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I get between 12.0 and 12.6 volts for the battery in flight. I thought you're supposed to always see 13 something on batt voltage when the engine is running so I stopped by a Rotax facility to run a test. They told me my battery voltage should always be in the 12 range and it's only the Lane A and Lane B volts that should be 13 something. Is this true? I get 13.8 volts on Lane and Lane B, but on my battery I only see as much as 12.6 and as low as 12.0 (which is where the yellow light comes on) when flying.

  • Re: What is the battery voltage supposed to be in flight?

    by » 9 months ago


    I see 14v on both FADECs (Lane A & B). And about 13.6v on the battery in flight. (912IS) Just after start, the battery reads 11.8v-12.2v. So shortly after start, we momentarily run the RPM up past 2500 which kicks in the generator A. Then we see 13.6v or so.

    I’m  still learning this machine. 


  • Re: What is the battery voltage supposed to be in flight?

    by » 9 months ago


    Albert Yowell wrote:

    I see 14v on both FADECs (Lane A & B). And about 13.6v on the battery in flight. (912IS) Just after start, the battery reads 11.8v-12.2v. So shortly after start, we momentarily run the RPM up past 2500 which kicks in the generator A. Then we see 13.6v or so.

    I’m  still learning this machine. 

    Hmmmm so that sounds about right. I usually see 12.2 battery volts at engine start, then after I run at 2500rpm for 5 seconds, my battery volts jump up just slightly to 12.6. Anyone have any ideas why my battery voltage would be jumping up but much less than expected after engaging the generator?

    Don’t you mean generator B though? It’s my understanding that gen A exclusively powers the engine and gen B handles all the ship power (avionics, lights, accessories). So when we spool up to 2500rpm for 5 seconds, we’re engaging gen b, right? 

    What would cause my generator B to be providing SOME power but much less than expected?


  • Re: What is the battery voltage supposed to be in flight?

    by » 9 months ago


    A fully charged UNLOADED battery in good condition, will rest at ~13.5 down to 12.6 depending on how long since the last charge.
    A healthy battery can take 24 hours or much longer to settle back to 12.6.
    A flooded Acid (Common Lead/Acid) Battery will float charge at ~ 13.8 volts.
    It takes more than 13.8 to overcome the chemical threshold to actually put power back into the Battery.

    The wires from the ECU/Fuse box to the battery might be 12 AWG or larger with 14 or 16 AWG extending to the Panel.
    The battery voltage needs to be measured directly at the battery or with dedicated wires to the voltmeter.
    It is not uncommon for the wire running to the panel to lose 1/2 volt or so,
    14,2v at the battery might end up as 13.6 at the panel.
    Battery Voltage and Panel Voltage may not be the same thing.
    Plenty to run everything just fine but not a true measure of the actual battery voltage. 

    - - -

    The Engine starts on Lane B and the B Generator only,  and the initial 12.0 to 12.5v is the starter-stressed battery powering the panel on its own.
    The Engine/ ECU switches to the A Gen once the RPMs reach 2400+ for 3+ seconds.
    At this point, the B gen is released to charge the Battery and power the panel.
    This is when the sudden voltage jump is seen.
    At idle, most of the available current is going to be used to recharge the battery and ~13 volts will initially be seen and it will take a little time for the voltage to creep up to the 14+ volt level.
    This is all normal operation.

    In flight, Lane A and B are powered by the A Gen in isolation from the Battery and Panel.
    The Battery and Panel are powered by the B Gen in isolation from the Engine/ECU
    You should see 13.8 to 14.2 v on the panel in flight with a little latitude depending on the wiring.
    Anything below 12.6 means the battery is discharging.


    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.


    Thank you said by: Jonathan, Max Marin Wirth

  • Re: What is the battery voltage supposed to be in flight?

    by » 9 months ago


    Bill Hertzel wrote:

    A fully charged UNLOADED battery in good condition, will rest at ~13.5 down to 12.6 depending on how long since the last charge.
    A healthy battery can take 24 hours or much longer to settle back to 12.6.
    A flooded Acid (Common Lead/Acid) Battery will float charge at ~ 13.8 volts.
    It takes more than 13.8 to overcome the chemical threshold to actually put power back into the Battery.

    The wires from the ECU/Fuse box to the battery might be 12 AWG or larger with 14 or 16 AWG extending to the Panel.
    The battery voltage needs to be measured directly at the battery or with dedicated wires to the voltmeter.
    It is not uncommon for the wire running to the panel to lose 1/2 volt or so,
    14,2v at the battery might end up as 13.6 at the panel.
    Battery Voltage and Panel Voltage may not be the same thing.
    Plenty to run everything just fine but not a true measure of the actual battery voltage. 

    - - -

    The Engine starts on Lane B and the B Generator only,  and the initial 12.0 to 12.5v is the starter-stressed battery powering the panel on its own.
    The Engine/ ECU switches to the A Gen once the RPMs reach 2400+ for 3+ seconds.
    At this point, the B gen is released to charge the Battery and power the panel.
    This is when the sudden voltage jump is seen.
    At idle, most of the available current is going to be used to recharge the battery and ~13 volts will initially be seen and it will take a little time for the voltage to creep up to the 14+ volt level.
    This is all normal operation.

    In flight, Lane A and B are powered by the A Gen in isolation from the Battery and Panel.
    The Battery and Panel are powered by the B Gen in isolation from the Engine/ECU
    You should see 13.8 to 14.2 v on the panel in flight with a little latitude depending on the wiring.
    Anything below 12.6 means the battery is discharging.

    Thanks, Bill! So since I get 12.6 battery volts or less when the engine is running, what does that mean for me? Bad battery? Bad stator? 


  • Re: What is the battery voltage supposed to be in flight?

    by » 9 months ago


    Any 12V lead-acid battery from your Cub Cadet to you Caterpillar 930 wheel loader has 2.1v per cell. 2.1x6=12.6. So a 12.6V means the battery is healthy, but not being charged. 


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