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Old 05-31-2014, 02:58 PM   #21
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The poster was asking if the black wire on the right side of the shunt was a problem since it was smaller in diameter than the larger cables on the left side of the shunt.

Cam is correct, this is a problem if the total amp draw of the coach plus the inverter exceeds the rating of the added "battery to shunt" cable.
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Old 05-31-2014, 03:13 PM   #22
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just so I know and learn something why can't it be added to the battery terminal it self? it is just a monitor?
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Old 05-31-2014, 03:17 PM   #23
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Out of curiosity, what amp rating is that shunt? Looks like a 100A. If so, better watch the current draw. Anything much over 1/2 of the shunt rating will create some heat.

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Old 05-31-2014, 05:34 PM   #24
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Originally Posted by RPAspey View Post
Out of curiosity, what amp rating is that shunt? Looks like a 100A. If so, better watch the current draw. Anything much over 1/2 of the shunt rating will create some heat.

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I believe it's stamped 500 amps.

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Old 05-31-2014, 05:40 PM   #25
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Originally Posted by gljurczyk View Post
just so I know and learn something why can't it be added to the battery terminal it self? it is just a monitor?
The shunt is only used to monitor amperage and not needed at all to monitor voltage. The way it works is like parallel resistors where one is very low resistance and the other is very high resistance.

So a 500/1 shunt will pass 500 times the current through the shunt than passes through the monitor.

The ammeter then only needs to be designed to carry 1/500th of the amperage of the true load. The display then multiplies the current reading by 500 on the "dial."

So for example if the inverter was pulling 200 amps through the shunt, the monitor would only "see" 200/500 amps or 0.4 amps. The display would still show "0.4 amps times 500" on the "dial" or digital readout.
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Old 05-31-2014, 05:42 PM   #26
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Originally Posted by DDC View Post
X2 reducing cable size is never a great idea.
Unless the original cable was way over-rated. Starters pull well over 250-300 AMPS, if I remember correctly. This is especially true on larger engines.

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Old 05-31-2014, 06:03 PM   #27
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The monitor wires do not pass current to the monitor. The monitor measures the voltage drop across the shunt. By knowing the resistance of the shunt and the voltage drop across it you can calculate the current. I=V/R.
Yes you need a larger cable going from the shunt to your battery, a car starter can draw a couple hundred amps buy this is for a short time so the cable does not have time to heat up. The other problem with the small cable is you will have more voltage drop across that cable so your devices will not get the full 12V.
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Old 05-31-2014, 06:20 PM   #28
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Originally Posted by Happy Vibe View Post
The monitor wires do not pass current to the monitor. The monitor measures the voltage drop across the shunt. By knowing the resistance of the shunt and the voltage drop across it you can calculate the current. I=V/R.
Yes you need a larger cable going from the shunt to your battery, a car starter can draw a couple hundred amps buy this is for a short time so the cable does not have time to heat up. The other problem with the small cable is you will have more voltage drop across that cable so your devices will not get the full 12V.
So, how does the monitor measure the voltage drop across the shunt without passing current? Just askin' - Inquiring minds and all...
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Old 05-31-2014, 06:34 PM   #29
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Inside the monitor there will be an OP amp. THis device has a very high input impedance so it does not draw any current. The OP amp takes the two wires, the one attached to the devices has a larger voltage than the the side connected to the battery. It amplifies the difference between the two wires and this gets sent to a voltage meter built into the the meter. I am guessing a little as I have not looked inside one of these monitors but the fact the monitor I use said to use shielded twisted pair to connect the shunt to the monitors leads me to believe this is how it works.
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Old 05-31-2014, 06:40 PM   #30
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Originally Posted by Happy Vibe View Post
Inside the monitor there will be an OP amp. THis device has a very high input impedance so it does not draw any current. The OP amp takes the two wires, the one attached to the devices has a larger voltage than the the side connected to the battery. It amplifies the difference between the two wires and this gets sent to a voltage meter built into the the meter. I am guessing a little as I have not looked inside one of these monitors but the fact the monitor I use said to use shielded twisted pair to connect the shunt to the monitors leads me to believe this is how it works.
Thanks, my monitor also uses twisted (but unshielded) pairs between the shunt and the computer.

I re-read the manual and here is what it says.
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Old 05-31-2014, 06:47 PM   #31
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The shielding is an extra step that may or may not be needed. The voltage coming from the shunt is very small, so by twisting the wires it helps with any electrical noise that me get on the monitor lines.
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Old 05-31-2014, 06:53 PM   #32
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Originally Posted by Happy Vibe View Post
The shielding is an extra step that may or may not be needed. The voltage coming from the shunt is very small, so by twisting the wires it helps with any electrical noise that me get on the monitor lines.
Yes, I did see it was micro-volts. Thanks.
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Old 05-31-2014, 07:08 PM   #33
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I just went out on the net an looked at a table for 6 Gage wire with 200 amp flowing in a 1 foot length of wire. It showed a .163 voltage drop. that is 32.6 watts of heat, and that is a lot of heat if the 200 amp is on/flowing for any length of time. 10 second start, you will never see/feel the heat.
One other thing , look at your nice new cable connector, nice an clean, what is it going to look like it a year or two? That 1 foot of 6 Gage wire has 0.0009 ohm, what do you think the connectors will have later?
OK 2 other things. Why did you put the terminal block in the plus side cables? Leave the 2 plus wire connected to the battery and run a wire to the terminal block from the battery.
All the breaks/connection worry me more the the small wire size.
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Old 05-31-2014, 07:54 PM   #34
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Why did you put the terminal block in the plus side cables? Leave the 2 plus wire connected to the battery and run a wire to the terminal block from the battery.
All the breaks/connection worry me more the the small wire size.
Because with THREE lugs on the positive battery post, I can only get the nut screwed on about half way. I don't like that nut just barely hanging on by a thread (no pun intended). I could not find battery post extenders anywhere.

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Old 05-31-2014, 08:17 PM   #35
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Herk:

Small correction, mv is millivolt (i.e. 1mv = 0.001 volts), 1 microvolts is 0.000001 volts. You need a very good DVM to measure microvolts.

Boowho:

One thing you could do is use two cables of the size you got between the battery and the shunt/connection block. That should be sufficient to carry the amps. We do that often in the shop, use two or three 2/0 cables instead of one 500 mcm cable. Much easier to move and bend the smaller cables than the thicker, stiff, heavy 500 mcm cable.

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Old 05-31-2014, 08:24 PM   #36
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Originally Posted by boowho View Post
I believe it's stamped 500 amps.

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Good. It makes no sense to have wire good for 200 amps but a low rated shunt. You'll also lose accuracy above 50% of current rating. Each shunt has a curve chart that shows the relationship between load and accuracy. Its pretty linear up to 50% then is almost expotential afterwards.

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Old 05-31-2014, 10:17 PM   #37
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a car starter can draw a couple hundred amps buy this is for a short time so the cable does not have time to heat up.
Well, I've decided to error on the side of caution and go with the larger cables.

I do understand what you are saying regarding starters only pulling high amps for 3-5 seconds. However, when I was a young man in -30 temps during most winters, the cars in Iowa could take MINUTES to get started, and I never saw a starter cable melt. Maybe they were bigger years ago??

Also, when in those temps I THINK the amps go WAY up from the usual 200-300. What you say is probably right, but just my observations over my lifetime. Again, I'm convinced going with the bigger cable is the best option for me. Or possibly use multiple smaller cables (in parallel) as Rick suggested,

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Old 05-31-2014, 10:48 PM   #38
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So, how does the monitor measure the voltage drop across the shunt without passing current? Just askin' - Inquiring minds and all...
Even a voltmeter passes a bit of current, but it's VERY, VERY, VERY tiny.

Maybe this will help - here's another way to think of it: Imagine you put an analog voltmeter (one with a face and a needle) across the shunt and read the voltage across the shunt. As the current through the shunt goes up, the voltage (drop) goes up. So now all you have to do (if you're smart enough) is erase the voltage readings on the meter face and replace them with current values. Wherever the needle was pointing when you were passing 1amp, mark the face "1 amp." Wherever it's pointing when you're passing 2 amps, mark the face "2 amps." (If you're "smart enough," this can be done using Ohm's law that was given previously without actually having to see where the needle was at 1 amp if you know the resistance of the shunt.)

Of course, these monitors all do this electronically; there IS no meter face.

(If you've ever read current with a "multi-meter," (a meter that can read volts, amps and resistance - ohms) this is basically what you're doing. The "shunt" is inside the meter and connected when you rotate the dial to read amps.) And if it's an analog meter, you can see on the face that they've marked the same face with 3 different scales, in volts, amps, and ohms. But in fact, no matter whether you're measuring volts, amps or ohms, the meter is actually showing volts; you're just reading the amps or ohms scale if that's what you're trying to measure.
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Old 05-31-2014, 11:56 PM   #39
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Originally Posted by gljurczyk View Post
just so I know and learn something why can't it be added to the battery terminal it self? it is just a monitor?
glj, I don't know if you gleamed the answer to your question from the discussions after you asked so I will take a shot at it. You asked why not just connect the the monitor to the battery (cables) the issue is that the shunt is a precise measuring device that the monitoring instrument is calibrated to measure across. The monitor requires the shunt for accuracy, You'll notice some shunts have more flat bars than others this is for handling larger loads and still meet the calibration requirement. The shunt could be attached to the battery post and still do it's job if it is designed to attach directly but I have not seen a shunt with a hole to go on the battery directly except for automotive testing ampmeters.
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Old 06-01-2014, 06:17 AM   #40
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One more question if I might....
does the engine starter current go thru the shunt or are we talking
about the house batteries and NOT the engine battery??

IF the starter current goes thru the shunt then I agree with many who
say the new cable needs to be as large as the original.
If we're talking about house batteries then I think the cable pictured in post 16
is likely OK as is.
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