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I am a student pilot training to get my PPL. My CFI asked me the following:

How can you know your alternator stopped working?

My first response was annunciator lights. Then the instructor said, what if you don't notice the annunciator lights or they themselves aren't work, what's your next check?

Check the amps. But then I started wondering, the plane will most likely have some amount of battery power. So the amps might not be fully gone.

Are there other ways to determine if the alternator isn't working?

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  • $\begingroup$ ‘Check the amps’ is an eyebrow-raiser; amps is not a thing that you either have or don’t have. There’s not much point in having an ammeter if you don’t know what it’s showing you. Typically it will indicate overall current to/from the battery, which should be slightly positive in normal operation but will go negative if the battery is discharging. However, an ammeter could show the output of the alternator, current being drawn by certain instruments ir equipment etc. $\endgroup$
    – Frog
    Commented Jul 31, 2021 at 21:58

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If you have a voltmeter installed, a failed alternator will cause it to read ~12V (or ~24V) while a working alternator should show ~14V (or ~28V)

The voltage regulator (wired in after the alternator) outputs a voltage higher than the nominal battery voltage so that the battery can charge. If the voltmeter reads the nominal battery voltage it may indicate a failed alternator. It can also indicate a bad voltage regulator.

Depending on the type of ammeter your aircraft is equipped with it may show discharge vs charge current in which case a discharge indication could indicate a failed alternator or an alternator that can no longer supply the required current to operate all of your electronics and you are operating at a loss.

Lots of good info here if you are curious.

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  • $\begingroup$ Load meters only show the actual output of the alternator. They don’t really show “total consumption”. If you turn on an item, like a landing light, it will show as a load on the ammeter, meaning the alternator is working. If there is no load, the alternator is not working, or there is nothing turned on. $\endgroup$
    – Mike Sowsun
    Commented Jul 31, 2021 at 11:33

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