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3 Answers

Alternatior vs Battery Voltage

Asked by: 848 views ,
Aircraft Systems

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

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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.

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  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.

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  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.

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