Batteries Flashcards
Purpose of an aircraft battery.
Provide source of power to start the engine or the APU.
Provides limited power should other sources become unavailable.
Associated with DC system, 12V or 24V (nominal).
Nominal voltage.
Represents an assigned value to a designate voltage class.
Describes the actual voltage with which a circuit operates whilst allowing a range that allows satisfactory operation.
Light aircraft has 12v nominal voltage circuit, lead acid battery will display around 12.5-12.7v off load, and 13.3-14.4 under charging conditions. Circuit will operate under these conditions.
Chemical cell.
Produces electricity through a chemical reaction.
Cell must compromise of 2 dissimilar materials electrically connected by some form of electrolyte.
Voltage and current can be increased by having a larger amount of plate material.
Primary cells - one time use
Secondary cells - rechargable.
Types of batteries.
Lead acid
Nickel Cadmium
Lithium iron
Battery construction
Number of cells linked in series to produce a terminal voltage. Measured at battery terminal.
In each cell, 2 dissimilar materials are placed in electrolyte. Creates chemical reaction (galvanic reaction).
Battery discharge
On load. (slightly lower than off load).
Change in chemical properties.
Each battery has slightly different discharge and current produced.
Part of the chemical change is through temperature.
Electron flow maintained by cell dividers.
Electron attracted to +ve cell and in the circuit pushed by electrons behind whilst being reattracted to the battery.
Use conventional flow +,- to describe battery output.
Battery charging.
Reverses current flow. Reverses chemical changes that occurred in during discharge.
To recharge, applied voltage must be higher than off load nominal voltage (around 15%).
14v for a 12v nominal battery
28v for 24v nominal battery
Should take 20 minutes to fully charge battery following engine start.
Battery capacity
Expressed in Ah.
10Ah = 10 amps for 1 hour.
Thermal runaway
Current increases, temperature increase, resistance increases, repeat.
Construction material of these batteries display a NTC.
Bartery charge limiters w=are the controllers which monitor the variable.
Aircraft batteries.
Static invertor for emergency or ground operation.
24V nominal batteries are rack mounted.
Connection methods.
Series or parallel.
Series - increase voltage with the capacity of any one battery available.
Parallel - Increase the capacity (current) available whilst retaining the nominal voltage of any 1 battery.
Battery monitoring.
At least the set voltage.
Lead acid.
Lead acid cell voltages decreases in a linear manner, negatively affected by temp and has a lower cell voltage when place on load.
2.2V off load, 12V battery has 6 cells.
‘Wet batteries’ due to liquid electrolyte.
During charging, a safety concern is off gassing.
Voltage drops with decreasing charge.
Nickel Cadmium, Lithium polymer, Lithium ion. (similar properties)
Nickel cadmium, lithium polymer, lithium ion, stable cell voltage, on and off load and maintain cell voltage for much longer, prone to battery runaway and thermal runaway.
1.3V (on and off load) 12v battery has 10 cells.
During charging, safety concern is thermal runaway.
Better power to weight ratio than lead acid.
Sustain correct charge for longer when discharging.
‘Constant’ output voltage during discharge.
Busbar
Substantial conductor, powered by an electrical source, which has attachment ports to allow connection of multiple loads.
