How to & Guides

Designing an Automatic Battery Charger

Have you ever wanted to create an automatic battery charger that charges the battery when its voltage drops below a certain level? In this article, we will guide you through the process of designing your very own automatic battery charger.

The Circuit Diagram

Circuit Diagram of Automatic Battery Charger

The automatic battery charger circuit consists of two sections: the power supply section and the load comparison section. The main supply voltage of 230V, 50Hz is connected to the primary winding of a center-tapped transformer, which steps down the voltage to 15-0-15V. The transformer’s output is then connected to diodes D1 and D2, which convert the low AC voltage to pulsating DC voltage through a process called rectification. The pulsating DC voltage is then passed through a 470uF capacitor to remove AC ripples, resulting in an unregulated DC voltage. This unregulated voltage is then regulated to a desired level using the LM317 variable voltage regulator. The output voltage of the regulator can be adjusted from 1.2V to 37V by varying the potentiometer connected to the adjust pin of the LM317. The regulated voltage is then applied to the battery through diode D5 and resistor R5. Diode D5 is used to prevent battery discharge when the main supply fails. When the battery charges fully, the zener diode D6 conducts, grounding the base of the BD139 NPN transistor and stopping the flow of current from the LM317 regulator. A green LED is used to indicate the battery’s charge status, and resistor R3 protects it from high voltages.

Circuit Principle

If the battery voltage drops below 12V, current flows from the LM317 regulator through resistor R5 and diode D5 to charge the battery. At this point, the zener diode D6 does not conduct, as the battery takes all the current for charging. When the battery voltage reaches 13.5V, the current flow to the battery stops, and the zener diode conducts, allowing current to flow through it. This provides the base of the transistor with sufficient current to turn it on, grounding the output current from the LM317 voltage regulator through the transistor Q1. As a result, a red LED indicates that the battery is fully charged.

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Charger Settings

When designing a battery charger, there are a few important settings to consider. The output voltage of the charger should be less than 1.5 times the battery voltage, and the charger’s current should be around 10% of the battery’s current. Additionally, the charger should have overvoltage protection, short circuit protection, and reverse polarity protection.

That’s it! Now you have a basic understanding of how to design an automatic battery charger. For more details and additional projects, visit OnSpec Electronic, Inc. Happy charging!

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