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Simple NiCad Battery Charger

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Suitable to be used onboard mobile robots

Questions about charging NiCad batteries have been asked here lately. This discussion outlines how to do a safe "overnight" charge on NiCad batteries. A safe charge is 0.1 x the battery pack's mAH rating for 14 hours. Note that this will not be the absolute maximum charge, but it will be so close that it won't matter. I may refine some values later.

The basic circuit (below) represents a regulated +12v source ( VS ) with a TO-92 LM317 adjustable voltage regulator connected as a current source, and a battery in series to GND (circuit common).


LM317x devices have three terminals: IN, ADJ, and OUT.

VS is a nominal voltage that can range up to about 30v, depending on the maximum ratings for the LM317x device.

Voltage VL (positive terminal of the battery pack under charge) is a test voltage used to sense (approximate) full charge. For NiCad batteries, use 1.65v per cell. Nominal NiCad voltage is 1.2v per cell.

Discharge load: a resistive load of 0.1 mAH rating of the pack.


(specific example)

Assume VS = regulated 12v and a three-cell pack ( 3.6v ) with 300mAH (milliamp Hour) capability. Discharge current ( 0.1 x 300mAH = 30mA ).

Resistors R1 and VR sense the output current; for the LM317, the voltage between ADJ and OUT is approximately 1.25v. Consult the LM317 data sheet for more details. Resistor R1 sets the maximum output current, and VR must be set for the particular battery pack being charged.

After constructing the circuit, connect the 12v supply and connect a voltmeter set to measure current between the terminals VL and GND Select the highest milliamp range (i.e., 250mA) before connecting the voltmeter. Set VR to the desired current ( 0.1 x 300mAH in this case). It may be helpful if VR is a multiturn trimmer potentiometer for easier setting.

Discharge cycle (recommended): Measure the open circuit voltage of the battery pack; if it is higher than 1.1 volts per cell (3.3volts), discharge the pack into a 30mA load (for this example, 3.6/0.03 = 120 ohms) until pack voltage = 3.3v.

Connect the + terminal of the pack to terminal VL, the -terminal to GND, and apply power for 14 hours. Or, monitor the pack voltage until it reaches 4.95 volts.

For NiCad batteries, the 14-hour limit isn't very critical; for NiMH batteries, the terminal voltage should be the cutoff point rather than time.

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