Carbon Cub EX-3 Engine Failure (N40DT)

[Tsquared]

Interesting discussion.

I appreciate you guys documenting this and developing potential solutions, but for those of us who lack an Electrical Engineering degree, is there a simple way I can check to see how long my ignition backup battery will last? Keep in mind my electrical knowledge starts and stop at plugging in things to a wall outlet.

For example, if I do an engine run at 1900rpm, on the right mag, with the right ignition battery set to emergency and run it for some set amount of time (5min, 10 min or whatever). Will that be a valid test of the emergency ignition battery and will it provide me with any useful data that could be useful in an emergency?

 

[Cactus Charlie]

For example, if I do an engine run at 1900rpm, on the right mag, with the right ignition battery set to emergency and run it for some set amount of time (5min, 10 min or whatever). Will that be a valid test of the emergency ignition battery and will it provide me with any useful data that could be useful in an emergency?

I have tried two methods to test my emergency ignition battery:

1. In flight test running at about 2100 rpm (my typical cruise) right ignition to emergency, left ignition normal. Monitor ignition voltage decay for 30 minutes. Return right ignition to normal. Take the SD card home and plot the voltage curve.

2. Observe ignition battery voltage during a 10 second emergency ignition test at every run-up. Periodically plot the voltage to see trend.

I only did method 1 once because I found the ignition battery charge current when the test was terminated exceeded the recommended maximum charge rate for the battery. I found that method 2 gave a good indication of battery health and did not abuse the battery.

The best way to test the battery is to remove it from the aircraft and run a loaded discharge test (at least 1 A discharge current) for at least 30 minutes. Record and plot the voltage. Then charge the battery and put it back in the aircraft. No one is going to do that as it's easier to replace the battery once a year. I have run this test on batteries I have taken out of service.

The problem with changing the battery once a year is that there is no guarantee that the new battery you fitted is any better than the one you removed. Fitting a new battery does not mean the run-up discharge test (test 2 above) is not needed.

You must shed non essential loads and carefully monitor MAIN bus voltage after alternator failure.

An EarthX battery will self isolate (cut off) at about 11 V and the engine will stop unless the emergency ignition battery is selected. An AGM battery such as the SBS-J16 should keep the engine running until MAIN volts drops below 7 V.

Edit - The fact that an AGM battery will keep the engine running down to 7 V does not imply that it will keep the engine running longer than an EarthX battery that cuts off at 11 V. The rate of change of voltage, and the shape of the voltage curve, is very different for these different battery chemistries.

 

[Neal]

An EarthX battery will self isolate (cut off) at about 11 V and the engine will stop unless the emergency ignition battery is selected. An AGM battery such as the SBS-J16 should keep the engine running until MAIN volts drops below 7 V.

I think LiFePO4's shutdown at 10.0 volts, not 11, but depend on the BMS programming. The voltages can't be compared but instead "state of charge" is the better number. LiFePO4 can use 90% of the available capacity whereas AGM's typically are damaged below 50% which can be around 11.9-12.1V depending on battery.

 

[Cactus Charlie]

I want to know at what MAIN bus voltage my engine will stop. That information helps me determine where I set the amber and red CAS alerts for the MAIN Volts scale.

I may estimate how long that will take based on rate of voltage decay and knowledge of the battery characteristics but I can't program that information into the alerting system.

The EarthX Lithium Battery User’s Manual does not specify the self isolation voltage but the discharge curves for the ETX680C all terminate at approx 11 Volts.

Earth X also says "EarthX ETX-Series batteries have over discharge protection which will disconnect the battery from the active load (drainage) when approximate 95-98% which is typically around *10.5-11V (double for a 24V battery) to protect the cells from damage. At this point, there are only seconds left of runtime left, which is equivalent to running out of fuel."

and "An over-discharged battery typically has an internal voltage less than 11.0V, but when the BMS disconnects, the voltage reading at the terminals of the battery will be zero volts."

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

 

[Neal]

I'm curious why CubCrafters, when they went to glass panels (G3X) they did not add a shunt for alternator current. Apparently this is common in certified aircraft and glass panels. I wonder if this is/was just an oversight? Some added one and I looked at this but decided it's beyond my desire to mess with. But the ability to get a CAS alert of an alternator failure seems like a critical alert we should have in glass panel aircraft.

Current shunt - CubCrafters Forums

CubCrafters Builder Assist program aircraft

forum.cubcrafters.com

 

[Cactus Charlie]

You don't need a shunt to add alternator current monitoring. Simply disconnect the alternator wire at the circuit breaker, slip a Hall effect sensor over the wire, re-connect the wire to the circuit breaker. That part is a 5 minute job.

What takes the time is fitting the sensor wires to the GEA 24 connector and configuring the custom GP input definition.

Some would argue that Battery Amps tells you if the alternator is working. Battery Amps will go negative immediately if the alternator fails. I prefer to have battery and alternator current displayed and recorded.

I used the Amploc Key100 sensor for alternator current.

 

[Cactus Charlie]

I think the new G3X conditional scale color bands and alerting would make it quite easy to issue a RED CAS alert with Aural warning if battery amps went negative in flight while the engine was running.

See "advanced gauge" in the current G3X installation manual. It takes a bit of experimenting to understand the concepts but it opens up all sorts of possibilities for customizing the displays and alerting.

 

[Neal]

Thanks. I'll review.

 

[Neal]

I think the new G3X conditional scale color bands and alerting would make it quite easy to issue a RED CAS alert with Aural warning if battery amps went negative in flight while the engine was running.

See "advanced gauge" in the current G3X installation manual. It takes a bit of experimenting to understand the concepts but it opens up all sorts of possibilities for customizing the displays and alerting.

Did a quick review, looks very interesting but yeah, hard to digest. If you happen to figure out a setup to make a red CAS alert for amps < 0 with engine running, I think it would help many others to know how to set this up and provide some protection via alerting we really need. Any setup steps, screenshots, etc. you care to share would be super great!

 

[Cactus Charlie]

Ok, but please move the discussion of alternator failure alerting to a new thread. We can talk about requirements and implementation there.

 

[Neal]

Question Thread 'How to get a red CAS alert for an alternator failure on the Garmin G3X?'

31 minutes ago

A new thread to discuss a specific topic from this thread from an engine failure situation that occurred due to an alternator going offline due to a wire terminal failure. This thread is to determine how to configure the Garmin G3X using the advanced gauge display functionality and logic controls to show a red CAS alert when the alternator amps go below zero with the engine running.

Thank you to @Cactus Charlie for helping to figure this out.

• Neal
• Replies: 0
• Forum: Avionics