Many years ago I purchased a battery charger from China in a local store. The charger is working well to this day.
I am not sure exactly how it works. However, the charger must consist of two phases:
1. Discharge phase
2. Charging phase
Those two phase especially essential for charging NiCad batteries that must be discharged before the next recharging phase of the battery charge and discharge cycle. However, the chances are that my cheap Chinese charger is only designed to charge the batteries and is not discharging them to almost zero before charging.
This is why I designed my own circuit.
Step 1: Design the Circuit
I created a circuit with four BJT (Bipolar Junction Transistors) transistor inverters. I used BJT transistors instead of MOSFETs to reduce the costs. However, you can implement this circuit with MOSFET inverters. MOSFETs could be more appropriate for this circuit because you do not need linear characteristics of BJT transistors for this circuit.
The switch is used to implement the initial battery charging state (1 V).
The feed back look initiates the discharging phase when the battery is connected. When the voltage drops to about 0.7 V the discharging phase is OFF and the charger begins to charge the battery.
Step 2: Simulations
There was an issue with simulations. However, I believe that my circuit can still work well even thought I never build it.