topic 5 dc source of electricity
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5. DC Sources of Electricity A battery is a device that stores chemical
energy and makes it available in an electrical
form. It composed of two or more cell that convert
chemical energy into electrical energy.
+ -+ -+ -+ -
Battery 12VSingle Cell
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5. DC Sources of Electricity (contd) The cell itself consists of two different conducting
materials, called electrodes, which are immersed
in an electrolyte.
A simple cell consist of; two electrodes (terminal)
positive and negative.
An electrolyte
a liquid which is
a conductor, either
acid or alkaline.
Electrode
Electrolyte
+ -
Loads
Electrode
Electrolyte
+ -
Loads
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
The chemical reaction between the
electrodes and the electrolyte results in a
separation of electric charges, in the form
of ions and free electrons.
Then the two electrodes have a difference
of potential that provides voltage outputfrom the cell.
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
The function of a battery is to provide a source
of steady DC voltage of fixed polarity.
The purpose of battery on the aircraft is for
starting small engine and for emergency use.
to supply power only to the essential load e.g. radio
communication in order for the pilot to communicate withthe ground tower, gyros (e.g. HSI, AH), escape light, etc.
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5. DC Sources of Electricity (contd) The cells of a battery are classified as
- Primary Cell.
- Secondary Cell.
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd) Primary cells (battery).
- To provide small / low power output e.g. the dry cell also known as
Leclanche Cell.
- They are called dry cell because the electrolyte is a paste rather than aliquid.
- They are not rechargeable. The cell will not function once their
chemicals are used up and the cells have to be thrown away.
- Examples are zinc-carbon cell, alkaline manganese cell and silver
oxide cell.
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
Zinc-Carbon cells
- The construction of Zinc-Carbon cell consist of;
+ ve plate (anode) used carbon rod.
ve plate (cathode) used zinc casing.
Electrolyte is ammonium chloride withmanganese dioxide in a paste form.
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
- The output of the dry cell is 1.5 V per cell regardless of their size.
- The size will determine the amount of current it will supply.
A manganese dioxide use as a depolarizer to ensure a chemical
reaction involving the constituents of both terminals; this reactioncauses a current to flow through a conductor that connects the
positive and negative terminals.
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd) Alkaline Manganese cells
- The purpose is to provide longer life
compare Zinc-Carbon cell.
- The construction of Alkaline Manganese
cell consist of;
+ ve plate used carbon rod
ve plate used manganese dioxide lined steel casing.
Electrolyte is potassium hydroxide and has a lower resistance than
ammonium chloride thus provide more load current.
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd) Mercury cells
- These cells have nearly constant voltage at low discharge currents
making them ideal for hearing aids, calculators, photographic
cameras and electronic watches.
- It has a high capacity in small size and very long shelf life, up to 10
years.
- Due to the content of mercury, and the resulting environmental
concerns, the sale of mercury batteries is banned in many countries.
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5. DC Sources of Electricity (contd)
- The construction of Mercury cell consist of;
+ ve plate used
mercury oxide.
ve plate used zinc
casing.
Electrolyte is potassium
hydroxide.
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
Lead Acid Cells
The purpose is to provide power supply to aircraft during emergency,
starting, etc (if the usages for aircraft).
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
The separator prevents +ve and ve plate from touching but allow
sulphuric acid to pass through (porous material).
6 or 12 cells, each cell rated 2.1 volts.
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2 volt x 6 cells are connected in series will give 12 volts
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5. DC Sources of Electricity (contd) Indication of fully charged condition.
Terminal voltage - 2.7 V/cell on charged
- 2.1 V/cell off charged- 1.8 V/cell discharged
or terminal voltage between 30.0 to 32.4 V (on charge), if
terminal voltage below 28.5 V, it should be withdrawn fromservice.
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Malaysian Institute of Aviation Technology
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Voltage Test
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
Specific gravity (S.G.) of electrolyte
- 1.275 and 1.300 at 80F or 27C reference).
- use hydrometer to check the S.G.
Gassing freely.
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
Capacity
- ability to produce a given amount of current for specific time e.g. 1A
for 1 Hour which is known as 1 Ampere/Hour (A/H) .- more than 80% efficiency fit for aircraft use
Efficiency (%) = Output capacity X 100
Rated capacity
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
Factor affecting capacity of a battery;
number of plate / cell
plate area
strength of electrolyte
temperature
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Malaysian Institute of Aviation Technology
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Note : Charging current is 4 A for
10 Hrs
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Malaysian Institute of Aviation Technology
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Note :
Total battery voltage must
not exceed charger voltage
Charging current is 2A for 10
Hrs
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Malaysian Institute of Aviation Technology
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5. DC Sources of Electricity (contd)
Constant voltage charging
-when battery connected in an aircraft, car, or aother vehicles.
- generating voltage system slightly higher than
battery voltage.
- the amount of current flow into a battery being
charge is determined by the state charge of
battery.- no need monitoring device
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Note :
When the battery is slightly weak, the charging current will increase. The
charging current decrease as the battery towards fully charged condition. Thus
no need monitoring device.
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5. DC Sources of Electricity (contd)
Capacity Test
- carried out after initial charge.
- 3 monthly.
- at any time if battery capacity in doubt.
- acceptable capacity for use on aircraft is 80% and above.
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5. DC Sources of Electricity (contd)
stored in clean, dry, well ventilated area. always stored in fully charged condition this is to
prevent sulphation take place.
give freshening charge every 2 to 4 weeks.
Storage
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5. DC Sources of Electricity (contd) Nickel Cadmium Cells
The purpose is to provide high current or capacity output within ashort period main used for turbine engine starting.
An improvement of the Lead Acid Battery.
The construction of NiCad Cell consist of;
- + ve plate used powdered nickel or sintered / nickel hydroxide.
- ve plate used cadmium hydroxide.
- The separator is using nylon and cellophane to keep plate apart
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Nickel Cadmium Cell
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5. DC Sources of Electricity (contd)
- The electrolyte is using potassium hydroxide (30%) and
distilled water (70%).
S.G. between 1.24 to 1.30 at roomtemperature.
No S.G. change during discharging or
recharging and this electrolyte act as a
conductor.
- The casing is made of polystyrene or nylon cell case and
sealed.
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5. DC Sources of Electricity (contd)
- For each cell the voltage is 1.55 to
1.80 V for open circuit voltage.
-For 9 to 10 cells are used for 12 Vaircraft system.
- For 19 to 20 cells are used for 24 V
aircraft system
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Adjust the electrolyte level
immediately after fully charge
condition with distilled water
Remove vent plug using vent
wrench to top up distilled water
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5. DC Sources of Electricity (contd)
Internal short circuit between the cells are indicated that the battery
wont hold charge. Check for leakage test. Check all connection any sign of corrosion or burning. The
connectors must be torque tight.
Adjust the electrolyte level immediately after fully charge condition
with distilled water only.
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5. DC Sources of Electricity (contd)Leakage Test
- check between terminal and steel casing using ammeter.
- leakage current must not more than 100mA, if more clean the steel casing.
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5. DC Sources of Electricity (contd)
- If cell unbalance still occurs, discharge cell voltage to
approximately 0.2 V/cell, then use shorting strap across each cell
for 3 to 8 hours to completely discharged them. This process
known as Deep Cycling (equalization of cells).
- Recharge with constant current charger. Each cell should have a
voltage of 1.55 to 1.80 V at temperature between 21C (70F) and
26C (80F).
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5. DC Sources of Electricity (contd)
Neutralizing agent or cleaning agent for
alkaline spillage is dilute boric acid,
vinegar, lemon juice.
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5. DC Sources of Electricity (contd)
Specific Gravity
Specific gravity is the ratio of the weight of a certain amount of a
given substance compared to the weight of the same amount of
pure water.
The specific gravity of pure water is 1.0.
Any substance that floats has a specific gravity less than 1.0. Any substance that sinks has a specific gravity greater than 1.0.
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5. DC Sources of Electricity (contd)
To measure a battery's specific gravity, use aninstrument called a HYDROMETER. Is a glass
syringe with a float inside it.
The float is a hollow glass tube sealed at both
ends and weighted at the bottom end, with a scale
calibrated in specific gravity marked on its side.
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5. DC Sources of Electricity (contd)
To test an electrolyte:
-Draw the electrolyte it into the hydrometer using the suctionbulb.
- Draw enough electrolytes into the hydrometer to make the
float rise.
- DO NOT draw in so many electrolytes that the float rises into
the suction bulb
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5. DC Sources of Electricity (contd)
The float will rise to a point determined by the specific gravity of
the electrolyte.
- If the electrolyte contains a large amount of active ingredient,
its specific gravity will be relatively high.
- The float will rise higher than it would if the electrolyte
contained only a small amount of active ingredient
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5. DC Sources of Electricity (contd)
NOTE:Hydrometers should be flushed with fresh water after each
use to prevent inaccurate readings. Storage battery
hydrometers must not be used for any other purpose.
To read the hydrometer, hold it in a vertical position and read the
scale at the point that surface of the electrolyte touches the float.
Refer to the manufacturer's technical manual to determinewhether or not the battery's specific gravity is within
specifications
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5. DC Sources of Electricity (contd)
When cells connected in series, there is an
increase in the voltage and current (ampere)
remains constant.
+ - + - + -
1.5V
0.5A
1.5V
0.5A
1.5V
0.5A
+ -+ - + -+ - + -+ -
1.5V
0.5A
1.5V
0.5A
1.5V
0.5A
Voltage = 1.5 V x 3= 4.5 V
Current = 0.5 A
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5. DC Sources of Electricity (contd)
Internal Resistance (IR).
The resistance present inside a battery while connected to aload.
Restricts the movement of current inside of any power source,
including battery.
As a cell is discharged, its internal resistance increases.
Therefore, its output voltage decreases for a given value of load
current.
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5. DC Sources of Electricity (contd)
The voltage drop across resistor in series circuit will slightly
less than the power source, hence due to IR or of the power
source.
In some applications, the changes in cell terminal voltage are
so small that they make no practical difference.
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5. DC Sources of Electricity (contd)
Thermocouple
Thermoelectric temperature sensor which consists of two
dissimilar metallic wires, coupled at the probe tip (measurement
junction) and extended to the reference (known temperature)junction.
The temperature difference between the probe tip and the
reference junction is detected by measuring the change in voltage
(electromotive force, EMF) at the reference junction.
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5. DC Sources of Electricity (contd)
The absolute temperature reading can then be obtained by
combining the information of the known reference temperature
and the difference of temperature between probe tip and the
reference.
HOT JUNCTION
COLD JUNCTION
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5. DC Sources of Electricity (contd)
Metal pairs most commonly used are Chromel / Alumel and
Iron / Constantan.
This principal is used for aircraft Cylinder Head Temperature
(CHT) and Exhaust Gas Temperature (EGT) Instrumentation.
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END OF TOPIC 5
DC SOURCES OF ELECTRICITY