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How to build Simple 12 Volt Car Battery Charger Circuit

We will create a ” 12 Volt Battery Charger Circuit” in this guide.

To charge batteries, we implement a voltage to the terminals and the battery begins to charge. The charging protocol is determined by the size and type of battery that is being charged. Some types of batteries have a high tolerance for overspending and, depending on the battery type, can be recharged by joining to a constant voltage or constant current source. When it comes to safe charging, fast charging, and/or maximum battery life, things get complicated. Here, we design a simple 12-volt battery charger circuit diagram using a few commonly available components, and this circuit is suitable for all types of 12 Volt batteries.

This simple 12-volt Battery Charger Circuit provides an outline design for a general battery charger, and you can add features like reverse polarity protection to this circuit by placing a diode at the output. (Diode anode to output positive supply and diode cathode as output positive terminal) and transistor-based overcurrent protection. The following charger circuit is a rough prototype that provides a 12 Volt output to the battery. This circuit is created to provide up to 3 amps of charging current.

Component Needed:

No Component Values Qty
1 Step down transformer 0-14V AC / 3 Amps) 1
2 Bridge Rectifier module BR1010 1
3 Electrolytic Capacitor 100µF/25V 1,1
4 Resistor 1KΩ/1W 1
5 LED   1
6 Ceramic Capacitor 0.01µF
12 Volt Car Battery Charger Circuit

Working Procedure:

As illustrated in the circuit, we have a power supply section consisting of a 0-14-volt AC step-down transformer, which is used to convert a 230V AC supply into a 12V AC supply, and for AC to DC rectifying, we have used bridge rectifier module BR1010, which provides a highly efficient DC supply with a high current rating. This Bridge Rectifier module will have four terminals, two for AC supply input indicated by a sign wave and two for DC output indicated by a positive and negative sign. C1 and C1 are smoothing capacitors. In this circuit, the C1 and C2 capacitors act as a filter, and the LED signals the presence of a DC voltage source there at output.

The RMS output of the transformer is 12 V in the simplest layout above. That is, after rectification, the peak voltage will be 12 x 1.41 = 16.92 V. Although this appears to be higher than the 14 V full-charge level of the 12 V battery, the battery is not harmed due to the transformer’s low current specification.

However, it is best to remove the battery as soon as the ammeter reads close to zero volts.

Auto Shut-OFF: You can easily make the above design auto shut off when the full charge level is reached by adding a BJT stage with the output shown below: We used a common emitter BJT stage with its base clamped at 15 V in this design, which means that the emitter voltage can never exceed 14 V. When the battery terminals exceed 14 V, the BJT becomes reverse biased and enters an auto shut down mode. You can adjust the 15V Diode value until the output voltage for the battery is around 14.3 V. This converts the first design into a fully automatic 12 V charger system that is simple to construct while remaining completely safe.

Why is Current Control Important?

Constant Current Setup:

Charging any type of chargeable battery can be critical and requires some attention. When the input current used to charge the battery is significantly high, adding a current control becomes critical.

We all know how smart the IC LM317 is, so it’s no surprise that it’s used in so many applications that require precise power control. The Current Controlled 12V Battery Charger Circuit Using IC LM317 presented here demonstrates how the IC LM317 can be configured with just a couple resistors and a standard transformer bridge power supply to charge a 12 volt battery with maximum accuracy.

How Does It Work?

The IC is essentially wired in its normal mode, with R1 and R2 included for the required voltage adjustment. The IC’s input power is supplied by a standard transformer/diode bridge network; the voltage after filtration via C1 is approximately 14 volts. The filtered 14 V DC is applied to the IC’s input pin. The IC’s ADJ pin is connected to the junction of resistor R1 and variable resistor R2. R2 can be fine-tuned to match the final output voltage to the battery. Without Rc, the circuit would behave similarly to a simple LM 317 power supply, with no current sensed and controlled.


When the power supply is turned on, the circuit begins to operate. The step down transformer steps down 230V RMS AC power to 15V RMS voltage. The bridge rectifier then rectifies this low voltage AC voltage to produce an unregulated DC voltage with AC ripples. The filter capacitor allows the alternating current ripples to pass through, resulting in an unregulated and filtered DC voltage across it. Two operations take place here: – 1. This unregulated DC voltage is fed directly to the DC load (in this case, the battery) via a relay. 2. The unregulated DC voltage is also fed into the voltage regulator, resulting in a regulated 12V DC supply.

Live Simulation: How to build Simple 12 Volt Car Battery Charger Circuit

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