I have a Z-14 inspired design (which is not depicted in the default VPX planner diagram). This is a two alternator and two battery design (again, not depicted here) that can be connected via a single X-Feed contactor. Both of the outputs of each bus contactor feed a Schottky E-Bus Diode to create an essential bus for my electronic ignition.
The Main alternator is a AL12-EE70/B while the backup is a FS1-14B. Only one alternator will be live at any given point in time and the switch over between the two is a manual operation via a physical panel switch that controls the VPX ALT1 and ALT2 power pins. There are actually two switched inputs into the VPX for ALT1 and ALT2 that are connected to a SPDT on the VPX, but the VPX does also manage the ALT lines to ensure only one alternator can be active at any given point in time.
The VPX is physically located above my left side rudder pedals on a custom bracket that spans the firewall to mid-panel. This turned out to be a great place for the VPX, however I will warn you, think about your connectors with the wires attached when planning where to put your VPX box. With the connectors and wires connected, there is not much room above the rudder pedals.
Trying to avoid single points of failure was the goal in my setup. I mention my alternator setup because that is one of the first things I struggled with that I was not fully excited about (but the best of the options I came up with). Having two batteries and two alternators is obviously for redundancy however while designing you have to balance redundancy with complexity. If you look at the VPX configuration you will see both ALT1 and ALT2 switches as well as ALT1 and ALT2 Field wires. The actual schematic for a given alternator to be enabled is not straight forward. A ground line comes into the my MASTER/ALTERNATOR switch. When the Master switch is moved to the ALT position, this connects ground to the input of a SPDT Alternator selector switch. This in turn tells the VPX-Pro to enable the alternator that is chosen.
From the pilots perspective this alternator setup is not bad. If the alternator appears to not be charging, flip from ALT1 to ALT2. If the system is still not charging at that point one could look at the MFD VPX configuration page to see if the ALT1 or ALT2 power lines have any current draw, but after that, there is not much you would want to do in the air. There are just too many variables behind the panel.
The second issue that did not excite me was my coil packs on my electronic ignitions. All three coil packs come to a single point and FlyEFII recommends keeping those three feeds together. The problem with this though is that if you loose a connector on your coil pack line your engine is no longer running. At least with the ignitions themselves there is redundancy. The coil pack is a single point of failure. I ended up really not liking the options I came up with on how to feed power to my ignitions and coil packs. Should I bring them back straight to the battery? Which Battery? Do I split the ignitions between AUX and MASTER? If I split up the ignitions on the busses, and the bus that powers the coil pack goes down, whats the point?
I ended up biting the bullet and putting an essential bus in on top of the 2 alternator 2 battery design. Each power contactor feeds a schottky diode making an essential bus. If either the AUX or the MASTER or both are on, the essential bus will have power. I ran a dedicated line from the rear or the aircraft forward for the essential bus and this ultimately goes to the only three physical breakers I have on the plane. A 15A for the Coil pack and 2 5As for the ignitions.
While again, the decision to add a third bus adds complexity, I feel better about the redundancy in the system for the critical components to keep the engine running.
Speaking of complexity... When I first setup my rear mounted avionics tray I forgot that my AUX and X-FEED contactors are reverse from each other, i.e. they are back to back. My MASTER contactor is in the stock location and feeds both the X-FEED contactor as well as the main power line running up to the starter and VPX.
On the left of this image is the AUX contactor with the right being the X-FEED contactor. When installed into the plane, the top right post of this photo gets a 2GA jumper to the output of the MASTER contactor. The top left post of this photo gets a 2GA jumper to the battery. Do you see the problem?
