Carbon Cub EX-3 Engine Failure (N40DT)

[Cactus Charlie]

I have chosen to use 11 V as the self isolation voltage.

EarthX replied to my question about self isolation voltage of the ETX680C -

"It will disconnect right around 10.8 volts depending on how much load is on the battery."

 

[Neal]

It's up to the NTSB as we know to look into all of that but we can all learn from accidents. It is normal in the Air Force (military) to study accidents and learn from them. It's apparent in GA no one wants to learn from accidents and just be cowboys. Very disappointed in some recent handling of this matter which I'll share someday in the future.

I'll check the diagram you pointed out, thank you. I just like the see the guidance in black and white. I know Klaus mentioned in the past the installation is not done per the manual such as with shielded grounds, etc. when digging into my coil pack failure saga. I get it that there are variables such as space available, size, weight, and climate that need to be factored into this battery decision.

 

[Neal]

Posts have been moved out of this thread to a new thread on topic of backup batteries for electronic ignitions so this thread can remain on topic of the accident and any lessons learned. For backup battery discussion please use:

Thread 'Choosing an ignition backup battery for an electronic ignition piston aircraft engine'

Oct 11, 2025

This thread is created to continue the discussion from this CubCrafters Carbon Cub EX-3 thread that had an engine issue and presumably defective ignition backup battery. It created a discussion on the suitability of the PowerSonic PS-1221S 2AH battery used by CubCrafters. Several are discussing options for a better replacement that aligns closer to what Light Speed recommends of a 4.5 AH battery size and also methods for testing the backup battery.

Posts below are being moved out of the referenced thread above into this one to keep the other thread on track with the accident...

• Neal
• Replies: 32
• Forum: Engines and propellers

 

[belchfire]

I can’t fracking believe that anyone allows an aircraft ignition system that relies solely on a battery for backup…and it is being discussed as if it is normal-my God people!

Of course I liked Bendix mags better than Slicks because the Bendix mags were built to be repaired and the Slicks got recycled...

I guess the newfangled fuel systems probably aren’t any better but seriously, a backup battery for the ignition system?

Oh well…us dinosaurs won’t be around much longer and we will leave the world to the likes of the idiot sales people and punk engineers at Boeing who think the 737 can replace the 757!

 

[Cactus Charlie]

"but seriously, a backup battery for the ignition system?"

Why would you think the shorting of two P leads is any less probable than loss of the alternator, full discharge of the primary aircraft battery, and then full discharge of the backup ignition battery?

I assume you have some experience of failure mode analysis and can offer a comprehensible explanation.

I have experienced loss of a magneto due to an incorrectly installed P lead. I have never had a failure in the redundant electronic ignition system of my aircraft.

 

[ve6yeq]

Instead of multiple backup batteries, it is better to just have a single IBBS and a backup power source such as a MZ-30 generator from monkworkz.com?

 

[Neal]

Instead of multiple backup batteries, it is better to just have a single IBBS and a backup power source such as a MZ-30 generator from monkworkz.com?

Cool concept!

Yes, a 6 AH IBBS for example would work. Or you could use two back to back smaller and independent batteries which is what I think CubCrafters for example should consider. Do you want a shared vs dedicated battery per task?

The concern that comes to mind with the currently installed PowerSonic sealed AGM style battery is how much of that battery can actually be used. Typically AGM's are considered dead (damaged) at 50%. It's rated at 2 AH but I'm not sure how many AH's I'd actually get. With a 1.2A power draw on a Plasma III ignition module it should last a good amount of time, "they" say we should be able to at least get 30 minutes. Assuming it's a healthy battery. I just hate how fast the voltage drops in just a 10 second runup test realizing though we're seeing charging current to non charging current (actual voltage of the battery) and then the drop under load. I'm curious what people are seeing their voltage stabilize at during the emergency ignition test. I think my last runup it was around 12.1.

 

[Cactus Charlie]

The concern that comes to mind with the currently installed PowerSonic sealed AGM style battery is how much of that battery can actually be used. Typically AGM's are considered dead (damaged) at 50%. It's rated at 2 AH but I'm not sure how many AH's I'd actually get.

A datasheet for the PS-1221S can be found here. As with all AGM batteries the capacity depends on the discharge current.

I also have a PowerSonic Technical manual on file but I have not been able to find it on-line.

 

[Neal]

A Ram Air Turbine (RAT) would be cool