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