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What to monitor in a dual alternator airplane

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PostPosted: Mon Sep 24, 2018 8:57 am    Post subject: What to monitor in a dual alternator airplane Reply with quote


It depends on what the purpose of the second alternator is. If it is a standby in case of primary alternator failure then you want the secondary to be on line when the voltage of the bus goes down a certain amount indicating either excessive draw or primary alternator failure.

If you have both set at the same voltage, they are both theoretically operating and you will not be able to tell if one fails. Set the way as suggested in this post,all you have to do as a pre-check is to alternately check the voltage each puts out at run-up. Further checking is unnecessary as if the bus voltage is lower than what you expect from the primary and is staying constant, you have a primary alternator failure. If it is lower than primary expectation and continues to decline you probably have failure of primary and secondary alternators.


In a message dated 9/23/2018 11:09:11 PM Central Standard Time, flyadive(at) writes:


On Sun, Sep 23, 2018 at 10:55 PM Art Zemon <art(at) (art(at)> wrote:

Over the years that I subscribed to this mailing list, there have been many conversations about monitoring voltage and current in an airplane. I want to approach this from a different perspective and see if I understand correctly.

The basic questions are:
  1. What should I monitor in my airplane?
  2. How do I interpret the indications, especially in flight?
My own airplane has a 12 volt system with two B&C alternators and voltage regulators. The primary regulator is an LR3C preset to 14.4 volts. The standby regulator is part of an SB1B-14 and is preset to 13.0 volts.

Barry - Why would you have the second Alt. set to ANYTHING less than the charging voltage?
13.0 VDC is NOT a Charging Voltage. Minimum Charging Voltage is 13.8 VDC with a Max around 14.5 VDC. For some reason people can not agree on the Max. Voltage. It is usually 14.2 to 14.5 VDC.


Here is what I think that I need to monitor:
  • Voltage (should pretty pretty much the same everywhere so I am just using my EFIS' bus voltage indicator)
  • Current (I strapped a solid state current sensor to the wire going from the master contactor to the fuse block)
  • Primary regulator overvoltage indicator
  • Standby regulator overvoltage indicator
  • Primary regulator/alternator ON (physical switch turned on)
  • Standby regulator/alternator ON (physical switch turned on)
And here is how I think I should interpret stuff:

  • Voltage approximately 14.4 volts: Everything is happy with the primary alternator and regulator doing their things.
  • Voltage approximately 13.0 volts: My primary alternator isn't providing power or my primary regulator isn't working properly.

Barry - Forget about the 13.0 VDC. It is too low. And only shows the output of your #2 Alt is not sufficient to charge a battery.
If you see BATTERY VOLTAGE anything around 12.5 VDC or Less you are running on battery and that is not good.

  • Check that the primary alternator switch is turned on. If the standby regulator does not have an overvoltage indication then it is operating normally and I do not have a flight urgency.

Barry - What is "normal" if you are NOT putting out a voltage that will charge the battery?

You are already below normal.
An idiot light is a back-up to a volt meter. If the bulb goes in the idiot light you are relying on something that will never work.
If the volt meter shows a failure you KNOW there is something wrong and cross check to the idiot light.

BUT! That is ONLY for a LOW voltage. What about OVER VOLTAGE?
If the volt meter failed and NOW it is time to check if the Idiot light says the same thing.
  • Voltage lower than 13.0 volts: I have problems with both regulators/alternators and I am depleting the battery. This needs attention.
  • If the voltage is lower than 13.0 volts, check that the alternator switches are turned on. If both are turned on and voltage is still low, check the current draw. At cruise RPM, I can expect 30 amps from the standby alternator.

Barry - Not correct! The Alt Systems has the ability to SUPPLY 30 Amps. BUT! That is ONLY if the Load on the plane is 30 Amps.
  • I need to intelligently balance load and amps available (governed by engine RPM) against battery capacity and remaining flight duration.

Barry - You do not have to balance anything. That is the job of ACU's.
What you should have done during designing and building your plane is:
Figure out ALL your electrical equipment draw and add a bit for future expansion AND THEN pick a Primary Alt that can supply 20% Greater than the Full Load.
  • Voltage higher than 14.4 volts: The overvoltage protection in the primary regulator has failed. Turn off the primary alternator.

Barry - YES! But, don't stop there. What if the over-voltage condition is coming from the back-up Alt?

Side Note: It is standard practice to crosscheck against at least two instruments.


"If you wash your hands before you go to the bathroom you might have the making of a crew chief."

Do I have it right?

-- Art Z.

--"We do not see the world as it is. We see the world as we are."

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