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Automatic 12V Battery Charger with Explanation

This article explains automatic battery charger. Components, connection diagram and simulation in proteus of 12V battery charger is also explained in the article.


Batteries are very important in electrical and electronics world. Batteries need to recharged once their charge is finished. This article will introduce about one of the simplest 12V battery charger with automatic cutoff. Automatic battery charger is built with easily available electronic components which are cheap and can be easily find.  The circuit is tested in Proteus ISIS for verification.

Typical Automatic battery charger arrangement

The minimum requirement for any battery charger is shown in the following figure

Aumatic battery charger arrangment

The voltage level of an AC source is brought down to a safe level by Transformer. The output of transformer is given to the rectifier. It converts AC into DC. The output of rectifier is unregulated so a voltage regulator is installed to get regulated voltage. Now regulated voltage can be given to the battery for charging however applying continuous voltage when battery is fully charged can damage the battery. For this purpose, auto cutoff arrangement is needed which should disconnect the battery from charger when charging is complete.

Components and circuit diagram for 12V battery charger.

  • Transformer (220/25V)
  • 1N4007 Diodes
  • Capacitor 470uF
  • Voltage regulator LM317
  • Resistors
  • Zener Diode ZPD12RL
  • Transistor BD139
  • 12V relay
  • 220V, 50Hz AC supply

The above components are connected in the following way to make automatic battery charger.

12V battery charger connection diagram

Working of Automatic Battery Charger.

Normal AC supply in most of the countries is 220V 50Hz. This high voltage cannot be converted directly into DC as it can damage the electronic components. For this purpose 220V/25V transformer is used to bring voltage down. For 12V battery charger, the voltage on the secondary side should be in between 15V and 25V so that it can be adjusted around 12V via regulator. The bridge rectifier circuit is used to convert AC into DC. The output of rectifier is pulsating DC which must be smoothed before applying to regulator. Smoothing capacitor is connected at output of bridge rectifier. Pure DC is than given to the voltage regulator LM317. The output of LM317 can be adjusted as follows.

Vo = VREF (1 + R2 / R1)

In series with R2, a variable resistor RV4 is connected so that the output voltage of regulator can be adjusted. The reference voltage is the input voltage.

A small capacitor is connected at the output of voltage regulator whose purpose is to stabilize the output of regulator. Its value is given in the datasheet of Lm317.

A diode D5 is connected after the resistor at the output of regulator. The purpose of this diode is obvious. It only allows the current to flow from regulator towards battery but not backward. In this way it saves the battery from being discharged through the regulator.

Zener diode along with transistor, two resistors and 12V relay makes the automatic cutoff arrangement for 12V battery charger. Zener diode starts conducting due to zener breakdown when voltage increases form 13V. When its start conducting, current is supplied to the base of transistor and it moves from cutoff region to conduction state. As a result the collector of transistor which was previously at 12V (due to cutoff) now at ground level (or near to ground level). As a result the 12 volt relay will operate (to learn how 12 V relay works click). The resistor at the connected at the base of transistor controls the sensitivity of the cutoff switch. Low value of this resistance will operate the switch as soon as zener starts conducting while higher value will operate the switch when voltage goes beyond 13V by good range.

When relay operates, battery is isolated from the automatic battery charger.


Automatic battery charger is simulated in ISIS Proteus and the result is shown int the following figure. Since the Proteus does not have any active model of Battery so simulation is done by keeping the battery terminals open. Initially the voltage is less 14.5V so the relay position is such that battery terminals are in the circuit for charging.

Automatic battery charger simulation 1

Now voltage is increased the relay has been operated as shown.

Automatic battery charger simulation 2

 Now the battery terminal are isolated from charger to prevent overcharging.

Simulation Video

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About Syed Noman ud din

Syed Noman ud din is an Electrical Engineer and working in Industry from last 3 years. He writes technical articles for electrical and electronic engineers. He has also published several research publications in renowned international journals.

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  1. What is the value of RV4, R1 and C4 capacitor? I have made this circuit but it is not operating.

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