Introduction: Cheap Battery Discharger
A NiCad battery needs to be fully discharged before you can charge it again to prevent permanent damage to the battery.
Previously I tried discharging the batteries with resistors, bright LEDs, or both. However, LEDs only turn ON at voltages above 2 V and you will need at least two AA/AAA/C/D batteries or one 9 V battery. When the total voltage of two 1.5 V batteries (connected in series) falls below 2 V the discharging of the LED discharger will stop and the battery will not be fully discharged. Using a resistor alone will not allow you to see how much power is still left in the battery.
This article shows you how you can make a simple battery and effective discharger. I used two light 1.5 V bulbs connected in parallel. But you can use just one.
You can also make a 9 V battery discharger with an old 12 V car light bulb.
It is important that you use a soldering iron because wires can rust over time. However, the circuit is very simple. You simply attach the two battery terminals to the light bulb not even considering the polarity that is relevant if you are using LEDs (Light Emitting Diodes).
You can use sticky, tape, plasticine/blue tack, or rubber band instead of battery harnesses to save money.
Parts: solder, light bulb, stick tape, cardboard.
Tools: Soldering iron, scissors.
Optional parts: battery harness, plasticine/blue tack, rubber bands.
Optional tools: wires stripper.