Battery Bus - before the Battery Contactor

[WilliamBatten]

To the collective,

I'm a new builder - waiting on my kit. Thought I would begin the journey of designing my electrical system. I've read all of the posts on this forum - but still learning. I know Bob Nuckolls is the guru - currently reading through his book. Having said all of that - I am confused by his "Battery Bus" in these diagrams.

https://bandc.com/reference-library/

These are his Z-11 & Z-13 Electrical System diagrams.

My question is: It seems to me that the Battery bus, connected directly to the battery ahead of the Battery Contactor, with no switch - would drain the battery down while sitting in the hangar. I've noticed that he has used a similar design on several other of his diagrams.

What am I missing?

Bill

[racaldwell]

Bill,

This is not needed and, IMO, not desired, unless you have some particular device that requires a small amount of power to be on continuously. So you need to know what you will plan to install as to whether you need that battery bus. I would guess he says to use a fuseable link as the overcurrent protection. That is just a short piece of smaller wire in the circuit that will melt first in an overcurrent event.

Rick Caldwell
Xenos 0057 37 hrs in Ph 1

[WilliamBatten]

Thanks Rick.

Any other thoughts as to why Nuckolls includes this bus on several of his diagrams?

Bill

[GordonTurner]

My impression is that it is for a few non-critical items that you may want power to without turning in the master. Another example might be comm two, so that you can get Atis clearance, plus a light for entering at night, and per Bob’s notes a clock. The catch is that anything on this bus that is LEFT ON will of course drain the battery.

Gordon

[Rynoth]

I have 2 things on my "hot" battery bus:

1) A small 1A fused wire to my multifunction display (I believe this helps from draining the internal button-cell battery in the unit)

2) A 7.5A fused 12v power socket in the cockpit. This socket has proven to be very useful, as it allows me easily plug in a 4A Battery Tender after every flight without having to open any panels or remove the cowling. During flight I can plug in a 12v-to-USB adapter if I need more plugs for powering gadgets (note that they will continue to drain the battery if left connected/charging after flight.) I have separate USB sockets that are on the avionics bus though so I don't typically need to do this.

I didn't actually create a "bus" for these 2 items. Instead I have an 18AWG wire attached to the battery side of the master contactor with an in-line fuse on the engine-side of the firewall to protect the wire, running to the 12v socket. From the 12V socket I attached a wire (with smaller inline fuse) to my MFD.

[WilliamBatten]

Ryan,

Thanks for the insight - makes sense.

Bill

[Dave Wolfe]

That is also intended for electrically dependent items needed to keep your engine running. Electronic ignition, fuel pumps, etc. By the time you go EFI, two batteries and their hot busses and a suitable architecture pretty much becomes a must from a failure analysis perspective.

[WilliamBatten]

Dave,

Can you elaborate a bit more? I am planning electronic ignition, fuel pump and TBI. However, I am planning a VFR only aircraft. What does the failure analysis look like in this scenario?

Thanks in advance,

Bill

[Dave Wolfe]

Its simple. If you remove a battery, will the engine still run? Battery failures are common, as are generator failures, open or short circuits, blown fuses, etc. When any of these occur, your engine must continue to run.

If your engine is equipped with dual ignitions, one if which does not requure external power to run, then the engine still runs when the electronic ignition is lost due to a battery failure, so you are OK safety wise and you can live with the simple electric system. If both ignitions require external power and are connected to a single batt, then a battery failure kills the engine. Thus, you need a redundant power source and appropriate electrical system to keep an ignition powered during all forseable electrical power failures.

The same goes for fuel pumps, efi systems, etc.

[WilliamBatten]

Thanks Dave - very helpful.

Bill