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View Poll Results: Best place to install a shunt
On the Alt B lead 14 40.00%
On the battery 11 31.43%
On both 5 14.29%
None 5 14.29%
Voters: 35. You may not vote on this poll

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Old 11-06-2019, 03:29 PM
Av8rRob's Avatar
Av8rRob Av8rRob is online now
Join Date: May 2005
Location: Rescue, CA. KROB
Posts: 343
Default Shunt location

Curious to best location for a shunt on a Garmin single alternator system and why?
Rob Lasater
Rv-14 flying baby, yeah.
Paid Dec 2019
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Old 11-06-2019, 03:34 PM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 5,372

On the battery. The health of the alternator to run the system and keep the battery charged is voltage. Amps will show what the system demand is if the alternator quits.

On the B lead, if the alternator quits, you can not manage the amps. In that event, volts will tell you the battery condition as it is used up.

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Old 11-06-2019, 06:28 PM
Bavafa Bavafa is online now
Join Date: Oct 2007
Location: Sacramento, CA
Posts: 3,259

For this discussion, and assuming only one Amp meter which I have, I used the following reasoning/logic for my setup.
- On the Battery, I would expected to read mostly about a negative reading close to zero. If the alternator fails, I would expect to read a positive reading which might be for the first time or possibly during a test flight when I had shut the alternator to see what my typical amp draw is. So, to a good degree I would be unfamiliar with typical amp draw at different stage of flight. Reading on the Batt may help me during a Alt failure situation but then again, I have the volt meter which probably is just as good of indication if not better for that emergency situation.

- On the Alt, I get a reading of my typical usage at any time when the engine is running and the Alt is working. If my Amp reading changes for any appreciable amount, it may be a good indication of possible issue and investigation might be on order. For instance, Amp suddenly goes up, volts comes down might indicate a short somewhere. If my draw goes up for any long period of time, I may want to check the health of my battery or find out what has changed. If it reads zero, I know my Alt has gone south but most likely I will have other indicator to confirm that.
So, I like measure my usage and for that I installed it on the Alt side. With that, I know how much my LL draw, what my heated seat use or the fuel pump.
But it is only $$$ so why not install two.
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Old 11-06-2019, 07:19 PM
Mich48041 Mich48041 is online now
Join Date: Apr 2008
Location: Riley TWP MI
Posts: 3,010

The voting choice "On the Battery" means to measure all current into or out of the battery EXCEPT starter current.
I vote for none because if it is not installed, it can not fail or make sparks during a forced landing.
A voltmeter reading of 13.5 or more indicates that the alternator is working. If you really want to know the current,
put a hall effect sensor around the alternator B lead to measure how hard the alternator is working.
The RV-12 ammeter was designed to measure battery current. Their reasoning is that in case of
alternator failure, the pilot could shut off loads based on current flowing out of the battery.
Why not just shut off everything that is not needed and fly the plane?
Joe Gores
RV-12 Flying
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Old 11-06-2019, 07:26 PM
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emsvitil emsvitil is offline
Join Date: Oct 2019
Location: SoCal
Posts: 222

What do you want to measure?

1. Amp output of alternator?
2. Amp load of lights, instruments, etc?
3. Amps to/from battery?

If you do 1 & 2, you can derive 3 in your head.
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Old 11-07-2019, 03:45 PM
AviatorJ AviatorJ is offline
Join Date: Feb 2015
Location: Oklahoma City
Posts: 901

I have two alternators, I did two alternator shunts off the B leads... This is mainly because I have a VPX and that gives me a good bit of info as well.
RV-10, N10JW
First Flight 2/14/2019
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Old 11-07-2019, 04:38 PM
Steve Iacoviello Steve Iacoviello is offline
Join Date: Apr 2015
Location: The Woodlands, TX
Posts: 33

I put mine on the B lead. In most of my flying it tells me load on the system. I can also see the condition of my battery right after start when additional amps are drawn for charging. If more amps or more time than typical is needed to charge the battery I conclude that the battery is getting weak. In the unlikely event I lose the alternator volts will provide what I need to know.
RV-9A completed 12/2011
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Old 11-07-2019, 07:40 PM
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GalinHdz GalinHdz is offline
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Location: KSGJ / TJBQ
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Originally Posted by emsvitil View Post
What do you want to measure?

1. Amp output of alternator?
2. Amp load of lights, instruments, etc?
3. Amps to/from battery?

If you do 1 & 2, you can derive 3 in your head.
And if you add #2 and #3 you get #1. At least for me, adding is easier than subtracting.

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Old 11-07-2019, 08:35 PM
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rswalden rswalden is offline
Join Date: Mar 2005
Posts: 121

Originally Posted by Av8rRob View Post
Curious to best location for a shunt on a Garmin single alternator system and why?
That always tells me the battery state, with or without the alternator working.

In normal flight with alternator online, I will see +14 volts with 0 amps drawing from the battery. Good. I know the battery is fully charged and the alternator is carrying the full load. How many amps is the alternator producing? Simple. Turn off the alternator and read the current draw from the battery that was provided by the alternator. Want to see if your landing light is working? Turn on the switch and you'll see a momentary load on the battery before amps returns to zero.

Now...If the alternator fails in flight, I again see the discharge RATE of the battery. That helps me reduce the load if necessary by turning off high-draw items... extending the time I can stay flying. If the shunt had been wired to the alternator, amps would show zero. No useful info. I have a red alternator off light that tells me that. No value added.

Also, before starting the engine, I always see the discharge rate of the battery which is useful for reducing the loads while waiting.

One final good reason: I use the experimental EarthX battery which handles huge surges of current during start. It draws a lot of current from the alternator to bring the charge back to zero. I like to keep close watch on the EarthX voltage and amps... In fact, that battery requires a blinking status light to troubleshoot malfunctions. Putting the shunt on the battery gives me a way to constantly monitor volts AND amps.

My opinion: There is no advantage to putting the shunt on the alternator, while wiring it to the battery gives me constant power monitoring from before engine start until after engine shutdown.

Of course, this a lot like our "Primer Wars" debates...lots of opinions!
Bob Walden (Waldo), CFII-ASMEL, Commercial Glider
KFFC "Falcon RV Squadron" Peachtree City, GA
RV-7A Tip-Up, IO-390, 600+ Hours
RV-6/7/9/12 Transition Training
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Last edited by rswalden : 11-09-2019 at 07:46 AM.
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Old 11-07-2019, 11:04 PM
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MCA MCA is offline
Join Date: Jun 2005
Posts: 689

I agree with Joe on this one.

Volts will tell you everything you need to know. One could argue that you don't need to "manage" the load if the alternator fails. Just turn off things you don't need and watch the voltage drop over time.

You should already know the high current items from the low current items as part of your electrical system planning. So knowing what to shut off is not too tricky.

While a shunt is not too complicated, it is one more thing to break.
Marc Ausman
RV-7 980 hours, IO-390, VP-X (sold)
RV-8 (flying a friend's)
Thinking about low and slow backcountry build.

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