Abstract:
A circuit utilized in a cell capacity grading machine for discharging a cell to a predetermined cell voltage which approximates the total discharge of the capacity of the cell. An indicator is provided for automatically indicating the termination of discharge of the cell and for initiating a cell ejection. Cells are capacity graded by ejection from a movable transport at spaced locations, each location corresponding to cells having a predetermined capacity range.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to machines for measuring the capacity of cells and, more particularly, relates to a circuit utilized in such machines for discharging a charged cell to a predetermined cell voltage indicative of discharge of the total capacity of the cell. 
     Electrochemical cells such as nickel cadmium are typically capacity graded as a means of quality control as well as to obtain matched capacity cells for making a battery of series-connected cells. 
     One prior art apparatus for capacity grading cells is a circuit board in which a plurality of cells are connected in series with a rheostat discharge load. The cells are graded using this apparatus by (1) inserting a plurality of cells in the board, (2) allowing the cells to discharge at constant current while periodically manually checking each of the cells with a voltmeter to measure cell voltage, (3) manually removing cells which have a voltage indicative of substantially total discharge of the capacity of the cell, (4) manually adjusting the rheostat to maintain a constant current discharge for the remaining cells, and (5) sorting the cells based on total discharge time. Such prior art methods require substantial manual labor and are, therefore, costly and unnecessarily complicated. 
     It is, therefore, an object of this invention to provide a circuit which automatically discharges cells and monitors the cell voltage for use in an automated cell capacity grading machine. 
     SUMMARY OF THE INVENTION 
     These and other objects are accomplished by a circuit comprising a constant current power supply connected to a cellholder such that a cell in said holder is discharged to a predetermined voltage indicative of substantially total discharge of the cell capacity. A switch means is connectible in parallel across the cellholder for bypassing current from said supply around the cell and terminating discharge of the cell. An indicator means is actuated responsive to the termination of discharge of the cell and initiates cell ejection from the holder. The cells and cellholders are located on a movable transport resulting in the ejection of the cell at spaced locations, each location corresponding to predetermined discharge times. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a circuit in accordance with this invention. 
     FIG. 2 is a graph of the voltage versus cell discharge time of one embodiment of the circuit of FIG. 1. 
     FIG. 3 is a schematic diagram of a method for capacity grading cells in accordance with this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the FIG. 1, a circuit is shown for discharging and monitoring the voltage of each of a plurality of cells. The circuit may be utilized in a battery grading machine to measure to individual cell capacities, for example, of nickel cadmium cells. The circuit is comprised of N (where N is an integer) series-connected subcircuits, each of which is associated with one cell. Two such individual circuits 101, 103 are shown in FIG. 1. The invention will be explained in connection with circuit 101, which is representative of circuit 103 and others not shown. Circuit 101 is comprised of a cellholder which is represented schematically by a positive terminal 105, and a negative terminal 107. The cellholder may be of any conventional construction such as a cupshaped housing. The housing preferably has an interior dimension corresponding to that of a cell and has a spring actuated latch closing the open end of the housing for latching a cell in place as will be explained in more detail below. The cellholder is shown herein with a cell 109 mounted therein. The cellholder is connected to a constant current power supply 110, with negative and positive terminals 111, 113. The power supply 110 provides a driving current which forces the cell to discharge through a pair of diodes 117, 119. Activation of the supply 110 is controlled by a switch 114. 
     Discharge of cell 109 is controlled by a parallel by-pass switching means or SCR 121, which has a cathode 122, gate 123, and anode 124. SCR 121 is connectable in parallel with the cellholder for abruptly changing back and forth between a non-conductive and a conductive state responsive to the voltage of cell 109. A bias resistor 127 and diode 128 are connected respectively between cathode 122 and gate 123 and between anode 124 and gate 123 for acting as a voltage divider to insure gate trigger control. When a charged cell is initially placed in the cellholder, the cell voltage commutates the switching means or SCR 121 in a non-conductive state. As cell 109 is discharged, the voltage drops across the anode 124 to cathode 122 and the gate 123 to cathode 122 increase. At the end of discharge or end of capacity, these voltages are sufficient to trigger SCR 121 into conduction to provide a by-pass current path around the cell for the driving current from supply 110. The switching of SCR 121 to a conductive state terminates discharge of cell 109. This condition is detected by an indicating means here shown as a light emitting diode (LED) 125 having an anode 129 connected to SCR gate 123 and cathode 131 connected to the negative terminal 107 of the cellholder. When the LED 125 is illuminated, this condition is detected by a photosensitive amplifier 141 which in turn provides an input signal to a solenoid 143. Solenoid 143 is connected by any conventional mechanical linkage 45 to the cellholder for ejecting the cell upon solenoid actuation. One preferred linkage is an air cylinder linkage which is comprised of an air cylinder for driving a reciprocating rod connected to the latch for the cellholder. In this linkage, the solenoid 141 operates an air supply valve to pressurize the air cylinder and in turn activates the rod to unlatch the cell in the cellholder. 
     The discharge time may be recorded in a number of ways apparent to those skilled in the art. However, one preferred method in accordance with the features of this invention, as shown schematically in FIG. 3, is to fix the circuit of FIG. 1, except for amplifier 141 and solenoid 143, on a movable transport means such as an intermittently driven closed loop belt conveyor system. The cells are automatically or manually loaded in cellholders one row at a time in rows which are disposed transversely of the conveyor belt. After loading one row, the belt is indexed and the power supply 110 is activated by closing switch 114 to start the discharge of the cells in that row. The loading and indexing steps are continued on an alternate basis. The first loaded row is thus progressively transported around the belt so as to approach a plurality of discharge detection stations which are located at spaced locations along the lower half of the conveyor belt. One or more photosensitive amplifiers, such as amplifier 141 (FIG. 1), are located above and transversely of the lower half of the belt. Alternative arrangements for the amplifiers are to have either one amplifier which is driven along the stationary row (between index steps) or a plurality of amplifiers equal in number and corresponding to the cellholders in the row and located above and transversely of the lower half of the belt. As the rows of cells are progressively indexed to each discharge detection station, the photosensitive amplifier(s) receives a light signal from any LED (such as LED 125, FIG. 1) which has been illuminated since passing the next adjacent discharge detection station upstream from the present station. Upon sensing of the LED illumination, the cell in the corresponding cellholder is ejected by release of the latch which permits the cell to slide downwardly due to the force of gravity on the cell into a bin located below each discharge detection station. As a result, each bin contains cells only which have been discharged, at a constant rate, within a predetermined range of discharge times. The cells in each bin are sorted based on their available capacity. The range of capacities or discharge times within each bin is determined by the indexing interval of the belt between each detection station (except for the first station which is positioned to correspond to the anticipated minimum discharge time of any cell). After ejection, the LED 125 is turned off and the current is continued to be by-passed through the SCR 121. 
     For an explanation of the operation of a specific example of an embodiment of the invention, reference is made to FIG. 2 which illustrates the voltage on each circuit components listed below as a function of discharge time. The values used herein to illustrate the operation are intended to be exemplary only and other arrangements and values will be apparent to those skilled in the art. 
     
         ______________________________________Component         Size______________________________________SCR 121           3 AResistor 127      470Diode 129         50 mA (silicon)LED 125           infrared, low             voltage forward drop             1.3 voltsDiode 117         3A (silicon)Diode 119         3A (silicon)Cell 109          1 Ah.-N/Cd______________________________________ 
    
     By reference to Curve A, it is seen that when a charged nickelcadmium cell 109 is inserted into the cellholder, it has an initial voltage of approximately 1.3 volts. The voltage drop across diodes 117, 119, approximately 1.5 volts, (Curve C) is opposite in polarity to cell 109. This places a net voltage of approximately 0.2 volt (Curve B) drop across SCR 121 (anode 124 to cathode 122) which places SCR 121 in a nonconductive state. 
     The constant current power supply 110 provides a two ampere driving current, the majority of which passes through cell 109 discharging in into diodes 117, 119. During the discharge of cell 109, LED 125 (Curve D) has a voltage drop across it of approximately one volt which is insufficient to cause light emission. As the cell capacity and cell voltage (Curve A) decrease, the voltage across the anode 124 to cathode 122 (Curve B) and gate 123 to cathode 122 (Curve F) of SCR 121 correspondingly increase. The voltage anode 124 to gate 123 (Curve G) of SCR 121 remains substantially constant during discharge. When the capacity of a cell 109 is near exhaustion at a value of approximately .5 volts, SCR 121 is turned on at this point with a voltage drop of 1 volt being developed across the SCR 121, anode 124 to cathode 122 and a .5 volt drop being developed across SCR 121, gate 123 to cathode 122. The voltage drop anode 124 to cathode 122 of SCR 121 is the sum of the cell voltage and the voltage drop on diodes 117, 119. The voltage drop gate 123 to cathode 122 of SCR 121 is developed across resistor 127, the remainder of the total voltage anode 124 to cathode 122 being developed across diode 128 (Curve G). 
     Also, at the end of cell capacity (0.5 cell voltage) cell voltage is clamped at approximately 0.6 volts which is due to the SCR 121 having a forward drop of 0.9 volts cathode to anode minus the forward drop of diodes 117, 119 of 1.5 volts. This prevents further cell discharge and insures that the cell cannot go into reversal (that is, the cell cannot be discharged beyond the available capacity of the plates of the cell). 
     LED 125 is turned on when the cell reaches 0.5 volts and the SCR 121 is in a conductive state. A voltage of 1.25 volts is required to turn on LED 125. This voltage is supplied by the SCR 121 forward drop (anode 124 to gate 123) which is 0.65 volts plus the &#34;clamped&#34; cell voltage of 0.6 volts. After LED is lit, the light is preferably electrically detected such as by a photosensor 141 which activates solenoid 143 to unclamp the cell and allow the cell to slip the cellholder due to the force of gravity on the cell. 
     After removal of the cell from circuit 101, the SCR 121 continues to be a conductive state and to by-pass current around the empty cell holder. In this condition only the SCR anode to gate voltage of 0.65 volts is applied to the LED and, therefore, it ceases to emit light. Thus, no further signals to eject a cell from this cellholder are generated. SCR 121 remains in a conductive state until fully charged cell is inserted into the cellholder which then reduces the voltage drop across the SCR 121 anode to cathode to a sufficient level to force SCR 121 into a non-conductive state. The inserted cell is then discharged in the manner explained above. 
     Other features of this circuit are that in the case of power failure a charged cell cannot self-discharge because its voltage is insufficient to force either the combination of diodes 117 and 119 or diode 125 and resistor 127 into conduction. 
     Another feature of the invention is that cell energy is dissipated in diodes 117 and 119 which allows a low power dissipation current regulated power supply to be used for constant current power supply 110. 
     A variation of this invention which will decrease leakage light current in LED 125 (FIG. 1) which may occur due to variable performance characteristics of LED&#39;s is to substitute a resistor for diode 128 (FIG. 1). This and other variations which will be apparent to those skilled in the art are intended to be within the scope of the appended claims.