Page 58 - Robots Androids and Animatrons : 12 Incredible Projects You Can Build

When building a charger for higher voltages, increase the value of
R1 proportionally to limit the current flowing through the LEDs.
For instance, for a 12V unit make R1 680 ohms; for a 24V unit
make R1 1.2K ohms.
At high voltages you may need a low-ohm-value, current-limiting
resistor connected to the DPDT relay. Measure the C/10 and C/30
current flowing to the batteries. These measurements will ensure
that the proper current is being supplied to the batteries.
Series and parallel charging
How the batteries are configured determines the voltage and cur-
rent of the transformer one should use. If you have eight C battery
cells in parallel, you need to multiply the current requirements of
each individual cell by 8. If the cell is rated at 1200 mAh, the C/10
requirement per battery is 120 mA. For eight cells in parallel, you
need close to 1 A (8 120 mA 960 mA 0.96 A) of current.
The voltage required is just 1.5 V. The ideal transformer’s output
would be 1.5V at 1 A. If the eight cells were held in series, the cur-
rent requirements would be 120 mA at 12V.
Fast charger
Many of today’s NiCd batteries are capable of accepting a fast 37
charge provided that the circuit can sense when the batteries are
fully charged and drop the current to C/30. Typically to fast-charge
a battery, you double the current for half the time. So you charge a
battery at C/5 for 7 h.

Although I haven’t tried the above circuit for fast charging, there is
no reason why it shouldn’t work. You may want to start with a C/10
charging current and adjust V1, and then switch resistor R2 for a
resistor with half the value.
Parts list
U1 LM317 voltage regulator
L1 DPDT relay (5V or 12V)
D1 Red LED
D2 Green LED
D3 1N4004
Q1 SCR
V1 5K-ohm PC-mounted potentiometer
1
R1 330 ohms, 4 W
R2 5 ohms, 2 W

Team LRN Power