Make Automatic 12V Portable Battery Charger Circuitat

How to make a Automatic 12 V Portable Battery Charger Circuit at Home

By: Alok kumar

Today we are going to learn how to make a Portable Automatic 12 volt battery charger circuit which starts charging Battery Automatically when the voltage of that Battery is below the specified voltage value. This charger will Automatically stops the charging process when Battery attains the full charge value. This process protect our battery from deep-charge and if voltage is below 12 V it start charging.

Here the main component is IC LM317. Here the power source is transformer but it can be any electronic supply ( for example SMPS power supply).

Components Required:

• 15-0-15V, 1A center tapped transformer 
• LM317T voltage regulator
• 12V DC battery
• BD139 NPN transistor
• LED’s – red, green
• 8.2V zener diode
• 1n4007 diodes – 3
• electrolytic capacitor – 470uF
• potentiometer or preset or variable resistor – 10k
• resistors (1/2 watt) – 100, 240 ohms
• 2 Resistors – 1k
• Resistor – 2.2k

 Circuit Diagram:

Portable Automatic 12V Battery charger circuit.

Here when the battery voltage goes below 12V, then the current from LM317 IC flows through the resistor R5 and diode D5 to the battery. At this time zener diode D6 will not conduct because battery absorbs all the current for charging itself.

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And when the battery voltage rises to 13.5V, the current flow to the battery stops and zener diode gets the sufficient breakdown voltage and it allows the current pass through it. Here now the base of the transistor gets the sufficient current to turn ON so that the output current from LM317 voltage regulator is grounded through the transistor Q1. As a result Red LED indicates the full of charge.

Now lets come to the circuit description

This automatic battery charger circuit design shown above involves two sections – power supply section and load comparison section.

The main supply voltage 230V, 50Hz is connected to the primary winding of the center tapped transformer to step down the voltage to 15-0-15V.

The output of the transformer is connected to the Diodes D1, D2. Here diodes D1, D2 are used to convert AC voltage to pulsating DC voltage called as rectification process. The pulsating DC voltage is applied to the 470uF filter capacitor to remove the AC ripples. Thus the output of the capacitor is unregulated Dc voltage. This unregulated DC voltage is now applied to the LM317 variable voltage regulator to provide regulated DC voltage. The output voltage of this voltage regulator is variable from 1.2V to 37V and the maximum output current from this IC is 1.5A Pre specified. The output voltage of this voltage regulator is varied by varying the pot 10k (Variable resistor 10kohm) which is connected to the adjust pin of IC LM317.

Now voltage regulator's ( IC LM317) output is applied to the battery through the diode D5 and resistor R5. Here diode D5 is used to avoid the discharge of battery when main supply is switched off or not charging situations. When battery charges fully, the zener diode D6 which connected in reverse bias starts conducting. Now the base of BD139 NPN transistor gets the current through the zener so that the total current is grounded. In this circuit green LED is used for indicating the charge of the battery to be full. Resistor R3 is used to protect the green LED from high voltages.

Here the output voltage of the battery charger should be less than 1.5 times of the battery and the current of the charger should be maximum 10% of the battery current (AH value of the Battery). 

For any further querry please ask them in comment section. I will be happy to answer them. If you find this circuit interesting please share this article.

Thank you.