GB892954A - Automatic battery charger - Google Patents

Abstract

892,954. Charging batteries. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 12, 1960 [March 5, 1959], No. 7749/59. Class 38 (4). Battery charging arrangement comprises means whereby when the battery is sufficiently charged the charging current is substantially reduced or cut off. A battery 18 is charged from a direct current source 11, 12 which is preferably stabilized and obtained from an alternating current source over rectifiers, the battery being charged over a normally conductive transistor T3. A further normally conductive transistor T1 enables a reference voltage to be developed by means of a Zener diode ZD and a variable resistor R4, this reference voltage being applied to the emitter 13 of a third transistor T2 to the base 15 of which is applied a potential, obtained from a potentiometer R2, dependent upon the battery terminal voltage. Whilst the battery 18 is being charged with transistors T1, T3 conductive the base and emitter potentials are such that transistor T2 is non-conductive. As the battery approaches its fully charged state the base potential of transistor T2 rises and transistor T2 begins to conduct and in a cumulative manner eventually becomes fully conductive thereby cutting off both transistors T1, T3 to cut off the charging current and reduce the reference potential produced by the diode ZD and resistor R4. Alternatively, the resistors R5, R7 may be given values whereby the conductivity of the transistor T3 may be reduced to a low level instead of being completely cut off, whereby a small charging current may be maintained. The reduction of the reference voltage when the charging current is cut off ensures that no continuous switching on or off of the charging current occurs due to the difference between the charging voltage and the battery open circuit voltage. Resistors R1, R3 may be of the type having positive or negative resistance characteristics. In a further embodiment a fourth transistor is situated between the transistors T2, T3. In yet another embodiment a fifth transistor is placed in the negative supply lead to provide a stabilized supply.