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Introduction
A caravan is a vehicle designed for living. It is a way of lifestyle. It is used as home. So it consumes electric current just like home. A caravan, of course, consists of home appliances and hence continuous supply of electric current is required. Even though caravan is a vehicle, it is equipped as a home. So the power supply becomes a problem for the caravan. It uses a battery as in the vehicles. Different power supply systems are used in the caravans.
Among these, some are through electric cable as in the home, or by the solar panel, or using generator, or by a rechargeable battery etc. In caravan a rechargeable battery is generally used for power supply. This battery provides electric current for the working of all devices in that caravan. But caravanning requires the use of high drain equipments such as televisions, fridges and pumps etc.
A plan of caravan is shown below. The family in the caravan uses the electronic devices in the convenient time.
Regular car batteries may be used for caravanning but they may not be very effective. Car batteries are designed to provide large amounts of power in short periods, mainly for starting the engine. They are then quickly recharged. But the battery should be charged in the requisite time. For the uninterrupted working of battery, recharging should be done according to the charge in that battery. Generally generators are used for this purpose.
But noise of the generator avoids its application in the caravan during night. So there is a need for other methods for finding convenient time for recharging of battery, which should be reliable and simple. The electric devices in caravan utilize high volume of electric current, due to use of high power electronic equipments and may be discharged quickly and hence it might be run out, possibly at night. By knowing the status of battery we can simply recharge the battery when needed. Status should expose the present state as well as life of battery. Ampere hour meter is the solution for this situation.
Purpose of an ampere hour meter
Purpose of this project is to solve the problem relating power supply of a caravan. Goal of the ampere hour meter in the battery is to get information about battery life. Ampere hour meter is the key instrument which shows battery capacity. The name ‘ampere hour’ shows two different units. So we can call it as derived unit. The ampere = coulombs/sec and the hour means 3600 sec. So the term ampere hour meter gives — (coulombs/sec) * (3600 sec) = 3600 coulombs.
The unit of electric current is ampere and for time is seconds (or 1/3600 Hour). Ampere is an SI unit of electric current. Ampere can be defined as the rate of change of charge. That is equal to coulombs/sec. Ampere hour of a power supply is expressed in coulombs, which in terms express in the unit of charge. An ampere hour meter can act as a battery charge indicator. For example, a battery with a 70 AH can supply a 70 ampere for 1 hour continuously. Or else it can be expressed as the battery can supply 1 ampere for a long duration of 70 hours. The charge quantity in the battery is expressed in coulombs.
Design of ampere hour meter
The ampere hour meter should alert the user about the amount of charge left in the battery. “Many types of ampere-hour meter have been manufactured in the past, the most important being electrolytic meters and motor meters. Theoretically the former are capable of very accurate registration but in practice the working results are not so good as with motor meters, and the latter are preferred by most supply authorities.” (Direct Current Meters).
In the past, the electrolytic or motor based ampere hour meters were used. So the size of device is bulky and also it is very tedious for trouble shooting. The conventional method for finding ampere hour or charge content in the battery is given below. Shunt resistor produces a voltage across it proportional to plating current. This voltage is then converted into frequency term which is proportional to voltage by using a voltage/ frequency converter.
There is a frequency divider circuit which is used for finding ampere hour (Ah) corresponding to summation current. Then this is fed to a LED, which indicates the reference of the measured voltage. Ampere hour meter can be designed in different methods. Here we are using the ADC (Analog to Digital converter) and microprocessor or microcontroller. An ampere hour meter is connected in series with battery. Ammeter is used to find the electric current in the circuit, which also connected in series with circuit. Resistance of ammeter must be very low. ADC is used in this circuit for measuring the voltage across the resistor. Analog to digital converter converts an analog data to its digital form. This conversion is done according to the reference voltage.
Battery voltage in caravan is generally 12V or 24V. The voltage is obtained by ADC in the circuit. ADC gives digital data. This data is then handled by microcontroller unit and calculates the ampere hour. This calculated value is sent to the display section. LCD is used for announcing the ampere hour value visually.
Components for the ampere hour meter
The main components used for the design of an ampere hour meter are ADC, microcontroller, LCD (Liquid Crystal Display) unit. ADC is available in various types. An ADC can be made by combining comparator circuits. Integrated circuits are also available. ADC acts as a volt meter in the direct current (DC) circuit. ADC0800 is an example for an 8 bit monolithic analog to digital converter using p channel ion implanted MOS technology. Microprocessor or microcontroller is a main unit in the ampere hour meter. PIC or 8085 can be used as microprocessor. 8085 is an 8 bit microprocessor. 8051 is the example for 8 bit microcontroller. A shunt resistor is connected parallel to circuit to find the parameters of that circuit. “A shunt is simply a resistor of very low value (frequently less than one ohm) that is used to help measure current”.
A shunt resistor can be made of a copper wire. The resistance of wire should be designed depending on sensitivity of instrument. Shunt resistor provides low resistance path and hence shrink the sensitivity of instrument to a known quantity. Shunt resistance should be designed according to the amount of current in the circuit. Here the current varies with 0 to 5 amps. Shunt can be calibrated and hence yield a more accurate one.
Principle of Ampere hour meter
“Ampere hour meter is an integrating meter similar to the what hour meter used to measure electricity usage in home. Typical ampere hour meters are digital indicators similar to odometer in automobile. It is direct current meter that will register in either direction depending on direction of current flow. Similar to ammeter, Ampere hour meter is connected in series.” (Ampere Hour Meter About).
The main principle in the design of an ampere hour meter is the dependency of the input or source impedance of battery with the charge content in the battery. Consider a caravan with a battery as a power supply. The whole electric devices are connected parallel to the battery as load. For finding ampere hour, we have to first find out the electric current reading. Ammeter for a caravan can measure up to 10 ampere. This is because of use of high current draining appliances in the caravan. A voltage is produced in the shunt resistor proportional to plating current. Below figure shows a phase detector. Here the reference switching signal and input signal are in phase.
Output of a phase detector gives a Direct Current proportional to Alternating Current signal level. Amplification of signal is generally done for the proper detection of signal. But here battery provides a high rate of current and hence the amplification of input signal is not required. A phase detector circuit provides a DC voltage proportional to amplitude of input wave. This is used to monitor changes in the input impedance of power supply battery.
By this way we can find out the state of battery. It gives the state of charge of that battery. The battery charge state is related to the source impedance of battery. Phase detector uses a low pass filter which cuts the signal having frequency more than its cut-off frequency. So at the output of low pass filter we get a direct current (DC) level. At discharged condition of the battery, it shows high impedance. And the impedance decreases according to increase in charge in battery.
The ADC is connected to the circuit to find the voltage. This converts the continuous form of signal from the ammeter to the digital form. Then this digital data is passed to a microcontroller. Microcontroller contains highly complicated circuits. But it can be used very easily. A micro-controller has different ports for digital communication with surroundings. Serial and parallel ports are in a micro controller to pass the information or data to or from the micro-controller.
If the analog to digital converter has the parallel output, then we can connect this output to the parallel port of microcontroller. A microcontroller contains many different units inside it. Arithmetic and logic unit, control unit and memory etc. are some examples of the internal peripherals in a microcontroller. So we can say that a microcontroller is a complex device according to the internal parts. But we can program a microcontroller simply by the assembly language. This program can be done in simulating software, so that we can find the errors and we can visualise working of our code. This reduces the cost of design as well as the time of design.
Generally simulator is in assembly language, but we can change this code into its hexadecimal format. A microcontroller comprises an internal memory to store this program. There is other kind of memory available in the microcontroller for doing calculations in the controller. This memory is volatile, and is called registers. This is a very fast memory and is compatible with other internal unit in the microcontroller. The program is stored to read only memory (ROM) of microcontroller. This can be erased by electrical method. So this kind of read only memory is called electrically erasable read only memory (EEROM).
Microcontroller is more reliable in the sense that it can be programmed, without changing the connections in the circuit. That means when we need to improve our device with present output, we may not change the connections just make changes in the program. The analog to digital converter acts as voltmeter, which is commonly used for measuring voltage in the circuit. This digital converter then samples the input level.
The sampled output is fed to micro controller unit and hence can perform various operations on this data. These data are then manipulated by the controller. The summation is done in the controller to get the ampere hour of the battery. There is a liquid crystal display to show the result visually. Liquid crystal display (LCD) should be firstly interfaced with the controller. The interfacing means connecting and programming the device LCD to controller. After interfacing the LCD to controller, microcontroller sends the result to display. The block diagram for the ampere hour meter is shown below.
Project analysis
Electric current from the battery is by passed through shunt resistor to get a medium sensitivity for the measurement of electric current. An ADC is connected across or parallel to shunt resistor. This ADC is used as voltmeter and also converts analog voltage data to digital. Combination of ADC and shunt resistor will act as an ammeter. So this unit gives current reading in digital form. Then this data supplied to any one port of microcontroller.
The microcontroller is programmed to get the Ah value. The programme should be done with that input of microcontroller is a electric current amount. Microcontroller gets power supply from the same battery. But the battery provides 12V and micro controller needs 5V. So we use an IC (integrating circuit) 7805. The value calculated by the microcontroller is then sent to LCD. The measurement is accurate because the data is handled digitally.
If the battery is full of charge then source impedance of battery is low. And if battery is reached to run out then the impedance is at its peak. So we can say that the impedance of battery is inversely proportional to the content of charge in the battery. Battery charge also depends on the load in the caravan. That means when we are using all the major current drawing devices at a time then ampere hour meter shows very low value. So reduce the load as the Ah rating decreases, and go to recharge. The ampere hour meter must be checked every evening so that we can avoid the complete discharge of battery at night. The use of microcontroller and analog to digital converter reduces error that may happen in the decision of ampere hour. Precision also increased due to the digital use ampere hour meter.
Cost of project
An ampere hour meter is designed and developed for finding status of battery in the caravan. The peripherals used in the circuit are easily available and cheap. Only microcontroller and ADC and LCD are the some complex unit in this circuit. These all can be designed and manufactured with an amount below 60$.
Conclusion
Ampere meter is fitted serial to the battery, for which the charge is measured. Lack of precision and reliability in the ampere hour meter makes its usage low. So nowadays the usage of ampere hour meters is reduced. There are other precise electronic devices which are available for finding the specific value of charge in the battery. The objective of ampere hour meter is, to warn about life of battery. The design of an ampere hour meter using a microcontroller is very reliable.
But in real case, the precision of an ampere hour meter is very low. It just shows the presence of flow of electric current in the battery. Invention of microprocessor is a milestone in the history of electronics. But microcontrollers with advanced features are available. So the design of highly precise, reliable, and error-free ampere hour meter is very simple. The digital world is growing with incredible speed. Electronic field provides more convenient and simple product for our life.
Works Cited
Ampere Hour Meter About. Engineers Edge: Solution by Design. 2009. Web.
Direct Current Meters. 2009. Web.
Shaw, Ian. Block Diagram of a Phase Detector. FAS: Phase Detector Utilising TRAC. 1998. Web.
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