Aircraft Electrical Systems
Objectives
Explain the difference between Primary & Secondary cells
(b) Compare Lead Acid & Nickel Cadmium batteries
(c) Check a battery for Specific Gravity
http://www.thisisnorthdevon.co.uk/news/Missing-battery-blamedplane-crash/article-1851031-detail/article.html
(a)
(d)
Aircraft Electrical Systems

The function of the aircraft electrical system
is to generate, regulate and distribute
electrical power throughout the aircraft
 New-generation aircraft rely heavily on
electrical power because of the wide use of
electronic flight instrument systems
Electrical Power Uses

Aircraft electrical power is used to operate:
– Aircraft Flight Instruments
– Essential Systems
– Passenger Services
Electrical Power Uses (cont.)

Essential power is power that the aircraft
needs to be able to continue safe operation
 Passenger services power is the power that
used for:
– Cabin lighting
– Operation of entertainment systems
– Preparation of food
Power Used

Aircraft electrical components operate on
many different voltages both AC and DC
 However, most of the systems use:
– 115 VAC @ 400 Hz
– 28 VDC

26 VAC is also used in some aircraft for
lighting
Aircraft Batteries
The aircraft’s battery (nickel cadmium/Lead
Acid) battery is final source of backup
power
 The battery provides 28 VDC
 It is also possible to change the 28 VDC
into 115 VAC 400Hz with the use of a static
inverter
 When using the battery, power usage is
limited by the short life of the battery

Primary & Secondary Cells

A primary cell is any kind of battery in which the
electrochemical reaction is not reversible. A
common example of a primary cell is the
disposable battery. Unlike a secondary cell, the
reaction cannot be reversed by running a current
into the cell; the chemical reactants cannot be
restored to their initial position and capacity.
Primary batteries use up the materials in one or
both of their electrodes.
Primary Cell
Secondary Cells

A rechargeable battery or storage battery is a group of
one or more electrochemical cells. They are known as
secondary cells because their electrochemical reactions
are electrically reversible. Rechargeable batteries come in
many different shapes and sizes, ranging anything from a
button cell to megawatt systems connected to stabilize an
electrical distribution network.

Several different combinations of chemicals are
commonly used, including: lead-acid, nickel
cadmium (Ni - Cad), nickel metal hydride
(NiMH), lithium ion (Li-ion), and lithium ion
polymer (Li-ion polymer).
Lead Acid Battery



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Positive material - Lead Peroxide
Negative – Spongy Lead
Container - glass or hard plastic
Filler cap – replenishment of distilled water
Vent hole – allows escape of hydrogen gas
Electrolyte – sulphuric acid
Specific Gravity – SG 1.270
Spillage - sodium bicarbonate
Alkaline Battery
(Nickel Cadmium, Ni-Cad)


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Mostly used on large aircraft
Positive Plate - nickel oxide
Negative plate - cadmium
Specific Gravity – SG 1.240 – 1.300
Electrolyte - potassium hydroxide - does not
change very slight for fully charged and fully
discharged
Spillage - Boric Acid
Worksheet 1 Task 1, 2 & 3
State of charge
SG – Specific Gravity

Can be determined by measuring the
strength of the electrolyte solution
 Hydrometer – Fully charged – SG 1.270

Discharged - SG 1.170
 Battery charging - Controlled
 To quickly can cause ‘gassing’
Battery performance

Low temperatures - rate of discharge is
decreased due to higher internal resistance
 Warm temperature – rate of discharge is
increased
 As lead acid battery discharges the SG of
the electrolyte reduces
 Care must be taken during winter operations
Lead Acid

Advantages

Less expensive
 Retains charge (during storage)
 Simple charger
 Cell voltage 1.5V (Ni Cad 1.2V)


Disadvantages
Only supplies half its rated capacity
 Size
 Electrolyte (Sulphuric acid)
Ni – Cad

Advantages

Smaller
 Lighter
 Capacity not affected by high discharge currents
 Provides nearly all rated capacity

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Disadvantages
Higher cost
Negative temp coefficient
Complicated charging
Loss of charge (1% per day)
Lower cell voltage 1.2
Maintenance
http://www.thisisnorthdevon.co.uk/news/Missing-battery-blamed-planecrash/article-1851031-detail/article.html
http://www.reuk.co.uk/Battery-Specific-Gravity.htm
Battery Specific Gravity


It is possible to test the state of charge of a lead acid battery (or
battery bank) with a multimeter simply by measuring the voltage
however this is not always an accurate indicator. More accuracy is
provided by measuring the specific gravity of the electrolyte in the
battery (assuming that the battery is not of the sealed type).
Measuring the Specific Gravity of a Battery
Specific gravity is measured with a hydrometer. Simply draw the
battery acid into the hydrometer so that the float is not touching the
sides, top or bottom of the barrel. Take the reading with your eyes
level with the surface of the drawn up liquid, and then subtract 0.004
for each 5°C above or below 25°C. Do this for each of the cells of the
battery.
Gill G25 Aircraft Equipment
Battery

Gill G-25 battery for GA single and
twin aircraft use. 12 volt

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Weight: 21 lbs.

Electrolyte: 2 Quarts./1.285 S.G.

http://www.transair.co.uk/pp+GillAircraft-Batteries-24V-DryCharged+3049
Battery Specific
Gravity
Below is a table of the specific gravity measurements to be expected at 75°F.
State of Charge
Specific Gravity
100% charged
1.265
75% charged
1.239
50% charged
1.200
25% charged
1.170
Fully discharged
1.110
Battery Testing

Measure terminal voltage

Check charge (using hydrometer)

Worksheet 1 Task 4