AUX Bus Fuse Tray

My AUX buss runs of a secondary ETX-900VNT battery. This buss supplies the backup power to the avionics which have AIRCRAFT PWR 2, and is controlled via a rear mounted contactor specifically for the AUX battery. The AUX battery gets charged via a X-FEED contactor.

I ended up with a second battery because something like the TCW IBBS did not provide me enough power to run things like my Engine Ignition with enough safety margin. So then I was looking at two IBBS, and at that point it was worth just putting in a secondary large battery and buss. When I originally designed this buss I had in my head that the buss would not draw over 30A because I was intending to not put a X-FEED contactor in, rather get power to the buss via a  Schottky Diode.  I still like that idea alot, but the main problem that I had with that approach was that I am really trying to use parts that I could get at any random airport with a maintenance shop.  I don't want to be lugging around my own spare parts everywhere I go.

My MAIN bus has a Vertical Power Electric Circuit Breaker, on the AUX bus I decided to go with fuses. Again, this is my backup system, by the time this thing is blowing fuses, I have some serious issues beyond thinking about reseting breakers. With the 30A max notional draw, I ran a 8GA from the rear mounted contactor up to a fuse block that I installed on my mid-panel. I looked at this fuse block for at least 3 months and every scenario I ran through in my head every time I saw it, this was a horrible idea (although it is a nice spot for it being out of the way). 

There were a lot of things I did not like about the fuse block from above. First, it is not accessible without removing the right panel. That alone makes this a bad idea. Second, I ended up mounting my GTX45R on a rack that I built coming up on top of the ignition brackets shown above. This obscured the fuse block even further. Also, the orientation in which the wires leave the fuse block put half of them rubbing against the GTX tray.

So I moved things around.

I removed the fuse block from the mid-panel and replaced it with my Manifold Pressure Sensors for my FlyEFII System 32. I wanted to keep these on the inside because, well, just because. I then milled out a 6160 1/8" bracket to connect the panel to the mid-panel which will hold the new fuse block.

This took me some time to figure out how to best mount everything, and I am really happy with it. I wanted to keep everything removable, I am not sure why, but it seems like a good idea. The tray itself is held to the mid and rear panels by #8 screws coming up from the bottom. The main wire is coming up through the tray itself inside of a locking grommet. The locking grommet is much bigger than it needs to be because you can fit the terminal (1/4") through the grommet itself.

The angles are hard to see in the photo, but I shielded the power cable with nylon braid, then built a small bracket out of 1x1 angle to bolt in an adel clamp that keeps the AUX power cable centered in the hole. This also isolates the cable from moving and putting stress on the terminal itself. The 8GA 1/4" hole un-insulated terminals have a pretty weak feeling to them...

You will notice that the AUX power does not go to the new sealed bottoe facing fuse block itself. It goes to the 1 and only standalone shunt I have in the plane. This will allow me to read my MAIN current draw from the VPX and the AUX current draw from this Shunt going through the GEA24 Shunt 2.

I am pretty happy with this setup. While it is no where near complete, I like how it is coming together. The fuse block has a sealed lid on it. That lid is facing downwards and you can actually remove the cover and touch the fuses from the co-pilot seat while in a normal seated position. You would have to use a mirror or something to make it easier to replace a fuse in flight, but it would definitely be possible.

Also, I am pretty proud of myself that I remembered my ELT and HOBBS meter are at the bottom right of my right panel. Before I drilled I discovered that the HOBBS meter was dead smack in the way of where I was going to put everything. 

To Shunt or not to Shunt

As I was wiring my GEA24 today I realized there was a decision to be made about where and if to install a shunt for measuring current. I already actually installed a shunt on the firewall to measure the current coming in from the alternator B-Leads to the Main Bus. (again, I will go over my bus decisions later, but this is a Z-14 inspired design where there are two alternators and two batteries). Thinking more about that though, I am not sure I want/need a shunt up there.

The decision impacts what information I will on my EFIS display. I am already pretty accustomed to monitoring Bus1 and Bus2 Amps and Volts from flying a G1000 and G500 system with dual busses. I like having that information available to me as a pilot.

The thing about my design is that the VPX-Pro already speaks to the G3X over RS-232 to relay the main bus voltage and current draw to the EFIS. So the EFIS can be configured to use that VPX information about the Main bus. The downfall of my setup is that the VPX current draw is not the full story for my entire electrical system. Some of the current is being sourced from the AUX bus which the VPX does not have visibility of.

So my original thinking was that I wanted to be able to see how hard the alternator was working. Only one of my alternators will be active at a time because they both feed the same bus. In thinking more about it though, I think it would make more sense to know how much current my AUX bus is using.

Sources of data:

• Main Bus Volts: VPX
• Main Bus Current: VPX
• AUX Bus Volts: GEA24 J244-Pin28 (attached to the aux fuse block)
• AUX Bus current: Rear mounted shunt (or maybe I am going to co-located this up front with the aux fuse block)

The thing that I find a little confusing about this is that the "Main Bus Current" is not the current that is going over the 2GA wire going to the battery box, it is what the VPX is outputting. So if I was flying and wanted to know how stressed my electrical system is in total, there has to be some math involved. That said, there was no way around that. Ideally I would talk to Garmin and they would provide a touch screen option to the EFIS page that when you tapped current you could flip flop between battery current and alternator current with both busses being depicted. I think for now though I am happy. (although I have to go remount my shunt to a new location)

Here are the relevant sections from both the G3X manual and the Vertical Power manual regarding sensing current.

G3X installation Manual

Current
The GEA 24 and GSU 73 have provisions to monitor bus current from two different sources. Current can be measured either using a shunt resistor such as the UMA 1C4 (50mV/100A type) or a Hall effect sensor such as the Amploc KEY100 series.

Shunt sensors: Shunt sensors are connected to the SHUNT 1 and SHUNT 2 inputs on the GEA 24 and GSU 73 (see Figure 30-2.2 and Figure 30-3.1). The SHUNT 1 input can be configured to display either "Bus 1 Amps" or "Main Bus Amps". The SHUNT 2 input can be configured to display either "Bus 2 Amps" or "Essential Bus Amps". 

Hall effect sensors: Hall effect current sensors are connected to the GEA 24 or GSU 73 general purpose (GP) inputs (see Figure 30-2.2 and Figure 30-3.1). Hall effect sensors can optionally be calibrated to adjust for installation differences (see Section 34.4.19.4). The supported configurations for Hall effect current sensors on GP inputs are similar to those supported for shunt current sensors:

• Bus 1 Amps (Hall)
• Bus 2 Amps (Hall)
• Main Bus Amps (Hall)
• Essential Bus Amps (Hall)

Vertical Power: When using a Vertical Power unit, configure SHUNT 1 to "Vertical Power Main Bus
Amps" or "Vertical Power Bus 1 Amps" to use primary bus current data from the Vertical Power unit.
Configure Shunt 2 to "Vertical Power Bus 2 Amps" to use secondary bus current data from the Vertical Power unit.

Vertical Power Installation Manual

3.7 Alternator Current Sensing (Shunt) When planning your aircraft electrical wiring you must consider whether to wire an ammeter (usually a shunt or hall effect sensor provided with the engine monitor) on the wire connecting the alternator(s) to the main bus. The ammeters indicate the amount of current the alternator is providing. 

A shunt is not required to tell if the alternator is working. It is very easy to tell if the alternator is working correctly by simply looking at voltage. If you see 14 (or so) volts with the engine running then it is working. If you see 12 (or so) volts it is not working or not turned on or the devices are drawing more current than the alternator can provide (note, engine must be running). If you set your low voltage alarm on the EFIS at 13 volts, then you will get a low voltage alarm if the alternator fails. 

Since the VP-X provides basically the same information as a shunt installed on the alternator b-lead, our position is that adding a shunt gives you no additional meaningful information. 

A shunt on the alternator b-lead shows the amount of current the alternator is providing to power the devices and charge the battery. The VP-X total current reading shows the total amount of current the devices attached to the VP-X are using. The delta between the two is the battery charging current, which goes to (basically) zero after re-charging any loss from starting the engine or charging a run-down battery. 

If the battery charging current is important to you, then you should install a shunt. If not, then simplify your wiring and don’t install it. Your call. And of course each builder’s needs are different so there is no absolutely right answer. 

The EFIS displays a VP-X page which shows individual device current as well as total current through the VP-X. The EFIS also has an ‘Amps’ gauge that is used to show the readings from the shunt. In some cases the EFIS ‘Amps’ gauge can be used to display total system current from the VP-X. Please check with your EFIS manufacturer for details. 

If you don’t install the shunt then the shunt wires on the engine monitor/ EFIS are not used

Dual Battery setup (ETX-900s)

Hidden deep in the schematic is the heart of the system, two EarthX ETX900-VNT Lithium batteries. The VNT versions of the batteries have vent ports that allow one to externally route the off gas with the supplied tubing. I have installed these in a Z-14 like configuration with one Main Battery and one AUX Battery with a cross-feed contactor between them. I am using my AUX bus as the Aircraft Power 2 feeder in lieu of running something like an IBSS system. So the start-up procedure will be like this:

1. Move the AVIONICS toggle from Off to AUX in order to power the backup power to the essential components. (I will make another post with my AUX power configuration). This will give me a Radio, PFD, Navigator, and a couple of other essential items.
2. Move the MASTER toggle to the "Batt" position. This will enable the main 2GA wire going forward to the starter.
3. Start the Engine
4. Move the MASTER toggle to the "Alt" position. This will enable the alternator.
5. Move the AVIONICS toggle to the "ON" position. This will let the VPX-PRO power up the rest of the avionics.
6. Move the X-FEED contactor to "ON". This will connect the MAIN BUS to the AUX BUS and allow charging of the AUX system.

The first thing to do to enable this system was to build a new battery box to accept 2 batteries. I kept the original location, but by using Lithium batteries, I was able to put to batteries in the place of the 1 original. I welded some 1"x2" 6160 Aluminum Angle to form a box with some .063 2024 to form the bottom of the box that rested on the 1" leg of the sides.

I then welded two lengths of 1"x2" facing forward and aft in order to give the box a couple of mounting points to the original 4 AN3 nutplates. This was in perfect alignment of the original two tie down points as well so I could use the standard hold down bracket.

The new batteries are a little shorter and a little wider, but they are really stable. Shown here is the right side without the electronics tray. See the other post about the avionics tray that I tied into the battery box.

Panel Layout

One of the most rewarding aspects about building your own plane is configuring the electrical systems and avionics just the way you want. While I am not Instrument rated right now, I fully intend to get my IFR ticket as soon as I free up some time by not building, but flying. Ideally, I would also perform my IFR training in my own 10. We will see how that works out. 

I will get into the 2 battery 2 alternator system I have installed in later posts, but I figured I would start with my actual avionics. I selected the following: Vertical Power VPX-Pro, x2 GDU460, 3 axis autopilot with a GMC307, GTN650, x2 GSU25, GMU22, GTR20, GTX45, GMA245, GEA24, ACK ELT and all of the components to have these packages work together. I am leaving my right panel open for a RAM ball mount for an iPad and I did install 3 mechanical backup gauges front and center.

These components are installed into a Aerosport 310 Panel with a throttle quadrant in the center console.

It is a bit of a mess right now, but the heart of this system is physically installed into N50412K.

Wiring

I have been a bit busy lately not posting, but I have been building! For the past couple of weeks I have been really straightening out my electrical system. I spend a lot of time in CAD software at work and for me, it made a lot of sense to build out my electrical system in an actual schematic editor. While Eagle is more geared around building PCBs, it's schematic editor is pretty straight forward. It is also free to use, so I figured it would be a good platform to build out a GARMIN parts library so other people can benefit for the hours spent building out the parts. If you are looking for Eagle it is now part of Autodesk. You can get it here:

https://www.autodesk.com/products/eagle/overview

Over the next couple of weeks I plan to get the rest of my wiring system into Eagle. I have most of it in a drawing right now, but I am working on cleaning it up. I am at the point where I just made the connections, as I build the wiring harnesses themselves, I have been putting in the wire colors into the schematic and marking them off as done.

If you want access to my EAA Avionics library I am keeping it here: https://github.com/karrelsj/eaa

If you want access to my actual schematic: https://github.com/karrelsj/N5412K