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  #1  
Old 04-07-2020, 07:18 AM
WingsOnWheels WingsOnWheels is offline
 
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Location: Plano, TX
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Default EI Power System Review

I am currently running one Mag and one EDIS-Based Electronic Ignition. In the future I am going to replace the mag with a second EI, but I first need to ensure that the electrons keeps flowing in the event of an emergency.

Goals for the system:
1) Ensure that the EI receives power in the event of a main buss or battery buss failure (or both), including a battery buss short to ground or battery disconnect.
2) Provide a minimum of 30 minutes reserve plus significant margin in the event backup battery condition is less than ideal.
3) The backup system should engage without any human intervention
4) Integrate with my existing electrical architecture without a significant rewire.

According to what I have read and been told on this forum, a backup battery of the same chemistry (lead-acid in this case) does not require a separate charge controller and that internal resistance is sufficient to limit charge current (please let me know if this is incorrect). Nominal current draw is ~1.5-2amps per ignition at cruise RPM. The battery I selected is a PC 1250 4ah SLA. This should provide at least 60 minutes of reserve. In the event that 60 minutes is insufficient, I can shutdown one ignition system and double that reserve.

The only downside I see with this arrangement is that the ignition buss is always hot, so it is extra critical for ensure the ignition switches are turned off after shutdown (of course that is always critical). Not included in this schematic, but in-work is a warning system for backup battery condition and a warning for leaving the ignition on with the main buss off (this can also act and the "backup in-use" warning.

Schematic (excuse the poor drawing, does the job though):
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Last edited by WingsOnWheels : 04-07-2020 at 07:23 AM.
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  #2  
Old 04-07-2020, 08:05 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is online now
 
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No, do not wire like this. You have set up for a single EI failure (internal short) to open both breakers. The fan will stop.

Have one EI on each battery, each on its own, single breaker. If your engine cannot run on one EI as a backup mode, pick a better EI.

My first RV started off life with a dual electrically dependent EI set up. Two hard inflight failures popped the associated breaker. So yes, this could happen.

Carl
I fly pMags....
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  #3  
Old 04-07-2020, 08:36 AM
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DanH DanH is offline
 
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What Carl said.

Do this instead, isolated feeds and grounds. One diode, not three, and replace the useless breaker with a fusible link for aux battery charge. Well, "useless" may not be fair, as you could pull it to preflight check the aux battery output.

The EDIS feeds really only need 5 amp protection, as the EDIS system only needs a bit over an amp.



Been flying this for a while now. Strip off the voltage monitor, push to test aux battery voltage, and ignition map select wiring, and it's pretty much what you see above. Both batteries are behind the baggage compartment in the RV-8, so there is no battery bus or main bus in the ignition system.



Aux features stripped away:

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Last edited by DanH : 04-08-2020 at 07:34 AM.
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  #4  
Old 04-07-2020, 09:13 AM
WingsOnWheels WingsOnWheels is offline
 
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Thank you both for the feedback, i was thinking about that same failure mode after I posted this and was discussing it with someone else. Eliminating the tie between the two systems looks like the best solution, simple and low component count. The other alternative was to add more fuses (after the EI buss), but that is just more to fail.

So would you suggest two smaller backup batteries instead only one? That would add the diodes back in to the one EI currently connected to the battery buss.

Thanks for the great advice.
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  #5  
Old 04-07-2020, 10:03 AM
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DanH DanH is offline
 
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Quote:
Originally Posted by WingsOnWheels View Post
So would you suggest two smaller backup batteries instead only one?
That goes to the question of "mission". For an IFR airplane I'd want two independent bus systems with two equal batteries. For VFR, it's one main battery and one little battery solely to support the second ignition if the main bus, alternator, and primary battery are all unavailable.
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  #6  
Old 04-07-2020, 10:12 AM
WingsOnWheels WingsOnWheels is offline
 
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Quote:
Originally Posted by DanH View Post
That goes to the question of "mission". For an IFR airplane I'd want two independent bus systems with two equal batteries. For VFR, it's one main battery and one little battery solely to support the second ignition if the main bus, alternator, and primary battery are all unavailable.
Good point. Right now the plane is just VFR. When I have another 10K to spend it may get an upgrade. For now, this backup is only for the EI, no other devices will be run off this backup. Sounds like one 4ah backup is enough for now. The EFIS and Mapping all have their own internal backup, so even on a bad day I can run all the important stuff for about an hour after a total electrical failure. I also have a portable Comm, so while it and even the EFIS are not critical for VFR, it doesn't hurt to have backup options.
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  #7  
Old 04-07-2020, 10:20 AM
WingsOnWheels WingsOnWheels is offline
 
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On a fun side note, i already made the battery holder, that was a fun project as well.



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  #8  
Old 04-07-2020, 10:28 AM
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Quote:
Originally Posted by DanH View Post
That goes to the question of "mission". For an IFR airplane I'd want two independent bus systems with two equal batteries. For VFR, it's one main battery and one little battery solely to support the second ignition if the main bus, alternator, and primary battery are all unavailable.
I have had two failures of my electronic ignition in flight. In both cases my single magneto saved my bacon. One was a wire failure and one was a Klixon breaker trip (no known cause). The fan kept spinning. Using Dan's wiring (simplified) is the ticket for VFR aircraft. Two independent ignitions without failure cross-coupling. No switches have to be actuated in an emergency.

I monitor both battery voltages with my EFIS. The breaker trip was called out as an under voltage alarm.
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  #9  
Old 04-07-2020, 11:31 AM
PhatRV PhatRV is offline
 
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Quote:
Originally Posted by DanH View Post
What Carl said.

Do this instead, isolated feeds and grounds. One diode, not three, and replace the useless breaker with a fusible link for aux battery charge. Well, "useless" may not be fair, as you could pull it to preflight check the aux battery output.

The EDIS feeds really only need 5 amp protection, as the EDIS system only needs a bit over an amp.



Been flying this for a while now. Strip off the voltage monitor, push to test aux battery voltage, and ignition map select wiring, and it's pretty much what you see above. Both batteries are behind the baggage compartment in the RV-8, so there is no battery bus or main bus in the ignition system.
Thanks for the diagram. I was looking for something like for my EI wiring.
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  #10  
Old 04-07-2020, 03:02 PM
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Carl Froehlich Carl Froehlich is online now
 
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Quote:
Originally Posted by DanH View Post
That goes to the question of "mission". For an IFR airplane I'd want two independent bus systems with two equal batteries. For VFR, it's one main battery and one little battery solely to support the second ignition if the main bus, alternator, and primary battery are all unavailable.
+1

Do not look at just power to your ignitions, look as total system design. Two equal batteries provides power for 2+ hours, a lot more than the typical EFIS wimpy backup battery maybe doing 30 minutes of backup power to keep your screen alive. And you get great cranking power.

Carl
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