CONTROLLED-CURRENT BATTERY CHARGERS

• US 3,624,481 A
• Filed: 05/23/1969
• Issued: 11/30/1971
• Est. Priority Date: 05/27/1968
• Status: Expired due to Term

• Current Assignee: James A. Macharg
• Original Assignee: James A. Macharg
• Inventors: Macharg, James A.
• Primary Examiner(s): Miller, J. D.
• Assistant Examiner(s): Gunther, John M.
• Application Number:
  US04/827,182
• Time in Patent Office: 921 Days
• Field of Search: 320/35,36,39,40,21,22,1,30 322/22T
• US Class Current: 320/153
• CPC Class Codes: H02J 7/007182 in response to battery voltage

Abstract

A battery charger more particularly for use in charging batteries of the nickel-cadmium type in order to avoid the undesired effects caused by overcharging, wherein a variable reference voltage is derived across a resistor from a constantcurrent device whose current may be controlled and this derived reference voltage is compared with the terminal voltage existing across the battery being charged, the result of this comparison causing the charging current to be switched off until the battery voltage drops below the reference voltage, whereby the state of charge of the battery is sensed through the ON/OFF ratio of the charging current and appropriate adjustments may be made to the reference voltage in order to achieve a substantially complete charging of the battery in the minimum time without any overcharging occuring.

Claims

1. A battery charger comprising reference means for deriving a reference voltage, comparator means for comparing said reference voltage with the actual voltage of the battery being charged, and switch means operable from said comparator means to switch off the charging current when the battery voltage reaches the reference voltage and to switch on the charging current when the battery voltage falls below the reference voltage, said reference means having a fixed resistance and means including two controllable constant-current circuits arranged in parallel with each other for passing and controlling a variable reference current through said fixed resistance, said reference means further having means including a switch for controlling one of said constant-current circuits.

2. A battery charger as in claim 1, wherein said resistance includes two resistors in series for forming two reference voltages, one of said reference voltages having means for connecting to the battery to supply a constant-potential charging current to the battery.

3. A battery charger as in claim 1, wherein one of said constant-current circuits includes temperature-sensitive means whose resistance rises with increase in temperature, and the other of said constant-current circuits includes current-limiting means to control the maximum current which may be passed.

4. A battery charger as in claim 1, wherein said switch means include means for determining whether or not the one of said constant-current circuits having means with a switch is conducting.

5. A battery charger as in claim 1, wherein one of said constant-current circuits includes temperature-variable electrical means for responding to the battery temperature and changing the current in the one of said constant-current circuits.

6. A battery charger comprising reference means for deriving a reference voltage, comparator means for compriSing the reference voltage with the voltage of the battery being charged, and switch means operable from said comparator means to switch off the charging current when the battery voltage reaches the reference voltage and to switch on the charging current when the battery falls below the reference voltage, said comparator means including a differential-pair transistor circuit to compare the variable-reference voltage and the voltage of the battery, said differential-pair transistor circuit having a pair of transistors with base electrodes, connecting circuit means in said comparator means for applying the voltages to be compared to the base electrodes of the transistors, said switch means having a Schmitt trigger responsive to the output of the differential-pair transistor circuit to produce an actuating current when the differential-pair transistor circuit senses that the battery voltage has reached the reference voltage, said switch means including a relay responsive to the actuating current of said Schmitt trigger for cutting off the charging current, and capacitive network means between the output of the Schmitt trigger and the relay for delaying return of said relay to the condition in which it reapplies the charging current.

7. A battery charger, comprising reference means for deriving a reference voltage, comparison means for comparing the reference voltage with the voltage of the battery being charged, switch means operable from said comparison means for switching off the charging current when the battery voltage reaches the reference voltage and for switching on the charging current when the battery voltage falls below the reference voltage, control means responsive to the switch means for forming a signal representing the ratio of the times during which the charging current is on as compared to the times during which it is off as a measure of the state of the charge of the battery, said control means being also coupled to said reference means for varying the reference voltage of said reference means in response to a reduction of the signal so as to cycle the charging current on and off.

8. A battery charger according to claim 7, wherein said control means includes capacitive means for developing the signal in the form of a descending voltage, circuit means in said control means for comparing the rate of drop of the actual voltage of the battery after cutoff of the charging current with the rate of drop of voltage across said capacitive means.

9. A battery charger as in claim 8, wherein said capacitive means includes a capacitor and voltage-forming means, said voltage-forming means applying a voltage to said capacitor during the period when the charging current is applied to the battery and removing the voltage when the charging current is removed from the battery, said control means having regulator means responsive to the voltage across said capacitor for causing said reference means to drop the reference voltage to a lower level when the rate of drop of said voltage across said capacitor is faster than the rate of drop of the voltage across the battery to prevent the charging current from being reapplied to the battery until the terminal voltage of the battery drops to a lower level.

10. A battery charger according to claim 7, wherein said reference means includes a resistor, a constant-current circuit supplying said resistor in order to develop the reference voltage thereacross, and at least one temperature-dependent resistor thermally associated with the battery and connected in the constant-current circuit to reduce the value of the reference voltage with rising battery temperature.

11. A battery charger according to claim 10, including at least one current-limiting diode connected in the constant-current circuit for modifying the effect of said temperature-dependent resistor.

12. A battery charger according to claim 7, wherein said reference means includes a resistor and a constant-current circuit supplying said resistor in order to devElop the reference voltage thereacross;

said constant-current circuit including a pair of complementary transistors, a pair of Zener diodes and a pair of temperature-dependent resistors;

the transistors, Zener diodes and resistors being arranged to form a ring-of-two circuit, said temperature-dependent resistors being thermally associated with the battery so as to reduce the value of the current supplied to the resistor and hence the value of the reference voltage with rising battery temperature.

13. A battery charger according to claim 12, wherein said comparator means includes a transistor, said transistor having a base electrode connected to one end of the resistor across which the reference voltage is developed and an emitter electrode connected to one terminal of the battery, the other end of the resistor and the other end of the battery being connected in common.

14. A battery charger according to claim 7, said reference means including a resistor and a constant-current circuit supplying said resistor to develop a reference voltage, said comparator means including a transistor for comparing said reference voltage with the actual voltage of the battery being charged;

said switch means including a relay for switching the charging current on and off and capacitive network means connected between the output of the transistor and the energizing winding of the relay for creating a delay in the reapplying of the charging current when the battery voltage falls below the reference voltage.

15. A battery charger according to claim 7, said reference means including a resistor, a constant-current circuit including a pair of complementary transistors, a pair of Zener diodes and a pair of temperature-dependent resistors for supplying a current to the resistor to develop the reference voltage;

said reference means further including the temperature-dependent transistor thermally associated with the ambient temperature and in parallel with the constant-current circuit, biasing means for biasing said temperature-dependent transistor so as to be normally saturated to augment the current through the resistor across which the reference voltage is developed.

16. A battery charger according to claim 15, wherein said switch means include charging current limiting means controlled by said comparator means to reduce the charging current during the ON periods as charging proceeds.

17. A battery charger according to claim 7, wherein said reference means include a resistor and first and second constant-current circuits supplying said resistor in parallel;

said reference means further including a transistor having a base electrode and an emitter-collector circuit, said transistor being in series with the second constant-current circuit, said switch means including means for sampling the ON/OFF ratio of the charging current to thereby control said transistor through its base electrode.

18. A battery charger according to claim 17, wherein the means for sampling the ON/OFF ratio of the charging current include a charging capacitor and means for charging said capacitor;

and means for comparing, during the OFF period, the rate of drop of voltage across the capacitor with the rate of drop in battery voltage.

19. A battery charger according to claim 7, wherein said reference means including a resistor, first and second constant-current circuits supplying said resistor in parallel to thereby develop a reference voltage thereacross, a switch in series with the second constant-current circuit, said control means having means responsive to the ON/OFF ratio of the charging current to control the position of the switch, said comparison means including a plurality of transistors forming a differential pair arranged so that one receives the reference voltage and the other the actual voltage of the battery on charge, said switch means including a relay for switching on and off the charging current whose energizing winding is actuated by the output from the differential pair.

20. A battery charger according to claim 19, wherein said switch means includes a Schmitt trigger circuit connected between the output of the differential pair and the energizing winding of the relay.

21. A battery charger according to claim 19, wherein the first and second constant-current circuits each include a pair of transistors;

a pair of Zener diodes and a pair of resistors arranged to form a constant-current circuit in each case.

22. A battery charger including a resistor, first and second constant current '"'"''"'"''"'"''"'"'ring-of-two'"'"''"'"''"'"''"'"' circuits supplying said resistor in parallel for developing a reference voltage;

a transistor acting as a switch in series with the second '"'"''"'"''"'"''"'"'ring-of-two'"'"''"'"''"'"''"'"' a differential pair of transistors arranged so that one receives the reference voltage and the other the actual voltage of the battery on charge;

a Schmitt trigger circuit connected to the output from the differential pair;

a capacitive network charged from the Schmitt trigger and controlling the transistor in the second '"'"''"'"''"'"''"'"'ring-of-two'"'"''"'"''"'"''"'"' circuit so as to control whether or not said second '"'"''"'"''"'"''"'"'ring-of-two'"'"''"'"''"'"''"'"' circuit augments the current flowing in the reference resistor; and

a relay having an energizing winding connected to the output from the Schmitt trigger and its contacts in series with the charging current, said capacitive network and said transistor forming sensing means for sensing the state of charge of the battery during charging on the basis of the ON-OFF ratio of the charging current and for continually modifying the ON-OFF ratio as charging progresses to thereby control the charging current.

23. A battery charger according to claim 22, wherein the first '"'"''"'"''"'"''"'"'ring-of-two'"'"''"'"''"'"''"'"' circuit includes a pair of temperature-sensitive devices which are thermally associated with the battery and whose resistance rises with increase of temperature.

24. A battery charger according to claim 22, including a second capacitive network, a transistor whose emitter-collector path is in series with the energizing winding of the relay, said second capacitive network being connected to the base electrode of the transistor wherein the first capacitive network is caused to charge slowly but discharge quickly.

25. A battery charger according to claim 14, including a diode arranged to be reversed biased with respect to the capacitive network and connected to the base electrode of the transistor which effects the fast discharge of the capacitive network but which is held inoperative while the capacitive network is being charged.

26. A battery charger according to claim 22, including a resistor across which a voltage is developed when the second '"'"''"'"''"'"''"'"'ring-of-two'"'"''"'"''"'"''"'"' in conducting, and at least one transistor whose state of conductivity is determined by the voltage developed across the resistor, said transistor being connected in parallel with a resistor in series with a capacitor of the capacitive network whereby the transistor acts as a switch to bypass the charging current to the capacitor when the second '"'"''"'"''"'"''"'"'ring-of-two'"'"''"'"''"'"''"'"' is nonconducting.