Patent GB-1202742-A (GB)
Title: Charging devices for electrochemical energy sources
IPC: H02J7/00
Filed: 1967-09-26
Abstract: 1,202,742. Charging batteries. SOC. DES ACCUMULATEURS FIXES ET DE TRACTION. 26 Sept., 1967 [26 Sept., 1966; 8 May, 1967; 26 May, 1967], No. 43845/67. Heading H2H. A system for charging a battery B from an A.C. source by way of a transformer Tr, rectifier Rd and a serially connected transistor T 1  comprises a circuit arrangement which includes means for terminating the charging operation when the battery is fully charged and, if desired, thereafter maintaining the charge by a trickle charging device. When the circuit is switched on to produce a voltage across a smoothing capacitor C 1  current flows in a circuit comprising a capacitor C 2 , resistors R 2 , R 3  and a Zener diode Z to produce a constant potential which is applied to the base of the transistor T 1 . If the battery B is fully discharged the potential at the emitter of transistor T 1  is such that the transistor saturates thus establishing the flow of charging current to the battery. Current flow in the resistor R 1  produces a voltage drop which is effective to render transistor T 2  conductive to maintain the potential developed by the Zener diode Z. Whilst the battery charge is low the transistor T 1  remains saturated, thus the battery is charged at a substantially constant current limited by the resistor R 1  as shown in the section I of Fig. 2. As the battery acquires a charge the voltage thereof and hence the emitter voltage of transistor T 1 -increases whereupon the transistor comes out of saturation and the increase in emitter voltage with the corresponding decrease of the base-emitter bias causes a gradual increase in the effective impedance of the transistor. Thus the battery continues charging at a decreasing rate of current as shown in section II of Fig. 2. As the charging current decreases it eventually reaches a value at which the voltage drop across the tapped portion of the resistor R 1  with the drop across the diode D 1  is insufficient to maintain transistor T 2  conductive. The bias developed by the Zener diode Z then disappears whereupon transistor T 1  goes into its high impedance state and the battery charging is terminated. If desired a resistor Re and a diode D 2  may be placed in shunt with the transistor T 1 , or the transistor T 1  and resistor R 1 , as shown, in order to maintain the battery charge by a trickle charge as shown in section III of Fig. 2. Large numbers of embodiments are described in which the various transistors may be either NPN or PNP types; a lamp may be provided to indicate charging of the battery; the resistor R 3  may be in parallel with the Zener diode Z to offset fluctuations in the supply voltage; negative temperature coefficient resistors included to provide temperature compensation; a third parallel transistor circuit whereby the transistor T 1  is controlled instead of by the circuit including transistor T 2  and means are included so that the trickle charging part of the operation is effected by way of the transistor T 1  in a partly conductive condition. The transistor T 1  may be formed by a Darlington pair.