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What's the point of having a different button for the battery? Why does the alternator turn on simultaneously with the battery? I need an easy and short explanation, if possible.


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    $\begingroup$ You want a short answer?! You have been on this forum before, right? $\endgroup$ – GdD Aug 4 '17 at 16:01
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I will expand a bit on @Noah Krasser's answer with some situations where you would want one or the other. FWIW under-volt scenarios are typically far less of a problem than over-voltage scenarios.

  1. Bad Voltage Regulator: Alternators vary their voltage output with RPM. To keep your system operating properly a voltage regulator is installed on the alternators direct output. This keeps your systems power supply at a constant voltage, typically at the cost of some power in the lower end. These things can and sometimes do fail, if they fail in such a way that the alternator is at a high RPM and putting out a voltage that could be potentially harmful to your avionics you are going to want the ability to cut it off while still drawing from your battery.

  2. Battery Short: More often than not batteries are stored in the tail section of GA planes or potentially anywhere. If the cable chase leading to the battery develops a short you may want the ability to cut the battery circuit out while still having the ability to power you aircraft from the alternator.

  3. Thermal Runaway: Batteries can runway on you and cause an over-volt issue. Again in flight you are going to want to be able to cut it from the system.

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  • $\begingroup$ The battery in your link wasn't a thermal runaway, just an overvoltage alternator that likely didn't disconnect from the bus (like #1) $\endgroup$ – selectstriker2 Aug 4 '17 at 17:55
  • $\begingroup$ @selectstriker2 updated it with the FAA's AC warning against it instead of that thread. Thanks for the heads up! $\endgroup$ – Dave Aug 4 '17 at 19:14
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Short and concise answer:

Imagine there is a problem with the alternator. You have to shut it down. Now there is only one button to turn off both the alternator and the battery. You just want to turn off the alternator, but you would also lose your radios, the GPS, everything. Is that what you want?

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    $\begingroup$ What's the point of turning off the alternator even if it's not working properly (or not working at all) ? How I can determine that it doesn't function as it should? $\endgroup$ – super jet 100 Aug 4 '17 at 16:19
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    $\begingroup$ @superjet100 Well to prevent it from being further damaged or damaging other components. This might be a bad example but if you find the alternator smoking you would have three choices instead of two: 1) cut the electricity then your engine dies, 2) keep everything running then the alternator catches on fire, then it burns the insulation and shorts the bus, then blows a fuse, then triggers a chain of bad things, 3) cut the alternator and run on battery for a while when you look for a safe solution. Same thing if the battery is faulty. $\endgroup$ – user3528438 Aug 4 '17 at 16:41
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    $\begingroup$ @superjet100 Also the failure modes of battery and alternator isn't as simple as "fried alternator is no alternator" and sometimes you need a switch to isolate the impact. $\endgroup$ – user3528438 Aug 4 '17 at 16:44
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    $\begingroup$ @user3528438: Most aviation engines will run just fine without electricity. $\endgroup$ – jamesqf Aug 4 '17 at 19:08
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    $\begingroup$ @user3528438 I think what jamesqf meant to say was that most GA engines run just fine without external electricity. They produce their own via the magnetos. They don't use power from the alt to fire the plugs. The only reason a GA engine needs battery power is to crank the starter. Even that isn't necessary if you get out and yank the prop, though. $\endgroup$ – reirab Aug 4 '17 at 19:34
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In addition to the other answers, most modern alternators have stator and rotor coils. One generates the electricity, the other consumes current to generate a magnetic field. When driven by the engine, the current consumed is a small fraction of the current generated.

However, when you first power up your machine you really do not want the batteries delivering current to the alternator till after the engine is started and running or you will reduce the amount of battery power available and eventually drain the battery.

As such, having the ability to completely disconnect the alternator is a good thing.

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