We all know that NiCad batteries need to be fully discharged before they are charged to prevent failure.

Thus I designed a circuit that does exactly that. 

This is how you use this circuit. 

1. Insert the battery.

2. Straight away turn ON the discharging button.

3. Wait until the battery discharges and then begins to charge on its own.

You just need to press the discharge button. The circuit will do the rest.

Step 1: Design the Circuit

I have drawn the circuit in PSpice simulation software student edition to save time.

You can see that I used a flip flop to switch between two states. Charge and discharge. You might need to use four transistor pairs instead of two transistor pairs for the battery driving circuit (Q2 and Q3 transistors). Also, you will need to use power transistors, not the general-purpose transistors that I used. The student edition does not include power transistors and allows only a limited number of components in the circuit.

If you making a 9V battery charger then you can use MOSFETs.

Step 2: Simulations

You can see how the battery is first discharging after the button is pressed and then begins to charge after the voltage falls to approximately 0 V:

Conclusion

I modeled the battery with two capacitors. This might not be true in real life.