Generator overvoltage scenarios

Glenn Martin wrote:

You really need to learn the difference between a "short circuit" and a "Puncture test".

here is a full comparison

https://www.youtube.com/watch?v=R9xZf4p8PkQ

The point is that LFP chemistry is one of the safest and abuse tolerant...and no acid burns

In what scenario in a light Aircraft will someone hammer a 3 inch nail through your battery ? and if you are worried about that being the thing that kills you when flying then perhaps it is time to take  up shuffle board instead...#stall#spin#midair#fuelstarvation#CFIT

You're misunderstanding me. I'm not arguing that LFP is less safe than PbA. I'm not worried about it at all. I'm going to buy an LFP battery just for the weight savings. I know it's safe and I trust it, more than lead acid for sure because it won't spray caustic juice all over me. I don't know why you're insinuating that I'm getting scared of a non-threat and suggesting that I "take up shuffle board instead". I find that mildly insulting. At least read what I'm saying before you assume that I don't understand what I'm talking about.

I opened this topic because I was curious about how the generator could get into an overvoltage scenario (just to learn how the system works in greater detail), not because I was worried about what would happen to the battery. Trust me, if I was scared of a dangerous mix of chemicals in a flying metal machine I'd be more scared of the gasoline than the battery chemistry!

Ken Ryan wrote:

Nevertheless, it does show what is possible, and serves as justification for EarthX requirement to either mount the battery outside the cabin or in a vented container.

Agreed

Fair enough I agree with you. I aologise if it came over as insulting...had a couple of bad days ! students doing their best to kill me..the galling ones are that two argued with the specs of the aircraft "because MS flight sim said that...." and "that I was wrong because he had read on the Internet that..."

Why bother having a POH when google knows all  !!

Anyway I have had an Earth X for two years and before that my own LFP in an aircraft for four years they are great batteries so far much better starts and no issues.  I am sure you will like them too.

Pilot Joe wrote:

Thanks Bill. I think I was misunderstanding it. So the overvoltage failure would be a problem with the regulator, not with the generator itself? So does the generator produce 40v (I guess only at max RPM?) and only if the MOSFETs fail closed will it overvoltage the rest of the system?

The Mosfets in the Regulator and the Battery BMS all perform the same function: Regulate Voltage.
The Alternator's Regulators are tying to maintain ~14v, and the BMS is trying to lower that into the lower 13s.

The alternator proper, is a dirt simple device.
It is a series of coils of wire surrounding a spinning multiple pole magnet.
The alternators voltage varies wildly with both Current load and RPM.
The voltage in the Alternators windings will Increase as the RPMs Increase or the Current Decreases.
The alternator could be at MAX voltage at Idle if the Current load were small enough.
It is the Regulators job to take this varying voltage and restrict it to the ~14 volts we all expect to see.

The battery is going to smoke if (Big IF) the Mosfets in the Alternators Regulator were to Short Circuit AND the Mosfets in the BMS were also to Short Circuit at the same time. 
It is Possible you are having a bad day, but as the Alternator can only muster 40+ Volts, and the Mosfets are rated to at least 60v, it is not very probable you will ever see this happen without an exterior event. (Lightnig Strike ???)

OK so what that website was saying is that the Rotax can only produce 40v at max RPM and low load so a failed regulator would let all that through to the BMS which can handle up to 60v? That clears things up.

Also is it really an alternator, not an AC generator? On my engine, a 912ULS it is not an alternator. It has a permanent magnet (with a self-excited field coil, otherwise it'd be a dynamo) which makes it a generator. An alternator has no permanent magnet and requires external power to the alt field ("ALT" switch) as well as rotation to start working, but a generator will produce power with just rotation.