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Old 05-31-2014, 03:58 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, 04:13 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, 04:17 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, 06:34 PM   #24
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Quote:
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.

2000 Cherokee 31BH, "flipped axles and raised",LEDs, solar, inverter, etc..aka "boondock ready". '07 Tundra 5.7L DC-LB, modded as well. C-co, 8/158th, AVN.
I believe it's stamped 500 amps.

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Old 05-31-2014, 06:40 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, 06:42 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, 07:03 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, 07:20 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, 07:34 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, 07:40 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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