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:

Presented here is a basic circuit for charging an automobile battery, complete with an indicator. It operates using a 230V, 50Hz AC mains supply to replenish the battery. The AC voltage is rectified and filtered to produce an unregulated DC voltage, which, in turn, is employed to charge the battery via a relay. A feedback mechanism, composed of a potential divider, a diode, and a transistor, consistently monitors the battery voltage. This circuit is powered by a regulated DC voltage, obtained through the use of a voltage regulator. The feedback arrangement is designed so that when the battery voltage surpasses the maximum threshold, the relay deactivates, discontinuing the battery charging process.

## 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 segment within the feedback and load section is structured by the careful selection of resistors. Since the diode’s conduction occurs at 14.4V battery voltage, the resistor values should be configured in such a way that when the battery voltage approaches its maximum level, the positive voltage applied to the diode remains at a minimum of 3V.

To achieve this, we opted for a 100 Ohm potentiometer and supplemented it with resistors, each having values of 100 Ohms and 820 Ohms. These choices were made with consideration for the desired outcome and after performing the requisite calculations.

## Car Battery Charger Circuit Operation:

Upon the availability of the power source, the circuit initiates its operation. Firstly, a step-down transformer converts the 230V RMS AC power into a 15V RMS value. Subsequently, the bridge rectifier transforms the lower-voltage AC into an unregulated DC voltage with discernible AC ripples. These AC ripples traverse through the filter capacitor, ultimately resulting in an unregulated yet filtered DC voltage across it.

At this juncture, two simultaneous actions occur:

1. The unregulated DC voltage is directly supplied to the DC load, in this case, the battery, through a relay.
2. The voltage regulator receives this unregulated DC voltage and transforms it into a regulated 12V DC supply.

In this particular configuration, the relay employed is a 1C relay, with the common point initially connected to the normally closed position, enabling the flow of current through the relay to charge the battery. As the current flows through the circuit, the LED illuminates, serving as an indicator of the ongoing battery charging process. Simultaneously, a portion of the current traverses through the series resistors, thereby dividing the battery voltage through the potential divider. Initially, the voltage drop across the potential divider is insufficient to forward bias the diode.

This voltage corresponds to the battery voltage and consequently governs the battery’s charging and discharging phases. The potentiometer is initially adjusted to its midpoint. As the battery voltage ascends, the voltage across the potential divider gradually reaches a threshold where it becomes adequate to forward bias the diode. As the diode commences conduction, the base-emitter junction of transistor Q2 is driven into saturation, causing the transistor to switch on.

The relay coil is energized by connecting the transistor collector to one of its terminals, resulting in the common contact point transitioning to the normally open state. Consequently, the power supply to the battery is severed, temporarily halting the battery charging process. After a predetermined duration, as the battery discharges and the potential divider’s voltage reverts to a level where the diode is either reverse-biased or off, the transistor is driven to cut-off. Subsequently, the timer switches to the off state, rendering no output. Consequently, the relay’s common point reverts to its initial state, specifically the normally closed position. The battery recommences the charging cycle, and this entire sequence repeats.

## 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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