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Dual Battery Engine (or Essential) bus question

BrianDC

Well Known Member
Question for the hive mind to see if I'm missing something. Starting to think about and design my electrical system for my Electric Ignition and Fuel Injection system. Looking through some of the aeroelectric diagrams I'm trying to see if there really is any difference between the following two options.

Option 1 - Both batteries feeding a single engine bus using Schottky diodes
Pro - If either battery is working, power is being fed to the bus
Con - May require more switches or breakers to "test" redundancy of the engine system?

Option 2 - Each battery has a dedicated engine bus
Pro - Easy (or easier) to test redundancy of engine system components
Con -installation of multiple buses?

More than willing to put myself out there to see if I'm missing something. I'm leaning towards option 2, but just not sure if there are good compelling reasons for Option 1?
 

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Uh oh...

There has been several threads on this; it got pretty ugly...

That said, I am flying my -10 with EFII and a modified Z-14 architecture...
 
Brian,

You are overlooking many cons associated with either option.

For an electrically dependent engine (not that I recommend having one) I suggest starting with:
- Two equally sized batteries.
- Each battery feeding an “Engine Buss” that is directly connected to the associated battery. The design must verify that the engine will run on only one of these busses.
- All other loads then come via a separate master relay (or smaller avionic buss relays) connected to each battery. Here again verify the engine still runs and you have whatever else you need to fly the plane if you open both master relays.

The point - when something bad happens you need to be able to quickly isolate most likely faults, this is done by opening both master relays. Having the engine still run and instrumentation to fly the plane after doing so is obviously required.

Carl
 
Brian,

You are overlooking many cons associated with either option.

For an electrically dependent engine (not that I recommend having one) I suggest starting with:
- Two equally sized batteries.
- Each battery feeding an “Engine Buss” that is directly connected to the associated battery. The design must verify that the engine will run on only one of these busses.
- All other loads then come via a separate master relay (or smaller avionic buss relays) connected to each battery. Here again verify the engine still runs and you have whatever else you need to fly the plane if you open both master relays.

The point - when something bad happens you need to be able to quickly isolate most likely faults, this is done by opening both master relays. Having the engine still run and instrumentation to fly the plane after doing so is obviously required.

Carl

Thanks Carl, all of those are good points and things I'm trying to take into account. (I was trying to avoid the electrically dependent engine debate). I did draw my example with the "e-bus" behind a contactor but understand the better design may be to have those busses on the battery side of the contactor.
Was mostly trying to understand if distinct busses vs diodes feeding a single buss posed any real difference besides personal preference.
 
Thanks Carl, all of those are good points and things I'm trying to take into account. (I was trying to avoid the electrically dependent engine debate). I did draw my example with the "e-bus" behind a contactor but understand the better design may be to have those busses on the battery side of the contactor.
Was mostly trying to understand if distinct busses vs diodes feeding a single buss posed any real difference besides personal preference.

Assume the fault is on your one buss, so getting power to the buss is not the issue. This simple example demonstrates why diodes, no matter how used, does not equate to separate busses.

A battery (if properly maintained) is the most reliable element for power distribution schemes. Having two batteries is not about “a battery going bad” is it about the stuff connected to a battery going bad. Thus my point, make sure you can still fly the plane if you loose one side.

Carl
 
Assume the fault is on your one buss, so getting power to the buss is not the issue. This simple example demonstrates why diodes, no matter how used, does not equate to separate busses.

A battery (if properly maintained) is the most reliable element for power distribution schemes. Having two batteries is not about “a battery going bad” is it about the stuff connected to a battery going bad. Thus my point, make sure you can still fly the plane if you loose one side.

Carl

Thanks Carl. This is why I'm asking the question! Hadn't thought of the bus itself having an issue.
 
where are the injectors?

Re post #1:

Keep in mind there is only one set of injectors and they are not powered by the ECU. You don't show injector power.
 
Re post #1:

Keep in mind there is only one set of injectors and they are not powered by the ECU. You don't show injector power.

Good point, and I was actually plowing through the manual regarding this. Don't know if I have the best answer yet. Either an IBBS or diodes to dual feed the injectors. Lots more research to do. No matter what I do understand that I have to accept some level of risk (or complexity).
 
While not flying yet, I did #2 for the reasons Carl stated.
Note: All engine loads are sourced from the battery side of the bus and not post contactor as you've shown. The idea here is that if there's ever smoke in the cockpit, you want to be able to switch off both masters and keep the engine running just as you're accustomed to today.

I was also originally torn on the diode thing mostly due to the injectors only having a single source of power and wanting to drive these redundantly.
Not to bring a specific vendor into the equation, but I happen to be using the SDS product and (for the 6 cylinder version anyways) they've made a modification to the Relay box that allows you to bring in 2 sources of power and in normal mode drive 3 injectors from one source of power and the other 3 from another source of power. Flipping the panel switch which allows either the primary ECU or the secondary ECU to drive all injectors will likewise power all of them from one supply or the other respectively.

To me, that was better than doing any Diode OR'ing and keeping things completely separate. It also allowed me to go back to #2 with separate feeds.
 
One setup that I'm exploring (and is running and working on a Murphy Rebel with SDS EFII) is:

- Main Bus + Main Battery + Alternator
- Emergency Bus + Backup Battery

Both buses feed the engine (possibly with an essential bus, need to check with the builder) and only one is active at any given time. This helps isolate the E-Bus from shorts or issues caused by a bad alternator or the like.

Combine this with an "Oh S***" switch and feed both battery voltages into an EFIS to keep an eye on charge levels and alternator failures.

Backup battery is charged on the ground. Gives you up to an hour of flying time to get on the ground again.

I like this setup mainly because it's pretty simple, but I'm interested to hear others weigh in on this and poke holes. I was thinking of simple ways to keep the backup battery charged like a small solar panel set up to trickle, but hangars ruin this idea.
 
To make sure I understand completely, are you saying that you have to flip a switch to keep the engine running in the event the source you're running on currently has a failure?

If so, I don't like that setup. It will fail at the most inopportune time (just after takeoff for instance) requiring a pilot response in an already critical phase of flight where you just want the engine to keep running with a single failure.

One setup that I'm exploring (and is running and working on a Murphy Rebel with SDS EFII) is:

- Main Bus + Main Battery + Alternator
- Emergency Bus + Backup Battery

Both buses feed the engine (possibly with an essential bus, need to check with the builder) and only one is active at any given time. This helps isolate the E-Bus from shorts or issues caused by a bad alternator or the like.

Combine this with an "Oh S***" switch and feed both battery voltages into an EFIS to keep an eye on charge levels and alternator failures.

Backup battery is charged on the ground. Gives you up to an hour of flying time to get on the ground again.

I like this setup mainly because it's pretty simple, but I'm interested to hear others weigh in on this and poke holes. I was thinking of simple ways to keep the backup battery charged like a small solar panel set up to trickle, but hangars ruin this idea.
 
While not flying yet, I did #2 for the reasons Carl stated.
Note: All engine loads are sourced from the battery side of the bus and not post contactor as you've shown. The idea here is that if there's ever smoke in the cockpit, you want to be able to switch off both masters and keep the engine running just as you're accustomed to today.

I was also originally torn on the diode thing mostly due to the injectors only having a single source of power and wanting to drive these redundantly.
Not to bring a specific vendor into the equation, but I happen to be using the SDS product and (for the 6 cylinder version anyways) they've made a modification to the Relay box that allows you to bring in 2 sources of power and in normal mode drive 3 injectors from one source of power and the other 3 from another source of power. Flipping the panel switch which allows either the primary ECU or the secondary ECU to drive all injectors will likewise power all of them from one supply or the other respectively.

To me, that was better than doing any Diode OR'ing and keeping things completely separate. It also allowed me to go back to #2 with separate feeds.

Just shot an e-mail to Ross about this. If this is an option, I'm quite interested. Agreed the Diode or other gymnastics isn't really my first preference.
 
To make sure I understand completely, are you saying that you have to flip a switch to keep the engine running in the event the source you're running on currently has a failure?

If so, I don't like that setup. It will fail at the most inopportune time (just after takeoff for instance) requiring a pilot response in an already critical phase of flight where you just want the engine to keep running with a single failure.

Yes. This was the hot topic in the other thread. The switch is pretty honking big and definitely a checklist memory item, so with proper planning and training, an intervention to this would take a few seconds at most.

That being said, I'm open to alternatives that afford me the same simplicity. Would diodes protect discharge on the backup battery? Would they protect the E-Bus from a short or voltage regulator failure from the alternator?
 
I have an electrically dependent Subaru in my RV-7A, with a dual battery single alternator system.

Each battery has a Hot Bus (no contactors) which powers a fuel pump and sends 2 power leads to the SDS ECU, which powers both ignition and fuel injectors. The redundant power leads go through diodes to feed the single ECU power leads (ignition, injectors).
So I have 2 Ignition switches and 2 Fuel pump switches (one from each battery).
I can take off with both on if desired - no action needed if a pump or ignition feed failure occurs.
This is pretty close to Z-14 (Aeroelectric Connection architecture) - but my system was designed before Z-14 existed :cool:

The engine electrical system is independent of the rest of the electrical system. I can turn off my Master and Aux (emergency bus) switches and it has no effect on the engine (just like a 'regular' airplane :eek:)
 
I have an electrically dependent Subaru in my RV-7A, with a dual battery single alternator system.

Each battery has a Hot Bus (no contactors) which powers a fuel pump and sends 2 power leads to the SDS ECU, which powers both ignition and fuel injectors. The redundant power leads go through diodes to feed the single ECU power leads (ignition, injectors).
So I have 2 Ignition switches and 2 Fuel pump switches (one from each battery).
I can take off with both on if desired - no action needed if a pump or ignition feed failure occurs.
This is pretty close to Z-14 (Aeroelectric Connection architecture) - but my system was designed before Z-14 existed :cool:

The engine electrical system is independent of the rest of the electrical system. I can turn off my Master and Aux (emergency bus) switches and it has no effect on the engine (just like a 'regular' airplane :eek:)

Very interesting.

Where are the fuses/breakers between the hot bus and the EFI? Before the diodes? After?

What does your key do? Is it just a simple starter contactor or do you connect the coils with it as well?

Is there anything you would change if you designed it again? Or what would you change if the alternator (or a bus) could somehow harm another bus?

My knowledge is light here so forgive me if some of the questions seem silly.
 
Hey Brian,

What is your continuous load with everything running? Injectors, ignition, fuel pumps, ECU(s), radios, EFIS, etc.

Not Total Connected Load but typical cruise current draw?


-Jeff
 
Not to bring a specific vendor into the equation, but I happen to be using the SDS product and (for the 6 cylinder version anyways) they've made a modification to the Relay box that allows you to bring in 2 sources of power and in normal mode drive 3 injectors from one source of power and the other 3 from another source of power. Flipping the panel switch which allows either the primary ECU or the secondary ECU to drive all injectors will likewise power all of them from one supply or the other respectively.

That’s a pretty big change in design, I can’t find anything about that in the latest sds docs. Because currently, the relay box just switches the signal pulse from the ECUs. I would think that adding another power source to the relay would just allow it to function on either source.

The real problem with removing the essential bus is - how do you fully power all 6 injectors and the LOP switch if one side goes out? Injector power does not go thru the relay box.
 
That’s a pretty big change in design, I can’t find anything about that in the latest sds docs. Because currently, the relay box just switches the signal pulse from the ECUs. I would think that adding another power source to the relay would just allow it to function on either source.

The real problem with removing the essential bus is - how do you fully power all 6 injectors and the LOP switch if one side goes out? Injector power does not go thru the relay box.

I know. There seems to be no mention or documentation of this feature on their site. I found out about it through the community and others doing similar things and looking for options/solutions.

With this modification, both power for the injectors and the ground switching (like what always was there) now goes through the relay box.

If one battery bus were to go out, you'd have to flip the Pri/Norm/Bkup switch which already exists in their architecture today. This is the same as what would need to be done if you lost an ECU.

Here's a blurb from a post on the Aeroelectric list.

Some of us with 6 cylinder engines and planning on Z-14 and SDS EFI+EI dual ECUs have been looking for a way to provide dual power paths to the fuel injectors without using a diode-fed engine bus. It is easy to power the primary and backup ECUs, coilpacks, and fuel pumps from separate buses but there is only one set of injectors.

I suggested to Ross and Barry at SDS that an arrangement similar to the relay box already supplied and required for the dual ECU 6 cylinder system would be a good starting point to provide a dual power path. This discussion has led to a slightly modified relay box with an additional daughter board and no new moving parts.

This new version relay box will provide singe-switch control of injector minus-side connections to the ECUs' drivers AND injector positive-side connections to separate power buses.

In the NORMAL switch position the injector relay coils are un-powered and injectors 1, 2, and 3 are driven by the primary ECU while injectors 4, 5 and 6 are driven by the backup ECU (this facilitates cylinder trimming for equalizing A/F ratios at each cylinder). In the PRIMARY switch position all six injectors are driven by the primary ECU. In the BACKUP switch position all six injectors are driven by the backup ECU. This scheme does not change with the modified relay box. Note that injectors are driven to ground by the drivers in the ECUs but are not powered by the ECUs.

The modification passes power to the injectors the same way, instead of having all six of them connected to an engine bus. When the switch (which will now be DPDT) is in the NORMAL position, injectors 1, 2, and 3 are powered by the #1 bus and injectors 4, 5, and 6 are powered by the #2 bus. When the switch is in the PRIMARY position, all six injectors are powered by the #1 bus. When the switch is in the BACKUP position, all six injectors are powered from the #2 bus.

So, no diodes and no additional relays beyond those already required for SDS.

If you want your relay box modified, reach out to Ross or Barry at SDS.
 
Very interesting.
Where are the fuses/breakers between the hot bus and the EFI? Before the diodes? After?
What does your key do? Is it just a simple starter contactor or do you connect the coils with it as well?
Is there anything you would change if you designed it again? Or what would you change if the alternator (or a bus) could somehow harm another bus?
My knowledge is light here so forgive me if some of the questions seem silly.
The hot busses are near the (rear mounted) batteries, and have all the fuses. Wires run from the busses to the front where everything else is located.
No key, just a start button which activates the starter solenoid. The master switch must be on to energize the starter of course.
To start, turn on a fuel pump and ignition switch, turn on the master, hit the start button :rolleyes:
Everything is based on AeroElectric principles, so virtually no chance the alternator (with OV protection) or any bus can harm anything else.
It has worked great for over 12 years. I would do the same thing again.
You can see my electrical system here: https://drive.google.com/file/d/1JsZfjcBrXs7lFKAu-uv8f16NEHnmma11/view?usp=drivesdk
 
injector drivers

... the relay box just switches the signal pulse from the ECUs...

Depends on one's definition of "signal". To me, signal would be something like low current TTL whereas the open collector injector drivers switch the injector coils to ground.
.
 

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The modification passes power to the injectors the same way, instead of having all six of them connected to an engine bus. When the switch (which will now be DPDT) is in the NORMAL position, injectors 1, 2, and 3 are powered by the #1 bus and injectors 4, 5, and 6 are powered by the #2 bus. When the switch is in the PRIMARY position, all six injectors are powered by the #1 bus. When the switch is in the BACKUP position, all six injectors are powered from the #2 bus.

Wow that has the potential to get rid of my ebus, and all the gadgetry that goes with it. Interesting.

Is there just one power path though that relay box? Put another way, will I need to put my 6 individual fuses after the power comes out of that box? Or are there 6 power paths through there so i can fuse them before the box.

How about the LOP switch? The diagram I saw showed a 2amp fused connection to a hot bus was needed for that. So how do you solve that one?
 
Wow that has the potential to get rid of my ebus, and all the gadgetry that goes with it. Interesting.

Is there just one power path though that relay box? Put another way, will I need to put my 6 individual fuses after the power comes out of that box? Or are there 6 power paths through there so i can fuse them before the box.

How about the LOP switch? The diagram I saw showed a 2amp fused connection to a hot bus was needed for that. So how do you solve that one?

There are 3 power inputs. 1) Power for Relay box 2) Main power input 3) Secondary power input
There are 2 power outputs: 1) To power injectors 1,2, and 3
2) To power injectors 4, 5, and 6.

When your Pri/Normal/Bkup switch is in the middle (normal)position:
The relay box is not powered
Relays are not energized.
Primary ECU switches grounds to Injectors 1, 2, and 3
Secondary ECU switches grounds to Injectors 4, 5, and 6
Primary power input passed to power output for Injectors 1,2, and 3
Secondary power input is passed to power output for Injectors 4, 5, and 6.

When your Pri/Normal/Bkup switch is in the Primary position:
The relay box is powered
Relays are energized.
Primary ECU switches grounds to Injectors 1, 2, 3, 4, 5, and 6
Primary power input passed to power output for Injectors 1,2, and 3 and power output for Injectors 4, 5, and 6.

When your Pri/Normal/Bkup switch is in the Bkup position:
The relay box is powered
Relays are energized.
Secondary ECU switches grounds to Injectors 1, 2, 3, 4, 5, and 6
Secondary power input passed to power output for Injectors 1,2, and 3 and power output for Injectors 4, 5, and 6.

PM me your email and I can send you a summary schematic of the configuration.
 
I know. There seems to be no mention or documentation of this feature on their site. I found out about it through the community and others doing similar things and looking for options/solutions.

With this modification, both power for the injectors and the ground switching (like what always was there) now goes through the relay box.

If one battery bus were to go out, you'd have to flip the Pri/Norm/Bkup switch which already exists in their architecture today. This is the same as what would need to be done if you lost an ECU.

Here's a blurb from a post on the Aeroelectric list.

Some of us with 6 cylinder engines and planning on Z-14 and SDS EFI+EI dual ECUs have been looking for a way to provide dual power paths to the fuel injectors without using a diode-fed engine bus. It is easy to power the primary and backup ECUs, coilpacks, and fuel pumps from separate buses but there is only one set of injectors.

I suggested to Ross and Barry at SDS that an arrangement similar to the relay box already supplied and required for the dual ECU 6 cylinder system would be a good starting point to provide a dual power path. This discussion has led to a slightly modified relay box with an additional daughter board and no new moving parts.

This new version relay box will provide singe-switch control of injector minus-side connections to the ECUs' drivers AND injector positive-side connections to separate power buses.

In the NORMAL switch position the injector relay coils are un-powered and injectors 1, 2, and 3 are driven by the primary ECU while injectors 4, 5 and 6 are driven by the backup ECU (this facilitates cylinder trimming for equalizing A/F ratios at each cylinder). In the PRIMARY switch position all six injectors are driven by the primary ECU. In the BACKUP switch position all six injectors are driven by the backup ECU. This scheme does not change with the modified relay box. Note that injectors are driven to ground by the drivers in the ECUs but are not powered by the ECUs.

The modification passes power to the injectors the same way, instead of having all six of them connected to an engine bus. When the switch (which will now be DPDT) is in the NORMAL position, injectors 1, 2, and 3 are powered by the #1 bus and injectors 4, 5, and 6 are powered by the #2 bus. When the switch is in the PRIMARY position, all six injectors are powered by the #1 bus. When the switch is in the BACKUP position, all six injectors are powered from the #2 bus.

So, no diodes and no additional relays beyond those already required for SDS.

If you want your relay box modified, reach out to Ross or Barry at SDS.

The above “blurb” was mine.

Ross and Barry worked hard to accommodate my desire for more redundancy and their solution is pretty simple and elegant in my opinion. I think they saw some advantages to making these changes for those of us hung up on dual power sources for the injectors.

There are a couple of other wiring changes required with this new relay box and as of now, none of these features are well documented. We just completed the wiring changes on our SDS equipped RV-10. Just hung the engine today so still a little ways away from flying.
 
LOP switch in Design 1 programmer

... How about the LOP switch? The diagram I saw showed a 2amp fused connection to a hot bus was needed for that. So how do you solve that one?

The Advance/LOP switch, programmer select switch, and the check engine light are built into the new Design 1 programmer that replaces the legacy round programmer.
 
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Does anyone know if this system change has eventuated? There’s nothing in the EM-6 documentation I can see.

In the below scenerio, there is still only one power wire running from the relay box, through the firewall to the injector. So is this really good redundancy? similarly, there’s only one ground running back to the relay box to be switched by the ECU. A short on either of these wires renders the injector dead (I think?).


There are 3 power inputs. 1) Power for Relay box 2) Main power input 3) Secondary power input
There are 2 power outputs: 1) To power injectors 1,2, and 3
2) To power injectors 4, 5, and 6.

When your Pri/Normal/Bkup switch is in the middle (normal)position:
The relay box is not powered
Relays are not energized.
Primary ECU switches grounds to Injectors 1, 2, and 3
Secondary ECU switches grounds to Injectors 4, 5, and 6
Primary power input passed to power output for Injectors 1,2, and 3
Secondary power input is passed to power output for Injectors 4, 5, and 6.

When your Pri/Normal/Bkup switch is in the Primary position:
The relay box is powered
Relays are energized.
Primary ECU switches grounds to Injectors 1, 2, 3, 4, 5, and 6
Primary power input passed to power output for Injectors 1,2, and 3 and power output for Injectors 4, 5, and 6.

When your Pri/Normal/Bkup switch is in the Bkup position:
The relay box is powered
Relays are energized.
Secondary ECU switches grounds to Injectors 1, 2, 3, 4, 5, and 6
Secondary power input passed to power output for Injectors 1,2, and 3 and power output for Injectors 4, 5, and 6.

PM me your email and I can send you a summary schematic of the configuration.
 
Does anyone know if this system change has eventuated? There’s nothing in the EM-6 documentation I can see.

In the below scenerio, there is still only one power wire running from the relay box, through the firewall to the injector. So is this really good redundancy? similarly, there’s only one ground running back to the relay box to be switched by the ECU. A short on either of these wires renders the injector dead (I think?).

As much as I (and others) have tried, the reality is we are not building a Boeing. We still only have one camshaft, crankshaft, propeller, etc.

If you want the redundancy of dual injectors, you will need to use a Rotax. Maybe the Lycoming iE2 does as well. With SDS (and I assume EFII) there is only one ground trigger and power wire running to each injector, but it's not too difficult to make sure they are well protected. I used high temp spiral wrap on the wires to each injector.

The Dual Bus architecture for the 6 cylinder engines is described in the EM-6 manual starting on page 20. There may be something coming from SDS for dual bus injector power for the 4 cylinder engines too. If not, reach out to me and I'll describe what I suggested to Barry as a fairly simple modification to accomplish this.
 
Dual bus injector power for 4 cylinder engine

Krea: PM sent. I'd very much appreciate learning about this.
 
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