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Measuring Amp draw

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speckter(at)comcast.net
Guest

Posted: Mon Nov 10, 2008 12:24 pm Post subject: Measuring Amp draw

I have my amp meter set up as a load meter measuring the output of the alternator.  However, it reads about 40 amps after the battery is recharged.  I don’t have 40 amps worth of fuses in my panel so I suspect that the shunt/EFIS aren’t giving me the correct reading.  Does anyone have a DC amp meter that they are willing to let me try to get an accurate reading so that I can confront the EFIS/engine monitor folks and get a correct correction factor applied?  

   

Or does someone have a better idea?  

   

Gary Specketer  

40274 Flying  

       [quote][b]

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jim(at)CombsFive.Com
Guest

Posted: Mon Nov 10, 2008 2:18 pm Post subject: Measuring Amp draw

Gary,I actually have a high current lab DC power supply that we could have used to check the shunt.  I know we had a busy weekend, but we could have just connected up the power supply and had a known calibrated power feed to the airplane (via the alternator lead).It's pretty heavy, or I would consider shipping it to you to use.The other choice is to make a second calibrated shunt and put in series with the alternator lead.   That's a lot easier to do.Jim C> I have my amp meter set up as a load meter measuring the output of the> alternator.  However, it reads about 40 amps after the battery is> recharged.> I don't have 40 amps worth of fuses in my panel so I suspect that the> shunt/EFIS aren't giving me the correct reading.  Does anyone have a DC> amp> meter that they are willing to let me try to get an accurate reading so> that> I can confront the EFIS/engine monitor folks and get a correct correction> factor applied?> > > > Or does someone have a better idea?> > > > Gary Specketer> > 40274 Flying> >   [quote][b]

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jim(at)CombsFive.Com
Guest

Posted: Mon Nov 10, 2008 3:06 pm Post subject: Measuring Amp draw

you might also try this:http://store.solar-electric.com/mka-100-100.htmlYou have two alternators, do you have two separate shunts?Jim CN312F, 40192   [quote][b]

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speckter(at)comcast.net
Guest

Posted: Mon Nov 10, 2008 5:24 pm Post subject: Measuring Amp draw

I don’t think the shunt is at fault, I think that the calibration in either the engine monitor or the EFIS is converting it to a wrong number.  The shunt is measured by the engine monitor and that info is fed to the EFIS.  Somewhere in there is the problem IMHO.  

   

Gary  

         

  

From: owner-rv10-list-server(at)matronics.com [mailto:owner-rv10-list-server(at)matronics.com] On Behalf Of Jim

 Sent: Monday, November 10, 2008 5:17 PM

 To: rv10-list(at)matronics.com

 Subject: Re: Measuring Amp draw  

  

   

Gary,

 

 I actually have a high current lab DC power supply that we could have used to check the shunt.  I know we had a busy weekend, but we could have just connected up the power supply and had a known calibrated power feed to the airplane (via the alternator lead).

 

 It's pretty heavy, or I would consider shipping it to you to use.

 

 The other choice is to make a second calibrated shunt and put in series with the alternator lead.   That's a lot easier to do.

 

 Jim C

 

 > I have my amp meter set up as a load meter measuring the output of the

 > alternator. However, it reads about 40 amps after the battery is

 > recharged.

 > I don't have 40 amps worth of fuses in my panel so I suspect that the

 > shunt/EFIS aren't giving me the correct reading. Does anyone have a DC

 > amp

 > meter that they are willing to let me try to get an accurate reading so

 > that

 > I can confront the EFIS/engine monitor folks and get a correct correction

 > factor applied?

 > 

 > 

 > 

 > Or does someone have a better idea?

 > 

 > 

 > 

 > Gary Specketer

 > 

 > 40274 Flying

 > 

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       [quote][b]

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speckter(at)comcast.net
Guest

Posted: Mon Nov 10, 2008 5:25 pm Post subject: Measuring Amp draw

Both Alternators feed into a common shunt.  Neal George has a clamp on amp meter, so I will try that first.  

   

Gary  

         

  

From: owner-rv10-list-server(at)matronics.com [mailto:owner-rv10-list-server(at)matronics.com] On Behalf Of Jim

 Sent: Monday, November 10, 2008 6:06 PM

 To: rv10-list(at)matronics.com

 Subject: Re: Measuring Amp draw  

  

   

you might also try this:

 

 http://store.solar-electric.com/mka-100-100.html

 

 You have two alternators, do you have two separate shunts?

 

 Jim C

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       [quote][b]

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marcausman

Joined: 08 Feb 2007
Posts: 70

Posted: Tue Nov 11, 2008 9:17 am Post subject: Re: Measuring Amp draw

Gary,



We saw the same thing with the shunts on my RV, prior to installing the VP-200. The current draw based on the shunts, once stabilized, read about double of the actual current (your mileage may vary). Once the VP-200 was installed (which is calibrated at the factory) and could compare the two, it was clear that the shunts needed calibrating. Problem is, most people don't have the capability to do this and the manual didn't say they needed calibrating. We saw this with a few beta customers too. So I suspect there are lots of people with shunts that are not calibrated. Of course with the VP-200 you don't need to install the shunts and this is a non-issue.    



Also, you have both alternators going into a single shunt, which implies that they are both wired to the same spot. Perhaps the switched side of the battery contactor?  My recommendation is to wire the backup alternator to the battery side of the contactor, and the primary alternator to the switched side of the contactor. Why? If you lose the battery contactor, then you can run the backup alternator which is able to "see" the battery (needed to regulate itself).

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_________________
Marc Ausman
http://www.verticalpower.com "Move up to a modern electrical system"
RV-7 IO-390 Flying

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