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06-10-2016, 08:53 AM
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Join Date: Feb 2008
Location: va.
Posts: 523
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Dual Batteries or Dual Alternators
For an airplane with all electric fuel pumps and ignitions, what are the pros and cons of each option?
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06-10-2016, 09:07 AM
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Join Date: Jun 2014
Location: oregon
Posts: 94
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It is best to isolate dual batteries (two bus system). This is essentially two seperate systems. The cons - expensive, more complicated, takes longer, double the weight as well. Dual batteries alone - double the available electrons, added weight. If you go dual batteries, you might as well do dual alternators as well.
I used one battery, dual alternators on my RV-10. All glass system. No problems todate.
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06-10-2016, 09:27 AM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,596
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There is nothing more reliable than a battery (assuming you have not abused it).
Two PC-625 provide ample cranking power. With single alternator and two master solenoids you run them in parallel for normal operations. If the alternator fails, you open both masters to split the panel loads between each battery (left side from left battery, right side from right battery, each connected to the battery via a 30 amp relay).
Two PC-625 batteries will provide 34 amp hours of reserve power. For practical terms, this is about two hours for most glass panels - assuming you don't have the landing lights, pitot heat and such on. On my RV-8A I mounted one battery on each side on the cockpit floor right behind the firewall. W&B worked well. On the RV-10 both batteries are mounted in the normal spot aft of the baggage compartment.
If you add a standby alternator, take the output via two 40 amp diodes and have one connection to left, one to right panel busses. Keep the rest rest the same. This way you can isolate most common fault modes to minimize the risk of loss of power to your vital loads.
Carl
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06-10-2016, 09:27 AM
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Join Date: Oct 2006
Location: Garden City, Tx
Posts: 5,145
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Single battery, dual alternators here. My backup alternator is a B&C 20amp on the vacuum pad, it can handle all flight-critical loads.
__________________
Greg Niehues - SEL, IFR, Repairman Cert.
Garden City, TX VAF 2020 dues paid 
N16GN flying 700 hrs and counting; IO360, SDS, WWRV200, Dynon HDX, 430W
Built an off-plan RV9A with too much fuel and too much HP. Should drop dead any minute now.
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06-10-2016, 09:40 AM
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Join Date: Feb 2008
Location: Largo, FL
Posts: 1,029
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Same as airguy, and
much simpler system/hookup.
__________________
RV-10 Co-built, maintained, flown (sold)
RV-8A Maintained and flown (Sold)
RV-6A Bought and Flying (N177RV), upgrades $$$
IO360, 180HP/CS, AFS 5600T, D10A, G650, G430, G327, ADS-B, VIZ385 AP
Very Happy Contributor
Comm, SMEL, CFII, A&P
Based at KCLW (Clearwater, FL)
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06-10-2016, 09:46 AM
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Join Date: Sep 2014
Location: silverdale, WA
Posts: 208
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Dual battery
and dual alternators for me. Installed cross tie solenoid. Accepted the weight to insure safe operation of a electrically dependent system.
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06-10-2016, 09:56 AM
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Join Date: Aug 2006
Location: santa barbara, CA
Posts: 1,682
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It is extremely unlikely that a battery will develop a fault mid-flight. The more common scenario is the alternator goes bad and develops an over-voltage condition or simply stops charging the battery. A small backup alternator on the engine accessory case corrects this condition and provides for safe continuation of flight to your destination, assuming your are not exceeding the amperage rating of your backup alternator. Careful consideration of your continuous use, 'must have' avionics and their collective electrical load and use of an e-bus for those items will take the stress out of such an event.
Check out Bob Knuckoll's Aero-electric Connection for the details.
erich
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06-10-2016, 10:17 AM
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Join Date: Dec 2007
Location: Fredericksburg, TX
Posts: 662
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+ 1 for what longline said. Secondary system is 8a alternator and small Life battery with crosstie. Came in handy when I accidently left main master on and discharged main battery.
__________________
Jim Averett
RV-8
TS36 - Silver Wings
Fredericksburg, TX
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06-10-2016, 10:28 AM
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Senior Curmudgeon
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Join Date: Sep 2005
Location: Dayton Airpark, NV A34
Posts: 15,420
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Quote:
Originally Posted by longline
and dual alternators for me. Installed cross tie solenoid. Accepted the weight to insure safe operation of a electrically dependent system.
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Quote:
Originally Posted by flyinga
+ 1 for what longline said. Secondary system is 8a alternator and small Life battery with crosstie. Came in handy when I accidently left main master on and discharged main battery.
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Yep, same here. Dual batt and alt. And cross tie for extra start boost.
Aux alternator and battery dedicated to the e buss and a Schottky diode allows main batt to power e buss if aux battery goes down.
__________________
Mike Starkey
VAF 909
Rv-10, N210LM.
Flying as of 12/4/2010
Phase 1 done, 2/4/2011 
Sold after 240+ wonderful hours of flight.
"Flying the airplane is more important than radioing your plight to a person on the ground incapable of understanding or doing anything about it."
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06-10-2016, 10:28 AM
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Join Date: May 2012
Location: Bowdoinham
Posts: 180
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Dual batteries, single alternator. One PC-680 for turning starter, and a little lightweight one to drive one of the CDIs.
VFR Day-Night Ops.
__________________
Nick "Hairball" Knobil
RV8 N80549 (First Flight 2003)
Aerosport O-360A1A, Dual LS CDIs, WW200C
Bowdoinham, Maine
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