Aircraft Electrical Power
Check the battery's state of charge by using a hydrometer. A hydrometer is
a device for determining the specific gravity of the electrolyte. Electrolyte is drawn into the hydrometer, and the specific gravity is indicated by the number on the scale that is aligned with the floating ball.
Each cell of a lead-acid battery is constructed of lead plates between a solution of sulfuric acid. The positive plate consists of
a lead alloy grid filled with lead dioxide. The negative plate, which stores the electrons, consists of porous lead. The sulfuric acid is mixed with water and changes in chemical composition depending on the charge of the cell.
The battery shop safety board contains items that should only be used in the event of an emergency. A deadman's stick is
a non-conductive hook that is used to pull or push a person away from a live circuit in the event of shock. Never directly grab a person being shocked or you will just add to the problem
The lead-acid battery is acid-based, and the Ni-Cd battery is
alkaline-based. The fumes and electrolytes of these batteries are diametrically opposed and will neutralize each other's contents if located together, most likely causing severe damage to the batteries.
Ni-Cd batteries consist of the case, external connections, internal cell connections,
and individual cells. The battery case holds the cells and is constructed of steel. The steel is protected by a rubber coating.
After the discharge cycle, if any cell remains above 0.5 volts, use a deep cycle load or dummy load to discharge the cell. Dummy loads
are energy-absorbing devices that draw current out of the cell. The dummy load can be anything from a locally fabricated light bulb with alligator clips to a sophisticated battery cover with load terminals.
New lead-acid batteries are often shipped to the battery shop dry. The electrolyte has not been mixed and the sulfuric acid is in a separate container. These batteries require the electrolyte to
be mixed prior to adding it to the battery. Again, read the instructions carefully prior to initializing a new lead-acid battery.
When the pulsed voltage is above battery voltage, current is flowing to the battery and the battery charges. When the pulsed voltage is
below the battery voltage, the negative current flow discharges the battery. The discharging action slightly depolarizes the plates with each cycle and keeps the battery cooler during charging.
Temperature affects the charging capacity of batteries. The most efficient temperature for charging batteries is
between 60 °F (15 °C) and 90 °F (32 °C). Charging should never be attempted below 20 °F (-7 °C) or above 120 °F (50 °C).
Flashlights have alkaline batteries; emergency locator beacons employ lithium, mercury, or silver-zinc batteries; and aircraft power is usually supplied by
by lead-acid or nickel-cadmium batteries.
Today's batteries are simply improved voltaic piles. Instead of piles of metal pieces, batteries are made up of
cells. Cells are single combinations of pile material consisting of two different metals separated by an electrolytic paste or fluid.
If the battery's specific gravity is less than 1.275, the battery should be
charged to full capacity before re-installing it in the aircraft.
A battery is a device that converts
chemical energy contained within its active materials into electrical energy by means of an electrochemical reaction.
Neutralizers are
chemicals that have a pH level which is opposite of the battery's electrolyte. Each shop should have neutralizer on hand in the event of a spill or splash. The neutralizer for a lead-acid battery shop is sodium bicarbonate (baking soda) and water.
If the battery has electrolyte overflow inside the case, disassemble the battery and
clean it. A full discharge is required before disassembly.
A battery rated at 40 Ah can provide 1 amp for 40 hours, 40 amps for 1 hour, or any
combination of amps per hour resulting in 40. If 5 amps are drawn by the load, the battery can provide current for 8 hours.
Specific gravity readings will vary by temperature, so
corrections must be made if the temperature of the electrolyte is either ≥ 90 °F (32.2 °C ) or ≤ 70 °F (21.1 °C).
When the battery is removed, inspect the battery compartment for signs of
corrosion, electrolyte spills, and indications of overheating. Ensure the drain tubes are free of debris and unobstructed.
In a battery shop, it is more likely that the constant
current method will be used because several batteries can be linked in series and charged simultaneously. The batteries can be in different states of discharge as long as they are all rated (Ah) the same.
When heat increases, the internal resistance of the battery
decreases. As the resistance decreases and the voltage remains the same, current increases. The increase in current causes more heat, which further decreases resistance. Y
Because the Ni-Cd battery maintains a constant voltage for most of its charge life, you cannot
determine the battery's state of charge by taking a voltage reading or testing the electrolyte. To determine the state of charge of a Ni-Cd, you must discharge the battery at a known current rate and monitor the time to full discharge.
Once the battery is completely discharged and equalized,
disassemble, inspect, and clean the battery cells, dividers, case, and hardware. Use fresh water and clean cloths to rinse and dry each component.
With the constant voltage method, current will be at its maximum when the battery is in a
discharged state. As the battery's voltage increases, the charging current will decrease. As the battery reaches full charge, the charging current will be reduced to a trickle.
If a battery needs to be charged, two methods can be used
either the constant voltage method or the constant current method.
The battery is primarily used to provide
emergency power in the unlikely event that all other power sources on the aircraft fail. However, it is also used as a short-term source of communications system power and to start engines or auxiliary power units.
Fumes from a Ni-Cd battery released during charging are potentially
explosive and must be vented to the outside; however, unlike lead-acid battery fumes, they do not normally cause corrosion in the battery compartment area of an aircraft.
The gases generated by batteries during the charging cycle are
explosive. A proper mechanical ventilation system must be installed in the battery shop. The ventilation system should provide at least 3 to 4 complete air changes per hour to prevent the buildup of explosive gas.
Inside a Ni-Cd battery, each cell is contained in its own individual case. A vent cap on top of the cell allows
gases to escape when internal pressure reaches 6 psi. At less than 6 psi, the cap remains airtight to prevent accidental electrolyte leakage. The cap can be removed for servicing the cell.
The cells can be connected in series to provide an increase in voltage, or
in parallel to provide an increase in current.
Connecting cells in series is one way to increase voltage , different types of metals and electrolyte combinations will also
increase voltage.
Thermal runaway can be prevented by monitoring the battery's internal heat during the charging cycle and discontinuing the charging cycle if the temperature
increases beyond the safe zone. A heat sensor is built into the battery case to monitor the internal temperature of the case.
Aircraft lead-acid batteries are constructed so that the cells are sealed in place as one
integral unit within a plastic or metal case. A servicing cap is provided on top of each cell.
Remember Ohm's Law: V = I x R. In the case of our aircraft battery, V is the battery voltage, I is the current the battery produces during a charging cycle, and R is the
internal battery resistance (plus the circuit resistance). The chemical reaction in the battery cells produces heat during charging.
Electrochemistry
is a branch of chemistry that, in part, studies the science of batteries. This science explains a battery's ability to store and produce chemical energy.
The first step in servicing a battery is to ensure the top of the battery
is clean. This prevents contaminants from entering the cells during cap removal. Use a nylon brush to loosen dirt and other debris, and then flush the top of the battery with fresh water.
when a Ni-Cd reaches its discharge point
it very rapidly loses its voltage.
A fully charged lead-acid battery will not freeze in moderately cold temperatures because
its electrolyte is mostly acid. However, as the battery discharges, it is more susceptible to freezing as the percentage of the electrolyte solution becomes less acidic.
a battery is considered discharged when it reaches a voltage that is equal to
its number of cells. The 24-volt, 12-cell lead-acid battery would be considered discharged at 12 volts.
A battery is considered discharged when
its terminal voltage equals 1.0 volt per cell times the number of cells in the battery.For example, a typical 24-volt, 20-cell battery would be considered discharged when the terminal voltage falls to 20 volts.
During constant current charging, the voltage is increased until a constant current level
l is reached. As the battery charges, the voltage is regulated to maintain the current at the constant rate.
When removing the battery for servicing, always remove the
lead to the negative (-) battery terminal first. This will eliminate a path to ground, preventing your tools from shorting the battery and you from getting an electrical shock while removing the positive (+) lead from the battery.
Check all of the cell connections for loose, damaged, or burned hardware. Pitted or burned hardware or connection links may indicate a
loose connection. Replace damaged hardware and tighten all connections.
Because Ni-Cd batteries are made up of individual cells, one bad cell will not
necessitate discarding the entire battery. Simply discharge the battery, replace the bad cell, and fully charge the battery.
The operation of a Ni-Cd battery during charging and use is similar to that of the lead-acid battery. During normal use, the electrons flow from the
negative plates through the circuit to the positive plates; during charging, electrons are forced back to the negative plates.
During battery charging, electrons are forced from the positive plates, through the electrolyte, and back to the
negative plates where they are stored. When the negative plates are saturated with electrons, the battery is fully charged and the electrolyte is most acidic.
Replace any cell identified as bad during the capacity check and reassemble the battery in reverse order of disassembly. It is a wise choice to use
new hardware when reconnecting the cell links. Ensure the battery is reassembled correctly before charging is attempted.
The positive plate of a Ni-Cd cell is made up of
nickel hydroxide.The negative plate is made up of cadmium hydroxide. The electrolyte in Ni-Cd batteries is a solution of potassium hydroxide (KOH) and distilled water.
Large multi-engine aircraft and most jet aircraft use
nickel-cadmium (Ni-Cd) batteries as their source of initial DC power. Ni-Cd batteries are lighter than lead-acid batteries; however, they require more maintenance and are more expensive.
The plates are separated by
nylon strips that allow the electrolyte to reach the entire plate material.
The varying current charging method, also called the
pulsed charge method, is used when a discharged battery needs to be fully recharged quickly. This method applies a pulsed DC voltage that surges for one second above the battery voltage for charging and drops slightly below the battery voltage for one second for cooling.
Deep cycling should be scheduled on a regular
r basis to ensure maximum battery life.
The aircraft battery is a portable and
rechargeable source of DC power to operate aircraft electrical and electronic circuits.
Servicing a Ni-Cd battery is a bit more complex than servicing a lead-acid battery. Because the state of charge of a Ni-Cd battery cannot be determined by checking the specific gravity of the electrolyte, the battery must be
removed from the aircraft and discharged under specified conditions to determine its charge.
A Ni-Cd battery is charged by increasing the voltage to a higher level than that of the battery. This will
reverse the current inside the battery and cause the battery to return to its original charged state. There are two methods of charging a Ni-Cd battery: constant current and varying current.
One disadvantage of a Ni-Cd battery is its tendency of thermal
runaway during the charging cycle, a possibility that increases after a long start or battery drain followed by a strong voltage charge. Thermal runaway is potentially hazardous to aircrew and maintenance technicians.
The neutralizer for the Ni-Cd battery shop is
s a 3% solution of acetic acid, vinegar, or lemon juice.
The charging potential is much greater than the discharge potential, creating a positive charging sum total during each cycle. The varying current method charges
s the battery to maximum capacity in approximately one hour.
A space is provided under the plates of the lead-acid battery to allow for the collection of
sediment. Collecting the sediment in this space prevents the battery from shorting due to any sediment particles touching the plates.
Inside the battery case are the individual cells and their connection jumpers. The jumpers are nickel plated and connect the cells in
series. A typical aircraft Ni-Cd battery has nineteen (19) 1.25-volt cells totaling 23.75 volts when fully charged.
Just as jewelry and belt buckles pose a shock and burn hazard, metal tools are also potential hazards in the battery shop. Use insulated tools when
servicing batteries and nylon brushes when cleaning batteries. Never use a wire brush to clean a battery. The wires in the brush can short the terminals and cause sparks.
After neutralizing any type of battery spill, always clean the area with
soap and water.
Clean the battery compartment with a solution of
sodium bicarbonate (baking soda) and water to neutralize any spilled electrolyte. Rinse with fresh water and dry the compartment.
When mixing the electrolyte for a lead-acid battery, ALWAYS pour
the acid into the water. Pour it slowly as you mix the electrolyte. NEVER pour water into the acid. Possible splatter may cause serious burns.
Check to ensure the battery fluid is at the proper level by removing the caps and visually checking the fluid level. The battery is properly serviced when
the fluid level is up to the indicator in the cell or just above the plates.
During normal battery operation, electrons flow from the electron-saturated negative plates through the aircraft circuit and back to
the positive plates.
The charge of a lead-acid battery can be determined by measuring the
the specific gravity of the electrolyte in each cell. The electrolyte in a fully charged cell will have a specific gravity of approximately 1.285 or greater, while the electrolyte in a fully discharged cell will have a specific gravity of 1.150 or less.
If the battery fails three capacity checks or is scheduled for a deep cycling, discharge the battery until each cell drops below 0.5 volts, and then place a shorting clip across
their terminals. Shorting out the low voltage cells will prevent reversal of the cell polarity as the battery continues to discharge.
Whether you charge the battery using the constant current method or the varying current method, monitor the case temperature using a shop
thermometer or the battery's overheat warning system during charging to avoid thermal runaway.
Lead-acid batteries are most common in light, general aviation aircraft because
they are relatively inexpensive and are easy to maintain.
The external connections on the Ni-Cd battery case include the
vent hose, battery heat sensor connection, and quick disconnect aircraft electrical connection.
The aircraft uses the constant
voltage method to maintain the battery's charge; it simply provides a voltage that is higher than the unloaded battery voltage. This forces the positive plates of the battery to give up electrons to the negative plate.
Ni-Cd batteries maintain a nearly constant terminal
voltage over a wide range of charge levels. Unlike the constant discharge curve of a lead-acid battery, the Ni-Cd remains near its fully charged state for 90%-95% of its charge life.
Two metals placed in a conductive and reactive chemical solution will produce a difference in potential, called
voltage. If you own a boat and are constantly battling corrosion on all the metal parts, you know this chemical reaction as electrolysis.
When a Ni-Cd battery is charging while installed on an aircraft, the sensor illuminates a
warning light in the cockpit if the battery's internal case temperature exceeds the safe zone.
The first step in servicing a Ni-Cd battery is to inspect and clean the battery. Use a nylon brush to loosen any dirt or deposits from the cell tops, and then
wipe the case and cover clean. Be cautious while cleaning a charged battery; avoid shorting out the terminals or creating sparks.
SAFETY EQUIPMENT
•Eye Wash Station•Protective Clothing (Face Shield, Gloves, Rubber Apron)•Neutralizers for Battery Electrolyte•Emergency Cut-Off Switch for Charging Units•Battery Servicing Chart•Safety Board (Deadman's Stick, CPR Instructions, First Aid Kit)•Fire Extinguisher
The constant current method is used:
•when a quick topping charge is required for a capacity check,•after the battery has been in storage for an extended period, or•as the initial charge after the completion of a deep cycle.
Generally, the capacity check uses the battery charger/discharger to discharge the battery at a specified rate for a specified time. If all of the cells remain above
0.95 volts for the specified time, the battery has passed the capacity check and can be recharged.
fully charged nickel-cadmium cells produce about
1.25 volts each
Removing a Ni-Cd Battery
1.Cut break-away wire on battery bus connector2.Remove battery bus connector3.Remove overheat switch connector4.Cut break-away wire on both hold down clamps5.Lift hold down clamps and turn counter-clockwise to release battery from tray
A correction of 0.04 should be added to the specific gravity reading for each
10 °F (5.55 °C) above 80 °F (26.7 °C), and a correction of 0.04 should be subtracted from each reading for each 10 °F (5.55 °C) below 80 °F (26.7 °C).
while fully charged lead-acid cells produce around
2.1 volts each.
Internally in a lead-acid battery, the voltage across each set of plates is approximately
2.1 volts. A typical aircraft lead-acid battery has 12 cells connected in series to produce 25.2 volts at the terminal with no load. Remember, due to internal resistance, the actual loaded voltage is approximately 24 volts.
The procedure for servicing a Ni-Cd battery compartment is similar to servicing a lead-acid battery compartment. Instead of applying a baking soda solution to neutralize any spills, a
3% mixture of acetic acid, vinegar, or lemon juice is used. Then the area is cleaned with soap and water followed by a freshwater rinse.
the battery's ampere-hour (Ah) rating.
Batteries are rated by their ability to produce a certain power for a given period of time. This is called the battery's ampere-hour (Ah) rating. A battery rated at 1 Ah has the ability to maintain a current flow of 1 ampere for 1 hour.
Memory effect is another phenomenon related to
Ni-Cd batteries. If a Ni-Cd battery is consistently recharged to 100% as soon as it discharges by only 20%, over time, it will eventually behave as if it is fully discharged at the 80% point. This effect on its memory radically reduces its charge life.
Spare batteries should be stored in the battery shop; however, never store lead-acid batteries in a
Ni-Cd shop or Ni-Cd batteries in a lead-acid shop. Total failure of all the batteries could result.
To determine the amount of internal resistance of a battery, we need to use
Ohm's Law. First, determine the internal voltage of the battery by subtracting the loaded voltage from the unloaded voltage. Next, using Ohm's Law, divide the internal voltage by the loaded current. The result is the battery's internal resistance.
All battery shops should meet certain safety requirements, including:
SAFETY REQUIREMENTS•Ventilation•Temperature•Service Condition•Personal Safety•Tools and Safety Equipment
battery shop.
The battery servicing area is usually called the battery shop. The lead-acid and Ni-Cd batteries are not compatible and should never be serviced in the same shop.
thermal runaway
Unless the charge (voltage) is discontinued, the battery will destroy itself and potentially explode. This is thermal runaway.
Once the battery is fully discharged (all the cells are at 0.5 volts or less and shorted), let the battery rest with the cells shorted for a minimum of
8 hours. This rest period will allow all the cells to equalize with each other.
Other advantageous qualities of Ni-Cd batteries over lead-acid batteries include:
: comparatively lighter weight, fast and simple charging, high number of charge/discharge cycles, good load performance, long shelf life, good low temperature performance, and exceptional ruggedness.
due to contamination.
It is never a good idea to use the same tools for servicing both lead-acid and Ni-Cd batteries . If you have no choice, ensure you wash the tools thoroughly with soap and water before use.
voltaic pile.
By adding layers to the pile, he could increase the voltage. This arrangement of layers is called voltaic pile.
Conduct a current leak test.
Current leakage occurs when current flows from the battery cells to the battery case. The maximum allowable current leakage for most Ni-Cd batteries is 50 mA, measured from the case to the terminals. If the battery fails the current leak test, discharge, clean, and service the battery.
1.0 V X N CELLS =
DISCHARGE VOLTAGE( EX 1.0V X 20 CELLS =20 V)
Count Alessandro Volta, way back in 1800, found that different metals placed together produced electricity under the right conditions.
He piled alternating pieces of zinc, saltwater soaked paper, and silver in a stack and found that when he measured across the two different pieces of metal, there was a voltage present.
deep cycling
The longer a Ni-Cd battery is used in an aircraft without being fully discharged, the more likely it is that the memory effect will occur. To avoid this effect, a process called deep cycling is used: the battery is removed from the aircraft and completely discharged to zero.
The next step in servicing a Ni-Cd battery is to conduct a capacity check.
This should be accomplished every time a battery is brought into the shop. The capacity check is the only way to determine a Ni-Cd battery's state of charge./1H
