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Old 05-02-2017, 10:19 AM
JHartline JHartline is offline
Join Date: Dec 2012
Location: Carrollton, GA 5GA2
Posts: 173
Default Battery backup versus standby alternator


Considering redundancy options for a G3X panel and I would prefer to use battery backups to give me roughly an hour of functionality before the screens go dark. What that functionality would be exactly is negotiable depending on complexity, cost, and design issues. I have sent a query to Garmin to see what they have to say but wanted to poll the community and see if anyone has gone a similar route.

I want to be able to have autopilot, comm/nav/GPS/ILS and at least some external lighting. Pitot heat is on the table.

If any builders have gone this route and would like to share thoughts I would love to hear about it. Stein will be doing my panel - I'm not an avionics guy or an electrician unfortunately.
James Hartline
Carrollton, GA 5GA2
RV-14 - Finish and FWF in progress
T-Bolt IO-390
Maule MX7-180
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Old 05-02-2017, 10:38 AM
KatanaPilot KatanaPilot is offline
Join Date: Feb 2015
Location: Locust Grove, GA
Posts: 333
Default Ask Stein

Stein and his crew are about as experienced as you will find. One of the things I really appreciate about Stein is his candor. He is not shy about giving his opinions and explaining the pros and cons of a particular component or configuration.

Working with Stein and his folks has been a great experience and both my son and I have learned a lot so far.

For the RV-7A that we are building, we are planning a single B&C externally regulated alternator, TCW backup battery and a G5 with its own backup battery.

For our RV-10, we will very likely add a second alternator and battery - as we are planning a fully electronic SDS EI/EFI system.
Krea Ellis

Locust Grove, GA
DA20-A1 Katana "Princess Amelia"
RV-7A Phase 1 under way, in paint at Evoke Aviation
RV-10 under construction
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Old 05-02-2017, 11:53 AM
Auburntsts's Avatar
Auburntsts Auburntsts is offline
Join Date: Jan 2005
Location: Dumfries, VA
Posts: 2,460

Well my philosophy is in the event of an alternator failure I don't want to load shed while in IMC -- IOW I want all of my tools at my disposal and don’t want to make a bad day worse by tying one hand behind my back when I need it the most. As a result, my design goal was to attempt to make fuel the only endurance variable to the maximum extent possible. The idea was that I wanted to be able to provide my system's normal operating load for as long as I had fuel. There's a number of ways to accomplish this but I chose to go the 2 alternator (belt driven primary, gear driven back-up) route with a single 28AH battery. Does my setup drive the risk to zero--nope, but I'm satisfied that it's been lowered sufficiently. YMMV....
Todd Stovall
RV-10 N728TT - Flying!

Last edited by Auburntsts : 05-02-2017 at 12:15 PM.
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Old 05-02-2017, 12:11 PM
vic syracuse vic syracuse is offline
Super Moderator
Join Date: May 2005
Location: Locust Grove, GA
Posts: 2,195

I, too, go the single battery and dual alternator belt. I think there is a better chance of an alternator failing/belt breaking, than a battery failure on one that has been properly cared for. The second alternator will even get you home, as the standby alternators today can pretty much carry the whole panel.

Vic Syracuse

Built RV-4, RV-6, 2-RV-10's, RV-7A, RV-8, Prescott Pusher, Kitfox Model II, Kitfox Speedster, Kitfox 7 Super Sport, Just Superstol, DAR, A&P/IA, EAA Tech Counselor/Flight Advisor, CFII-ASMEL/ASES
EAA Homebuilt Council Chair/member EAA BOD
Van's East Coast Representative
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Old 05-02-2017, 12:20 PM
Bevan Bevan is offline
Join Date: Sep 2007
Location: BC
Posts: 1,506

Backup alternator provides a known amount of current for remainder of your flight. A backup battery will have an unknown quantity. The main battery is your backup reservoir (of unknown quantity). No need for another. My 2 cents worth and this is what I've done.

RV7A Flying
O-360-A1F6 (parallel valve) 180HP
Dual P-mags
Precision F.I. with AP purge valve
Vinyl Wrapped Exterior
Grand Rapids EFIS
Located in western Canada
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Old 05-02-2017, 01:14 PM
jfrank71 jfrank71 is offline
Join Date: Jun 2014
Location: Cotulla, TX
Posts: 167

Recent panel rebuild for me included G3X Touch (2), G5 and a new B&C up front belt driven and a new B&C out back on the accessory pad.
Frank Leone
RV-10 Purchased
RV-12 Under Construction by my kids, wife & me
VAF Dues Paid 7/2016
Contribution Made 10/2016
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Old 05-02-2017, 01:34 PM
Aggie78 Aggie78 is offline
Join Date: May 2014
Location: Fort Worth, Texas
Posts: 331

Backup, vacuum-pad mounted B&C alternator, single battery.

Very comfortable with that decision...get outta the goo on the backup, then either find a fix for the bad primary or wait and fly home when in VFR on the backup.

Edited to add:

I completely agree with what Todd wrote above about having the max amount of tools available if I'm battling a major systems malfunction while single pilot IFR. I'm going to stack as many chips in my favor as possible in that situation.

Among other gear...Pitot heat, VOR/ILS, Primary GPS, landing lights all need a lot of electrons flowing to function and will flatten a backup battery perhaps much quicker than I'm comfortable with/capable of getting the airplane on the ground...

Great peace of mind with the extra alternator available.
Rob Schroer
T67-Hicks Airport, Texas

Last edited by Aggie78 : 05-02-2017 at 01:46 PM.
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Old 05-02-2017, 02:29 PM
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airguy airguy is offline
Join Date: Oct 2006
Location: Garden City, Tx
Posts: 4,056

Dual alternators and single battery here also, I don't want to give up capability in IMC just because my primary alternator takes a vacation. I have the B&C 20 on the vacuum pad and it will handle all normal flight loads except lights - so in the event I lose my primary alternator I simply go dark until making the approach to the airport and let the battery supply the minor additional amperage for the lights for the duration of the approach.

I also test my backup alternator every flight, or nearly every flight, for several minutes to make sure it's all happy.
Greg Niehues - VAF 2018 dues paid
Garden City, TX
N16GN flying!
Built an off-plan 9A with too much fuel and too much HP. Should drop dead any minute now.
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Old 05-02-2017, 03:14 PM
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CubedRoot CubedRoot is offline
Join Date: Sep 2013
Location: Ooltewah, TN.
Posts: 524

Like others have said, a good backup alternator is a good solution. I am planning on using one of these B&C units on my build:
Lynn Dixon
RV-7 Slider -
Tail kit Completed - March 2017
Wing Kit Started - June 2017
My Build Log:

VAF Donation paid for 2017
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Old 05-02-2017, 03:58 PM
penguin penguin is offline
Join Date: Jan 2005
Location: England
Posts: 1,030

A second battery is always a high maintenance, pay many times, decision. A second alternator is a low maintenance, pay once, route.

Before you make any decisions write down the equipment that you must have after primary alternator failure, and the stuff you would like. Next write their current draw at 12.5v and 14.5v (12.5v for the battery and 14.5 for standby alternator). Add up the numbers to find out what how much current you need to keep you from sweating. Expect to have to do a little research to find accurate numbers. Question why you must have the more current hungry devices.

Now figure how much battery is required to keep everything running (use 70% of nameplate capacity as a first guess), and what kind of stand-by alternator would be required. When you have the data it will be straight forward to make a decision.

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