What is a shunt?

[cagey]

I am prepared for a deluge of funny answers, but I'm serious I want to fit 2 ammeters to monitor both battery banks and instructions say fit a shunt. What is a shunt, what can I fit and where can I get one?
Thanks

 

[cliff]

[ QUOTE ]
I am prepared for a deluge of funny answers, but I'm serious I want to fit 2 ammeters to monitor both battery banks and instructions say fit a shunt. What is a shunt, what can I fit and where can I get one?
Thanks

[/ QUOTE ]A SHUNT in this case is a resistor.
Maplin Electronics sell them.

In your case you need to go back to the ampmeter supplier/manufacturer and obtain the correct ones to suit your application / meters.

The little 60-0-60 or 100-0-100 meters have built-in shunts which carry the bulk of the current. the meter actually measures the voltage drop across the resistor and hence indirectly gives the current from V=IR or "voltage is proportional to current for a fixed resistance".

If you try to draw more than 100amp through a 100 amp meter you will burn it out ( the moving coil voltage meter that is although it reads amps remember it is actually volts oit is measuring).

I think both Sterling and Adverk do remote shunt meters. Remember engine starting can pull 500amps or more (peak) from your battery so any shunt must be capable of carrying 500amps unless you just want to monitor charge rate in which case an ordinary 60-0-60 or 100-0-100 meter will do without external shunts.
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[JKay]

It's when the crew cock up completely!!!

regards Joe

 

[halcyon]

A shunt is a either a resistor of very low value, or a electronic module that measures the magnetic field around the cable. In both cases they produce a voltage proportional to the current that drives the ammeter. The voltage drop depends on the meter spec, but could be 75 millivolt for full meter travel, this voltage equating to say 50 amp. It depends on the meter you use, and the value you want to measure. In the case of electronic shunts the output could be 2.5 volt for 0 amp and 3.5 volt for 100 amp, this voltage is feed up to a unit by the meter that converts it to a voltage to drive the meter.
Resistive shunts fits inline with the cable and carries all the current, thus to measure current in a engine feed cable must be able to carry the starter stall current. Can lead to problems with shunt size and matching to meter, also you are only carrying a few millivolt, so can be effected by connection faults.
Electronic shunt fits on battery terminal and measures all current into and out of the battery, as they cannot be overloaded, and limit output voltage can carry starter motor loads without problem.
Electronic shunts are a little dearer , but mor flexiable, if you want advice all forum members with a Sealine have one.

Brian

 

[BigART]

A shunt is a device to assist the measurement of current in a circuit. There are a number of ways to measure current, the two most common are an ammeter or a shunt with voltmeter.

The ammeter measures the current directly by running the circuit through the meter. This is simple but the disadvantage of this method is that if you want to measure large currents from a remote location, long runs of heavy cable, expensive are required.

The way round this is to put a shunt in the circuit. A shunt is normally a resistor of low value capable of taking high currents. Whan a current runs though the shunt, it produces a small voltage across it (Ohms law, R=V/I). This voltage is directly elated to the current running in the circuit and can be measured remotely via small wires using voltmeter.

Merlin etc can provide the shunt

 

[lockwood]

I have always thought Jamie Oliver was a bit of a shunt.

 

[Anonymous]

Shunts are quite expensive and you'd need two for two ammeters. Have you considered installing a Link 20 or the BEP battery monitors - both available from Merlin. I have the BEP and am delighted, the Link 20 is well known and thought of. These monitors give, in effect a 'fuel gauge' type display for the batteries that is reasonably accurate both on charge and discharge. The BEP throws in a free bilge pump monitor as well.

 

[halcyon]

They both needs shunts as well !!!!

Brian

 

[VicS]

You may well find that the "Electrics course notes" in the Reading college course on boat maintenance will be helpful to you.

http://www.reading-college.ac.uk/marine/

Go to the section on "Instruments and warning lights" and read the subsection on ammeters.

 

[pvb]

Start by going one step backwards...

You'll have gathered from other replies that a shunt is a device which lets you measure high currents without having dirty great cables feeding the ammeter. But I reckon you should start by asking yourself whether you actually need to measure currents on 2 battery banks. Assuming one is a dedicated starter battery, an ammeter on this will show a small charge immediately after starting, and then virtually nothing. Waste of an ammeter.

If you agree with this idea, consider fitting a Link 10 or similar meter. It'll measure voltage, current and amp-hours, so it'll tell you much more than a simple ammeter, it comes with a 500A shunt, and it's easy to install.

If you disagree, and really need to see how little current is going into your starting battery, there's a Link 20 which measures 2 banks. Or, as Lemain said, there's the BEP range, which has a good reputation too.

 

[Anonymous]

Yes, but compare the cost of two shunts and two ammeters with a Link 20 and ask whether a little extra isn't worth it. He certainly ought to give it some thought.

 

[cagey]

Thanks everyone BEP seems best solution

 

[William_H]

Now for the cheapskate approach. All wiring has a finite volt drop determined by the resistance of the wire and the current through it. So a sensitive voltmeter can measure the volt drop of the feed wire generator to battery say 20 amps over 2 metres of typical cable as about 10 millivolts. .02 volt. Quite ok for a digital multimeter on 200mv max range. If you fit a wire to one end of the positive lead to the battery ie at splitter diode VSR or the switch and take another wire from the terminal itself you will find this small voltage is directly proportional to the current. Sure it is not calibrated in actual amps but if you can borrow an ampmeter to fit in circuit you can get a comparison. You can get a variable resitor each wire to each end of the variable resistor with the meter going from one end to the middle to reduce the voltage reading for a given current to a usable ratio. Many of the oled fashioned amalogue meters have a sensitivity suitable for this purpose for a permanent fixture.
The digital modules usually require 200 mv drop to give full reading 20 amp or 200 amp. the 200 mv can be an excessive drop in a dedicated shunt in addition to the wiring volt drop. Yes almost all ampmeters have a resistive shunt often in the meter case. As stated a remote shunt is far better in reducing the wiring run to the meter. In using any shunt or even the section of wire it is important to fit fuses to both sensing wires near the shunt because any connection to negative will permit very high currents to melt the wiring and the meter. Don't spend moneeeeeeey will

 

[VicMallows]

As a born cheapskate, I've always fancied Australia ... so been right all along!. Agree, why throw away power (and money) in a fancy resistor.

Vic

 

[halcyon]

Do not forget to build in the variation in resistance due to temperature changes in the cable. The other point is that you are only measureing current of the charge source, you need to measure battery terminal current to get a true reading. It works, but only give a rough guide.
Spent a lot of time in the 80's trying to use cable volt drop to make a useable system, to many variables, had to revert to a shunt, then develope a Hall Effect shunt.

Brian

 

[VicMallows]

Assumed we were measuring the volts-drop across a cable from the battery to some 'common point'. Personally use an ancient moving-iron ammeter!

Vic