DC-operated battery charger

EE February 1986 Mains-operated NiCd chargers are in plentiful supply, but a NiCd charger that operates from a car battery and enables fast charging is something special The one described here can charge 9; 12; or 15-volt batteries. DC OPERATED BATTERY CHARGER Lowering the e.m.f. - electromotive force - of a car battery is easily done with the aid of a resistor, zener diode, or voltage regulator, but rais- ing it is rather more difficult. The method chosen here is the familiar one of voltage doubling. How this is done in this charger is illustrated in Fig. 1. In Fig. la, switch S connects the negative terminal of electrolytic ca- pacitor C3 to earth, so that both C3 and C4 are charged to the (car bat- tery) supply voltage Ub: Uo=UC4=UD2+UC3= UD2 +Ub-UDI =Ub (1) negative terminal of C3 to Ub, so that the output voltage, Uo, becomes: Uo=UC4=Ub+UC3-UD2= 2Ub-UD2 (2) When the switch is returned to earth as in la, the potential across C4 re- mains at Ub, because C3 cannot dis-...