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  #1  
Old 06-10-2016, 09:53 AM
wilddog wilddog is offline
 
Join Date: Feb 2008
Location: va.
Posts: 311
Default 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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  #2  
Old 06-10-2016, 10:07 AM
Evolution10 Evolution10 is offline
 
Join Date: Jun 2014
Location: oregon
Posts: 91
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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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  #3  
Old 06-10-2016, 10:27 AM
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Carl Froehlich Carl Froehlich is offline
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 1,137
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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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  #4  
Old 06-10-2016, 10:27 AM
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airguy airguy is offline
 
Join Date: Oct 2006
Location: Garden City, Tx
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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.
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Garden City, TX
N16GN flying! http://websites.expercraft.com/airguy/
Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
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  #5  
Old 06-10-2016, 10:40 AM
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RONSIM RONSIM is offline
 
Join Date: Feb 2008
Location: Largo, FL
Posts: 772
Default Same as airguy, and

much simpler system/hookup.
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RV-10 Co-built (sold)
RV-8A Maintained and flown (Sold)
RV-6A Bought and Flying (N177RV)
IO360, 180HP/CS, AFS 5600T, G650, VIZ385
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Based at KCLW (Clearwater, FL)
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  #6  
Old 06-10-2016, 10:46 AM
longline longline is offline
 
Join Date: Sep 2014
Location: silverdale, WA
Posts: 179
Default 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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  #7  
Old 06-10-2016, 10:56 AM
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erich weaver erich weaver is offline
 
Join Date: Aug 2006
Location: santa barbara, CA
Posts: 1,468
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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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  #8  
Old 06-10-2016, 11:17 AM
flyinga flyinga is offline
 
Join Date: Dec 2007
Location: Fredericksburg, TX
Posts: 459
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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.
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RV-8
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Fredericksburg, TX
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  #9  
Old 06-10-2016, 11:28 AM
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Mike S Mike S is offline
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Location: Dayton Airpark, NV A34
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Quote:
Originally Posted by longline View Post
and dual alternators for me. Installed cross tie solenoid. Accepted the weight to insure safe operation of a electrically dependent system.
Quote:
Originally Posted by flyinga View Post
+ 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.
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.
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VAF 909

Rv-10, N210LM.

Flying as of 12/4/2010

Phase 1 done, 2/4/2011

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"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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  #10  
Old 06-10-2016, 11:28 AM
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YellowPeril YellowPeril is offline
 
Join Date: May 2012
Location: Bowdoinham
Posts: 164
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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.
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RV8 N80549 (First Flight 2003)
Aerosport O-360A1A, Dual LS CDIs, WW200C
Bowdoinham, Maine
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