The battery in a general aviation aircraft serves three main purposes. First, it is used to crank the engine until it starts; second, it is used as emergency electrical power in case the alternator or generator system fails and third, it is used to filter out noise from the electrical system. The first two purposes are pretty clear, but the third is not. Until recently, most general aviation aircraft were fabricated from aluminum that is riveted together. The electrical system uses this aluminum as a return path for many of the systems in the aircraft. (no sense running an extra wire for the return current when an excellent conductor is already in place-this saves weight and gives the aircraft additional usable payload).There are electrons travelling in many different directions during a normal flight as different electrical systems are used. The electrical engineering reasons are not clear to me, but I know a battery helps filter out much of the "noise" created by the electrical system. If there is an unusual noise in the radios, many times it is an alternator with a diode going bad, a magneto generating RFI (radio frequency interference) or an ignition lead that has broken radio shielding. If nothing is found wrong with the alternator, magnetos or ignition wires, many times changing the aircraft battery will eliminate any noise in the radios. Occasionally, troubleshooting an electrical problem can be very frustrating and time consuming. The general aviation maintenance technician receives training in electrical systems, but sometimes intuition and experience are the only way to determine the problem.
Twelve volts? Fourteen volts? Twenty four volts? Twenty eight volts? There are people that call a general aviation aircraft electrical system 12 volts and the same system is called 14 volts by other people. They are both correct because when the alternator is not on line, the battery is operating at 12 volts. As soon as the alternator is on line, the system shifts to 14 volts in order to charge the battery and make sure it is ready in case of an alternator failure. The same relationship occurs when the system uses a 24 volt battery. The 24/28 volt system is used because the higher voltage allows less current to do the same job. If less current is required, wire sizes can be reduced and aircraft total empty weight is also reduced.
Don't apply the incorrect voltage on the system-12 for 24 (or) 24 for 12. Motors, light bulbs, electronics, etc. are designed to work properly and for a long time using the correct voltage, but will not work or burn out quickly using the wrong volts.
Checking for correct bonding between components is part of the general aviation maintenance technician's responsibilities. Some aircraft designs incorporate a position light on the rudder. The rudder is designed to move on hinges. If a bonding wire is not connected between the airframe and the rudder, the hinge will not provide a good path for electricity and the position light may not operate properly.
The engine of a general aviation aircraft is mounted on rubber shock mounts to reduce vibrations. If there is no bonding wire or the bonding wire is defective between the engine and the airframe, the engine will be effectively electrically isolated from the aircraft. Probes for engine gauges may not have a complete return path and may not indicate correctly. The engine starter uses a lot of current to crank over the engine. If the bonding wire is absent or deteriorated the current may try to find another path. There are reports of a small copper tube full of fuel used to prime the engine had carried the full starter current, burst and caught fire.
Radio systems often use a seperate ground system to isolate them from the main aircraft electrical system. All this means is that the radio system uses another wire to complete its circuits, rather than using the airframe structure.
Newly designed aircraft are becomming more and more an electrical machine. The general aviation aircraft technician requires continual training to "keep up" with the engineers. An update for the radio package including GPS, Autopilot, XM weather, collison and terrain avoidance, traffic alerts, synthetic vision, etc. using a laptop computer may take up to two hours. Better have an external power supply connected to keep from running down the aircraft battery.