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

[Neal]

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 discussion vice aircraft owners considering options as we learn from the incident.

Link to the new EarthX backup battery -

ETX104 Back Up Battery

The ETX104 back up battery provides power for critical loads with current limiting built in to protect from excessive discharge/charge current.

earthxbatteries.com

It seems this battery is not new. I googled it and there are discussions from quite a while ago, maybe 2020? Why won't this work? Sure seems like a far better option than what we have now. I'll have to check with Pete and see if CC has looked into this although they are slower to change as we know.

 

[Cactus Charlie]

I had not seen this battery before when browsing the EarthX site. Thanks for the correction on its newness.

My only reservation about using this battery is that it likely will not reach full charge. I would still expect the useful duration to be far more than the 2 AH AGM battery even if that was in good condition.

If I did not have a near new IBBS waiting to be put to good use I would probably try the EarthX option. The mod is as simple as removing the existing battery and carrier, fabricating a mount (3D printed tray with strap?), and fabricating a short adapter cable. Probably also should wire the warning LED.

Completely reversible if results are not as expected.

 

[Neal]

I ordered one this morning as I intend to try it and design a 3D printed mount. I'm not sure why it would not charge to 100% as the BMS controls the charge and discharge. There was mention on the Van's forum I believe about a diode setup needed. I'm not smart on that, not sure if that's valid or not. Seems if there is charge current provided and nothing is cutting that off then it should go to full until the BMS stops the charge. Correct me if I'm wrong.

I also saw discussion of using the same battery as the IBBS uses, the TCW battery. Only problem is it's far more expensive and I'd prefer LiFePO4 due to the longer discharge capability. While you could piggy back the two on the same battery it was discussed a dedicated battery should be used for ignition backup. Imagine a higher capacity TCW battery which I think you just bought from someone or what's used in the XCub?

While people will say "We've used this ignition backup battery for years and never had an issue" - the problem is rarely has anyone needed to rely on it. If things go south where I am now, it's not so bad but in hostile terrain like Idaho, you have a problem.

I don't trust the current battery in use to be running my engine. I'm surprised it hasn't been improved in all this time, hence, I ordered the EarthX this morning.

 

[Cactus Charlie]

The diode is required for the existing AGM battery and is already present. I described the required modification.

I have not measured the diode voltage drop. I do not know if CubCrafters used the diode recommended by Lightspeed and I have not studied the data sheet for that diode.

Any diode will have some forward voltage drop and this reduces the voltage available for charging the battery.

Some internet sources say LiFePO4 requires 14.6 V for full charge. I could not find a specific reference on the EarthX site. MAIN bus voltage is normally no more than 14.2 V and the charging voltage will be reduced by the diode.

We do know that 14.2 V is enough for the EarthX main battery. What I don't know is the diode voltage drop.

I simply do not know if the battery will reach full charge. I do not say that it will not.

 

[Neal]

One other note: If I go with the EarthX backup battery, I will no longer be changing it annually like the AGM option. There is a reason the AGM needs to be changed annually - it's trash. That battery needs to go.

 

[Cactus Charlie]

There would be no change in maintenance for me. I have always changed the AGM battery "on condition", never on a calendar schedule.

I strongly encourage anyone using the AGM battery to run a 10 second discharge test at every run-up. The rate of voltage decay is a good indicator of battery condition.

The one currently in my FX-3 is not likely to last a year -

 

[Neal]

LiFePO4 batteries are typically 10 year batteries. I'd consider replacing it at 5 years but obviously monitor performance regularly. I do not think it warrants annual replacement like the AGM from the factory.

Does anyone know the draw of the Light Speed ignition? I'm trying to understand how CubCrafters sized a 2AH battery like this as an engine ignition life critical backup.

 

[Cactus Charlie]

The current draw of the ignition system depends on voltage. As the voltage reduces the ignition module voltage regulator demands more current. Current also increases with RPM (more rpm, more sparks, higher average current to produce the sparks).

Current is typically about 1 A but I have seen it as high as 1.4 A. I have emergency ignition battery current monitoring and recording on my FX-3.

You have a 2 AH emergency ignition battery because CubCrafters is obsessed with weight saving. Lightspeed suggests 4.5 AH.

Despite the small capacity and the short lifespan the ignition battery will, if in good condition, run the engine for over 30 minutes. That is based on in-flight test on my FX-3.

It's not the capacity that is the problem. It's the lifespan. This is compounded by the fact that many owners don't know how to test the battery. That's a problem with the AFM template checklists.

 

[Cactus Charlie]

I also saw discussion of using the same battery as the IBBS uses, the TCW battery. Only problem is it's far more expensive and I'd prefer LiFePO4 due to the longer discharge capability.

I didn't understand this comment. The TCW IBBS internal battery is LiFePO4. The battery pack is 4 standard LiFePO4 cells in series but each cell terminal is wired to the battery pack connector so the cells can be charged individually rather than as a series cell battery. This means the IBBS can be fully charged with a much lower input voltage than would be needed for a standard series cell battery pack.

 

[Neal]

I didn't understand this comment. The TCW IBBS internal battery is LiFePO4. The battery pack is 4 standard LiFePO4 cells in series but each cell terminal is wired to the battery pack connector so the cells can be charged individually rather than as a series cell battery. This means the IBBS can be fully charged with a much lower input voltage than would be needed for a standard series cell battery pack.

So would a dedicated TCW 4 or 6 AH battery be a better option for ignition backup? Better than the AGM? More expensive than the EarthX backup battery but which is the best option over the AGM?

 

[Cactus Charlie]

I don't think you need the best ignition backup. You just need an ignition backup that will reliably keep the engine running for whatever time you think is necessary for the missions you fly.

When this was discussed on SuperCub.org there was one person who was insistent that the emergency ignition must last as long as a full fuel load. I disagreed. There is nowhere I plan to fly that is more than an hour from a safe landing spot.

You also have to consider when you start the clock. I would know immediately if my alternator failed as I have alternator current monitoring and alerting. If my main battery was in good condition I expect to keep the engine running on the main battery for over an hour with appropriate load shedding. I have demonstrated over 2 hours of normal G3X system functionality running it on the IBBS so shedding G3X load from the main battery is one way to keep main battery capacity for ignition.

The duration of the ignition backup only comes into play after the main battery dies. I expect to be on the ground long before then.

 

[Neal]

@Cactus Charlie I skimmed the Light Speed installation manual and did not see anything regarding sizing of the ignition backup battery. Can you find the reference please? I'd like to see the guidance from Light Speed.

 

[Cactus Charlie]

I'd like to see the guidance from Light Speed

The suggested diode type and the battery (4.5 AH) are shown in "FIGURE 1: DUAL POWER SUPPLY DIAGRAM".

I see no need for a battery that big but you have to decide how long you want the engine to run on that battery and size it appropriately.

I don't think the accident that started this thread was caused by the ignition battery being too small. As I have stated previously, I have demonstrated 30 minutes in flight engine run time on a good 2 AH battery.

 

[Neal]

For reference for the thread here is Figure 1 from Light Speed:

 

[Neal]

I received my EarthX backup battery today. It fits in my hand, compact and lightweight. I'm going to put it on my Optimate charger as I've learned from my motorhome LiFePO4 install to install a LiFePO4 battery fully charged for cell balancing.

It will be a week or two before I can install this as my plane is in an alternate hangar due to hangar area construction. I'll check the fit and see how this is going to work and report back when that time comes.

 

[Cactus Charlie]

For reference for the thread here is Figure 1 from Light Speed:

Just to be sure there is no misunderstanding - The FX-3 and EX-3 emergency ignition battery is not wired like this.

In the EX-3 and FX-3 switching to emergency disconnects MAIN bus from the ignition battery, and disconnects MAIN bus from the right ignition module, and connects the ignition battery to the right ignition module.

There is no configuration in which MAIN bus and the ignition battery can power the right ignition module at the same time.

 

[Neal]

The relevant information per Figure 1 was to point out the battery size. It's also technically irrelevant. Size it to what you want. Light Speed tells me the plasma III at cruise draws 1.2A.

Use the battery you trust. Size it to the duration you want.

 

[Neal]

It was brought to my attention from a friend via email that the EarthX ETX104 4AH backup battery is being phased out and replaced with a new offering coming in January. It's larger, heavier and 8AH but at 2 lbs not bad. If you want the ETX104 I suggest ordering ASAP.

EBBS Backup Battery: COMING SOON

EBBS is a backup battery for avionics and critical electronics controls for your aircraft. Delivering uncompromising reliability when it matters most.

earthxbatteries.com

 

[Neal]

I emailed TCW and inquired about their IBBS battery which we have for our G3X to see if it's a viable option. It seems two of these mounted back to back would be an ideal setup. One for G3X, the other for Light Speed.

Integrated Battery Back-up System (IBBS) - TCW Technologies

 

[Neal]

I put the EarthX battery on my Optimate charger so when it's time to install I know it's fully charged. I will keep tabs on the voltages to see if it gets to full charge from routine aircraft charging. I am considering also adding the same optimate pig tail I have on my starter battery and see if I can route it over the top under the panel so I can plug my optimate charger into it to let it maintain the battery a few times a year.

You can get the dimensions from the EarthX website but it fits in my hand, here is a picture of it on the floor of the hangar charging. It's tiny. But I don't know about fitment yet behind the panel.