Ignition backup battery failure on run up - FX3

[Tsquared]

Earlier this summer while performing a engine run up prior to a flight I had the Right Mag emergency backup battery fail.

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Video taken after aborting the flight and returning to the hangar


The PS-1221S battery had been replaced in October of 2025 during my conditional inspection. Normally I fly my FX3 about 120 hrs a year, but I am down to about 50 hrs a year due to work, weather and an elderly mom who has needed a lot of care recently. Prior to this attempted flight on June 6th, the plane had not flown for 3 weeks and had less than 15 hrs since January. Whether this is a factor or not, it’s worth noting. Also note the battery had a date sticker of 06/20/25 and this failure occurred on 06/06/2026.

During the run up the voltage dropped to 5.9 before I shut off the RT IGN Battery. I considered this a failed runup and returned to the hangar for diagnostics.

The solution to this was to replace the PS122S battery with a new one. My new SOP will be to replace that battery one year after the battery manufacturing date and not during the conditional inspection time. I have also changed my engine run up check list to include a 10 sec run on the emergency battery.

if you haven't already changed the alerting on for Alternator/Amp to give you a red and audio alert I strongly recommend doing so. This is aligned with what Cactus Charlie recommended in some earlier threads (see the N40DT accident threads and the ignition battery threads).

I’m posting the videos and pictures as most people have never seen (and hopefully never will) this failure before. Hopefully this thread and the other discussions on the ignition system will educate folks and prevent another N40DT situation where the emergency backup system failed when it was needed the most.

 

[Cactus Charlie]

You are fortunate if you can be shipped a PS 122S that is within 1 year of manufacture date. Read the embossed debossed date stamp. The sticker was likely not put there at manufacture.

Earlier this year I had a battery fail less than 2 months after if was shipped to me.

I have data for all my ignition batteries used over the last 6 years and it's obvious to me that elapsed time (since built, since shipped, or since fitted) is not a good predictor of current condition.

 

[Cactus Charlie]

Read the debossed date stamp.

20241218-JA indicates a manufacture date of December 2024.

The battery in my plot above had date code 20240619-JA. The current battery is a warranty replacement with the same date code. It is performing better.

 

[ve6yeq]

When I bought my most recent PowerSonic PS-1221S, I found the local distributor with the highest volume and explicitly waited until a new shipment arrived. That way, it was only 2 months from manufacture to my hands. If I had gotten the battery from the cheapest place, then it would have been sitting on the “shelf” for a long time...

That said, I am not happy with the battery’s performance, and I have been looking for a local shop with the right crimp tool so I can rewire the system to run off the IBBS, but it appears that I will have to break down and buy the crimp tools...

 

[Sntry]

I understand Neil modified his system to use an EarthX battery with more capacity and reliability. Is this an option anyone else is considering?

 

[ve6yeq]

I understand Neil modified his system to use an EarthX battery with more capacity and reliability. Is this an option anyone else is considering?

Neil replaced the PowerSonic with an EarthX to improve reliability without changing the system’s behavior. Personally, I see the current default FX-3 design as a legacy leftover from before the Garmin G3X Touch was used with its IBBS. If you have the IBBS, which includes automatic failover, you don’t need PowerSonic or the manual, pilot-controlled failover complexity.

 

[Cactus Charlie]

I would not want my ignition on the G3X Touch IBBS. The additional approximately 1.3 A load will shorten the time IBBS is able to drive the DU, GEA, and GSU.

If you had a 6 AH IBBS maybe having one IBBS power G3X and ignition would be viable but I'd still prefer a completely independent emergency ignition power source.

 

[lrobichaux]

I am no electrical or electronics expert, so this is just my humble "jack of all trades" opinion. IMHO, someone (either CC or some smart person on this forum) should publish a safe, tested solution to upgrade the backup battery system on the FX/EX-3 to match the X/NXCub. From the various threads on this forum, the PowerSonic battery is a known, proven weak point that's begging for a solution. The X/NXCub uses a dual IBBS system where one 3AH IBBS is completely dedicated to the right electronic ignition. It's controlled by a 3-position switch that automatically provides power to the right mag when main battery fails. I've upgraded my NXCub with a 6AH IBBS for the right ignition. The other separate 3AH IBBS drives the G3X Touch. Neal's EarthX solution is a good partial solution, but doesn't make the FX-3 as robust as the X/NXCub.

Anyone here on the forum care to suggest a solution? Maybe with all the various posts flying around I missed here someone detailed a solution. I would have my local avionics shop immediately upgrade my EX-3 if there is a safe, tested solution.

Many thanks!

 

[Neal]

Unless people knock on CubCrafters' door for a solution it won't happen. They are very resistant to change for some reason. The squeaky wheel concept applies, people need to pressure them for change and the recent (last year?) accident was enough for me.

 

[Cactus Charlie]

Anyone here on the forum care to suggest a solution?

If you are willing to pay an avionics shop, and it you are willing to change the ignition battery switch, you could just give them the XCub schematic and tell them to implement that IBBS solution.

My design goal was to keep the same switch and the same switch functionality so any future owner of my FX-3 would not be confused by non-standard operation.

I can't get too excited about the ignition battery any more. I believe that, with immediate alerting of alternator failure, I can fly at least an hour without using emergency ignition.

I did recently purchase an EarthX ETZ5G and intend to run some tests with it as a substitute for the AGM battery.

 

[Neal]

I did recently purchase an EarthX ETZ5G and intend to run some tests with it as a substitute for the AGM battery.

Interesting. I did a compare to learn more:

 

[Cactus Charlie]

Did you search include product availability? I cannot find ETX104 on EarthX site although some distributors still have stock.

 

[Neal]

It was being discontinued, I asked them about it when I spoke with them for my warranty replacement of my dead ETX-900. I think if you call them you could get one, I believe they will keep some on hand. It's a six year battery from what I recall.

 

[lrobichaux]

@Neal how would you say your EarthX ignition backup battery solution is working out? Are you having to keep it on a battery maintainer? Would you recommend it?

 

[Neal]

@Neal how would you say your EarthX ignition backup battery solution is working out? Are you having to keep it on a battery maintainer? Would you recommend it?

Outstanding. I could not be happier with it.

 

[lrobichaux]

Outstanding. I could not be happier with it.

Could you be talked into producing another 3D printed tray for a modest fee?? HA!

 

[Neal]

Could you be talked into producing another 3D printed tray for a modest fee?? HA!

I could do that. First see if you can get the battery. Modest? That depends on the plane(s) you own haha.

 

[Neal]

Are you having to keep it on a battery maintainer? Would you recommend it?

I do have a pigtail connected to it but have not used it. I've been away on a RV trip and just got back and after two months the voltage matches the starter battery (ETX-900) exactly which has been on the Optimate charger so there is nothing draining it.

As I'm in my condition inspection now I did pull the panel off to inspect my mount. The current mount I have is not a solid 3D print, I do have a solid one printed in the drawer as I wasn't sure how this one would hold up. It is perfectly fine. I will no longer be inspecting the battery compartment now that I see it is sitting perfectly, no damage from vibration, etc.

I'm not an A&P as you know, it's my best alternative for the battery I had zero trust in, the PowerSonic AGM. I hope to never have to use it and with knowledge gained from @Cactus Charlie here I hope I can get where I need to go managing loads, etc.

So yes, I'm thrilled with it, I can't recommend it as I'm not qualified to do so. But would I did it over again for myself? Absolutely.

I'd call EarthX to see what they have left of these and if they are willing to sell them.

 

[lrobichaux]

I could do that. First see if you can get the battery. Modest? That depends on the plane(s) you own haha.

I'll call and see what they have. Just to confirm, you have the ETX104?

 

[Neal]

I'll call and see what they have. Just to confirm, you have the ETX104?

Affirmative - ETX-104.

MOD Thread 'Upgrading the ignition backup battery in a Carbon Cub E/FX-3 (EarthX ETX104)'

Dec 20, 2025

A few discussions on the forum about choosing an ignition backup battery but this thread will document the actual mod of switching to a different ignition backup battery from the PowerSonic AGM installed in my FX-3. Anyone else that wants to contribute with their mod to their Carbon Cub of any flavor feel free to use this thread as well.

This evening I decided to pull the right forward panel off and begin the brainstorming process of changing out the PowerSonic battery to another option of higher capacity and likely LiFePO4. I didn't take any pictures but here are my discoveries from the...

• Neal
• Replies: 98
• Forum: Aircraft accessories and mods