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.

 

[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.