Patent GB-1225802-A (GB)
IPC: H02J7/00
Filed: 1968-05-27
Abstract: 1,225,802. Automatic voltage and current control. J. A. MACHARG. May 22, 1969 [May 27, 1968], No.25323/68. Headings G3R; G3X. A battery charger comprises a comparator for comparing the battery voltage with a reference voltage, which varies according to the state of charge of the battery, and for controlling a relay to switch the charging current on or off according to whether the battery voltage is below or equal to the reference voltage, the arrangement being such that the relay cycles according to the state of charge of the battery after the first operation of the relay to switch the charging current off. The basic embodiment shown in Fig. 1 uses a transistor TR3 as the comparator to control the coil of relay R through another transistor TR4. After the first operation of the relay, a capacitor C ensures that subsequent ON periods of the relay R are of constant time duration, whereas the OFF periods are dependent on the rate of fall of battery voltage and hence on the state of charge, thus producing a tapering average current until the battery is fully charged. The reference voltage is obtained from a voltage stabilizer R3, Z3 and a constant current circuit supplying a reference resistor R f , the former circuit comprising a self-stabilizing arrangement of transistors TR1, TR2, Zener diodes Z1, Z2, current limiting diodes D1, D2 and temperature dependent resistors R1, R2. The latter are mounted in thermal conductivity with the battery B so that the reference voltage is dependent on the battery temperature. The response to temperature variation may be improved by interposing a further resistor between the constant current source and resistor R f  and adding further transistors, resistors and current limiting diodes, of which the transistors may be mounted to be responsive to ambient temperature, Fig. 2 (not shown). In this modification, the value of charging current is made dependent on the reference voltage by using the latter to additionally control a transistor connected in series with the relay contacts. To speed charging, the reference voltage may be raised to delay the first operation of the relay and thereafter reduced during the cycling period. To this end the constant current circuit may be connected in parallel with one or more such circuits, Fig. 3 (not shown) or may have an over-riding voltage applied to its emitter resistances. Fig. 4 shows an embodiment using two parallel-connected constant current circuits to supply the reference resistor R f . The circuit containing the components TR22, 23; Z22, 23 and R22, 23, 24 provides a fixed reference voltage, whilst the circuit TR24, 25; Z24, 25 and R25, 26, 27 provides an additional current to raise the reference voltage by a predetermined amount. This additional current is controlled by circuit members TR26, 27 and R28 in accordance with the voltage across a capacitor C22, which voltage is in turn dependent on the rate of fall of battery voltage during the OFF periods of relay R and on the charge and discharge of the capacitor C22 through respective resistors R42 and R43. The comparator comprises a differential pair of transistors TR28, 29 and controls a Schmitt trigger TR31, 32 which acts as a level sensitive switch to determine the ON and OFF periods of the relay R. After the first operation of the relay, the additional current is progressively decreased to lower the reference voltage to that provided by the fixed constant current circuit, at a rate determined by the voltage across capacitor C22. Charge and discharge of the latter tends to further reduce the tapering average current by lengthening the OFF periods of the Schmitt trigger. The relay driver circuit may be modified, Fig. 5 (not shown) to advance the relay de-energization relative to the Schmitt trigger operation. In a further modification, Fig. 6 (not shown) the resistor R42 is inserted or by-passed according to the value of the reference voltage, to ensure resumption of charging after the reference voltage has fallen to its lowest limit. A second reference resistor may be provided in the embodiment of Fig.4 to produce a voltage which is used to control the output voltage of the battery charging source.