Introduction to 12v charger with auto cut off

A 12v charger with auto cut off is a device that can charge a 12-volt battery, by supplying a constant current and voltage. However, overcharging a battery can damage it and reduce its lifespan. Therefore, a 12v charger with auto cut off is a better option, as it can automatically stop the charging process when the battery is fully charged.

A 12v charger with auto cut off can be designed and implemented using various components and circuits. One of the common methods is to use a step down transformer to drop the voltage from 220v to 15v. A preset and a relay can be used to sense the battery voltage and switch off the charger when it reaches a certain level, such as 14v.

Some LEDs can be added to indicate the charging status and the auto cut off feature. In this project, we will learn how to build a 12v charger with auto cut off using some of these components and circuits. We will test and evaluate the performance of our charger and compare it with a conventional charger without auto cut off.

Hardware components used in this project

S/NO	Component	Value	QTY
1	Transformer	15v	1
2	Diode	–	5
3	Capacitor (C1)	1000uF	1
4	Capacitor (C2)	10uF	1
5	Resistor (R1)	10k	1
6	Resistors (R2,R3)	1k	2
7	Preset (VR1)	10k	1
8	Relay	–	1
9	Transistor	BC547	1
10	LED (Red, White)	–	2
11	Lead acid battery	12v	1

Relay: A relay is a device that can switch a large electric current with a small one. It uses an electromagnet to move a metal arm that connects or disconnects two contacts. Relays are useful for controlling high-power devices with low-power signals, such as sensors, microcontrollers, or switches. Relays can also isolate different parts of a circuit and protect them from damage. The state of a relay depends on whether the electromagnet is energized or not. When a small current flows through the coil, the electromagnet attracts the arm and closes or opens the contacts, depending on the type of relay. Here is a link to relay datasheet.

Transformer: A 15v step down transformer is a device that can reduce the voltage from a higher level to a lower level, such as from 230v AC to 15v AC. It consists of two coils of wire, called the primary and secondary, wrapped around a core of magnetic material. The primary coil is connected to the input voltage, and the secondary coil is connected to the output voltage. The ratio of the number of turns in the coils determines the amount of voltage reduction. A 15v step down transformer can be used for powering low-voltage devices, charging batteries, or isolating circuits.

Diode: The diodes in this system are used for rectification and to prevent the backflow of current from the battery.

Circuit diagram for 12v charger with auto cut off

How the 12v charger with auto cut off works

From stepping down the voltage from 230VAC to 15VAC by the step down transformer, the rectifying circuit converts the voltage to DC. The filtering capacitor ensure fine DC voltage passes. The preset is set such that when the voltage of the battery reaches 14v, it will send high voltage to then base of the transistor therefore turning the transistor on and energizing the relay.

To set the preset to turn on when the battery reaches 14v: after implementing the above circuit, connect a power source of 14v directly to the NC terminal of the relay and turn the preset till it trigger the relay switches to the NO. Once that is done, the charger is ready to be used. The capacitor at the base of the transistor prevents noise from triggering the transistor.

Applications of the 12v charger with auto cut off

Inverters: An inverter is a device that converts DC power from a battery to AC power for household appliances. A 12v charger with auto cut off can keep the battery charged and ready for use in case of power outage.

Computer UPS: A computer UPS (uninterruptible power supply) is a device that provides backup power to a computer in case of power failure. A 12v charger with auto cut off can maintain the battery level of the UPS and protect the computer from data loss or damage.
Kids automotive rechargeable buggy cars: These are toy cars that run on rechargeable batteries and can be controlled by kids. A 12v charger with auto cut off can ensure the safety and fun of the kids by avoiding overcharging and overheating of the batteries.
Auto-motives (transportation vehicles): These are vehicles that use 12v batteries for starting, lighting, and ignition. A 12v charger with auto cut off can extend the battery life and performance of the vehicles by preventing sulfation and corrosion.

Engineering projects: These are projects that involve the use of 12v batteries for various purposes, such as robotics, solar panels, LED lights, etc. A 12v charger with auto cut off can simplify the design and operation of the projects by eliminating the need for manual monitoring and switching of the charger.