DESIGN AND CONSTRUCTION OF 12V BATTERY CHARGER

ABSTRACT

This project presents the design and construction of a battery charger. A battery charger is an electrical/electronic device used to put energy into a secondary cell or rechargeable battery by forcing an electric current through it. The system consists of a step down transformer, an AC to DC converter and a DC voltage regulator. The circuits are designed using copper wire, rectifier diodes, electrolytic capacitors, resistors with other passive and active component of electronics. A battery charger is an indispensible device for battery users because battery is an essential element that powers any electronic products like UPS inverters system, photographic equipments, hand-held lamps (flashlight or torch), computer-memory standby, toys, novelties and automobiles, etc. These products will become useless if there is no charger to refill their battery when it runs down. In this project, a sophisticated class of battery charger is designed not just to refill rechargeable batteries but to conserve the battery life because wronghandling and charging of a battery can permanently damage the battery even if is brand new. With this project, people can recharge both alkaline, NiCad and lead acid batteries by themselves and then save money on battery purchases.

CHAPTER ONE

INTRODUCTION

1.1 Background of the Study

Few modern electrical appliances receive their power directly from the Utilitygrid like National Electricity Power Authority (NEPA) or Power Holding Company of Nigeria (PHCN), while a growing number of everyday electronic and electrical devices require electrical power from batteries in order to achieve greater mobility and convenience. Rechargeable batteries store electricity from the utility grid for later use and can be conveniently recharged when their energy has been drained. According to Woodbank Communications Ltd in their 2005 battery chargers and charging methods review in http://www.mpoweruk.com, battery charging involves three key functions: getting the charger into the battery (charging), optimizing the charging rate (stabilizing) and knowing when to stop (terminating). Appliances that use rechargeable batteries include everythingfrom low-power mobile cell phones to high-power industrial fork lifts. The sales volume of suchproducts has increased dramatically in the past decade. Hundreds of millions of theseproducts are sold annually to businesses and consumers, with close to a billion in U.S and Nigeria.

While designers of battery chargers often maximize the energy efficiency of their devices to ensure long operation times between charging.  They often ignore how much energy is consumed in the process of converting ac electricity from the utility grid into dc electricity stored in the battery. In this project design, significant energy savings are possible by reducing the conversion losses associated with charging batteries in battery-powered products. We could save a lot of electric power using new electronic technology in our charging system and then highlight several design strategies for improving the efficiency of other chargers. We introduced a smart or intelligent battery charger that does not only recharge batteries but conserves AC electrical energy and as well save the battery life. Most battery chargers require human attention say Chu, Kim-Chiu the writer ‘development of intelligent battery charge’ (1989), from the University of Hong Kong, but in this project, an automatic battery monitor is used to reduce human attention to about 85 per cent to eliminate overcharging of batteries.

1.2 What is A Battery Charger?

A battery charger is a system that draws energy from the grid, store it in a battery, and release it to power a device is called a battery charger system.

A system designer in ADACC(Engr. A.A Ndubuisi) in his 1999article describes a battery charger is an electrical and electronic device that is used to put energy into a secondary cell or rechargeablebattery by forcing an electric current through it.The charging protocol of a battery charger depends on the size and type of the battery being charged. Some battery types have high tolerance for overcharging and can be recharged by connection to a constant voltage source or a constant current source; simple chargers of this type require manual disconnection at the end of the charge cycle, or may have a timer to cut off charging current at a fixed time. Other battery types cannot withstand long high-rate over-charging; the charger may have temperature or voltage sensing circuits and a microprocessor controller to adjust the charging current, and cut off at the end of charge. A trickle charger provides a relatively small amount of current, only enough to counteract self-discharge of a battery that is idle for a long time. Slow battery chargers may take several hours to complete a charge; 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 overcharge.

Christine T. Bryant, (1990) say not all chargers can recharge alkaline batteries. It makes sense to use alkaline batteries while powering electronic systems even though they are difficult to recharge but they do not have a self-discharge. This is because alkaline batteries have long shelf lives and do not suffer the ‘memory effects’ of Nickel-cadmium batteries. The term ‘memory effects’ refers to the batteries becoming weaker with continued use, particularly when the batteries have seen light use and do not respond well to further charging. The problem stems from low battery currents which flow from only a small part of the active anode area of the battery. If higher current had been drawn or if the battery had been completely discharged, the whole active area of the anode would have been involved. The unused area essentially ‘films over’ and acts as a barrier to current flow. Further charging does not restore the active area. This is a chemical change causing the electrodes to degenerate in Nickel-metal hydride and Nickel-cadmium batteries. It is reversible by charging and discharging several times. Batteries that are not recharged before use will not supply the full amount of stored energy. None of the above happens with common alkaline batteries. The rate of self discharge in Nickel-cadmium is about 2% per week, in Nickel-metal hydride it is about 3% per week. At temperatures higher than room temperature, these rates increase. Using the wrong charger on alkaline batteries can be downright dangerous. If you read the warning labels printed on ordinary batteries, you will observe that a NiCad charger should never be used on alkaline batteries. Such a charger would supply currents in excess of safe values, would not turn off automatically when battery voltage exceeds safe limits, and would continue unchecked until the battery was damaged.In order to achieve ten times extension of the normal life of an everyday alkaline battery, you will have to recharge it frequently, many more times than ten. With this project it is possible to recharge both alkaline and NiCad batteries because the system has anautomatic battery monitor which monitor and adjust the charging rate of batteries. Charge rate is often denoted as C or C-rate and signifies a charge or discharge rate equal to the capacity of a battery in one hour. For a 1.6Ah battery, C = 1.6A. A charge rate of C/2 = 0.8A would need two hours, and a charge rate of 2C = 3.2A would need 30 minutes to fully charge the battery from an empty state, if supported by the battery. This also assumes that the battery is 100% efficient at absorbing the charge. A battery charger may be specified in terms of the battery capacity or C rate; a charger rated C/10 would return the battery capacity in 10 hours, a charger rated at 4C would charge the battery in 15 minutes. Very rapid charging rates, 1 hour or less, generally require the charger to carefully monitor battery parameters such as terminal voltage and temperature to prevent overcharging and damage to the cells.

1.3 Aims and Objective

The major aim and objective of this project is to design and construct a battery charger that can be use to charge any kind of 12v rechargeable batteries including alkaline, NiCad or lead acid batteries. With the lack of centralized power grids, car batteries have taken the place of oneof the main energy sources available in developing countries. With this in mind, ourobjective will be to design a cheap, versatile and efficient lead acid car battery charger which will interest and appeal to the “cost-minded” customer.One of our main incentives in developing this project is for a low-cost charger (affordable) to integrate or combine with otherdevice. After carrying out our project research on the targeted users, they have been able todevise a list of particular features that would be essential in the project, like designing   battery charger to be universal. Using a standard AC power, ability to charge a typical 12 Volt lead-acid (automotive) battery.We used these requirements as guidelines to implementing our product as well as include additional features that we thought are important to the functionality.

1.4  Scope of the study

The scope of this project is to have a direct current battery charger of 12 Volts and 10 Amp maximum which can be suitable for car batteries and electric vehicles. Electric vehicles need high-rate chargers for public access.

1.5 Significant of the Study

The significant of this project is not just to recharge batteries but it works as a D. C power adapter for experimentation. It may, however, require an externalcapacitor to be connected across its output terminals in order to “smooth” the voltage sufficiently, which may be thought of as a DC voltage plus a “ripple” voltage added to it.  The project is also significant because children will be attracted to charging their own batteries in their toys and possessions. Managing their own batteries is fun, and they know it helps the environment by not having to throw batteries away when they can recycle them.

1.6 Limitations of the study

The project is limited to 12V batteries. It is not advisable to use it on rechargeable batteries above 12V.  Also there is no internal resistanceconnected in the battery charger to limit the short circuit current.

1.7  Constraints of the study

The biggest setback experienced during the course of this project is financial difficulties in funding the design of the project. Secondly, sourcing of materials and components used was not easy. Finding the rectifier diode and DC regulator for this charger was not an easy task. We had to destroy many electronic devices for solution. Some of the components we used for this project were very difficult to find. Furthermore, we received a severe electric shock while trying to plug in the workbench power supply to test our design and that made us to almost give up.

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