Due to partial discharge, Ni-Cd batteries have a memory effect that is not good. The solution is to fully drain the battery before charging it again.

Fig. 1: Circuit to keep Ni-Cd away from memory effect A simple resistor will do a good job of discharging the cell, but if the cell discharges past a certain voltage, the polarity of the cell could change. Here is a simple circuit that discharges the cell to a level of 600 mV. This makes sure that the cell is correctly discharged without the risk of polarity reversal. The current going through the battery is not always the same. This gives the battery a chance to recharge during the breaks, which makes it last longer. As shown in Fig. 1, the circuit is a simple flyback oscillator with a frequency in the kHz range that depends on how X1 is wound. Wrap 30 turns of 36SWG wire around a bobbin core for the primary and 15 turns for the secondary. Figure 2 shows more about the size of the bobbin. It's easy to work. When the circuit is turned on, current flows through inductor X1 and through transistor T1. When the switch is off, the LED1 lets out the energy stored in the inductor. Once the end-of-cell-discharge voltage is reached, the LED stops glowing because the voltage at the base drops below the forward conduction voltage (0.6V) of the transistor.

Fig. 2: Winding details If you think a cell has the memory effect, you should empty it and then charge it a few times in a row. This will bring the cell back to its full size. Connect a few LEDs in parallel to up the load and speed up the discharge.