when under test. The sockets themselves are mechanically and electrically interconnected by the fixture in the manner shown in detail in connection with FIGS. 2 and 3 now described. 
     Referring now to FIG. 2, there is shown three adjacent sockets, such as socket  20  shown in FIG. 1, and adjacent sockets comprising socket  21  having a cell  30  mounted therein and a next adjacent socket  22  of the array of sockets. As can be seen, with reference also to FIG. 3, each of the sockets include a holder  24  with a central recess  25  for receiving a cell adapting insert  26 . The generally rectangular recess  25  of the holder  24  includes a floor  27  (FIG. 2) with an opening  28  allowing a configured electrical contact  40  to be mounted and extend within the opening  28  of the holder  24  and opening  29  in adapter  26 . Adapter  26  can take on different forms, however, generally comprises a generally U-shaped or rectangular block having opening  29  formed therein with a rounded end  31  generally conforming to the diameter of the cell  30  to be tested such that the cell can easily fit within the opening  29 . Other adapters, such as  26 ′ in FIG. 3, having different dimensions for receiving different sized cells, can be employed by insertion into recess  25  of holder  24 . The sockets  20 , including holders  24  and inserts  26 , are made of a suitable insulative material and can be molded of, for example, a polymeric material such as A.B.S., P.V.C., acrylic or the like. 
     Holder  24  also includes a pair of downwardly projecting tabs  32  extending downwardly from opposite sidewalls  34  for snapping into associated slots in the panel  12 . The panel opening  13  provides a sufficient open rectangular area for contacts  40  to be easily accessible from behind the panel  12  and yet receive and hold sockets in place. The socket block  24  also includes a forwardly extending land  36  with downwardly projecting L-shaped corner tabs  38  and an aperture  39  for receiving a fastener  41  for fastening contact  40  to the block. Each of the sockets  20  includes an electrical contact  40  which is adapted to provide a fixed contact associated with the individual socket and a movable contact which selectively engages the fixed contact on an adjacent socket when the sockets are assembled onto the fixture as illustrated in FIGS. 1 and 2. For such purpose, contact  40  includes a base  42  having an aperture  43  through which fastener  41  extends with the width of the base sufficient to bridge the land  36  and position an upwardly projecting end  44  of contact  40  into the aperture  29  of adapter  26  such that it contacts the conductive sidewall  33  of cell  30 . For such purpose, the vertically projecting end  44  is bent downwardly at an angle to define a first leg  45  and an integral inwardly curved leg  46  to provide an apex  47  for making contact with the cell sidewall. 
     Contact  40  also includes, extending from base  42  in a direction opposite contact  44 , a U-shaped configured contact  50  defined by spaced vertically extending legs  48  and  51  integral with base  42 . Contact  50  engages, as best seen in FIG. 2, the end terminal  35  of a cell  30  when positioned within aperture  29  of a test socket. Contact  50  is movable, as illustrated by the cantilevered mounting through fastener  41  in the central area of base  42  and includes a contact end  53  joined to leg  51  by extension  52 . Contact  40  is made of a suitable conductive material, such as beryllium copper, which has excellent conductive properties and yet allows the movable contact  50  to flex, as seen in FIG. 2, in association with cell  30  when the cell is positioned within the aperture  29  and return to a position in which contact end  53  engages the corner  46 ′ of the adjacent contact, as shown in FIG. 2, when the cell is removed, thereby providing a continuous current path across the opened test socket  20  when it does not contain a cell. The opening  28  in the floor  27  of each of the blocks  24  associated with the socket  20  allows the free flexing of movable contact  50 , while the fastener  41  holds the apex  47  of the fixed contact in position with respect to the sidewall  23  of aperture  29  and compresses contact  40  into firm mechanical and electrical engagement with the cell wall  33  of a cell, such as cell  30  shown in FIG.  2 . 
     As also shown in FIG. 2, the sockets interfit and nest to provide the array with adjacent contacts  40  engaging both the side of the cell fitted within its socket and the center contact of the cell in an adjacent socket. Thus, as seen in connection with FIGS. 2 and 3, the fixed contact  44  extends upwardly within aperture  29  of a socket and is supported against the end wall  23  of an adjacent socket block while its movable contact  50  extends upwardly through the opening  28  in the same adjacent socket block. 
     The electrical conductors coupling the test fixture to the circuitry shown in FIG. 4 are terminated by coupling them to contact  40  by a suitable lug position between fastener  41  and aperture  43  in the base  42  of contact  40 , thus, providing an electrical contact with each of the cell terminals. The circuitry for providing a reference voltage and loading the cells of the test fixture is shown with reference to FIG.  4 . 
     Shown in FIG. 4 for illustrative purposes are three cells  30 ,  30 ′ and  30 ″, which are coupled by the test fixture as shown in FIG. 2 in series with one another and the remaining cells of the test fixture across a load resister  60  of FIG.  4 . Each of the cell terminals are coupled to an isolation amplifier circuit  70 . It being understood, for example, that input  2  of amplifier circuit  70  is coupled to the same fastening screw  41  as input terminal  3 ′ of amplifier circuit  70 ′ utilizing a pair of separate conductors fastened to the same contact, which engages both the negative terminal of cell  30  and the positive terminal of cell  30 ′. Amplifier circuit  70  provides an output voltage at terminal  72  which represents the cell voltage to which the amplifier is coupled and applies this voltage to one input of a voltage comparator  74 . Comparator  74  has a second input coupled to an adjustable reference voltage source  75  set to a threshold of, for example, 1.0 VDC. Comparator  74  receives the voltage from terminal  72  of amplifier circuit  70  and a reference voltage at input terminal  76  from reference voltage source  75 , and, when the voltage at terminal  72  falls below the reference voltage at terminal  76 , comparator  74  applies a positive voltage at output terminal  77  to power LED  15  associated with the cell  30  and socket into which cell  30  is inserted. Thus, when the voltage across cell  30  falls below a predetermined threshold, LED  15  will be illuminated, and the operator can remove the cell from the test fixture. When this occurs, the movable contact arm  50  moves up such that contact  53  engages apex  46 ′ (FIG. 2) of the cell holder, short circuiting the input terminals of the removed cell location such that the remaining cells continue to discharge. 
     Thus, with the test fixture  10 , illustrated in FIGS. 1-4, up to 100 cells can be simultaneously tested and a visual indication is presented to the operator such that the operator can remove the cells when their voltage falls below a predetermined level. The method by which the cells are graded comprise inserting the cells into the test fixture, switching a load  60  into the series circuit of interconnected cells, and recording the time at which the LED  15  became activated for each cell when removing these cells from the fixture upon activation of the LED indicating the cell has fallen below a predetermined threshold. Utilizing such testing sequence, the relative capacity of the cells can be graded as accurately as desired and individual cells can be grouped for subsequent assembly into battery packs in which it is desired to have cells of substantially similar capacity electrically coupled. As can be appreciated, the array of sockets  20  can be lessened or increased to provide a greater or fewer number of individual testing units for cells and 100 is representative only. 
     A test fixture according to a second and more preferred embodiment is shown in FIGS. 5-9. Test fixture  100  is similar in principle to the first embodiment shown in FIGS. 1-4 except that cell blocks  24  are integrated into a single upper plate  110  that includes a plurality of holes  111  (FIG. 8) for receiving a plurality of transparent socket inserts  120 . As shown in FIGS. 5 and 8, each row of holes  111  in the bottom surface of upper plate  110  has an elevated ridge  116  to which electrical connectors are secured that are similar in construction to those used in the first embodiment. Specifically, as shown in FIG. 6, electrical connector  140  includes a vertically projecting end  144  ( 144 ′,  144 ″) that is bent upwardly to define a curved contact shoulder  146  ( 146 ′,  146 ″). Vertically projecting end  1144  is also bent downwardly at an angle to provide an apex  147  ( 147 ′,  147 ″) for making contact with the side wall  33  of a cell  30 . Connector  140  also includes a U-shaped configured contact  150  ( 150 ′,  150 ″) that, as best shown in FIG. 6, engages the end terminal  35  of cell  30  when positioned within an aperture  129  ( 129 ′,  129 ″) of a test socket  120  ( 120 ′,  120 ″). Contact  150  is movable as illustrated by the cantilevered mounting through a fastener  141  ( 141 ′). Further, connector  140  includes a contact end  153  opposite vertically projecting end  144  for contacting curved contact shoulder  146  of an adjacent connector  140 . 
     As shown in FIGS. 6 and 9, sockets  120  ( 120 ′,  120 ″) include a vertical central slot  125  for straddling electrical connectors  140  that run down the length of each row of holes  111  in upper plate  110 . Each of inserts  120  includes an aperture  129  into which a cell  30  may be inserted. The size of aperture  129  may vary depending upon the size of the cell to be inserted and graded. Aperture  129  preferably includes a bottom shelf  128  which contacts the crimp area  34  of a cell  30  when fully inserted into aperture  129 . In this manner, bottom shelf  128  acts as a stop to limit the depth of insertion of a cell  30  into socket  120 . 
     Sockets  120  also preferably include a dome-shaped upper surface  122 . By forming each socket  120  out of a transparent material and forming upper surface  122  in a dome shape, light may be projected upward from light sources  115  (FIG. 6) through each socket  120  so as to be projected outward from the top surface  122  of socket  120 . Thus, when one of light sources  115  is illuminated, the operator will have no difficulty determining which light sources are associated with which of the cells being graded. In addition to having a dome shape, upper surface  122  of each socket  120  may be etched so as to diffuse the light transmitted therethrough. 
     The diameter of the upper portion of each of sockets  120  is preferably larger than the diameter of each of holes  111  such that a ledge  121  formed about the bottom periphery of the upper portion of each socket  120  rests upon the upper surface  112  of upper plate  110 . The upper portion of each socket  120  also includes a flat horizontal projection  124  having an aperture that lines up with an aperture  113  formed in upper plate  110  for receiving a fastener  141 . Fastener  141  may be a bolt or screw that not only serves to hold sockets  120  in place on upper plate  110 , but also holds electrical connectors  140  up against the bottom of ridge  116 . To secure fastener  141 , a nut  145  may be provided, or the apertures in one or more of upper plate  110 , socket  120 , or electrical connector  140  may be threaded. 
     The assembly of the upper plate  110 , sockets  120 , and electrical connectors  140  is then preferably placed over a bottom plate  160 , which has a plurality of corresponding holes  162  for receiving a bottom portion  127  of each socket  120 . As shown in FIG. 9, the bottom portion  127  of each socket  120  may have a smaller diameter than the remaining portion of socket  120  so as to define a shoulder  123  that may rest upon an upper surface  164  of lower plate  160 . Preferably, the electrical circuitry shown in FIG. 4 is mounted beneath lower plate  160  with one or two light sources  115  per hole  162  mounted below lower plate  160  so as to project light upward through holes  162  and through a corresponding socket  120 . To enable electronic circuitry to read the voltages across each cell  30 , pogo pins  165  are provided that extend through an aperture provided between each of holes  162  so as to contact a bottom end  142  of fastener  141 . Alternatively, pogo pins  165  may directly contact portions of electrical connectors  140 . Pogo pins  165  are then coupled to the amplifier circuits ( 70 ) of the circuitry shown in FIG.  4 . 
     By utilizing a combination of an upper plate  110  and a lower plate  160 , the upper plate assembly  110  and sockets  120  may be loaded with cells  30  prior to being loaded upon lower plate  160 . In this manner, cells may be loaded into the sockets  120  of one upper plate  110  while another set of cells in another upper plate assembly are being discharged and graded on the lower plate assembly  160 . Then, when the cells are finished being graded, one may simply lift the upper assembly off of lower plate  160  and drop a different upper assembly onto lower plate  160  so as to speed up the grading process of these mass-produced cells. 
     Also, load  60  (FIG. 4) may be provided as part of the lower plate assembly  160  so as to become connected across each row of connectors  140  when an upper plate assembly is placed on the lower plate assembly. Further, by not permanently connecting load  60  to connectors  140  as part of the upper plate assembly, the cells placed in the upper plate assembly may be simultaneously charged by placing the upper plate assembly on a charging lower plate assembly. 
     The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or zuse the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the Doctrine of Equivalents.