Question on G3X, shunt, Alternator B-lead?

[Dbro172]

Need help wrapping my head around this. I understand my plane power 60 amp alternator needs a fuse. I didn't have one put in my panel so I ordered a 60amp ANL and holder. I gather that the ANL and Garmin supplied shunt should be mounted on the firewall near the batt.

Can I run the B-Lead (in series) from the alternator, to the ANL, from the ANL to the Garmin shunt and then from the Garmin shunt, to where?? Where does the B lead go from the shunt? (To the panel buss bar, master solenoid, starter solenoid) I'm confused.

[Pat Hatch]

Derek,

From the shunt (or ANL in my case), run a #4 wire to the Starter Contactor. See my schematic below. This is a basic Bob Nuckolls design on my RV-6, and only one of many ways to do it.



Here's a larger crop:



Everything from the alternator to the right is on the firewall. Hope this helps.

[MarkW]

Derek,
Just as Pat says. I just thought I would add a picture.
The B-Lead from the ANL can be a copper strap for convenience and can connect to the starter or Master solenoid as long it is on the load side of the master.

Here is mine before I connected the B-lead to the top of the shunt.

[Dbro172]

Thanks Guys, That helps.

[BillL]

There are two major was ways to wire the shunt to measure amps.

1. on the b lead, it measures the total alternator output.

Pros - one can tell if the alternator is overloaded due to direct measurement of output.
cons - since it measures all output from alt, then it sums battery charging amps, and system load. You don't know exactly what your systems demand is

2. mount shunt in line from battery to system. this always gives amp demand by system.
pros- always gives system demand on start up so you will have a consistent reference for demand.
cons - dont know what the actual battery charging amperage is. but voltage on G3X will give voltage to tell you it IS being charged, just not the quantity.

I chose #2, since the voltage will be measured and by reading amps I could manage power load if the alternator quit. Also, I will be able to remember what the load is for a typical configuration. Battery charge drops after a few minutes to a low number anyway. Digital voltage readouts are so good these days, I am confident that it will tell me if the output of the alternator is substandard, or non existent.

I read up on this and either is an acceptable practice per aviation standards, so both are "right"

What do you want to know, and why, will determine the selection.

[Dbro172]

Originally Posted by BillL

What do you want to know, and why, will determine the selection.

I'm not sure exactly what I want to see? But it seems that once the ANL and shunt are mounted on the firewall, relatively close to each other and close to the master & starter solenoids it would be pretty easy to swap wires & change configuration of whether im monitoring alternator output via the B-Lead or system demand through the panel power.

[BillL]

Originally Posted by Dbro172

I'm not sure exactly what I want to see? But it seems that once the ANL and shunt are mounted on the firewall, relatively close to each other and close to the master & starter solenoids it would be pretty easy to swap wires & change configuration of whether im monitoring alternator output via the B-Lead or system demand through the panel power.

Absolutely! It is not a big deal, just a minor change in wiring. I just passed by this same situation a few months ago and decided to share the potential paths.

Like Yogi said: "when you come to a fork in the road, take one" (extra points for knowing why he said it)

[Bill.Peyton]

I can't help but notice in the posted photo that the alternator bus is exposed. I am only posting this in the interest of safety, so please don't take these comments wrong.
I don't know if this is your final photo, but it is not a good practice to expose any bare wires, more importantly, high current lines, especially on the engine side of the firewall. Boots should be used on all terminations/ terminals, copper bus bar, while easy to install, should be replaced with properly sized aircraft appropriate wire. The ANL fuse should be enclosed in an insulated holder, as should the shunt. Fuses need to be installed as close to the shunt as practical on both wires leaving the shunt going to the measuring device (EIS).
IMHO the ammeter should always show the current draw from the power source. The power source is the alternator, not the battery. If the alternator quits, you should have a preplanned list of avionics to shed to reduce your power. Looking at the current draw from your battery as it slowly dies, is not a good use of the ammeter. Monitoring the current draw from your power source is.

[BillL]

Bill,

So, when your alternator dies, how will you know what your amperage draw is from your battery? Amps will read zero if the shunt is in the b-lead. When the voltage drops this tells me the same thing - no amps from the alternator. Granted, you can still read battery voltage, but it only tells you energy level (capacity), not power draw.

Is there some reason to quantify the battery charging amperage? or is the current draw of the system set up to potentially demand more than the alternator can provide? Wouldn't that be an undersized alternator?

I am not disparaging the B-lead shunt, I just don't see your implied advantage.
Admittedly, the b-lead shunt will give definitive information during the time after start-up and when the battery charging demand approaches its float demand. Does that information tell me something to prevent a failure or aborting a flight?

[Bill.Peyton]

Bill,
Let's say that you have your ammeter on the power source (alt output). You know from many hours of flight that your normal current draw is 18 amps. You set an alarm on the EFIS or EIS to let you know when the current reaches 90% of your alternator capacity. The alarm sounds, you look down and see this unusually high current demand. You land and investigate. If your ammeter is on the battery, you wait until there is a fire or smoke, because you will never know until the current exceeds the alternator capability and the battery begins supplying current to supplement the alternator and feed the short.
Why would you want a meter to help you only if you have already had a failure. I would much rather continually monitor the health of the alt. rather than the aforementioned.
In either case, if your alternator fails you would probably be best served by landing immediately to investigate, even if you have a standby alt. If you have an alt. fail, just shed everything but a previously chosen list of essential equipment and find the nearest airport to investigate.
The great thing about these aircraft is you can build them any way you desire