Means for controlling battery chargers
In battery charging, there is no one fixed voltage at which a given group of batteries achieves fully charged status. Each battery voltage at fully charged condition varies from all other similar batteries and no one voltage will satisfy all batteries of the same type. The only fact that is true of all batteries is that once batteries are fully charged, their terminal voltage will stay fixed. The ideal battery charger control, therefore, is one which senses the point at which any batteries being charged thereon develop a fixed terminal voltage and which thereafter starts the process of terminating the charging cycle. 
     It is the object of this invention to provide such an ideal charging control device. 
     Reference is hereby made to my copending application for Letters Patent, Ser. No. 825,943, filed Aug. 19, 1977. 
     With the above and other objects in view, which will become immediately apparent upon reading this specification, studying the accompanying drawings, and digesting the claims, this invention resides in the unique and novel form, construction, arrangement and combination of the various parts and elements described therein. 
    
    
     IN THE DRAWINGS 
     The drawing presents a schematic diagram of a voltage following control circuit which embodies my invention. 
    
    
     In the drawings, A represents a Voltage Following Control Device comprising a positive terminal 10 and a negative terminal 12 across which is removably connected the respective terminals of a battery B. Connected the terminals 10, 12 and separated therefrom by a blocking diode 14 is a terminal voltage following circuit 16 comprising a Zener diode 18, and three series resistors 20, 22 and 24. Connected in parallel with the voltage following circuit 16 is a relay energizing circuit 26 including the coil 28 of a relay 30, the collector of a transistor 32, and the emitter of the transistor 32. The base of the transistor 32 is connected to the cathode of the diode 14 through a resistor 34 and a second blocking diode 36. The relay 30 also includes a set of normally open contacts 38 which supply operating current to a battery charger 40 where the relay 30 is energized. Also connected to the resistor 34 through a third blocking diode 42 is the gate of an PUT 44, the cathode of which is connected to the negative terminal 12. Connected in parallel across the gate of the PUT 44 and the negative terminal 12 are a resistor 46 and a capacitor 48. The gate of the PUT 44 is also connected to the common connection of the resistors 20, 22, through a resistor 50 and a fourth blocking diode 52. The anode of the PUT 44 is connected to the common connection of the Zener 18 and the resistor 20 through two resistors, 54 and 56, in series. The common connection of the resistors 54 and 56 is connected to the common connection of the resistors 22, 24, through a fifth blocking diode 58 and is also connected to the negative terminal 12 through a capacitor 60. 
     The components of the circuit A are selected that when a battery B is connected across the terminals 10, 12, the voltage imposed between the base and the emitter of the transistor 32 will place transistor 32 into conduction causing energizing current to flow through the relay coil 28 energizing the relay 30 and closing the contacts 38 which causes the battery charger 40 to start charging the battery B. As long as the battery B is in a not fully charged condition the PUT 44 remains non-conducting. 
     The voltage following circuit 16 follows the terminal voltage of the battery B which increases as the battery charge increases. The Zener 18 reduces the battery terminal voltage by a predetermined amount and that reduced voltage is apportioned between the three series resistors 20, 22, 24, according to their respective magnitudes. The voltage at the common connection of the resistors 20 and 22 is transferred to the gate of the PUT 44 through resistor 50 and the voltage at the common connection of the resistors 22 and 24 is transferred to the anode of the PUT 44 and the capacitor 60 which develops a charge rapidly equal to that voltage. It should be here noted that the capacitor 60 also starts developing a charge independent of battery B terminal voltage through the resistor 56; however, the magnitudes of capacitor 60 and resistor 56 have been preselected to make this a comparatively long period of time with respect to the charging current from resistor 24. It should be further noted that the voltage presented to the gate of the PUT 44 depends on the respective magnitudes of resistors 50 and 46. 
     The characteristics of PUT 44 have been preselected so that when its anode gets at least 1/2 volt more positive than its gate, it will fire, which event will occur some finite time after (1) the battery B terminal voltage has exceeded the limit set by the Zener 18, and (2) the voltage at the gate of PUT 44 has stopped rising (the terminal voltage of all batteries stops rising when the battery is fully charged), and (3) the voltage at the anode of PUT 44 has been increased to a value which exceeds the gate voltage of PUT 44 by at least 1/2 volt by the charging of capacitor 60 through resistor 56. Once PUT 44 goes into conduction, it is held in conduction by the valley current, and transistor 32 is biased out of conduction by additional current flowing through the resistor 34, which opens the relay 30 and stops the charging of the batter B. PUT 44 remains in conduction until the battery B is removed from the terminals 10, 12, or until the terminal voltage of the battery B drops substantially below the rated voltage of Zener 18. 
     The following table presents a suggested component usage for a Circuit A which is adapted for use in 36 volt battery charging circuits: 
     
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Resistors         (ohms)                                                  
20                680                                                     
22                82                                                      
24                820                                                     
34                8.2K                                                    
46                15M                                                     
50                150K                                                    
54                220                                                     
56                4.7M                                                    
Capacitors                                                                
48                .027nf                                                  
60                4500nf                                                  
Diodes                                                                    
14                1N2069                                                  
18                1N4121 (Zener, 330)                                     
36                1N2069                                                  
42                1N4154                                                  
52                1N4154                                                  
58                1N4154                                                  
Transistor 32     2N3858A                                                 
PUT 44            2N6028                                                  
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     It should be understood that changes and modifications in the form, construction, arrangement and combination of various parts and elements shown herein may be made without departing from the nature and principle of my invention.