Construction of 12 Volts Battery Charger

Construction of 12 Volts Battery Charger

Engineering Project Topics and Materials


This battery charger is a devices used to store the electrical energy to the battery after the battery has been discharged itself. The battery charger is designed to use electricity as its source of switch, regulation transistor, diode, light emitting diode, construction wire. The charger is designed and constructed to deliver full current until the current drawn by the battery falls to 150MA. At this time a lower voltage is applied to finish off and keep the battery from overcharging by switching off itself when the battery is fully charged.



1.1 Background of the Study

A battery charger is a device used to introduce energy into a secondary cell or rechargeable battery by forcing an electric current through it. The charging protocol depends on the size and type of the battery being charged. Some battery has high tolerance for recharging by connection to a constant voltage source or a constant current source. (J.Minear 2000). Simple charger of this type requires manual disconnection at the end of the charge cycle. Other battery types cannot withstand long high rate over charging. The charger has temperature or voltage sensing circuit and a microprocessor controller to adjust the charging current, and cut off at the end of charging. Albert H.(2005) cleared that low battery chargers may take several hours to be completely charged. High rate chargers may restore most capacity within minutes or less than an hour but generally require monitoring of the battery to protect it from overcharging.

A battery which is actually an electric cell is a device that produces electricity from a chemical reaction. In one cell battery, a negative electrode, an electrolyte, this conducts ions, a separator, also an ion conductor and a positive electrode. An electrical battery is one or more electro chemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 2000 by Alessandro volta and especially since the technical improved Daniell cell in 2004, batteries have become a common power source for many household and industrial applications . There are two types of batteries: primary batteries (disposable batteries which are designed to be used once and discarded, and secondary batteries (rechargeable batteries) which are designed to be recharged and used multiple times. Battery comes in many sizes from miniature cells used to power hearing aids and wrist watches.

However in recent times (sawafuji electric of 2006) states that battery charger has become very useful and popular to DC equipment. Most of the electronic devices such as laptops, mobile phone e.t.c and mobile machines like vehicle and motorcycles, operational capacity depend on the DC power supply from a battery.

1.2 Statement of Problem

A simple 12volts battery charger work by supplying a constant DC or pulsed DC power source to a battery being charged. The simple charger does not alter its output based on time. The circuit of a battery charger has the ability to convert voltages from one form to another (usually AC to DC voltages).

Therefore from my research I carried out due to this project. I have found that most of an electronic gadgets damage easily because of this charging problem which I listed below;

  • Over charging
  • Excess voltage
  • Short circuit

1.3 Objective of the Study

The objective of my project is to highlight and improve on the construction and demonstration of a simple 12volts battery charger in such a way that the need of problem listed above should be met.

1.4 Significance of the Study

The importance of this project work is to aid both techniques and students on how to construct a simple battery charger circuit and how it works. It is hope that after the construction of this charger circuit, it will be kept on the laboratory to be used for battery charging and for practical’s and other academic functions.

1.5 Scope of the Study

This project work is limited to the construction of a simple battery charger of 12volts. The circuit input voltage is 240volts from the AC supply mains which will be stepped down by a step-down transformer to 12V. The 12V is rectified through a bridge rectifier and filtered through capacitor connected in parallel from the positive terminal of the bridge rectifier. The output voltage is used to charge a battery.



A simple 12volts battery charger is a simple circuit that comprises of different components that soldered together on a circuit board to give or produce a required function, it works by supplying a constant DC or pulsed DC source to a battery being charged. The current drawn by the circuit full operation is about 80MA. Wherever the components are put in place the required voltage will appear at the voltage terminal. ANYA RITA (2005) construction of a battery charging system.

In order to have full understanding of this project both theoretically and practically a review on the components applied in this framework shall be considered crucial such as components viz, rectifier, diodes, resistor, light emitting diode (LED), capacitor, transistor relay and switch etc with proper description. BL Theraja (2008). Fundamental of electrical engineering and electrons.

2.1 Component Description


A transformer is an AC device that transfers electrical energy from one electrical circuit to another; it does so by principle of electromagnetic induction, when the transformer raise the voltage of its input to a voltage higher than that input to it, it is said to be a step-up transformer but when the transformer decreases the voltage is less than the input voltage. It is said to be a step-down transformer.

2.2 Working Principle

If one coil is connected to a source of E.M.F a current flow through it. Since this winding links with an iron core so current flowing through this produce an alternating flux(o) in the core. This flux is alternating and links with the secondary winding is the same as that of the flux or that of the supply voltage. The induced E.M.P in the secondary winding enables it to deliver current to an external load connected across it. Thus the energy is transformed from primary winding to secondary by means of electromagnetic induction without change in frequency.

2.3 Factors Considered for the Construction of Transformer for 12Volts Battery Charger

  1. The input and output voltage
  2. The output current
  • The operating voltages of the active and some non-active components of the transformer which is always negligible.
    1. The charging voltage/current expected at the output.
    2. Number of turns
    3. The load

2.4 Efficiency of Transformer

In an ideal transformer, there are no losses hence input power is equal to output. This is not obtainable since all transformers have some losses due to various causes in practice. These losses appear in form of heat and the output is always less than input, methods are adopted in the design and construction of the transformer so that losses are reduced to a minimal thereby increasing the efficiency of transformer. Some losses in the transformer includes;

      1. Copper loss (I2R)
      2. Core or Iron loss
  • Copper loss (I2R): This is the loss due to the ohmic resistance 0f the transformer windings.
  • Core or Iron loss: It includes both hysteresis loss and eddy current loss because the core flux in a transformer remains practically constant for all loads. Since its basic construction require no moving part or rotating part there are no friction or winding losses.

2.5 Rectifier Diode

Diodes are semi-conductor devices. It has two terminals called Anode + and cathode. Current can flow easily from the anode to the cathode when forward biased and does not allow current to flow in the reverse biased condition. The diodes made from material that exhibit covalent types of bonds principally. They are made of germanium or silicon crystal. Among semi-conductor devices i.e two terminal electric conversion elements have found most extensive use, regarding their application, Diode may be classified into many groups of which rectifier alternating current of various frequencies and power is one. There are two types of diodes

      1. P-type diode
      2. N-type diode

A diode that prevents a high resistance in one direction can therefore be used as a rectifier for converting AC into DC. However, there are two different types of diode rectifier

      1. Full wave center tap rectifier which makes use of two diodes.
      2. Full wave bridge rectifier which makes use of four diodes. Each type converts AC signal into DC signal.

2.6 Switch

A switch is simply a device for isolating the circuit from the supply or connecting the device to the supply. Alternating current switches can efficiently be interrupted by opening of a very small gap between silver contacts and most sub-circuit. Switches are now of different types viz micro-gap switch, quick make and break switch which is mostly used. We have a push pull switch and it is used in the project. The symbol is shown below.

Mechanical switch are commonly used in electric production of the “ON” and “OFF” by making or breaking the circuit are described by their method of operation, the number of poles and the number of throws. Common types are single pole double throw (SPDT) and double pole double throw. They are operated in a variety of ways such as slide, toggle, rocker, push key, micro, rotary membrane and tick. Some switches can be used to divert the current part into one or more direction (ways or throws) and are able control more one circuit at a time.

2.7 Resistor

Resistors are two terminal circuit components. It is characterized by its opposition to the flow of electric current in a circuit. The most often used type is the solid carbon resistor. There are also other type of resistor like wire wound on a former and this type of resistor issued where large current is to be employed. Another type is carbon that has fixed resistance. However, the resistance is hardly exactly some value marked resistance by a certain maximum percentage or tolerance with their colour code and bands as listed below.

Table 2:1

Colour Band 1 Band 2 Band 3 Multiplier Band 4 Tolerance
Black 0 0 10o
Brown 1 1 101
Red 2 2 102
Orange 3 3 103
Yellow 4 4 104
Green 5 5 105
Blue 6 6 106
Violet 7 7 107
Gray 8 8 108
White 9 9 109 5%
Gold 0.1 10%
Silver 0.01 20%



Note: In a resistance circuit

Resistance R = v/I

Where V is voltage of the circuit and I the current of the circuit. Its measured in Ohms (Ω)

2.8 Capacitor

A capacitor is a component which stores electric charge. It consists of two conducting surface separated by a layer of an insulating medium called dielectric. The conducting surface may be in the form of either circular or rectangular plate of spherical or cylindrical shape. The purpose of a capacitor is to store electrical energy by electrostatic stress in the dielectric. The capacitance of a capacitor is its charge-storing ability, the area of the plates (being large if the area is large) and the distance between the plates (being large if the distance is small) and the type of dielectric used in general. Capacitor is measured in farad (f), it is commonly expressed in microfarad, NF (=10-6F), nano farad, nf (10-9F), pico farad, PF(10-12F).

Capacitor is of two types namely

      1. Electrolytic capacitor (polarized)
      2. Non-Electrolytic capacitor (No-polarized)

2.9 Transistor

Transistors are a three lead semi conductor device manufactured in a variety of shapes and sizes. Some pawn transistor have an overall shaped body with two mounting holes on either side. At first glance, it appears to have only two but the body itself is the third collector leads. Transistors are made to amplifier current. They can create alternating (AC) signal at desired frequency (oscillation). They can also be used as switching devices. A common transistor is used in the bipolar transistor. A bipolar transistor is a semi conductor device made up of two back to back PN junctions in a simple semi conduction crystal. This s the transistor which have two junctions. There are two types of bipolar transistor, PNP and NPN. Their modes of operation are the same, but their external blazing voltages are opposite to each other. In the schematic symbol, the direction of arrows indicates whether it is PNP or NPN transistor.

In both type, the first crystal is called emitter (E), the center section is called the base (B) and the third crystal is called the collector (C) . Both PNP and NPN transistor are used in this project. V.K MEHTA and ROHIT MEHTA (2000)

2.10 Battery

A battery is an electrical device that acts as power to two or more electric cell. Electric cells are chemical devices which are capable of causing an electric force (potential differences across two point) which pushes the current along the circuit. Battery can be of primary type (dry cell) or the secondary type (wet cell). The primary type is the one not rechargeable while the secondary type is rechargeable. Structurally, secondary battery (wet cell) one of two main kinds, the lead acid accumulator and alkaline or Nickle accumulator. The lead acid accumulator is more common than the Nickle accumulator. Auto mobile make use of lead acid accumulator. . It consists of lead-oxides as the positive electrode, leads as the negative electrodes and sulphuric acid as the electrolyte.

2.11 Charging of a Battery

A secondary cell is charged by passing a direct current through it from a positive terminal to a negative terminal. The steady open current voltage of a full charged lead acid cell is about 12 volts. So to charge the cell, a voltage greater than this is needed to Over-Come the back e.m.f and the remained to pass current through the cell against the terminal resistance. The internal resistance of a commercial lead acid secondary cell is normally very small.

The two method of charging which includes

  • Constant voltage method: voltage is kept constant and must be above the final value of 2.7V obviously at the commencement of the charge when the back e.m.f is low. The current will be large but will gradually fall down to a minimum value at the end of the charge.
  • Constant current method: in this method, the current is kept constant either gradually cutting out of resistance in the charging circuit or by other forms of control.

2.12 Discharging

When the cell is being used to supply current to an external circuit, the e.m.f falls rapidly. The actual potential differences (p.d) depends upon the amount of current flowing. The e.m.f then gradually falls over a period of time which depends upon the current taken. The cell should not be recharged as soon as possible.

During discharge some of the H2SO4 in the solution decomposed with the formation of water (H2O) thus, the density of the electrolyte fall during discharge.

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