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04-06-2018, 11:26 AM
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Join Date: Oct 2014
Location: Cedar Rapids, IA
Posts: 232
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Quote:
Originally Posted by Zuldarin
In this scenario I have a much smaller backup battery on the firewall. This allows me to have a reliable battery backup without having to run multiple long heavy power leads to the tailcone.
I added a relay in the main feed to the Engine bus but I want it to fail closed so I am planning on using a normally closed solenoid that is operated via a momentary switch. This allows me to test the backup power to the engine bus still have power in most failure modes.
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This is pretty cool, I've got nearly the same thing in my plan. I was thinking of using a traditional small battery so I could use it inside the cockpit, closer to CG. Maybe something like Tim Olson's, but probably less capacity and less weight. His added to over 20 lbs.
I'm still not convinced of the need for the switch/relay in the connection between Main bus and Engine bus. Yes it has to be tested, but couldn't that be solved with a startup procedure? - Throw the Engine Bus switch, make sure the ENG bus is powered
- Throw the Main switch, commence startup
- At run-up, kill the Aux to ensure the fan keeps spinning
I plan on powering my backup Attitude indicator thru the engine bus. This saves the weight/cost of the backup battery and would be an easy indicator of bus power in this startup procedure.
Great discussion. Keep it going guys, super helpful.
Last edited by unitink72 : 05-10-2018 at 01:57 PM.
Reason: Wrong bus on step1
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04-06-2018, 11:54 AM
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Join Date: Mar 2013
Location: Snoqualmie, wa
Posts: 393
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Quote:
Originally Posted by Carl Froehlich
Darin,
Some thoughts:
- Both batteries available for engine start? If so, the rear battery need not be as big.
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I hadn't really considered the backup battery as available for start but it in effect is available in this design. I will have to ensure the wires will support the current. Thanks!
Quote:
Originally Posted by Carl Froehlich
- Don?t forget to do a W&B calculation. My guess is you will be nose heavy.
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I expect to be a little nose heavy but I always carry extra weight in the baggage area in the form of a tool/spare parts bag. Its saved my bacon a time or two. I also plan on having my remote transponder mounted aft which should help. I will keep that in mind though because I don't want be out of CG without that tool bag.
Quote:
Originally Posted by Carl Froehlich
- Consider a 70amp or so breaker on the output of the primary alternator. 60amp alternators have a nominal output of 60 amps. It is not uncommon for them to put out a little more if it is trying to bring up a depleted battery. A 70 amp breaker will help eliminate nuisance trips and still provide the protection you want.
Carl
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Sounds reasonable. Thanks.
__________________
Darin
Snoqualmie WA
RV-9A Sold
RV-10 Building - FWF
www.DarinAnderson.com
2020 donation sent!
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04-06-2018, 11:59 AM
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Join Date: Mar 2013
Location: Snoqualmie, wa
Posts: 393
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Quote:
Originally Posted by unitink72
I'm still not convinced of the need for the switch/relay in the connection between Main bus and Engine bus. Yes it has to be tested, but couldn't that be solved with a startup procedure? - Throw the Aux switch, make sure the ENG bus is powered
- Throw the Main switch, commence startup
- At run-up, kill the Aux to ensure the fan keeps spinning
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That is a good observation. The EFI systems I am looking at have a controller/display that would allow for this option. Good thinking.
__________________
Darin
Snoqualmie WA
RV-9A Sold
RV-10 Building - FWF
www.DarinAnderson.com
2020 donation sent!
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04-06-2018, 12:01 PM
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Join Date: Mar 2013
Location: Snoqualmie, wa
Posts: 393
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Quote:
Originally Posted by majuro15
I like the idea of using a momentary open switch to test feeds. You could have a "push to test" button that is rated for whatever current going through that alternate power path that would then test the primary path. I would be an alternate to having multiple switches to power on the aircraft power.
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I actually like Josh's idea of using procedure rather than adding another potential point of failure. If we can achieve the same end result with procedure why add a switch?
__________________
Darin
Snoqualmie WA
RV-9A Sold
RV-10 Building - FWF
www.DarinAnderson.com
2020 donation sent!
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05-07-2018, 01:28 PM
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Join Date: Jul 2014
Location: West Covina, Ca
Posts: 52
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More or less reliable?
I reluctantly am going to jump in to this thread. I too am struggling with an EFI power design. I am partial to an engine bus, switched off of two batteries. Both batteries will be independent with their own alternator.
No fuses or such in this rough diagram, but conceptually, here is an idea. Would having two diode modules increase reliability or not? What I am looking at is:
http://ixapps.ixys.com/DataSheet/DSS2x121-0045B.pdf
Low Vf drop, extremely robust for our application. But I also can't figure out how I could test each diode module without the switch after the module, but that would leave the diodes always powered up. I am spitballing here, so no worries about criticism. Have at it.

__________________
Cliff Langlois
Meridian, Idaho
Cliffsrv10.blogspot.com
Project Sold 9/2018
RV-10
Empennage finished!
QB Fuselage and wings in hand
Taking a bite of the elephant every day.
Last edited by Pukauma : 05-07-2018 at 01:35 PM.
Reason: wrong pic
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05-07-2018, 03:54 PM
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Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,597
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Perhaps you can explain what the diodes do for you. As both sets of diodes provide current flow for a fault on the engine buss, a straight wire would do the same. In other words, a fault on the engine buss will be reflected on both batteries. If you have separate alternator for each battery, then perhaps I can see a function, but that would be several down the list of stuff to worry about.
If you have a single engine buss, any fault on it will stop your EFII engine, no matter how many feeds you have going to it. Can you split your EFII loads into two busses, each buss adequate to keep the engine running? If so, you will have new opportunities for design.
No reason for each battery to have a dedicated alternator. A primary (60 amp) feeding both and vacuum pad (20 amp) standby alternator will do more than you need.
While you work this I recommend you include all the other loads and how you will feed them. Doing it by section then splicing it all together may not yield what you want.
Carl
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05-07-2018, 05:50 PM
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Join Date: Apr 2008
Location: Riley TWP MI
Posts: 3,070
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That diode will be one of the most reliable things on your airplane. Mount it to an aluminum surface with heat conductive paste. If kept cool and protected against excessive current, it will likely never fail. Having two diodes will give you peace of mind, but will cost money and add weight. Other than that, it will not hurt to have two diodes.
__________________
Joe Gores
RV-12 Flying
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05-07-2018, 05:53 PM
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Join Date: Jul 2014
Location: West Covina, Ca
Posts: 52
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Diodes
My desire for diodes is to keep my two electrical buses as separate as possible. The diodes prevent current running back to a battery/bus, which in an emergency could be a problem. And yes, each battery it's own alternator. Completely independent systems.
Without an emergency situation, (or even with), the diodes would hopefully guarantee me with at least one source of current to the engine bus.
It becomes difficult to split my EFI (EFII) onto two separate buses. For instance, the third coil pack for 6 cyl. runs cylinders 4 and 5 top and bottom. If this coil pack were on one bus, it could potentially completely go down.
My intent here is to have one dual fed engine bus, off of which all engine electrical components can be fed. Everything else I can probably survive without. I am only addressing a concept for feeding EFI components, the remainder is another story.
__________________
Cliff Langlois
Meridian, Idaho
Cliffsrv10.blogspot.com
Project Sold 9/2018
RV-10
Empennage finished!
QB Fuselage and wings in hand
Taking a bite of the elephant every day.
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05-07-2018, 06:31 PM
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Join Date: Aug 2010
Location: Newport News, Va
Posts: 325
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Un-necessary SPOF
Quote:
Originally Posted by Pukauma
It becomes difficult to split my EFI (EFII) onto two separate buses. For instance, the third coil pack for 6 cyl. runs cylinders 4 and 5 top and bottom. If this coil pack were on one bus, it could potentially completely go down.
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Check with SDS, they don't have this single point of failure.
It's not only that the coil pack's bus could lose power but its wiring could fail or it could have an internal failure. Why not two triple coils, one for top and one for bottom plugs?
Last edited by johnbright : 05-12-2018 at 06:30 PM.
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05-08-2018, 11:30 AM
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Join Date: Jan 2008
Location: Upland, CA
Posts: 286
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EFII Bus Manager
This concern was alleviated many moons ago with our Bus Manager.
12 years on the oldest installs, hundreds in service - rock solid reliable!
https://www.flyefii.com/products/bus-manager/
And if you happen to look at our website, don't forget to look at System32:
https://www.flyefii.com/system-32/
System32 ECUs have a built-in high end power supply that operates from 3 volts to 40 volts and can absorb a 4 Joule energy hit from alternator load dump or a lightning strike. I doubt any other electronics in the plane are this robust.
Bottom line - with a Bus Manager and System32, it's pretty hard to have the engine turn off unintentionally.
Robert Paisley
EFII
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