The airplane's electrical system powers all of the electrical equipment onboard the airplane, such as lights, radios, pumps, fuel gauges, electrically driven flight instruments, and electric motors.
Electrical Buses
The electrical equipment on the airplane will be connected to one or more buses. A bus is simply a central connection point for multiple items, which simplifies wiring to the power source.
Battery
The buses draw electrical power from the battery, which chemically stores electricity. The battery is useful for starting the airplane, but can only power the electrical equipment for a short time if not recharged. This time will vary from airplane to airplane and battery to battery, but may be as little as 20-30 minutes.
Alternators and Generators
An alternator or generator is driven by the engine and used to recharge the battery when the engine is running. The exception is that at low engine speeds a generator usually will not produce enough electrical power to power the airplane's electrical equipment. During these times of low RPM, power is drawn from the battery. The pilot is usually able to tell whether power is being drawn from the battery by noticing the electrical equipment. For example, lights may dim slightly at low RPM.
Alternators will usually charge the battery even at low engine RPM.
The alternator or generator voltage is set higher than the battery voltage, so that the battery will be charging when the alternator or generator is operating (and engine RPM is sufficient). Airplanes will either have a 12 volt battery charged by a 14 volt generator or alternator or a 24 volt battery charged by a 28 volt generator or alternator.
Master Switch
A switch in the cockpit allows the pilot to turn the battery on or off, connecting it or isolating it from the rest of the electrical system. A similar switch allows the pilot to connect or disconnect the alternator or generator to or from the electrical system. These two switches are often combined into one master switch.
Fuses and Circuit Breakers
To prevent electrical equipment from overloading and overheating, fuses and/or circuit breakers are used as protection. A circuit breaker will pop out as a result of an item of electrical equipment being overloaded, while a fuse will overheat and melt. A fuse must be replaced, while a circuit breaker may be reset by pressing in back in. However, if a fuse blows or a circuit breaker pops, it is an indicator that there is a problem with the associated electrical equipment or part of the airplane's electrical system.
Voltage Regulator
A voltage regulator is usually incorporated into the alternator or generator. The voltage regulator assures constant voltage is supplied to the electrical system as the engine power is varied.
Ammeters and Load Meters
An ammeter or load meter is a gauge provided to the pilot. An ammeter shows the pilot whether the battery is being charged, while a load meter shows how much demand, or load, the electrical system is demanding from the generator or alternator.