Battery Charger

# Car Battery Charger Circuit

Outline

• Car Battery Charger Circuit Working Principle:
• Car Battery Charger Circuit Diagram:
• Car Battery Charger Circuit Design:
• Car Battery Charger Circuit Operation:
• Applications of Car Battery Charger Circuit:
• Limitations of this Circuit:

## Car Battery Charger Circuit Working Principle:

This is a basic automobile battery charger circuit that includes an indicator. A 230V, 50Hz AC mains supply is used to charge the battery. This AC voltage is rectified and filtered to produce an unregulated DC voltage, which is then utilised to charge the battery via a relay. A feedback circuitry made up of a potential divider, a diode, and a transistor constantly monitors the battery voltage. A regulated DC voltage powers the relay and feedback circuitry (obtained using a voltage regulator). The feedback circuitry is constructed in such a way that when the battery voltage exceeds the maximum, the relay is turned off and the battery charging stops.

## Car Battery Charger Circuit Design:

To design the entire circuit, we first design three different modules- the power supply section, the feedback and the load section.

### Power Supply Design Steps:

1. The desired load in this case is a car battery with a rating of around 40AH. The required charging current would be roughly 4A because the charging current of a battery should be 10% of the battery rating.
2. The needed secondary current for the transformer is now roughly 1.8*4, or about 8A. We can use a transformer with a 12V/8A rating because the needed load voltage is 12V. The needed AC voltage RMS value is now around 12V, with a peak voltage of 14.4V, or 15V.
3. Because we’re utilising a bridge rectifier, each diode’s PIV should be greater than four times the peak AC voltage, or more than 90V. We’ll use 1N4001 diodes with PIV values of around 100V.
4. Because we’re making a regulated power supply, the maximum permissible ripple is equal to the capacitor peak voltage minus the regulator’s required minimum input voltage. A voltage regulator, the LM7812, is used to provide a controlled 5V supply to the relay and the 555 Timer in this example. The ripple would then be roughly 4V. (Peak voltage of about 15V and input regulator voltage of around 8V). As a result, the filter capacitor value is estimated to be around 10mF.

### Feedback and Load Section Design:

The voltage divider section of the feedback and load section is designed by selecting resistors. Because the diode will only conduct when the battery voltage reaches 14.4V, the resistor values should be such that when the battery voltage is near its maximum, the positive voltage provided to the diode is at least 3V.

We chose a 100 Ohm potentiometer and additional resistors of 100 Ohms and 820 Ohms apiece, keeping this in mind and performing the necessary calculations.

## Car Battery Charger Circuit Operation:

When the power source is available, the circuit operation begins. The step down transformer converts 230V RMS AC power to a value of 15V RMS. The bridge rectifier then rectifies the low voltage AC voltage to produce an unregulated DC voltage with AC ripples. The AC ripples flow through the filter capacitor, resulting in an unregulated and filtered DC voltage across it. Two operations take place here: – 1. Through a relay, this unregulated DC voltage is delivered directly to the DC load (in this case, the battery). 2. The voltage regulator receives this uncontrolled DC voltage and converts it to a regulated 12V DC supply.

The relay in this case is a 1C relay with the common point connected to the typically closed position, allowing current to pass through the relay to the battery, charging it. The LED begins to conduct as current goes through it, showing that the battery is being charged. A portion of the current also passes via the series resistors, dividing the battery voltage via the potential divider. The voltage drop across the potential divider is insufficient to bias the diode at first.This voltage is equal to the battery voltage and hence controls the battery’s charging and discharging. The potentiometer is first adjusted to its middle. As the battery voltage rises, the voltage across the potential divider rises to the point where it is enough to forward bias the diode. When the diode begins to conduct, the transistor Q2’s base emitter junction is forced to saturation, and the transistor is turned on.

The relay coil is powered when the transistor collector is connected to one end of it, and the common contact point moves to the usually open state. As a result, the power supply is disconnected from the battery, and the battery’s charge is halted. After a period of time, when the battery discharges and the voltage at the potential divider returns to a point where the diode is reverse biassed or off, the transistor is forced to cut off, and the Timer is now in the off position, with no output. The relay’s common point returns to its original state, i.e. the ordinarily closed state. The battery begins to charge once more, and the procedure is repeated.

## Applications of Car Battery Charger Circuit:

1. This circuit is portable and can be used at places where AC voltage supply is available.
2. It can be used to charge toy automobile batteries.

## Limitations of this Circuit:

1. It is a theoretical circuit and may require some practical changes.
2. Battery charging and discharging may take longer time.

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