It really does actually. Thinking through failure scenarios is difficult! (For me)
I agree that 2 battery failures is probably pretty rare to happen in the air. In the case of one battery failure, I guess you would close the ecu power switch on that side?
What would happen if the alternator regulator went haywire and one alternator suddenly put out 100 volts. Could both earth x batteries come off line in that case at the same time? If that happened, you might have unstable power from the alternators but worse if you turned off the batteries your engine would stop?
I think in theory the b&c regulators are supposed to act faster in an over voltage scenario to shut off the alternator than the battery would shut itself off but I’m not certain on relying on this theoretical function for an electrically dependent engine - what if the regulator totally fries? (I assume you lose this OV protection). Or if using earth x batteries is there a seperate OV protection device to the regulators?
I wonder if the Monkworx alternator needs a battery in the system? If it didn’t then it would be a good candidate perhaps for a direct feed to the EFI/EI bus should both earth x batteries come offline?
Or perhaps one pc680 and one earth x is a good idea?
Some thoughts (and I suspect I”m repeating myself):
- Assuming good maintenance and no abuse (e.g. master left on) AGM batteries are very reliable. I suggest they represent the most reliable component in your electrical systems. The rub is the stuff between the battery(s) and where you want power to go. This is the Achilles’ heel in many systems I review and I suggest the overriding reason two batteries are required for electrically dependent engines.
- Most dual battery designs demonstrate advantage in backup power modes if two identical batteries are used. Small backup batteries can provide a false sense of security.
- For any electrically dependent engine no engine electrical power should come from the downstream side of the master solenoid(s). I build my panels (pMag engine) so that the avionics also come from the battery side of the master solenoids. The test is can IFR flight be continued after you open your master(s) in the event of smoke in the cockpit?
- If running EarthX battery(s) and buss voltage went high (as you your OV protection failed to protect) the batteries would isolate themselves from the buss, the buss voltage would spike at whatever it does while you fumble to find the alternator switch, and your panel will be fried. Here is a VAF thread describing such an incident.
https://vansairforce.net/community/showthread.php?t=154137
The lesson learned is verify your OV protection and know how to turn off your alternator(s).
- The MonkWorX generator (not alternator) does not need any external component to come on line. Personally I would be uncomfortable running it without a battery but I’m good to list this as another backup mode in my plane.
Some common themes:
- A two battery, single alternator design provides for more redundancy backup modes than a two alternator, single battery. Battery size is based on your mission profile. I have ~3 hours of IFR flight on battery power alone.
- I consider my standby generator as a means to get home if the primary alternator fails. This was demonstrated last week by the current owner of my old RV-10.
- Carefully examine the path power goes to get to your critical components (avionics for the IFR mission and electrically dependent engine). Assume any single component (switch, relay, buss, connection, etc.) fails and evaluate what happens. Is this acceptable? If not then you need a mitigation plan.
Carl