How do Shunts work? - SailNet Community
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post #1 of 38 Old 09-18-2012 Thread Starter
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How do Shunts work?

Just installed a Microlog battery monitor in my boat. I balked at the $90 price of the shunt until I received it. It looks like hardware a power company would use, with 3 large blocks (Brass?) connected by two (Brass?) bars. As I understand it, the Shunt steps down the current for the instrument, to allow the current to travel 15 feet to the monitor, through 22 guage wire. I expected this to take place on the Positive connections. However, all connections to the shunt were Neg.. The only Pos. connections being from each battery to the monitor (all via 22Ga wire).
Can someone explain how the shunt works? For example: My alternator is grounded through my motor, and is connected to the shunt only through the Neg cable between the shunt and the batteries. Yet, the monitor provides the charge rate from the Alt (amongst several other mesurments).
In basic terms (if possible), how does this work?

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post #2 of 38 Old 09-18-2012
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Re: How do Shunts work?

A shunt is just a resistor that produces a voltage drop when current passes through it. The voltage drop is measured and displayed as current.

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Re: How do Shunts work?

- and if I can add to the above -

It's better to do that on the neg/ground side

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post #4 of 38 Old 09-18-2012
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Re: How do Shunts work?

All a shunt does is measure a mV voltage drop across it. Most used for battery monitors are a 50mV shunt. So a 500A rated shunt would see a 50mV drop at 500A and 0mV drop at 0A. The "gauge" transposes the voltage drop to amps or current.. They don't really read "current" they read voltage drop and the meter transposes that reading to current.

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post #5 of 38 Old 09-18-2012
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Re: How do Shunts work?

A shunt is simply a precision resistor, of a low resistance value (i.e. one ohm) that has been carefully calibrated so that the voltage drop across it will be a precise number of millivolts per amp of current. (If that's all greek, look up Ohm's Law and voltage drops in resistors.)

Non-techie bottom line: By carefully machining bits of specific metals, you wind up with a shunt that has a voltage drop of 50 millivolts per amp (of the current passing through it) or sometimes 75 millivolts per amp, or another number. Let's say for simplicity that the shunt had a drop of ONE millivolt per amp.

In that case, you could simply hook up a voltmeter across the shunt, read the number of millivolts that were being dropped in the shunt, and read that as amps. Since the typical shunt is dropping 50mV per amp, you now need to add a voltage divider to the voltmeter, so that when the voltmeter sees 50mV, it says "one". 100mV, it says "two".

A voltage divider can be really trivial, so it doesn't really affect construction. The point is that the meter and the shunt must be matched, so they are counting the same quantities as "one" amp.

Some of the things you might not consider: The metals used in the shunt shouldn't oxidize or rust. They should be dimensionally stable as the ambient temperature changes, and their resistance also shouldn't change within the designed load range. If the shunt heats up and changes resistance--all the readings will be off.

So it is really a trivial bit of engineering and machining, but only trivial if the guys who are building it know what all the rules are. And if it is properly made, it works "forever" with zero maintenance or calibration required.
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post #6 of 38 Old 09-18-2012
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Re: How do Shunts work?

Very misleading. The resistance value of a 100 amp 50 millivolt shunt is .0005 ohms, well well below 1 ohm. A 1 ohm shunt would cripple a 12 volt system very quickly. Most shunts are rated at 50 millivolts per maximum current rating, not per amp. All shunts have a derating factor with 66% being the most common. That means a 100 amp shunt shouldn't be operated over 66 amps continuously because it will overheat.

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Re: How do Shunts work?

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Originally Posted by hellosailor View Post
So it is really a trivial bit of engineering and machining, but only trivial if the guys who are building it know what all the rules are. And if it is properly made, it works "forever" with zero maintenance or calibration required.
Yep, if someone really wanted to be cheap you could use two meters, one measuring current through a 5-10A load (10A is a common max for multi-meters) and another measuring the resulting voltage to calibrate your own shunt from almost anything - a length of wire or a piece of metal. But you'd have to be mindful of the power dissipation at full load and be sure it could handle it. Power goes up exponentially as current increases.

Or try digikey. This 0.0005 ohm resistor would be 50mV at 100A and dissipate 5W.
TGHGCR0005FE Ohmite | TGHGCR0005FE-ND | DigiKey

However like with all boat things, at the end of the day it's probably worth it to just bite the bullet and shell out the $ for the proper marine version.
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post #8 of 38 Old 09-18-2012
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Re: How do Shunts work?

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Originally Posted by L124C View Post
It looks like hardware a power company would use, with 3 large blocks (Brass?) connected by two (Brass?) bars.
The shunt you have is actually a dual shunt (two back to back shunts). The large blocks are brass, the bars are soldered to the brass blocks and are a precision resistance alloy called manganin which is approximately 86% copper, 12% manganese and 2% nickel.

Eric
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Re: How do Shunts work?

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Or try digikey. This 0.0005 ohm resistor would be 50mV at 100A and dissipate 5W.
The "standard" types used on boats are more accurate than that resistor (1% tolerance and 60ppm/C). Typical rating for marine shunts are resistor accuracy of less than .25% and 20ppm/C resistance drift.

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post #10 of 38 Old 09-19-2012
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Re: How do Shunts work?

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Originally Posted by L124C View Post
My alternator is grounded through my motor, and is connected to the shunt only through the Neg cable between the shunt and the batteries. Yet, the monitor provides the charge rate from the Alt (amongst several other mesurments).
In basic terms (if possible), how does this work?
Be careful with this installation. There are cautions in the installation manual about this. Depending on your setup, you could be sending full starting current through the shunt which could be problematic.

Eric
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