By Rick Farmer
A question I have heard asked many times, by those who want to become a commercial pilot or private pilot or just like flying, is “how long will the battery last if the alternator fails?” Well, this question is very hard to give an accurate and simple answer to, so let’s dive into some of the complexities of our aircraft battery and charging system. I like to explain electrical theory in terms of flowing water. Think of a garden hose attached to a tank of water with a spray nozzle attached at one end. The tank of water, in this case, can be thought of as a battery, and the weight or “pressure” of the water can be related to the voltage of the battery. This pressure is what gives the “push” needed to move the water (or in this case electrons). When you pull the spray handle, (turn on electrical equipment) water flows from the source, through the hose and out of the nozzle. This flow of water can be related to the flow of electrons in an aircraft system and is known as amperage. A higher amperage drawing on the system means a higher flow of electrons from the battery. The result is a faster draining battery that will be seen as a drop in voltage on a voltmeter or a negative indication on an ammeter.
Most IFR certified aircraft manufactured since 1990 are 28-volt systems. That means the battery has an internal voltage of 24 volts. So why is the alternator output 28 volts and why does this matter? The first 24 volts provides the voltage required to run all the aircraft appliances and the last four volts provide the push necessary to trickle charge the battery keeping it at a peak charge ready for the next aircraft start (or restart) or to stand ready as the reserve for running the systems should the alternator or its support systems fail.
Batteries are rated in terms of amp-hours which is a measure of how many amps the battery will supply for one hour at its rated voltage. The average 24-volt GA battery provides about 15 amp-hours of power at a full charge (usually until a preset voltage limit such as 20 volts for a 24-volt battery). However, that number begins to diminish as soon as the battery is placed into service and is affected by many factors such as frequency of use, maintenance practices, temperature, charging/discharging cycles… etc.
The state of charge of the battery at the time of the alternator failure will determine the number of minutes the battery can keep the systems going before electrical equipment starts to die. The pilot should be wary, as the amp-hour rating (even at a full charge) is not necessarily linear. As the battery starts to work to power everything in the aircraft, such as might occur with a failed alternator, the voltage also starts to drop and many radios and other appliances drop offline at particular voltages, but long before the battery is actually dead.
So regardless of whether you’re already an established career pilot or seeking to become a commercial pilot or private pilot, it is important to closely monitor the state of your charging system in flight and during run-up and follow the checklist if you receive a low voltage indication. Turn off all non-essential electrical equipment as soon as possible, and remember that you can also quickly determine what electrical equipment draws the most power by looking at the amp rating on the related circuit breaker.