Electrical system design - Electronic ignition

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I used an Aeroelectic Connection resource to help design the electrical system below. It has no standby battery to power the Surefly Ignition Module. Power is always supplied via the battery. The master switch can be turned off with the battery powering the ignition for a longer duration than the aircraft has fuel with full tanks. The direct battery connection to the L ignition complies with the Surefly Ignition Module installation instructions. The R side was moved to the Relay to avoid a single point of failure.

I don't like a host of extra backup batteries. But the main battery needs to be fully charged and maybe even replaced annually because its so critical to system integrity. Trying to keep it simple.

What are your thoughts?

This is the simplest way I know to add backup power. From a standard main bus, power the ignition breaker. Connect a backup battery to the breaker as shown, through a toggle switch. For normal operation, the main bus will supply power, through the diode, to power the ignition. And, if the toggle switch is closed, also maintain a charge on the backup battery. If the power is removed from the main bus (for any reason) the backup battery will power the ignition, regardless of the position of the master switch/relay. This means that you can fly with ships battery only and as it's voltage decreases, the backup battery will take over. The blocking diode insures that power from the main bus will be available to the ignition and backup battery but will prevent the backup battery from providing power to anything other than the ignition. If you run electronic fuel injection, simply run an EFI breaker in parallel to the ignition breaker in the drawing and increase the size of the backup battery.

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By the way, in the OP, 'single point of failure' refers to one item that can cause a system failure. In this pic, the ships battery failing would be a 'single point of failure' as there is no source of backup power.

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wireweinie said:

This is the simplest way I know to add backup power. From a standard main bus, power the ignition breaker. Connect a backup battery to the breaker as shown, through a toggle switch. For normal operation, the main bus will supply power, through the diode, to power the ignition. And, if the toggle switch is closed, also maintain a charge on the backup battery. If the power is removed from the main bus (for any reason) the backup battery will power the ignition, regardless of the position of the master switch/relay. This means that you can fly with ships battery only and as it's voltage decreases, the backup battery will take over.

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I consider this design to be less than satisfactory, at least for the Light Speed Engineering system. With manual selection of emergency battery the main battery can be run down to about 6 V before selecting emergency. In the posted design the main battery can only feed the ignition while its voltage exceeds backup battery voltage plus the diode drop. The posted design does not make best use of the available main battery capacity.

Opening the backup battery switch stops its premature use but it also prevents it being charged.

Light Speed Engineering publishes a schematic that shows how they recommend the emergency battery should be connected.

Well, if you want manual 'selection' it doesn't get much more manual than a toggle switch. If you want to 'save' the charge on the backup, open the control switch for the backup. The point of having that toggle in that position, is to close it during normal ops to insure a charge on the backup and provide seemless change over from ships battery to the backup battery as the ships battery loses charge. It also has a secondary feature of isolating the backup battery in case the backup develops a fault, thus preventing lack of backup battery from not allowing flight to maintenance.

Your statement about the backup not allowing 'use' of the ships battery to full extent makes no sense. If the ignition module depletes the backup voltage below ships battery voltage, the ships battery will take over powering the ignition until it's voltage drops below the backup voltage, back and forth. In real life, battery/charging system failures are, many times, not noticed until lights are dim or coms stop working. Without a backup power source the EI engine will stumble or quit. With the backup battery protected by the blocking diode, electrical equipment may be used until the ships battery fails entirely and the backup will continue to power the ignition without interuption.

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wireweinie said:

Your statement about the backup not allowing 'use' of the ships battery to full extent makes no sense. If the ignition module depletes the backup voltage below ships battery voltage, the ships battery will take over powering the ignition until it's voltage drops below the backup voltage, back and forth.

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In that scenario you have no idea how long the engine will run. Both batteries will be exhausted at the same time and the engine will stop. On the other hand - if the main battery is run to depletion, and then the emergency ignition battery is manually selected, you should know that you have a minimum of 30 minutes engine run time remaining.

Absolutely wrong. Two batteries connected in parallel to one load, do NOT both deplete at the same rate as a single battery. If the ignitions draws .5 amps, it draws .5 amps from one battery or a total of .5 amps from two batteries. And since the two batteries are separated by the blocking diode the ships battery will stop supplying power any time it's voltage falls below that of the backup battery.

A backup battery needs to power the ignition system for a period of time equal to a full load of fuel as fuel is the ultimate limiting factor. A 30 minute run time for some of us is a non starter. It's just not enough time to get back to a safe place.

You claim that you'll be starting out with a fully charged battery, but as pointed out above, most charging system failures are noted by electrical equipment shutting down. That means you can never assume that you'll have a good condition ships battery when you note the trouble beginning.

Also, Some backup systems only power one ignition system, either left or right. It needs to be able to power both. What happens if the only system to get emergency power is the problem to begin with? Back to single point of failure.

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wireweinie said:

Absolutely wrong. Two batteries connected in parallel to one load, do NOT both deplete at the same rate as a single battery.

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I did not say that they did. I said they would both be exhausted at the same time. That same time will be later than if either battery was feeding the ignition system alone.

wireweinie said:

A backup battery needs to power the ignition system for a period of time equal to a full load of fuel as fuel is the ultimate limiting factor. A 30 minute run time for some of us is a non starter. It's just not enough time to get back to a safe place.

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You can set whatever emergency battery duration you want. CubCrafters sizes their emergency ignition battery for 30 minutes minimum duration but that does not define the time the engine will run after an electrical system failure. There are many single failures that will not allow flight to continue for full fuel duration. Most of them have no reversion.

wireweinie said:

You claim that you'll be starting out with a fully charged battery, but as pointed out above, most charging system failures are noted by electrical equipment shutting down. That means you can never assume that you'll have a good condition ships battery when you note the trouble beginning.

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I made no such claim.

wireweinie said:

Also, Some backup systems only power one ignition system, either left or right. It needs to be able to power both.

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You can set whatever design requirements you wish. There are many aircraft flying in which a single failure will result in only one ignition system being functional. This does not apply only to aircraft with electronic ignition systems. A short to ground of a P lead in a dual magneto aircraft will cause total loss of one ignition system.

wireweinie said:

What happens if the only system to get emergency power is the problem to begin with? Back to single point of failure.

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The first failure in this proposed scenario is that there is a problem in the ignition system that has the emergency backup. The engine continues to run fine on the other system. It takes a second failure to deny use of the other ignition system. Two independent failures are not an example of "single point of failure".

I suggest you run a more careful failure modes and effects assessment on your proposed system. If the first sign of an electrical system problem is that systems start shutting down then you will have partially discharged the main and emergency batteries before you know you have a problem. I would prefer to have a fully charged emergency ignition battery when I become aware of an electrical system problem.

- two separate power supplies

- ability to select manual or automatic control of power supply selection

- ability to power left, right, or both ignition systems

- able to continue normal flight even if the backup battery inop/removed

- can be installed with three components; toggle switch, power diode, and backup battery

- if desired, EFI can be powered off the backup system by simply connecting the EFI breaker in parallel to the EI breaker

This system does, in fact, give the pilot the maximum number of options possible for keeping the ignition systems in operation during major electrical failures. Deigned, built and several are flying.

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wireweinie said:

By the way, in the OP, 'single point of failure' refers to one item that can cause a system failure. In this pic, the ships battery failing would be a 'single point of failure' as there is no source of backup power.

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During flight if our Battery is the single failure, do we have a single point of failure that takes out power to the ignition? It seems to me that the alternators both remain functional and supply power to the ignition. On the ground, with the engine off then You are right, we have no backup.

Edit....
I suppose an earthX battery thermal runaway overheat would call for turning off the battery master switch to stop the overheat. So if I move the right ignition power supply to the main bus side of the Battery Master Relay we have eliminated that single point of failure issue, and can finish my donut and coffee.

An alternator without a battery connected is unstable at best. If you try to run it, over time, the voltage 'decays'. That means that any time the reg reduces voltage, it does just that. But it can't increase the voltage. It could take seconds or minutes, but without a battery in the circuit, alternator voltage will go away.

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wireweinie said:

In real life, battery/charging system failures are, many times, not noticed until lights are dim or coms stop working. Without a backup power source the EI engine will stumble or quit.
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This happened to me once on a dark stormy night. It seems to me that well designed electrical system will indicate a charging system failure and avoid this scenario. Maybe a Volt Meter or a low voltage light. Seems to me like B&C has this function designed into their alternators and Voltage Regulators, and the the CGR-30P.

Low voltage lights are available on charging systems like Plane-Power and B&C. Also, as you noted, in many instruments from Electronics International, JPI, etc. I like them as they grab your attention and make you look at the panel.

If you have a low voltage light installed, you should check out how they trigger on. The newer systems will turn the light on if system voltage goes under OR over a set limit. But if you have an older system such as the Cessna factory charging system, the light will turn on when the voltage drops below the set point, but, if an over voltage event happens, the OV module will trip the field breaker and the light won't come on until the battery voltage drops down to the set point.

And a note on voltage instruments. Some have a switch that needs to be set for 12 volt or 24 volt systems. If it's in the wrong position, the light will never turn off.

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