Aircraft Electrical Systems Objectives (a) Explain the difference between Primary & Secondary cells (b) Compare Lead Acid & Nickel Cadmium batteries (c) Check a battery for Specific Gravity (d) plane-crash/article detail/article.html plane-crash/article detail/article.html

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: – 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 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 Spillage - sodium bicarbonate

Alkaline Battery (Nickel Cadmium, Ni-Cad) Mostly used on large aircraft Positive Plate - nickel oxide Negative plate - cadmium Specific Gravity – SG – 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 Discharged - SG 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 Disadvantages Higher cost Negative temp coefficient Complicated charging Loss of charge (1% per day) Lower cell voltage 1.2

Maintenance crash/article detail/article.html

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 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 Weight: 21 lbs. Electrolyte: 2 Quarts./1.285 S.G. Aircraft-Batteries-24V-Dry- Charged Aircraft-Batteries-24V-Dry- Charged+3049

Battery Specific Gravity Battery Specific Gravity State of ChargeSpecific Gravity 100% charged % charged % charged % charged1.170 Fully discharged1.110 Below is a table of the specific gravity measurements to be expected at 75°F.

Battery Testing Measure terminal voltage Check charge (using hydrometer) Worksheet 1 Task 4