Critique my Electrical System

I'm working on my electrical system design. My plan is to install the EFII system using two alternators and one battery. The primary ECU, injector, fuel pump, and ignition will be powered off the always hot battery bus with the secondary set of the above powered off of the main bus.

I've chosen to install a couple of fusible links in areas where the aeroelectric connection book recommends wire runs 6" or less and I was unable to achieve that. I'm not sure if a run of 2-3 ft would require a fusible link... still scratching my head on that. I haven't completed the diagram quite yet as I haven't decided on which alternator/regulator combo to go with so that portion is left mostly out at this point.

Avionics haven't been purchased yet but I'm leaning towards the Garmin G3x suite of instruments.

Any feedback would be very appreciated.

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Do you have a PDF copy to enable reading the fine print?

Never mind, I found a better copy of your drawing HERE.

Here's my feedback.

I find it difficult to read your schematic, which probably means that it is more complicated than it needs to be.

The most unreliable things in an electrical system (after the alternator) are switches, relays, connectors and contactors. Adding these to 'improve' reliability or increase fault tolerance may not be the best strategy.

Out of curiosity, what did you use to create the doc?

electrical system

We always recommend using a two battery setup when you have an electronically dependent engine.

Alternators do not work without a battery attached. If you have a problem with one of the connections to a single battery, this will be a big issue.

The progression of a safe electrical system is:
1. Two batteries, one alternator.
2. Two batteries, two alternators.

You want to have multiple sources to feed your electronics.
In a single battery, two alternator system, the battery becomes the critical item and the weak link of the system.

The simplest way to implement strategy 1 or 2 above is to use our Bus Manager. This is a one box (safe) solution to the power distribution for the entire plane.

Robert Paisley
EFII

I don't understand the advantage of a two battery system on a single engine aircraft. The battery is simply a reservoir for electrical power. It requires wiring, switches, contactors etc to intergrate into the electrical system so having two batteries means twice as much infrastructure. The batteries too need to be replaced periodically. Why not have a bigger single battery than two smaller batteries?

It is my understanding that the 8-amp backup alternator (permanent magnet) that mounts to the vacuum pad does not require much if any battery voltage to get it going. That could be the backup (unlimited time duration) for the main alternator, should that fail. Once the aircraft engine is running, the main battery has little to do as the alternators should be supplying all the power needs.

Bevan

Bevan,

There is no element in your airplane electric power distribution as reliable as the battery (assuming you don't take off with it flat). Two smaller batteries in parallel provide the large starting current of one large battery. For casualty conditions two batteries provide independent, and configurable power to the panel.

How you carefully design the distribution will determine if you are successful in having a redundant system that is not prone to single point failure.

There are a lot of ways to achieve the objective. I offer that the two battery single alternator approach is a reasonable compromise between complexity and reliability.

Carl

batteries

The batteries don't get connected in parallel.
They are two separate power sources with charge isolation and their own separate wiring.
Alternators put out a rectified AC voltage. This is still an AC voltage. The battery serves as a giant capacitor to smooth out the pulses from the alternator.
There are many ways to lose a battery. Most of which have nothing to do with battery reliability. There are several wire failures that could cause the battery to go off line. The second battery system is the redundant solution to prevent this from becoming a big problem.

Robert

Thanks for your critique. I can see the two battery solution is a popular one. In my mind, as I was reading through various diagrams, the two battery solutions were more confusing and seemed overly complex.

I'm aware of the option of the Bus Manager. I realize you consider it to have triple redundancy and a protected source of power for the EBUS but I just can't get past the fact that all of that is still in one box. What happens of there is a major failure of some sort inside the box?

The other thing I like about the two alternator plan versus the two battery plan is in the event of an alternator failure with the two battery system, you basically just hit the timer on your remaining battery power and have to get it to the ground before that time runs out. With the secondary alternator, a loss of a primary alternator becomes a non event.

These thoughts aren't offered as an argument, I really do appreciate the criticism that has been offered. I only mention this as an explanation of my thought process.

In response to how I created the doc.. it was drawn up with the online schemer from digikey.com. Shemeit is the name. I wish I could figure out CAD but it seems to be beyond my immediate grasp.

Looks pretty complicated to me, but I don't have EFII so that may necessitate more complexity.

Couple of thoughts...do you need an avionics master? Lots of people don't have them, and modern avionics can handle the voltage sag on starting no problem (plus you want your EFIS on during starting, no? assuming it includes engine monitoring).

The starter button on the stick has always bothered me...the possibility of hitting hit while the engine is running is one factor, but so is the possibility of someone hitting it inadvertently on the ground. If you use it, make sure you have adequate safety interlocks to keep curious fingers or fat-fingered friends from hitting it and swinging the prop onto someone's head. IMO, better to have it on a switch or button on the panel.

That said, I'd probably do a FMEA on the system, listing each component and what happens if it fails, and then make sure it's mitigated somehow. E.g., which of your avionics has its own backup battery (e.g., Dynon SV does, as does their D6/D10 units, but my GNS-430W and SL40 don't). Thus, an alternator failure means likely one of those has to be on the essential bus to ensure communications. And so on...

Plan A for ( Day night VFR )I am in the middle of my EFII wire install, I have three busses, A for one ECM, IGN, pump one, buss B for the other ECM IGN and pump two, lights and other stuff split between these. an avionics buss for everything the Dynon system needs, then the big expensive 30 amp back up battery switch for the backup battery that will THEN connect to the ECM buss. this back up battery is charged with a schotsky heat sink diode, the two batteries pass through the firewall at two different locations, separate 8 Ga wires, and redundant battery grounds. I can lose either buss or either battery and still turn the fan.......not completely done but close....

Couple of thoughts...do you need an avionics master? Lots of people don't have them, and modern avionics can handle the voltage sag on starting no problem (plus you want your EFIS on during starting, no? assuming it includes engine monitoring).

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That one has been brought up quite a bit so I'm guessing I can find a better solution to what I'm trying to do. One part of having the avionics master switch was to have the avionics shut of for engine start ( I figured I could power the EFIS through a backup battery or put it on the battery bus). The second idea was to have an elegant way to disable to infinity stick mounted start button so I don't do something boneheaded like hit the starter accidentally in flight. I figured I would never take off without hitting the avionics master switch and wiring the starter through a DPDT switch seemed like a good way to do both at the same time. If anyone has an elegant solution on that one, I'm all ears.

The starter button on the stick has always bothered me...the possibility of hitting hit while the engine is running is one factor, but so is the possibility of someone hitting it inadvertently on the ground.

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Wouldn't the master switch have to be on as well as the start button hit for any action to the prop to occur?

Plan A for ( Day night VFR )I am in the middle of my EFII wire install, I have three busses, A for one ECM, IGN, pump one, buss B for the other ECM IGN and pump two, lights and other stuff split between these. an avionics buss for everything the Dynon system needs, then the big expensive 30 amp back up battery switch for the backup battery that will THEN connect to the ECM buss. this back up battery is charged with a schotsky heat sink diode, the two batteries pass through the firewall at two different locations, separate 8 Ga wires, and redundant battery grounds. I can lose either buss or either battery and still turn the fan.......not completely done but close....

Click to expand...

Thats an interesting way to do it. One option I was considering was putting one half of the engine related items on the battery bus and the other half on the essential/avionics bus wired though a hefty switch to a backup battery. Do you have a diagram on how you are wiring this system? I'd be interested to see it. I'm trying to figure out how the diode is wired to keep the battery charged and what type of diode is required. I guess the next problem would be to figure out where to mount that second battery in an -8. My main battery is aft mounted. I've got that section completed with and avionics shelf in the back for a remote mounted transponder and ADSB. I'd rather not modify the battery tray. I've seen others put a battery in the front baggage compartment on the floor. I'd like to keep that open for baggage or possibly a smoke tank if I decide further down the line. That leaves the firewall I guess. Maybe that's my best option for a backup battery.

jump4way said:

Wouldn't the master switch have to be on as well as the start button hit for any action to the prop to occur?

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Yes, but go read some of the horror stories about what people (or their kids) have done at fly-ins and airshows and such. That's why I have a KEY which must be inserted and turned to ON before the starter will engage, in addition to the Master.

People do dumb things...plan for it.

RV-12 kits come with a momentary key switch to operate the starter. That is its only function. The master switch is separate. I turn the key with one hand, operate the throttle with the other hand, and hold the control stick with my knees.
If you do not want to use a key, how about a panel mounted momentary switch located near the throttle and wired in series with the stick switch? Or if that location is impractical, how about under the panel where it could be pressed with your knee?
Sooner or later, a maintained starter-enable switch will be left in the enabled position, waiting for an accident to happen.

jump4way said:

That one has been brought up quite a bit so I'm guessing I can find a better solution to what I'm trying to do. One part of having the avionics master switch was to have the avionics shut of for engine start ( I figured I could power the EFIS through a backup battery or put it on the battery bus). The second idea was to have an elegant way to disable to infinity stick mounted start button so I don't do something boneheaded like hit the starter accidentally in flight. I figured I would never take off without hitting the avionics master switch and wiring the starter through a DPDT switch seemed like a good way to do both at the same time. If anyone has an elegant solution on that one, I'm all ears.



Wouldn't the master switch have to be on as well as the start button hit for any action to the prop to occur?



Thats an interesting way to do it. One option I was considering was putting one half of the engine related items on the battery bus and the other half on the essential/avionics bus wired though a hefty switch to a backup battery. Do you have a diagram on how you are wiring this system? I'd be interested to see it. I'm trying to figure out how the diode is wired to keep the battery charged and what type of diode is required. I guess the next problem would be to figure out where to mount that second battery in an -8. My main battery is aft mounted. I've got that section completed with and avionics shelf in the back for a remote mounted transponder and ADSB. I'd rather not modify the battery tray. I've seen others put a battery in the front baggage compartment on the floor. I'd like to keep that open for baggage or possibly a smoke tank if I decide further down the line. That leaves the firewall I guess. Maybe that's my best option for a backup battery.

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I am using this: http://www.bandc.aero/essentialbusdiodew8wattheatsink.aspx

Not sure maybe someone can correct me if I am wrong but I think this will handle 15 amps? but it will not be passing that amount of current to a fully charged stand by backup battery, my mission is VFR so if bat one goes off line I will have plenty of time to land under power on bat two..somewhere....I hope...testing in phase one will confirm or not.....I am using lots of NKK quality 15 amp S series switches, trying to keep it simple.....VFR...VFR...VFR!
I have a window and paper map if I need to shut down the color TV thing in front of me ;-)

I'm a total newbie to all of this and have never built a plane in my life, so take what I say with a few grains of salt.

Having the DPDT that serves on one side as an avionics master and on the other as a hot wiring for the starter push button. What happens when you lose an engine in flight and want to attempt a re-light? Do you have to shut down your avionics for it cuz the DPDT has to be moved to the bottom position to energize the start push button?

Maybe I totally misread the plan... i am new after all

I think that most aircraft engines will continue to rotate due to the prop windmilling. So if the pilot forgets to fuel switch tanks and runs one tank dry, then all he has to do is lower the nose and switch to the other fuel tank. If lucky, the engine will start running on its own before reaching the ground.
There is also an alternate source of avionics power from the battery bus through a relay (relay controlling switch is not shown).