Alternator failure - checklist discussion

[Andy]

The other question I have on this topic is what are the real loads and what is the alternator really supplying?

I was curious too so I added alternator current monitoring to my FX-3. For my configuration in flight with strobes and wig/wag landing lights, and after the battery has recovered from cranking, the alternator current is about 11 A and the battery charge current about 4 A.

Your alternator load will be maybe an amp greater as you have the second COM, remote audio panel, and GNC 355 instead of GPS 175.

[hawgdrvr]

I was curious too so I added alternator current monitoring to my FX-3. For my configuration in flight with strobes and wig/wag landing lights, and after the battery has recovered from cranking, the alternator current is about 11 A and the battery charge current about 4 A.

Your alternator load will be maybe an amp greater as you have the second COM, remote audio panel, and GNC 355 instead of GPS 175.

It's odd that the G3X doesn't show that current for the ALT. Why does it only show what seems to be battery charging? We aren't seeing the real load on the alternator.

[Andy]

It's odd that the G3X doesn't show that current for the ALT. Why does it only show what seems to be battery charging? We aren't seeing the real load on the alternator.

CubCrafters uses one current sensing shunt and it is measuring battery current. You could add a second shunt, or a Hall effect current sensor, if you wanted a direct reading of alternator output. You could also re-wire the shunt to measure alternator current instead of battery current.

The hard part of the mod to add current sensors is working on the GEA 24 connectors.

[kshunz]

Here are voltage traces to go along with the trend plot I posted previously. These are all from the same battery, spread out over about 13 months. They were all at approximately 1700 RPM. I presume the current varies linearly with RPM. I hadn't really thought about that in terms of the voltage drop until now, but I can't imagine my small variations in RPM could account for the spread in voltage drops I'm seeing.

ignition_voltage_drop_traces.jpg

[hawgdrvr]

I'd like to know what the current draw is (amp hours) on 2 x lightspeed ignitions, assuming both remain active in emergency mode.

Then we can deduct from the amp hour size of the battery how long is should last.

My understanding though is when that battery dies it then goes to the starter battery?

[Andy]

Here are voltage traces to go along with the trend plot I posted previously. These are all from the same battery, spread out over about 13 months. They were all at approximately 1700 RPM. I presume the current varies linearly with RPM. I hadn't really thought about that in terms of the voltage drop until now, but I can't imagine my small variations in RPM could account for the spread in voltage drops I'm seeing.

Thanks for preparing those plots. Can you reduce the data density by showing only plots for new battery, mid life, and oldest. I think that would show any trend better.

One thing I noticed about your plots is the quite large variation in voltage before the tests start. My ignition battery usually runs about 0.1 V less than Main bus and is almost constant at 14.0 to 14.1 V. (I would have expected ignition battery to be 0.2 to 0.3 V lower than MAIN due to the diode drop but none of these voltage readings is calibrated and they probably all have at least 0.1 V error.)

I have not attempted to analyse the ignition module current. Like you, I expected current to increase with rpm but it isn't zero at zero rpm unless the module is not enabled by the ignition switch. In a spot check of a recent log I saw -1.3 A at 11.9 V and 1,930 rpm for one test . In another log I found -1.3 A at 12.1 V and 1,810 rpm.

Maybe I'm curious enough to run an rpm sweep while running on the ignition battery.

[Andy]

I'd like to know what the current draw is (amp hours) on 2 x lightspeed ignitions, assuming both remain active in emergency mode.

Then we can deduct from the amp hour size of the battery how long is should last.

My understanding though is when that battery dies it then goes to the starter battery?

Only the RIGHT ignition module can be powered by the emergency ignition battery. It will draw no more than about 1.5 A.

When the emergency ignition battery is exhausted the RIGHT system stops producing sparks.

Nothing automatically reverts to the main aircraft battery but the LEFT system will continue to use it if the switch is on BOTH, the battery is not cut off (EarthX), or discharged.

When attempting to deduce battery useful time from current and AH rating you must consider that the battery AH rating is typically for a load of 0.1 C. (2 AH battery rated for discharge at 0.2 A for 10 hours.) The ignition system draws about 0.6 to 0.7 C so the battery will be exhausted far more quickly than the AH rating would suggest.

I have demonstrated 30 minutes engine run time, in cruise flight, on a good ignition battery. The problem is that they don't stay good for very long.