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Thread: Alternator size

  1. #11

    Join Date
    Dec 2019
    Location
    Idaho
    Posts
    117

    Default Re: Alternator size

    Quote Originally Posted by efwd View Post
    Hi Victor
    I hesitate to reply, in fact I have written this reply three times. The reason, I don't have the calculations to refer too and as with many of us, electrical was not my favorite. Anyhow, for what it's worth I have everything on your list except the tail light but I do have aft facing white lights on both wing tip position lights. Some of our numbers don't match such as the trim motor. I calculated my amps (but I don't have the data here at home) then, while at the Kitfox Factory, I spoke with Judd. On his advise, I am not running any addtl alternators. My 912iS electrical system is operating all those items on your list. I do have the backup battery as Justin mentioned but I have no reason to believe it has ever kicked in to keep the PFD or MFD screens on. Im running the VPS as well and I have a mechanical Master switch. I run a lot of stuff on startup too. Landing lights, strobe, position lights, Radio, Transponder, both PFD and MFD etc etc.
    Thanks for the info. So it looks like you turn everything on before you hit the start button? I think it's
    not obvious when the battery backup kicks in to keep your avionics alive during start. You could verify
    by disabling the battery backup and measuring the voltage at the avionics power terminal during
    start to see what it does and then turn the backup battery on and see if it's different. Not sure that will
    work though since there are 2 power inputs to each avionics device.

    One important difference between the 912is you have and the 914 I am planning to use is that the 912is has
    2 internal alternators. One is 16 amps and the other is 30 amps. The 914 only has a single 18 amp alternator
    which is pretty wimpy considering the engine needs 8 amps to run and the radio can use up to 7.5 amps
    during transmit.

    Victor

  2. #12
    Senior Member PapuaPilot's Avatar
    Join Date
    Nov 2013
    Location
    Nampa, Idaho
    Posts
    1,226

    Default Re: Alternator size

    Another thing to consider is that you only need to account for continuous loads to determine the electrical load, and the alternator size required. The trim motor, transmitting radios and fuel boost pump do not need to be counted. Larger intermittent loads can use the ship battery. I counted my nav and strobes as a continuous load, but not my landing lights because I consider my plane a day/VFR aircraft. For radios like COMs or transponder that transmit you need to find out what the current draw is in standby (continuous), not when transmitting (intermittent).
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  3. #13
    Senior Member
    Join Date
    Dec 2017
    Location
    Mapleton,UT
    Posts
    1,224

    Default Re: Alternator size

    Based on what you said you are going to have, you will not regret adding the 32 amp alternator but very possible you will regret not getting it.
    Dustin Dickerson

    Building 7ss STI x 2
    Oratex
    29" shock monster
    EP912STI 155hp
    Garmin
    N33TF......FLYING!
    N53TF......FLYING!

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