12V Automatic Battery Charger With Auto Cut Off

In this project, I will be going to show you a 12V automatic battery charger with auto cut off when the battery level is below the maximum threshold voltage. When the battery gets its maximum voltage then the automatic battery charger circuit will automatically cut-off the charging of the battery. Thus it prevents by overcharging of the battery.

Here is the full details how automatic battery charger can build and how it works.

Circuit Diagram

automatic battery charger

Components Required

  • 12-0-12 1A Step-Down Transformer
  • LM317 Voltage Regulator
  • 1N4007 P-N Diode
  • 12V, 1W Zener Diode
  • Resistors ( 1 KΩ, 2.2 KΩ, 222 Ω, 100 Ω )
  • 1000 uF/25V Electrolytic Capacitor
  • LED (Green, Red)
  • 100 KΩ Potentiometer
  • BD139 NPN Transistor
  • 12V Battery

Circuit Connection

In this automatic battery charger, A 12-0-12 1 A step-down transformer is used here. The main voltage 220V, 50Hz is connected to the primary part of the transformer.

The output of the transformer i.e from the secondary part is connected to the diodes D1 and D2. Here diodes D1, D2 are used to convert the AC voltage to the pulsating DC voltage. This is a rectification process. A 1000uF/25V electrolytic capacitor is connected across the pulsating DC voltage to remove the unwanted ripples.

Thus we can get an unregulated DC voltage. This unregulated DC voltage needs to be regulated by using the LM317 voltage regulator to drive a regulated DC voltage.

The output voltage from the voltage regulator can be regulated from 1.2V to 37V and the maximum output current from this IC is 1.5 A. The output voltage of the voltage regulator has operated by a potentiometer which value is 10 KΩ which is connected to the adjust pin of the LM317.

The output of the LM317 voltage regulator is applied to the battery through the diode D5 and resistor R5. Here the diode D5 is used for avoiding the discharge of the battery when mains failure.

When the battery gets full charge, the Zener diode D6 which is connected in reverse bias is conducting the current. Now the base of the BD139 NPN transistor gets the current flow through the Zener diode. So no current flows to the battery and all the current are being grounded then.

In this automatic battery charger circuit, the green LED indicates the charging of the battery. There is a protector resistance i.e R3 for the green LED from high voltage.

Working of Automatic Battery Charger

In this automatic battery charger if the battery voltage level is getting low at the specified voltage level, then the current is flowing from the LM317 IC through the resistor R5 and diode D5 to the battery. At this point, the Zener diode D6 is not conducting any current because all the current is taken place for charging the battery.

When the battery voltage reaches the maximum threshold voltage i.e 13.5V, then the current flowing to the battery is stopped and the Zener diode then gets the sufficient breakdown voltage and it can allow the current flowing through it.

Presently the base of the NPN transistor gets the adequate current to turn on so the output current from the LM317 voltage controller is grounded through the transistor Q1. Subsequently, Red LED demonstrates the loaded with charge.

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