Electrical System - Decisions

The two most important systems in this aircraft is the fuel system, and the electrical system.  If something happens to either one, the prop will stop turning.  I have taken a lot of time on these two designs.

All UL power engines are electrically dependant.  If the ECU loses power, the engine will stop turning.  Power to the ECU runs both the computer, and the 2 coils.

There are many designs (and opinions) of how to harden a 12V system with an electrically dependant engine.  The most common is a main / essential bus scenario with 2 batteries.  The essential bus is fed by both batteries for redundancy.  This system can use either 1 or two alternators.  The UL power engine (520T specifically) only has provisions for a single alternator.

Design goals:

1. Simplicity (within reason).  Needs to be as simple as possible while meeting design goals
2. Multiple batteries for both redundancy and goal 3.
3. 1 hour plus runtime in the event of an alternator failure.  6 cylinder UL engine needs 
4. Start engine from either battery, or both

Failure modes.  The system needs to be resilient against:

1. Alternator failure (keep engine running for 1 hour minimum)
2. Device short circuit (protect battery from over discharge)
3. Over voltage (in case of regulator failure)

Jumping to the end, below is the output of the process:

Capacity Requirements:

Design Process:

Throughout this process, I looked at the following configurations:

• Option A:  Two independant batteries, 1 bus, 1 alternator
• Option B:  Main / Essential bus, 1 battery per bus, 1 alternator
• Option C:  EFII Bus Manager (Main / Essential bus, 1 battery per bus, 1 alternator)

Below is a table of each option and how it should react in a specific failure condition.

So after months of research, discussions and pondering I am going with option A.

• Simpler
• No diode loss
• All systems can be run of either battery, or both
• Unlike standard lead acid or AGM batteries, the EarthX have their own monitoring - in theory you know what is happening
• With the UL system (permanent magnet generator, not an alternator requiring voltage to excite the field coil) you can run without batteries (if capacitor installed on bus side).  In the event of complete battery failures, the engine will still run.
• EarthX has built in over voltage rejection
• Does require more manual intervention... pilot will have to monitor battery status, charge current / generator status, etc.