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Keep Away NI-CD From Memory Effect

Ni-Cd batteries suffer from an undesirable memory effect due to partial discharge. The remedy is to completely discharge the battery before recharging.

A simple resistor will do the discharging job nicely but there is the risk that the cell may discharge beyond a certain voltage and cause polarity reversal in the cell. Here is a simple circuit that discharges the cell to a level of 600 mV, ensuring that the cell is correctly discharged without the risk of polarity reversal. The battery is not discharged at a constant current. This allows the battery to recover during the intervals, to extend its useful life.

Fig. 1: Circuit to keep Ni-Cd away from memory effect
Fig. 1: Circuit to keep Ni-Cd away from memory effect

As shown in Fig. 1, the circuit is a simple flyback oscillator that oscillates at a frequency in kHz range depending on the winding of X1. Wind the primary with 30 turns of 36SWG wire and secondary with 15 turns round a bobbin core. Details of the bobbin size are shown in Fig. 2.

Fig. 2: Winding details
Fig. 2: Winding details

Working is simple. When the circuit is powered on, transistor T1 conducts and current flows through inductor X1. During the switched-off state, the energy stored in the inductor is discharged via LED1. Once the end-of-cell-discharge voltage is reached, the LED stops glowing, as voltage at the base goes below the forward conduction voltage (0.6V) of the transistor.

If you suspect a cell is suffering from the memory effect, discharge and recharge it a couple of times in succession. This will restore the capacity of the cell. For faster discharge, increase the load by connecting a few LEDs in parallel.

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