Page 493 - Battery Reference Book

47/24 Constant-current charging

Newer selenium bridge rectifiers and Mylar dielectric also Table 47.6) can also be used to provide a con-
capacitors make practical charge currents possible in stant current to batteries of series connected cells. The
excess of 6.1 mA/cm ’. circuit values shown will provide a charging current
The value of R1 in Figure 47.19(c) is not critical of approximately 4.5 mA for two B50 cells connected
since it is a current-limitingqesistor to restrict ini- in series. This charger will not be damaged if it is
tial surge when the circuit is energized. The value connected to the a.c. power line with the batteries
of R1 may range from 25 to 250C2. This resistor removed. By changing the resistor values, single cells
is often omitted when more recent improved com- or three to five cells in series could be charged. Higher
currents requiring high wattage resistors would not
ponents are used. The bleeder resistor, Rz, provides be practical because of the excessive heat that would
a discharge path for the capacitor, removing possible be produced. Small button cells, such as those used
shock hazard across a.c. prongs when removed from in hearing-aids, may be readily charged from flash-
the receptacle. For circuits having large capacitance light cells or lantern batteries. Figure 47.19(e) (see
values (higher current ratings) the rectifier may be also Table 47.6) is shown by way of example; this cir-
burned out if the battery is removed when the cir- cuit will charge one B50 cell at approximately 5 mA.
cuit is connected to the 120V ax. supply. The res- Additional cells may be charged as shown by the
istance type charger shown in Figure 47.19(d) (see broken lines.