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Battery Charger

Battery Charger

This circuit is an AC-powered current source designed for recharging batteries. It can crank out as much as 1 amp and can be modified to go even higher by choosing different devices for Q1. Since this circuit uses AC line voltages and currents, please exercise extreme caution during assembly, turn-on, and test.

NiCAD batteries have a capacity specification called milliamp-hours. This value called "C" is a measure of how much total current they can provide in one hour. Milliamp-hours is another way to express the energy contained in the battery. To recharge a NiCAD battery conservatively, it is common practice to pump a current of 0.1 C into the anode or positive terminal for about 12 hours.. Therefore, if you had a D-size NiCAD with a capacity of 4000mAh, you would want to charge it at 400mA for about 12 hours. Another advantage of this charging technique is that it is gentle on batteries and doesn't cause them to lose capacity as quickly as the fast charge techniques.

The output current of this device is controlled by the summation of the bandgap reference diode and the base-emitter junction of the PNP transistor. The PNP transistor provides negative feedback to the gate of the MOSFET. As noted in the schematic, the batteries being charged can have a total of 12V which is equivalent to about 8 NiCAD's in series. The output current is determined by the value of R1 which is determined by:

R1=3.2Volts/Iout

The power dissipation of R1 will equal:

Pr1=3.2Volts*Iout

Be sure to provide pleanty of heatsink for Q1 and choose an appropriately sized resistor for R1. The following table summarizes some of the resistor current combinations that are possible:

Iout Resistor Value Resistor Power
100mA 33 ohms 1 watt
500mA 6.2 ohms 2 watt
1Amp 3.3 ohms 5 watt

The power dissipation of Q1 as a function of Output current and load voltage can be shown as:


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