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  #1  
Old 12-30-2015, 04:05 PM
togaflyer togaflyer is offline
 
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Location: Cleveland Ga
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Default Shunt question

I sent an e mail to the G3X folks and got an answer that may not have completely answered my question. Being so, I wanted to toss it out to find out if what Im thinking on doing is a safe idea or not. So here it is: I will be running a Plane Power 60 amp for the primary and a Plane Power 30 amp back up on the accessory pad. I will have a field switch for each one. Additionally, the back up only kicks in when the current drops below 13.2 volts. I will be attaching the primary alternator B lead to the shunt. Im wondering if I can also attach the back up B lead to the same shunt terminal. The back up would not be producing any current until activated, which means the primary has already failed. The question is would this arrangement create some inaccurate readings. Or are there issues that would be created by this set up. Reason for doing this. If Im running on the back up, I can monitor the total amp demand, allowing me to manage the load.

Here is the response from the Garmin folks.

Hello Richard,

A current shunt only knows how to measure the current that's flowing through it. If you connect two alternator B-leads to a single shunt, you will simply be measuring the sum of the total current flow. Whether that is an acceptable result for your particular application is a question only you can answer.

- Matt

From: Richard Gurr [mailto:carriageco@gmail.com]
Sent: Wednesday, December 30, 2015 9:57 AM
To: G3Xpert <g3xpert@garmin.com>
Subject: Re: amp shunt

I'm installing a back up alternator along with the primary. I plan to run the B lead of the primary alternator to the shunt. Is it possible to also run the back up (Plane Power gear driven) to the same shunt. The back up will be on a field switch and inactive unless the primary alternator fails. With the back up also strapped to the shunt I could monitor the output of the back up, if activated and with the primary off line (either by failure or the field switch). Will this create erroneous data from the shunt or some other issue if both alternators B leads are attached to the same shunt.

Thanks,

Richard
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  #2  
Old 12-30-2015, 04:40 PM
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Jesse Jesse is offline
 
Join Date: May 2005
Location: X35 - Ocala, FL
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Default

If they are attached to the same side of the shunt, then you will get what it sounds like you are wanting. The reason the backup alternator is set at a lower voltage is so you can have both on all the time and the backup will only put out current if the primary fails or if it can't keep up (which should never be the case). The information you will be reading is what the alternator is putting out, not what you are using. Once the battery is charged fully, then the number should be very close to the same.
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  #3  
Old 12-30-2015, 06:55 PM
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Walt Walt is offline
 
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Quote:
Originally Posted by togaflyer View Post
I sent an e mail to the G3X folks and got an answer that may not have completely answered my question. Being so, I wanted to toss it out to find out if what Im thinking on doing is a safe idea or not. So here it is: I will be running a Plane Power 60 amp for the primary and a Plane Power 30 amp back up on the accessory pad. I will have a field switch for each one. Additionally, the back up only kicks in when the current drops below 13.2 volts. I will be attaching the primary alternator B lead to the shunt. Im wondering if I can also attach the back up B lead to the same shunt terminal. The back up would not be producing any current until activated, which means the primary has already failed. The question is would this arrangement create some inaccurate readings. Or are there issues that would be created by this set up. Reason for doing this. If Im running on the back up, I can monitor the total amp demand, allowing me to manage the load.
You will not get any erroneous readings, this is the same way I have my own dual alternator system hooked up. I would suggest installing current limiters between the alt and shunt.
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  #4  
Old 12-30-2015, 09:30 PM
togaflyer togaflyer is offline
 
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Location: Cleveland Ga
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Walt, when you say a current limiter are you talking about an ANL fuse. I plan to run the B lead to ANL fuse then to the shunt.
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  #5  
Old 12-31-2015, 08:03 AM
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Walt Walt is offline
 
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Quote:
Originally Posted by togaflyer View Post
Walt, when you say a current limiter are you talking about an ANL fuse. I plan to run the B lead to ANL fuse then to the shunt.
Yes same thing.
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  #6  
Old 12-31-2015, 02:07 PM
togaflyer togaflyer is offline
 
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Thanks......
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  #7  
Old 07-10-2017, 10:16 PM
dbaflyer dbaflyer is offline
 
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Thinking about this for my own system. If you attach alt 1 and alt 2 B-Lead to the same shunt how do you setup the G3X monitoring to tell you Alt 1 has failed? I like the simplicity of only one shunt but want to be able to tell that the amps running through the shunt are coming from alt 1 or alt 2.
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  #8  
Old 07-10-2017, 10:30 PM
BobTurner BobTurner is offline
 
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Quote:
Originally Posted by dbaflyer View Post
Thinking about this for my own system. If you attach alt 1 and alt 2 B-Lead to the same shunt how do you setup the G3X monitoring to tell you Alt 1 has failed? I like the simplicity of only one shunt but want to be able to tell that the amps running through the shunt are coming from alt 1 or alt 2.
Buss voltage = 14+ volts, alternator 1 is on-line
Buss voltage =13.2 volts, only alternator 2 is on-line
Buss voltage less than 12.8 volts, both alternators are off.
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  #9  
Old 07-10-2017, 10:31 PM
crabandy crabandy is offline
 
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With 1 shunt I would assume you "fail" the main alt by turning off the field and seeing if #2 is outputting amps into the system during run-up before takeoff.

I opted to use 2 shunts for my Plane Power/SD8 since my amps are nil below 1800rpm. Easy to wire in a second shunt and configure the G3X to display both Amps.
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  #10  
Old 07-11-2017, 06:05 AM
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Pat Hatch Pat Hatch is offline
 
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I would do two shunts and monitor both on the G3X. Bringing both B leads to one shunt also sets up a single point of failure scenario, which could wipe out both alternators. Shunt failures are unlikely, but possible. Something to think about in your planning.
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