NiCad Battery Charger
This charger is designed to charge a wide range of NiCad/NiMH battery packs which are commonly used in portable tools. The charging current is adjustable to meet various charging requirements. It can deliver 3A to a 19.2V battery pad. Once set, the charging current is kept constant. There is a Delta-T cut off circuit. It has two temperature sensors. One measures the surface temperature of the battery pad, another measures the air temperature. It cuts off the charging current when the temperatures differ by 10 degree C.
Figure 1 is the temperature sensing and current cut off circuit. It uses the micro power OpAmp LMC6482 to compare the voltages presented by two sets of diodes. D1 and D2 serve as the hot sensor which is attached to the surface of the battery pad and measures its temperature. D3 and D4 are hanging in the air and measure the air temperature. When both sensor sets are at the same temperature, the voltage at pin 3 is slightly higher than that at pin 2 such that pin 1 is high. Q2 is ON and energizes the relay. The relay contacts closes and charging current is delivered to the battery. When the battery is fully charged, there will be a temperature rise so that the voltage at pin 2 starts to ramp down. When it becomes lower than the voltage at pin 2, pin 1 goes low, which turn Q2 OFF (de-energize the relay) and Q3 ON (turn ON the LED and Q1). The function of Q1 is to latch the state so that it won't resume charging even the battery temperature returns to air temperature. To resume charging (say, another battery pad). the push button must be pressed to reset the state.
D1 through D4 are silicon diodes manufactured in the same batch so that their temperature coefficients are close. Q1, Q2 and Q3 are general purpose small signal transistors. The resistance of R2 needs to be selected so that the circuit functions properly. The relay coil is rated 5V 50mA and the relay contact is capable of breaking 5A at 30VDC.
Figure 1 Temperature sensing and cutoff circuit
Figure 2 is a switching mode constant current source. Trickle charging is not implemented with this charger. Most battery pads can be fully charged in less than an hour. D2 and D3 are 3A fast diodes. D1 is rated 2A or above. Q1 is installed on an heatsink. L1 is a custom part. I wound 30 turns with 18AWG magnet wires on a toroid core. Its dimensions are: outer diameter 1-1/8", the inner diameter 1/2" and the height is 1/2".
Figure 2 Switching Mode Constant Current Source
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