ECM connection to two batteries

06 Apr 2004 ECM connection to two batteries

The Eggenfellner subaru engine has only one ECM. This will seem very strange to folks used to the dual ignition systems of conventional aviation engines. These auto ignitions are pretty reliable, from what I hear. I hope so!

One thing I want to be able to do is to run my ignition from either or both batteries, without having to worry about one of them failing. I believe it is possible to do this, and I have proposed a possible solution, but I need to get some expert advice about this solution.

Eggenfellner suggest using the Control Vision “EXP-Bus” product, which I don’t really like. It seems like a nice product, but I would prefer to “roll my own”, for reasons articulated clearly by Bob here:

http://www.aeroelectric.com/articles/expbusad.html

Another possiblity is to put the ECM on the “Endurance Bus”, which will be powered by both batteries. Good idea, but in the extremely unlikely event that I get smoke in the cockpit, I want to be able to power off everything without causing the big fan in the front to stop turning. That’s why I want both the ECM and the fuel pumps to be powered directly from the battery bus.

Here is a drawing that shows what I think would work. The ECM is connected to both battery busses, with a fuse protecting the wire after they are joined together. There are diodes on each side to ensure that current does not flow from one battery bus to the other via this connection. I’ll have a switch to turn on/off the ECM somewhere, probably on the ground side, I think.

Please click on the image for a larger view. Any comments, suggestions, or flames are most welcome!

Here is version 1:

Here is version 2 that shows two fuses, upgraded to 10 amps, and two switches. I could use one, but I want to eliminate any single point of failure.

Update: 20051217 – Closer examination of the ECU (ECM) shows that it has two physical connections for the power. I assume this is to elminate mechanical single points of failure. Very cool!

Now, since I really don’t know what’s going on iinside the computer, I need to make sure that I’m not bridging two different electrical busses via the ECU (ECM). I’ll need to find the best way to do this.

Here is another way to do this. This allows both ECM connections to always have power, if either switch is on.

Update: 20210918 I ended up removing the Eggenfellner Subaru engine and replacing it with a Lycoming. Any post that references the Eggenfellner or the Subaru should be there only as an example of what not to do. Listen to the designer, and install the engine that that designer recommends – which for the RV-8 is a Lycoming IO-360.