Electrical System - RV-9, Dual G3X, GTN, G5, SD-8

EDIT: Please scroll down for latest diagram version.

Trying to sort out my electrical system and hoping for some feedback. Below is my first draft, and is very subject to change.

PlanePower 60A main alternator
B&C SD-8 backup
IBBS backup battery
Garmin G3X system w/ all the goodies
GTN-625
2 p-mags.

Please take a look and let me know what looks off or recommended changes. In particular I have the following questions:

- Does it make sense to use the keep-alive on the GAD 27? Would be nice to keep the endurance bus live during startup but I can't figure out what amperage it can handle. Clearly in the current setup it doesn't work as there is no power from the main bus during startup so it has no power to keep-alive with. How are people wiring the GAD 27?

- What is the preferred method to set up the sd-8 to come on by itself? I have heard setting it's regulator voltage low is one way? Can I set some sort of alert so I know when this happens? Maybe a Master Caution on the G3X based on voltage drop to the setpoint?

- The G5 has it's own battery but should I also have it on the IBBS so it is the battery of last resort? i.e. when the ibbs dies I still have the internal G5 battery?

- I'm wondering if I even need the ibbs backup battery anymore with the spare alternator and GAD 27 providing keepalive power. Granted the SD-8 can't keep everything running indefinitely but it would add a good bit of time to my main battery runtime. I can get things down to under 8a fairly easily by load-shedding the e-bus if need be.

- What's the right spot for the starter contactor? Should it be wired to the e-bus so I can start the engine only when that is energized?

- What else am I missing?

Hmm, Im wondering if I should have the GAD 27 entirely on the main bus and not have a landing light on the e-bus. Would be nice to turn on a landing light right before touchdown to see the runway but it's unlikely to be crucial.

Can the GAD 27 have its keepalive power and also it keepalive output on the same bus?

FWIW, dump the PP and get a B&C primary alt.

Usually the starter contactor is connected to the other side of the battery contactor. Then if the starter contactor sticks closed, the master switch can be shut off to kill power to the starter.
Suggest that an endurance bus relay be installed near the battery so that all power can be shut off close to the battery without having an always hot wire entering the cockpit. Doing that will be appreciated if there is smoke in the cockpit or to minimize sparks in the event of an imminent forced landing.

Updated the starter relay.

Not sure I'm sold on the relay. I see the benefit but at the same time I like the idea of a low-complexity switch for the e-bus. Also, how much power does a relay like the S704-1 take to stay closed?

You might want to add of the current loading for all of your devices on your endurance bus. It's seems to me that you are over the amount of output for the sd8. Also I'd put the servos on the main. They can draw quite a bit. I put my pmags on pullable breakers. May not be required but I liked the option.

Added current load and breaker sizes. Also broke out the "Cabin Power" and "LRUs" sub-buses.

Amperages in brackets are the breaker sizes, amperages by themselves are typical current usage for the device.

what program did you use for your schematic?

Thanks already playing with it.

Protect that keep-alive circuit with a properly sized fuse or breaker. Bad things can happen with unprotected circuits, even a "little 1 amp" keep-alive circuit. Seen it. Not pretty.

Brownout Prevention

During engine cranking, the small brownout battery will prevent voltage sag on the E-Bus. View PDF

This is starting to look like a whole lot of breakers. Any thoughts on doubling some of them up?

I was thinking Nav+Strobes, Servos+GMC, AudioPanel+GTR20. That brings me down to 28, assuming I use fuses for the alternator protection. I see pictures of breakers on the right side of a panel and ~30 seems like a huge number.

And breaker/switch layouts...

I was wondering if I needed an avionics "master" but that's effectively the e-bus bypass switch. I wonder if I should rename it.

Do not name the switch "Avionics" because it does not shut off the avionics. Simply call the switch "E-Bus". An avionics switch is not desired because it will be an unnecessary failure point. An avionics switch will defeat the advantage of having two power paths (relay and diode).

i would re-think your E-bus. there is a lot of load on a bus that is counting on a 8 amp alternator to keep you flying. the Idea of a E-bus is that you don't have to load shed, when you are down to the E-bus only the bare minimum is powered by it. spark, fuel, one radio, and one nav source, thats about it. I don't even have the efis on it, that's is what the backup battery is for. just my .02

bob burns
RV-4 N82RB

Moved some stuff around. Still wondering about the need for some sort of avionics switch for engine start but can't quite wrap my head around how to do it.

Any thoughts on what that e-bus relay fuse should be? I'm thinking 20amp which would be below the relay heat shedding capacity (25A) and match the peak current while the radio is transmitting.

Items on that bus & typical current:
- GTN 1.6A
- Servos 0.7A (3.6A)
- GTR200 0.6A (3A)
- Boost 3.8A
- G5 0.2A
- PFD 2A
- EIS 0.4A
- GSU 0.2A
- GMC 0.2A
- IBBS 5A

This gets me a typical of 14.7a with a peak of 20A if transmitting and moving servos at the same time. This also has the IBBS charging and the Boost pump on all at once.

If a fuse is loaded to capacity, it could eventually nuisance blow. How about a 25 amp fuse and size the wire accordingly? Not to worry about the relay. Its contact current rating is based on opening or closing a circuit. Once a relay is energized and its contacts stop bouncing, it can handle intermittent current greater than its rating.
I suggest that you post your schematic and question on the AERO-ELECTRIC LIST. See what they say about the desirability of an avionics switch.
Alternator fuses should be located close to the battery. Suggest that the fuse be located on the other side of the shunt. Consider connecting the 60 amp fuse to the other side of the battery contactor.

Been waiting since Tuesday for them to "approve" my membership in that forum...

I'm thinking about moving the boost pump off the e-bus. Any thoughts? Is this a bad idea? It assumes I wont have a mechanical pump and alternator failure at the same time.

Hmm. I think it makes more sense to move the G5 off the e-bus. It has its own 4-hour battery and if I'm not on the ground 4 hours after a main alternator failure I am likely out of fuel and have bigger problems.

New breaker layout on the right.
Changes:
- e-bus relay switch moved to main switch area
- moved g5 down to main bus