Aircraft Electrical Diagram
Power Generation, III
Most aircraft require some form of electrical power to operate navigation-, taxi-, landing-, strobe lights, one or more COM and NAV radio's, transponder, intercom and other electronic systems. The electrical system consist of a battery and an alternator or generator on older aircraft. All of this is connected through several meters (kilometers in large aircraft) of wire.
All matter on earth is made up from molecules and they basically consist of atoms. These atoms are made of electrons, protons and neutrons. And electricity is about the flow of electrons attracted to protons and repelled by other electrons.
Alternators and generators generate electrons for us to make use of, but these itmes do nothing if the engine is not operating. To get them going we need some form of storage to be able to start up our engine so that electrical power can be generated.
And as alternating current is impossible to store, we will discuss direct current combined with the basic secondary battery. As this model can be recharged in contrary to the primary battery.
For an aircraft engine to be able to start (ie. not by handpropping) there is a need to store energy and release that in a controlled method. That is usually done in a chemical form in the battery, just like reservoir, and being topped up by an alternator or generator when driven by the engine. In light aircraft it is usually a 12 volt type, as in a normal car.
More sophisticated aircraft use a 28 volt system because they need more electrical power (for starting turboprops or turbines) without the need for using a larger and heavier 12 volt battery and thicker wires. With a 28 volt system you can carry twice the amount of amps in the same wire without any problems. More about this in our article which electrical system to choose.
Rechargable batteries are normally the lead acid type (Flooded or AGM) or NiCAD (Nickel Cadmium) battery. As in a car, the flooded lead acid battery also generates hydrogen (very explosive) during charging and this needs to be vented overboard to prevent any accidental explosion. The acid in the battery is very corrosive. Hence the use of NiCADs in larger aircraft which do not have these disadvantages, but these need current limiting and temperature sensors as they can get warm during recharge and a thermal runaway must be prevented.
Lithium batteries can also be used but they must be charged so that each cell receives the same amount of energy. This is called balanced charging with a dedicated profile (CCCV). They also need protection against rapid discharge as some chemistries can heat up very quickly causing a fire hazard or even an explosion. The Boeing 787 Dreamliner suffered from this anomaly, early 2013.
There are several Lithium battery technologies available these days: Cobalt (Li-Co), Ion (Li-Ion), Polymer (Li-Po) and Iron Phosphate (LiFePO4). Each type has their own load and discharge characteristics (constant current and constant voltage, CCCV) and voltages ranging from 3.3 to 3.7 V per cell and it is chemistry dependant.
There could be a potential safety problem with storage of so much energy in a lithium battery. For example: suppose you need to store 1 GW for a day (which not unusual within the energy supply industry) this totals to: 1.000.000.000 x 86400 seconds = 8.6 x 10^13 joules = 86 TJ. Which a wee bit more than the nuclear bom on Hiroshima (Little Boy, 67 TJ or 16 kT).
Thus, if such a battery ever experienced a sudden catastrophic dielectric failure due to a mechanical issue, or if the batteries overheated and exploded, the resulting energy release would be the same as a 16 kiloton nuclear explosion. Liquid fuel energy is dangerous too, but compared to what could happen in a battery disaster (thermal runaway), they seem like the safest option as fluids easily pour away.
Fuel - Battery
The difference here is that the battery contains the fuel and oxidizer in one package where the fuel tank only contains the fuel. Should anything happen to the battery, a puncture during an accident is not unlikely, the resulting fire and explosion are next to impossible to prevent.
Where a standard battery delivers and takes in a charge rather slowly, supercapacitors can do this very quickly and without any degradation for millions of times even under extreme temperature conditions and vibrations. The reason for this behavior is their low internal resistance compared to a chemical battery. As such they operate as a high performance buffer delivering high energy for short periods of time. Ideal for starting engines. These capacitors are not cheap but virtually indestructible and, like lithium, light weight. To learn more about these follow the next link to the Battery University.
The main difference between batteries and supercaps is the way energy is stored, energy density and discharge profile. Where the battery has relatively flat low voltage drop during discharge the supercap is a linear line down. This result in a large amount of energy being unused as the voltage is too low at that point for any device connected to it.
The capacity of a battery is expressed in Ah, amperes per hour. Theoretically, a battery of 35 Ah is capable of delivering 35 A for one hour or 1 A for 35 hours or any other combination. But it needs to be fully charged to be able to do that, hence the need for recharging during flight. A capacitor is expressed in Farads, which is equivalent to 1 Ampere second per volt or F = As / Volt. At 12 V, 1 Ah is equal to 300 F.
The energy (watt per second or Joule) stored in a capacitor is 1/2 C x V^2. For a battery it is Voltage x Capacity (in A-hr) x 3600 sec/hr.
Most modern aircraft use an alternator because these are able to deliver more power and are able to charge the battery with the engine idling, were the generator is unable to do that (less power per RPM) and has a heavier construction. The main design difference between these two is that an alternator has a rotating electromagnet (rotor) which is energized by the field current (ALT switch) and a stationary coil (stator) delivering the power.
The generator has a stationary permanent magnet and rotating coils within the fixed magnetic field generating the power through a commutator which rectifies the alternating voltage/current (AC) to direct voltage/current (DC). The alternator uses diodes to convert the AC to DC.
Both devices are voltage regulated and deliver 13,8 volts to the aircraft electrical system and they need an over-voltage protection and indicator for the pilot. Over-current protection is by design with an alternator but not in the generator, this one needs a separate current limiting device.
After the engine has started, the alternator (or generator) will provide the electrical power for the aircraft and recharges the battery. The battery is thus only needed to start the engine, supplementing the alternator in a high load situation and as an emergency source of power in case of engine or alternator failure.
The generator needs at least 1200 engine RPM to supply enough electrical power to recharge the battery, where the alternator can do that from idle RPM (below 900).
Some aircraft have a ground power receptacle, useful when testing the electrical system without running the engine and draining down the battery. Can sometimes also be used for starting the engine when battery capacity is not sufficient (cold weather). You will find a ground power switch near the master switch with a 'ground' and 'flight' mode. You have to make sure that the ground power unit is of the same voltage and polarity as the aircraft system.