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Wire size, battery to starter

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jrichard(at)ccser.com
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Posted: Tue Jan 09, 2007 8:37 am Post subject: Wire size, battery to starter

I would appreciate anyone’s real life experience with this seemingly simple decision of what wire size to use connecting battery to starter.  After reading Bob’s Aero-Electric Connection chapter 8 and calculating voltage drops using numbers from Figure 8-3 (AWG wire table), #4 AWG wire is more than adequate in a 24 volt system with battery to starter run of about 10 feet starting a 6 cylinder engine.  In my case, the airframe is composite, so I can not use the airframe as the return conductor and must run a second wire back to the battery from the starter.  Bob discussion on page 8-9 “WHEN IS AN OVERLOAD NOT AN OVERLOAD” helps me lean toward #4 AWG vs the larger #2 AWG.    

   

Have any of you wired your 24 volt, aft battery location, starter system with #4 AWG wire and if so, how does it crank when you first hit the starter switch?   And does anyone know the wire size used on newer production single piston aircraft with aft mounted batteries?  

   

I’m sure this issue has been discussed in the past, but I haven’t seen it during my involvement with the Aeroelectric list.  I apologize to anyone seeing this for the Nth time.    

   

John Richardson  

Lancair Legacy  

   

   

        [quote][b]

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nuckollsr(at)cox.net
Guest

Posted: Tue Jan 09, 2007 12:59 pm Post subject: Wire size, battery to starter

 	  Quote: 	
		  At 08:32 AM 1/9/2007 -0800, you wrote:



I would appreciate anyone s real life experience with this seemingly 

simple decision of what wire size to use connecting battery to 

starter.  After reading Bob s Aero-Electric Connection chapter 8 and 

calculating voltage drops using numbers from Figure 8-3 (AWG wire table), 

#4 AWG wire is more than adequate in a 24 volt system with battery to 

starter run of about 10 feet starting a 6 cylinder engine.  In my case, 

the airframe is composite, so I can not use the airframe as the return 

conductor and must run a second wire back to the battery from the 

starter.  Bob discussion on page 8-9 WHEN IS AN OVERLOAD NOT AN OVERLOAD 

helps me lean toward #4 AWG vs the larger #2 AWG.



Have any of you wired your 24 volt, aft battery location, starter system 

with #4 AWG wire and if so, how does it crank when you first hit the 

starter switch?   And does anyone know the wire size used on newer 

production single piston aircraft with aft mounted batteries?



I m sure this issue has been discussed in the past, but I haven t seen it 

during my involvement with the Aeroelectric list.  I apologize to anyone 

seeing this for the Nth time.


	


    I've seen some 14v LongEz wired with a pair of 4AWG where

    the round trip was on the order of 24 feet. They reported

    "adequate performance". But they never ventured out of

    the California style weather.



    The question becomes one of maximizing performance when

    trying to crank an engine in cold weather where battery

    impedance goes up as does cranking currents. Our cold weather

    test parameters for getting STC to put RG batteries into

    TC aircraft assumed 300A for 15 seconds and a cold soaked

    BATTERY (about -15C) voltage had to stay above 9V for the

    15 second interval.



    The 24 a.h. RG battery passed with flying colors. Now the

    question becomes how much of that 9V are you willing to toss

    of at the terminals of the starter for the same cranking

    scenario?



    I know this is a very broad brush . . . I'm aware of very

    few detailed cold weather cranking studies done on our

    piston singles . . . and all those airplanes have the

    battery mounted up front in an all metal airplane. That

    data would not be very helpful to us here.



    The risks are as follows: You may find that 4AWG "get's

    'er started" most if not all of the time. But I'll suggest

    it will crank better and start sooner if wiring drops

    are minimized. Short cranking cycles translates to longer

    battery and starter brush life. Finally, it may make the

    go/no-go difference in an unusually cold cranking environment.



    2AWG is 4 oz/foot, 4AWG is 2.5 oz/foot. The delta weight

    for your proposed 20' round trip run is 1.5 x 20 = 30

    oz. The real question before you is are you willing to

    trade about 2 pounds of empty weight for cold-cranking

    insurance?



    Bob . . .

      ---------------------------------------------------------

      < What is so wonderful about scientific truth...is that >

      < the authority which determines whether there can be   >

      < debate or not does not reside in some fraternity of   >

      < scientists; nor is it divine. The authority rests     >

      < with experiment.                                      >

      <                            --Lawrence M. Krauss       >

      ---------------------------------------------------------

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