Welcome Guest. Sign in or Signup

3 Answers

Alternatior vs Battery Voltage

Asked by: 907 views ,
Aircraft Systems

Can you tell me why the alternator on a Cessna 172R has 28 volts while the battery is only 24 volts?

Ace Any FAA Written Test!
Actual FAA Questions / Free Lifetime Updates
The best explanations in the business
Fast, efficient study.
Pass Your Checkride With Confidence!
FAA Practical Test prep that reflects actual checkrides.
Any checkride: Airplane, Helicopter, Glider, etc.
Written and maintained by actual pilot examiners and master CFIs.
The World's Most Trusted eLogbook
Be Organized, Current, Professional, and Safe.
Highly customizable - for student pilots through pros.
Free Transition Service for users of other eLogs.
Our sincere thanks to pilots such as yourself who support AskACFI while helping themselves by using the awesome PC, Mac, iPhone/iPad, and Android aviation apps of our sponsors.

3 Answers

  1. RickS on Sep 08, 2016

    Although I am an A&P, an electrical guru I am not. Let me say this, however…. I suspect it\’s for the same reason your car\’s alternator runs at 14v but your battery is 12v.

    The battery voltage is a nominal value, but the electrical system runs on the voltage of the alternator output. And there is some tolerance/variation in the voltage as well.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes

  2. Best Answer

    MasterFlight on Sep 09, 2016

    The higher voltage when the alternator is on causes electricity to flow back into the battery, which recharges it after engine start and keeps it charged.

    The 24V battery to 28V alternator voltage ratio is standard, and is usually referred to as a 28 volt electrical system, but sometimes as a 24 volt system, depending on who you are talking to.

    14/12 volt airplanes would have 14 volt alternators and 12 volt batteries.

    +2 Votes Thumb up 2 Votes Thumb down 0 Votes

  3. crossaint on Sep 12, 2016

    The 24 volts is a nominal, easy to calculate number, 2 volts per lead-acid cell. Actually, the voltage varies with temperature, current and charging state between 1.75 and 2.4 volts per cell. The battery consists of 12 cells which adds up to 28.8 volts, when full.
    The alternator controller adjusts to 28.5 volts to keep the battery neraly full, butnot overloaded.

    In fact, when the ALT switch is closed, there is no difference between battery and alternator voltage; they are connected.

    +1 Votes Thumb up 1 Votes Thumb down 0 Votes

The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.