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The battery discharger published on this page can be improved by adding a Schottky diode (D3). This ensures that the NiCd battery is not discharged to 0.6-0.7V but only below 1V as recommended by the manufacturer. An additional effect is that the light emitting diode D2 will blink when the battery connected to the terminals is exhausted. The circuit in the diagram is based on an unstable general-purpose oscillator operating at a frequency of about 25 kHz.
When transistor T2 conducts, a current flows through the inductor L1, after which energy is stored in the resulting electromagnetic field. When T2 is cut off, the field collapses, then a reactive EMF is generated that exceeds the forward voltage (about 1.6 V) of D2. Then a current passes through the diode so that it lights up. Diode D1 prevents current from flowing through R4 and C2. This process is only interrupted when the battery voltage no longer provides enough base voltage to the transistors.
In the original circuit this happens at about 0.65 V. By adding forward bias of D3 (about 0.3 V), the battery end discharge voltage is increased to 0.9-1.0V. Additional resistors R5 and R6 ensure that enough current flows through D3. When the battery has discharged to the recommended level, it should be removed from the discharger because unlike the original circuit, a small current continues to flow through D3, R2-R3 and R5-R6 until the battery is fully discharged. The fact that D2 blinks when the battery is close to the recommended discharge level is due to the increased internal resistance of the battery, lowering the terminal voltage below the threshold level.
If no current is flowing, the internal resistance has no effect because the terminal voltage rises to the threshold voltage taking some of the energy from the battery. When the discharge is complete to the recommended level, the LED will turn off. Therefore, it should be noted that the battery has been sufficiently discharged when the LED starts blinking.