This Automatic Battery charger turns On only when the battery demands charging current. It can be used to charge 12 Volt Lead Acid or Tubular batteries .The automatic switching helps to keep the battery always in top condition. The overcharge and over discharge cut off facilities are included so that the charger can be left unattended for long periods.
The switching action through the relay is achieved by by sensing the terminal voltage of the battery under charge. Relay contacts break the AC supply to the charger transformer when the battery voltage rises above 14 volts. When the terminal voltage drops below 11.5 volts, relay contacts complete the Neutral path of AC supply to the charger transformer.
Operational Amplifier IC1 is used as a precision voltage comparator to monitor the voltage level of the battery. Its Inverting input gets a reference voltage of 1.8 V from the junction of Red LED and R3 while the Non inverting input gets slightly higher voltage of 2 volts ( as set by VR1). This makes output of IC1 high. PNP transistor then remains off to keep the relay de-energized. Since the Neutral line is connected through the Comm and NO contacts of the relay, AC path remains cut off and no charging takes place.
When the battery voltage drops below 11.5 volts, voltage at the Non inverting input (pin3) drops below that of the Inverting input (pin2) and the output of IC1 turns low. T1 then conducts to actuate the relay. Relay contacts completes the neutral path and charging process starts. Green LED indicates the charging process.
1. Discharge the battery using a 12 volt Car bulb till the battery voltage drops to 11.5 volts.
2. Measure the terminal voltage using a multimeter. It should be around 11.5 volts. Connect the battery to the outputs of the circuit.
3. Adjust VR2 till the relay clicks and Green LED turns On.
4. Charging processes starts and measure the terminal voltage at an interval of 15 minutes.
5. When the battery voltage increases to 14 volts, adjust VR1 till relay turns off. At this point, Green LED also turns off.
6. Careful setting of VR1 and VR2 gives 3 volts difference between the On and Off states.
7. The Charger is now ready to use.