Page 1 of 2 12 LastLast
Results 1 to 10 of 20

Thread: Rapid voltage drop when backup ignition tested during runup?

  1. #1
    Senior Member
    Join Date
    Jul 2020
    Location
    FL001
    Posts
    282

    Question Rapid voltage drop when backup ignition tested during runup?

    TSquared sent me a video of his FX-3 runup a week ago showing a rapid voltage drop when doing a runup with key to R and backup ignition ON. We figured his backup battery was bad so he has a new one coming. Today I went for a short flight after installing a new starter battery and noticed the same. His plane is 2 years old, mine is essentially new, battery has been in for about a year now but not flown until this past June.

    So the question is, why when turning on the backup ignition is the voltage (right side of the two voltages) drops like a rock?

    https://youtu.be/mPhzREYnc70

  2. #2
    Senior Member ceslaw's Avatar
    Join Date
    Sep 2013
    Location
    Carterville, IL
    Posts
    589

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    Does it drop to about 12 V and hold steady? If so, probably the difference between charging voltage and non charging voltage when checking backup function. Drop to 12 volts normal. Dropping steadily below 12 volts suggests bad backup battery. Couldn't tell the voltage level from the video.

  3. #3
    Senior Member
    Join Date
    Jul 2020
    Location
    FL001
    Posts
    282

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    I stopped it, next time I'll see how low it goes. Thanks.

  4. #4
    Junior Member
    Join Date
    Dec 2017
    Location
    Plymouth, MN
    Posts
    25

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    Quote Originally Posted by ceslaw View Post
    Does it drop to about 12 V and hold steady? If so, probably the difference between charging voltage and non charging voltage when checking backup function. Drop to 12 volts normal. Dropping steadily below 12 volts suggests bad backup battery. Couldn't tell the voltage level from the video.
    in my case the voltage dropped to 11.9 and likely would have continued to drop if I hadn’t stopped. New battery should arrive late this week.

  5. #5
    Senior Member Andy's Avatar
    Join Date
    Dec 2019
    Location
    Glendale, AZ
    Posts
    714

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    Quote Originally Posted by hawgdrvr View Post

    So the question is, why when turning on the backup ignition is the voltage (right side of the two voltages) drops like a rock?

    https://youtu.be/mPhzREYnc70

    Ever since I have owned an FX-3 I have advocated testing the emergency ignition battery to evaluate its performance rather than replacing on calendar date.

    I hold the emergency ignition for at least 10 seconds for each run-up test and have built a history of battery performance. It's trivial to plot the logger data to see how the battery is holding up. An unloaded battery on charge can be expected to show a rapid voltage drop when taken off charge and placed under load. The plots of voltage against time will show any trend in loss of capacity.
    Last edited by Andy; 04-06-2022 at 07:00 AM.

  6. #6
    Senior Member
    Join Date
    Jul 2020
    Location
    FL001
    Posts
    282

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    I think what we have to remember is that this battery has not been on charge while sitting in the hangar. So at runup you're seeing the "uncharged state" as mentioned above. What would be a good check is to perform this test AFTER flight so the battery has had some time to charge and see where the resting voltage is at that point.

  7. #7
    Senior Member Andy's Avatar
    Join Date
    Dec 2019
    Location
    Glendale, AZ
    Posts
    714

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    Quote Originally Posted by hawgdrvr View Post
    I think what we have to remember is that this battery has not been on charge while sitting in the hangar. So at runup you're seeing the "uncharged state" as mentioned above. What would be a good check is to perform this test AFTER flight so the battery has had some time to charge and see where the resting voltage is at that point.
    There is no load on the emergency ignition battery when the aircraft is parked. It should hold full charge for a long time. My FX-3 is probably unique in that I display and log emergency battery current. It does not show a large charge rate after engine start. It would if had discharged.

    Data from Jan 7 2022:
    Volts before start 12.8
    Volts after start 14.0
    Max charge current after start 0.2 A
    Min voltage during ignition test 12.1
    Max discharge during ignition test 1.1 A
    Peak charge current after 10 second ignition test 0.6 A
    Time to drop to 0.1 A charge 68 seconds

    If the emergency ignition battery voltage drops below 12 V during a 10 second test it's probably time to change it.

    Edit to add - G3X logged data is heavily lagged and will not show any short term peaks.
    Last edited by Andy; 04-06-2022 at 06:45 AM.

  8. #8
    Senior Member
    Join Date
    Jul 2020
    Location
    FL001
    Posts
    282

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    This morning the thought came to mind as to how long should I expect the ignition backup battery work? I was told long ago "45 minutes". So to take an engineering look at this we need to know the aH draw on the ignition. From what I can determine it appears the lightspeed ignition draws ~2 aH. The PowerSonic battery's capacity, fully charged *is* 2 aH. Technically that means a 2A draw should last 1 hour. That battery has to be perfect and fully charged obviously.

    Now the question is, why isn't the ignition wired to the starter battery instead of a small battery in a sub-panel? Wouldn't the starter battery be a better option which appears to offer 12-16 aH? as Lightspeed mentions in their FAQ you'd run out of gas before running out of battery.

    https://lightspeed-aero.com/Products/FAQs.htm

    https://www.aircraftspruce.com/catal...dysseydry7.php

    https://www.power-sonic.com/product/ps-1221s/

    Edit: I'm no electrician, I just want to make sure my engine will keep running for the longest time necessary.
    Last edited by hawgdrvr; 04-07-2022 at 07:24 AM.

  9. #9
    Senior Member Andy's Avatar
    Join Date
    Dec 2019
    Location
    Glendale, AZ
    Posts
    714

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    Quote Originally Posted by hawgdrvr View Post
    So to take an engineering look at this we need to know the aH draw on the ignition. From what I can determine it appears the lightspeed ignition draws ~2 aH. The PowerSonic battery's capacity, fully charged *is* 2 aH. Technically that means a 2A draw should last 1 hour.
    The measured current draw of the Lightspeed ignition is just over 1 amp at 12 volts. It may increase as the voltage reduces depending on the characteristics of the internal voltage regulator.

    A 2 AH battery will not supply 2 A for 1 hour! The AH capacity is usually specified for a discharge current of C/10 i.e. 0.2 amps for this battery. The capacity reduces with discharge current and will be far less than 1 hour at 2 amp discharge.

    I have demonstrated and shared my data for an inflight 30 minute use of emergency ignition. 30 minutes is certainly possible with a good battery. I doubt that 1 hour would be possible.

    Use of the emergency ignition battery in flight should be a desperate last resort. At the first sign of electrical system malfunction shed all non essential loads and run ignition on the main battery until it drops to about 6 volts. Then, if you have not found a place to land, hope for the emergency battery to keep you going a bit longer.

    I hope use of the emergency ignition battery was covered in the TacAero course. It (running as long as possible on main battery before selecting emergency battery) was included in my discussion with DAR at airworthiness inspection.

  10. #10
    Senior Member ceslaw's Avatar
    Join Date
    Sep 2013
    Location
    Carterville, IL
    Posts
    589

    Default Re: Rapid voltage drop when backup ignition tested during runup?

    There are three power sources. First is the alternator. If it dies the voltage will drop to around 12 volts and everything will continue to function off the second power source, the main battery. That will certainly provide some good flight time. The ignition will continue to function well below 12 volts, as low as 8 volts (if memory is correct).

    If both the alternator and the main battery fail, then flipping the emergency battery switch will disconnect the ignition from the main power source, connect the back up battery to only one of the two ignitions, and keep the engine running for a theoretical 45 minutes, which is the third ignition power source. It powers only the one ignition so all the other electronics would quit.

    The emergency battery is not directly connected to the main battery when activated. Separate circuits.

    Worded differently, with a power failure the emergency battery is the back up to the main battery which is the back up to the alternator. Lots of back ups!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •