12V Battery Charger with Auto Cut-Off Using a UPS Transformer

Building a 12V battery charger with an automatic cut-off using a UPS transformer is a practical and efficient way to charge batteries safely. This charger ensures that the battery gets the required voltage and current while preventing overcharging, which extends battery life.

Components Required:
UPS Transformer (12V-0-12V output)
Bridge Rectifier (4x 6A diodes or a ready-made rectifier module)
Voltage Regulator IC (LM317 or LM338)
Relay (12V, SPDT)
Zener Diode (for voltage regulation)
Transistor (BC547 or equivalent)
Capacitors (1000µF, 470µF, and 100µF)
Resistors (1KΩ, 10KΩ, etc.)
LED Indicators (Charging & Fully Charged)
Heat Sink for Regulator
Working Principle:
AC to DC Conversion: The UPS transformer steps down 220V AC to 12V AC, which is then converted to DC using a bridge rectifier and filtered with capacitors.
Voltage Regulation: The LM317/LM338 regulator maintains a steady 13.8V output to safely charge a 12V lead-acid battery.
Auto Cut-Off Mechanism: A relay circuit monitors the battery voltage. Once the battery reaches full charge (13.8V-14.2V), the relay disconnects the battery from the charger, preventing overcharging.
LED Indicators: A red LED indicates charging, and a green LED signals full charge.
Advantages:
Prevents Overcharging: Ensures battery longevity.
Efficient Charging: Delivers stable power.
Budget-Friendly: Uses easily available components.
Safe Operation: Includes voltage regulation and relay control.
This DIY charger is perfect for 12V lead-acid batteries used in UPS systems, solar power setups, and vehicles.