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

 

[ve6yeq]

I have been thinking about the "backup" power situation on the Carbon Cub EX-3/FX-3 and the different panel options that are commonly used. From my limited testing of the PS-1221S ignition backup battery during engine runups (voltage often drops to 11V within seconds) and my experience with AGM batteries from my amateur radio days, I am not relying on it for more than 5-10 minutes of engine runtime.

For a simple "steam gauge" panel, there is no IBBS and only an Ignition backup battery. In this case, after the alternator has failed and the main battery is discharged, the ignition backup battery will provide 5-10 min of engine runtime, and with static system "powered" airspeed, it will turn a day VFR "emergency" power-off landing into a "precautionary" landing. However, there is no margin for a "diversion" to land elsewhere that is more convenient.

To maintain the same level of redundancy for a "glass panel", a 3 AH TCW IBBS was added to keep enough flight instruments operational for turning a day VFR "emergency" power-off landing into a "precautionary" landing without changing the ignition backup battery. Even though the IBBS will keep the panel alive for some time (helping prevent stalls/spins), the ignition backup battery situation does not allow any margin for "diversion".

Now, along comes the "IFR" panel (e.g., G3X Touch PFD, G5 BFD, GNC navigator + autopilot) and the EarthX main battery, and the available level of power redundancy is very "muddy" at best. Personally, the combination of a small IBBS for avionics and the separate ignition backup battery does not match up to my expectations of being able to divert if the weather allows. I do have the optional battery fitted to the G5 BFD, which is useful in case of a PFD failure, but it does not address the ignition limitations.

I want to be able to divert and not just perform an immediate "precautionary" landing, as I plan to fly in areas without any suitable "precautionary" landing opportunities in the future. I am thinking of a single larger backup battery (e.g., a 6AH TCW IBBS or the new 8AH EarthX EBBS) for the right ignition and core flight instruments, along with a secondary power source (e.g., a Monkworkz MZ-30). This way, if I lose the primary alternator and/or the main battery fails, I will have the time to divert, not just perform a "precautionary" landing ASAP.

 

[Neal]

Good info and I look forward to what you come up with. I agree, the powersonic is not something I want to rely on and this accident (N40DT) has pushed me to the point of it's time to do something about it.

I haven't pulled my right interior panel yet to explore the EarthX backup battery option to see if it will fit. But I think we/I need to also not be forced into placing an ignition backup battery in that location. We should discuss alternate location ideas. I'd say there's plenty of room behind the panel as the EarthX backup battery I have fits in my hand. It could sit right behind the GDU with ease. The question is will it cause any type of interference? I'm not sure where the wires are running from to get to the right interior panel and how hard it may be to change that wire path.

I'll add that I think I owe full transparency in that I posted a question to the CubCrafters "Official Product Support" Facebook group on October 15th asking about the EarthX backup battery as an alternate ignition backup battery and to date the message has not been approved and I don't expect that it will. I have a screenshot of my post which I don't feel the need to share "yet" but I am beyond disappointed that CubCrafters has chosen not to share this situation for lessons learned and help address concerns and alternate options for an ignition backup battery. I am a customer and this kind of treatment is absolutely unsatisfactory. I can't answer why they haven't approved my post and this is not the first time they've done this as they did with my coil pack situation a year ago which I expressed my extreme disappointment to them about. I needed help diagnosing the situation, no ranting was involved, I just explained the problem yet they wouldn't approve my post. This is why I invite all to come here to Flightinfo.com to share tips with one another to avoid censorship. The Facebook group is for owner support. If they are allowing non-owners in and they are fearing marketing concerns, they failed by allowing non-owners in a private group which they created to become their official support channel for owners. An aviation company relying on Facebook for support is baffling but Pete via email has always been outstanding and remains so.

This forum is not private and topics here have already come up by prospective owners asking about situations they are learning about here and YouTube. I will continue to ask questions and share as we learn this lesson in the Air Force to learn from accidents to prevent others. It seems some want to hide them and not address them. Very disappointed.

So let's continue to work together to see what we can come up with as we all know we CANNOT depend on the quality of battery CubCrafters uses for the ignition backup battery. While spec wise a 2 AH battery should suffice, this battery has proven to degrade fast and not knowing if we are ever recharging it fully to maintain battery health is another concern. This is why I plan to add a pigtail that I already purchased to my EarthX backup battery so I can put it on a charger a few times a year as cell balancing is crucial to LiFePO4 batteries and I can't trust the aircraft charger to be properly maintaining it.

 

[Cactus Charlie]

You guys are missing the essential fact that if you are alerted immediately for alternator failure, and manage the electrical systems properly, you have at least an hour for a diversion before needing to consider the emergency ignition battery.

That said, I plan to fit 3 AH TCW IBBS instead of the existing 2 AH emergency ignition battery. If for no other reason than I am confident it will have a far longer life if it is never used.

It is not trivial to fit an IBBS instead of the AGM battery. I have been considering the design options for a while and have decided on, and bench tested, the configuration I plan to use. However, I will not publish anything until it has been tested in the aircraft.

 

[Neal]

You guys are missing the essential fact that if you are alerted immediately for alternator failure, and manage the electrical systems properly, you have at least an hour for a diversion before needing to consider the emergency ignition battery

You’re assuming a healthy and fully charged battery. The health of these concerns me the most. Paranoid? Maybe. I’ll take extreme caution over complacency.

 

[Cactus Charlie]

You’re assuming a healthy and fully charged battery.

Perhaps you misunderstood. I assumed your paranoia was directed at the AGM emergency ignition battery. I am only assuming a healthy and full charged MAIN /STARTER battery.

Do you willingly fly with a weak, uncharged, main battery? Some people do and get way with it because they have magnetos. Not wise with electronic ignition.

 

[Neal]

My starter battery is a fully charged and well maintained EarthX ETX-900. My concern is the powersonic.

 

[Cactus Charlie]

You lost me. You said "You’re assuming a healthy and fully charged battery. " and then you say "My starter battery is a fully charged and well maintained " which was exactly my assumption.

Given those conditions you have at least an hour to land even if the ignition battery is worthless assuming you follow a reasonable load shedding plan.

 

[Cactus Charlie]

assuming you follow a reasonable load shedding plan.

Do you know how to force G3X Touch system to use IBBS rather than main battery. I'll bet that most owners do not.