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EI Power System Review

WingsOnWheels

Well Known Member
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):
image001.png
 
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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....
 
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.

image001.png


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.

Ignition%20Power%20Supply.jpg


Aux features stripped away:

Ignition%20Power%20Supply%20Stripped.jpg
 
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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.
 
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.
 
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.
 
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.
 
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.

image001.png


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.
 
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
 
+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

I agree for an IFR aircraft. In my case, the modification is for a flying VFR airplane. I don't see significant value in adding a second full size battery at this point. The EFIS backup is good for 45-60 minutes (assuming the main battery is gone), but I don't *need* the EFIS anyway. My tablet which is used for navigation is good for about 1.5-2hrs on battery. If I do add IFR capability down the road, it will be a different story. Then a second PC680 would make a lot of sense.
 
Here is the wiring diagram that I used for a back up battery for my Dynon HDX and my new SDS CPI-2 EI. I used an Earth X 6.2Ah ETX-18B for my back up battery which should keep the EI and essential Dynon components active long enough to land. Please feel free to comment if there is a problem with this setup, or if I left anything out.



My only concern with that layout:
Do you leave your backup battery switch on all the time (at least when running), or do you have to turn it on?

Also, if you have a fault in your Avionics/Critical buss, you will lose both ignition systems.
 
Mike, are you sure you wish to drive both ignitions from the avionics bus?

A reasonable design goal might be to drive both ignitions from separate and independent sources while everything else is shut down, preferably by flipping one switch (the master).

Another might be to label switches for what the really do. Here we have switches identified as "avionics master" and "batt 2", which result in a stopped fan if moved to OFF.

Why a 70 amp breaker on the B lead? It means you need to run a hot B lead through the firewall to the instrument panel. An ANL or similar firewall forward ends that bit of 1950's silliness; the hot lead in the panel is protected.

I see a "backup" label just to the left of module B. What is going on there?

You realize a short anywhere between the backup battery and the two 15 amp ignition breakers will kill both ignitions?
 
The Standby Battery

Since Five days have passed with no activity on this thread, I think its okay to share two ideas from Bob Nuckolls on standby batteries that some might consider hi-jacking a thread.

(If the name Bob Nuckolls does not ring a bell, the kit planes link below gives a little info for you.)

https://www.kitplanes.com/bob-nuckolls-and-morrie-hummel-inducted-into-eaa-hall-of-fame/

Batteries have been an essential component
of aircraft electrical systems from day-one.
The first airborne electrics were battery powered
by devices recharged after every flight given
that engine driven power sources were not
yet prominent fixtures. Indeed, wind-driven
generators were more prevalent (read low power
and DRAG!).

Batteries were 'wet', produced fumes when
charged. dribbled acid over themselves and
the airplane and had relatively short service
lives. I.e. they were a commodity like tires,
oil and gasoline. Their reliability as a
power source depended on constant monitoring
of condition.

While secondary (rechargeable) batteries have
become much more efficient, environmentally
friendly, more per pound and cubic foot, they
are still a commodity. You cannot install
a battery into any function expecting it to
perform as needed for more than a tiny fraction
of the airframe service life. Further, if loss
of a battery-supported system adversely impacts
probability for comfortable termination of
flight like fuel and oil . . . the battery's
condition demands reliable monitoring of
condition for continued airworthiness.

This is a burden on cost of ownership. It
expands the operator's work load for periodic
and capable preventative maintenance peppered
with the expense of replacement on condition.

Design goals for elegant electrical systems
architecture includes minimizing the need
for batteries in both number, size and expense.
The ultimate achievement for reduction of
batteries might be (1) a battery capable
of but one start cycle and (2) sufficient
capacity to operate landing lights during
the last 60 seconds of a dead-stick landing
at night.

THAT battery might indeed be quite small and
its cost of ownership very nominal. It might
even be cheap enough to simply replace it at
every annual . . . or perhaps every 100 hours
just as you might change the oil. Thus you've
eliminate the need for time, effort and test
equipment necessary to conduct preventative
maintenance on ship's battery(ies).

This line of reasoning sets the design goals
for an electrical system tailored to a project's
engine, airframe, capabilities and planned
missions. One goal is to minimize singling
out any particular set of loads as critical
but to consider the machine, pilot and
flight environment as a system.

Over the past 30 years, a family of architectures
have been proposed, refined, published and
applied to projects in the OBAM aviation
community. Some have served the builder's
design goals well . . . others not so well.
It's an iterative evolutionary process that
strives for demonstrable success.

The proposed Z01 architecture takes into account
the pronounced shift in aircraft designs that
incorporate multiple engine driven power sources,
electrically dependent engines and yes . . .
a desire to shed as much of the battery weight
and cost of ownership as practical.

Discussions here on the List and elsewhere
are needed to ferret out any failure to meet
a design goal . . . not as SUPPLIERS of any
particular electro-whizzy but as USERS
of those devices as components of the whole
FLIGHT SYSTEM.


Bob . . .


This is a thought from Bob about a second standby alternator.

An SD8 standby alternator would weigh
less than an 8Ah battery, requires
close to zero preventative maintenance
and carry 8+ amps of load as long as the
fan is running. .

 
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.

image001.png


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.

Ignition%20Power%20Supply.jpg


Aux features stripped away:

Ignition%20Power%20Supply%20Stripped.jpg
Dan,
On the bottom simplified drawing what?s the specs on the diode to the aux battery if it is a 12 amp hour battery
 
Dan,
On the bottom simplified drawing what?s the specs on the diode to the aux battery if it is a 12 amp hour battery

Honest answer is "I dunno". Reviewed them many moons ago, and since lost the package.
 
Honest answer is "I dunno". Reviewed them many moons ago, and since lost the package.

Many options but I?d use a 30amp dual package Schottky diode, run the diodes in parallel and mount with a heat sink. These are just a few dollars.

Carl
 
There is a nice, small 3aH rechargeable back-up battery on the aviation market. Is there any reason not to use such a battery as the primary battery for ei's that require battery power to function? It would be independent from the ships electrical system, except for recharging.

George
 
Probably regret it, but I'm going to post some block diagrams, the end result being primary power for a VFR bird with dual SDS CPi. The diagrams are not intended to be a CPi2 endorsement, nor are they the only way to do things. I simply got curious about applying the system to a wiring installation.

Why post these? To some extent, it's about method. Too often builders approach wiring design like some kind of crazy connect-a-dot. The result can look like a bowl of spaghetti, and be very difficult to check for gotcha issues. Block development allows logical progression from fundamental architecture to a practical system. It's easy to understand, and more important, easy to analyze for surprises.

Examples, progression of basic architecture:

Architecture.jpg


And onward to basic VFR architecture for dual CPi2. Red line is the firewall, cyan blocks are panel switches. "KLSW"= kill switch, a momentary or toggle ground. "CP" = circuit protection, which may be an ANL (or similar), a blade fuse, or a fusible link.

Please review the CPi2 manual, as it was designed with built-in backup battery management and warning systems:

http://www.sdsefi.com/cpi2mannov19.pdf

Yes, I know, three batteries. Times change. There is very little weight penalty with lithium-iron, as compared to lead/acid AGM, and for VFR, two of them can be quite small. BTW, one CPi feature is selectable charging management for lead or lithium backup.

Systems%20A%20and%20B.jpg


Click here to download a basic short-or-open review in Excel:

https://www.danhorton.net/VAF/EI Power System Review/Short-Open A and B.xlsx

Short%20or%20Open%20Snip.JPG
 
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I finally got around to making a mount for the backup battery to power the EI in an emergency. The mount also incorporates the fuse and diode.

20200926-013639.jpg


Current plan is to put it on the front of the firewall. Under the panel is a better environment, but difficult to access. Plus I am running out of good spots. I also want to keep it forward for cg reasons.
 
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I think you should mount it on the panel where everyone can see it!

Seriously, a nice cool place is really desirable for a backup battery.
 
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