Match: 
#6
Message: 
#58841
Date: 
Jul 08, 2013
From: 
"Robert L. Nuckolls, III" <nuckolls.bob(at)aeroelectric.com>
Subject: 
Re: Low voltage indicator with dual       alternators



>My operational goals for the Z-14 included being able to run much of 
>the panel
>on one battery while still having a fully charged battery for starting.  The
>idea being that I wanted to be able to enter flight plans and even 
>check weather
>before engine start.  That's worked out well.  I can connect the 
>buses for starts
>if needed and that works well as well.

    Okay, but consider the ENERGY needed to do these two tasks.
    Suppose you turn on three electro-whizzies for a total drain
    of 3A (a lot for solid state gizmos) and take 5 minutes to
    herd all the ducks into place. That 12v x 3A x 300 seconds
    for a total of 11,000 watt-seconds.

    Now, lets crank the engine at 200A x 12v x 10 seconds
    for a total of 24,000 watt-seconds.

    How much energy is available for use in a 18 a.h.
    SVLA battery?  At a 3A rate will give it up over 5 hours
    at an average voltage of 11.5 for a total of about 650,000
    watt-seconds.

    In other words, the task of conducting pre-flight activities
    and starting the engine are not big demands on a well
    maintained battery.

>
>However, I normally start with the buses separated because if I go 
>parallel, the voltage drop causes my 3 MDFs to re-boot.

   How much 'stuff' do you have to turn on before engine
   start? On the Beech products we used to offer a mini-ebus
   switch that would let you fire up a comm radio directly
   from the battery. Your comm radio probably needs 0.2A receive,
   and maybe 1.5A transmit. A long winded pre-flight activity
   might need 1000 watt-seconds.

   I'm thinking that the bus from which all your 'kitchen
   sink' accessories is powered also powers the comm radio
   and you have no way to power up the comm radio independently?
   You might want to consider moving a comm radio to a
   battery bus and adding . . .

   COMM RADIO . . . . . Check OFF

   . . . . to your shutdown checklist.

   Bottom line is that with two batteries on board, you've
   got perhaps a million watt-seconds to burn. Doing your
   starts in parallel will reduce relative abuse to the batteries
   and extend their service life. Letting the 'kitchen sink'
   accessories clock in after engine start may not be
   a significant delay in your departure plans.


   Bob . . . 

End Msg: 
#6 

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