If the alternator or generator fails, the entire electrical system of the aircraft will run off of the battery alone. The ALT or other warning light should come on, indicating that the battery voltage is now greater than the alternator or generator voltage. Alternator or generator failure can be confirmed by looking at the ammeter or the loadmeter. The ammeter shows current flow, so if it indicates a discharge (the needle is below zero), the battery is supplying all the power. A loadmeter shows how much current the alternator or generator is putting into the system, so if the needle is on zero the battery is doing all the work. However, it is important to know the normal position of the loadmeter needle, since it indicates the amperes of load being placed on the alternator by each electrical component which is turned on, including the battery. A lower than normal needle position could mean there is a problem. The battery can only supply enough current to run the basic components of the electrical system for about 30 minutes. 

Load shedding, or turning off unnecessary electrical equipment, will be necessary at this point. Any components not that are not critical should be turned off by pulling the circuit breakers—basically anything that spins, heats, or lights (gyros, autopilot servos, flap and gear motors, fuel and gear pumps, pitot heat, and all external lighting). The ALT switch should also be turned off, since leaving it on can cause as much as 1.5 amps of power to flow into the device’s coil windings, control boxes, and regulators. Although use of the primary radio may be necessary (the second NAV/COM can be turned off), transmitting should be kept to a minimum as it uses a lot of current. Some modern aircraft make the process of load shedding easier by providing an essential bus switch. When activated, all of the less critical electrical components will be disabled. Even if the battery dies, the engine will keep running, thanks to the magnetos, which are independent and self-powered.

At no time should a pilot consider continuing a flight once the electrical charging system has failed—A landing should be made at the nearest suitable airport.