This transformer-based battery charger circuit can be used to charge any battery, provided the transformer rating is higher than the battery rating. Normally, a linear regulator IC is used for charging. But this charger circuit uses a MOSFET switch, which effectively draws current from the transformer when its voltage is close to the desired battery charging voltage.
The full-wave bridge rectifier rectifies the transformer output and the combination of diode D5 and capacitor C1 provides gate bias voltage to the MOSFET (T2). Zener diode ZD1 maintains the gate voltage at a maximum of 12V relative to the source. The rectified AC potential is also divided by the combination of resistor R1 and 5-kilo-ohm preset VR1 to set the charging voltage.
When the rectified AC voltage is below the threshold of IC1 (TL431), the MOSFET conducts. Filter capacitor C2 at its output charges through diode D6. As the rectified voltage decreases from peak to 0V, IC1 turns off at approximately 13.6V. Transistor T1 conducts to provide another pulse current to charge capacitor C2. Thus power drain from the transformer occurs in short pulses and the battery charges between the pulses.
To limit the charging current to the battery, the current through the battery is sensed by resistor R3. Once the voltage drop across R3 reaches the base-emitter threshold of transistor T1, it conducts to turn on IC1, which, in turn, switches off MOSFET T2. This process continues, limiting the charging current to the battery.