Ammeters and shunts. Help needed

[Norman_E]

My boat has separate ammeters for charging current and consumption current. The former is disconnected and the latter clearly under reads by some margin. Both have 0 to 100 amp scales. I will have to take the panels off to see how the meters are rated, but I think they are 50 ohm moving coil meters. They can be seen in the photo below, on the left hand panel, which has a voltmeter, charge current meter, and a meter for a generator, and on the second panel which has the consumption ammeter.

The charge current meter was disconnected and the shunt removed because an uprated alternator was fitted. Here is where I need help, assuming it is a 50 ohm coil meter what shunt do I need to make the 100 amp meter show 100amps when 125 amps charging current is present. In other words make the meter read 20% low. All I want is to be able to see what the 120 amp alternator is putting out without putting the meter right off the scale. It would not matter if the meter showed half the actual current provided I knew that was the case. I might even be able to mark the scale. All the shunts I can find on ebay seem to be 100, 150 or 200 amp shunts also marked as 75 mV. What is the significance of that? I thought voltage was irrelevant, only the relationship between the resistance of the shunt and the resistance of the meter coil determines how the meter will work.

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[pvb]

The meter is a simple voltmeter. If it's a 75mV voltmeter, it'll show full-scale deflection when supplied with 75mV. The shunt is calibrated to supply a certain voltage at a certain current. So you first need to determine the rating of the meter. Then choose an appropriate shunt. If your 100A meter is a 75mV meter, and if you buy a 200A 75mV shunt, then the meter will read full-scale at 200A current, so you'd just need to double the apparent reading to get a true current.

However, do you regularly get 125A charging current? If not, there's no problem having a 100A shunt - it will cope with higher current, and you'll just get full-scale deflection on the meter, which won't damage it.

 

[halcyon]

My boat has separate ammeters for charging current and consumption current. The former is disconnected and the latter clearly under reads by some margin. Both have 0 to 100 amp scales.

What you need to do is to fit a shunt that gives net amps to domestic bank on one meter, what is actually going into / out off the battery. With your current set up you need to take two readings and do maths. How to do it depends on you wiring layout.

Brian

 

[Norman_E]

The meter is a simple voltmeter. If it's a 75mV voltmeter, it'll show full-scale deflection when supplied with 75mV. The shunt is calibrated to supply a certain voltage at a certain current. So you first need to determine the rating of the meter. Then choose an appropriate shunt. If your 100A meter is a 75mV meter, and if you buy a 200A 75mV shunt, then the meter will read full-scale at 200A current, so you'd just need to double the apparent reading to get a true current.

However, do you regularly get 125A charging current? If not, there's no problem having a 100A shunt - it will cope with higher current, and you'll just get full-scale deflection on the meter, which won't damage it.

Thank you for that. Clearly I must look at the meters and hope they are marked with their rating. Actually I doubt that I ever get even 100 amps charge. I am not certain whether the alternator is 120 amp or 110 amp, as I have two documents on board that quote it differently. The boat inventory says 110 amp, but some other piece of paper says 120. If it proves to be a 50mV meter would a 150 amp 75mV shunt give the same effect as a 100 amp 50mV one. I ask because 75mV shunts seem much more common, but I remember seeing a 50 (though 50 what I don't know) when I looked at the backs of the meters some time ago. The boat is in Turkey so I can't check anything until I go there in September.

Would it damage the meter if it was given an occasional 10% more voltage than its rating?

 

[Norman_E]

What you need to do is to fit a shunt that gives net amps to domestic bank on one meter, what is actually going into / out off the battery. With your current set up you need to take two readings and do maths. How to do it depends on you wiring layout.

Brian

I actually rather like the idea of seeing charging current and domestic consumption separately. I was going to fit a Nasa BM2 battery monitor but have sent it back as the instructions make it clear that it would not work properly with my battery setup. I am thinking of buying the Merlin Smartgauge which gives voltage and battery capacity reading but not current, hence I want to revive the ammeters.

 

[pvb]

If it proves to be a 50mV meter would a 150 amp 75mV shunt give the same effect as a 100 amp 50mV one.

Yes, a 150A 75mV shunt will give 50mV at 100A. Slight over-voltage won't damage the meter. The fact that you have a 110/120A alternator is almost irrelevant - unless you have a huge battery bank the alternator is unlikely ever to reach its maximum output.

 

[William_H]

As said in practice 100 amp meter is probably very suitable for a 125 amp rated alternator. As you will seldom if at all see 125 amps going into load or batteries. Any 25% overload of the moving coil meter should not hurt it just pin the needle until the current drops back to 100 amps or less.
However you can reduce the meter current by fitting a series resistor in one leg of the wires shunt to meter. So if it is a 50 ohm meter then adding a 50 ohm reistor will half the indication. So a 12.5 ohm resitor in series should make the meter read 125 amp at FSD. (full scale deflection.) The actual shunt should be able to cope with 25% overload easily.
You could even have a push button switch to bypass series resistor to give correct reading.
Actually calculating ressitance of the shunt would require that you also know the current in the 50 ohm meter at FSD. Typically 1 ma somtimes 500ua or even 50 ua. You could check this with your multimeter on current scale using a variable resistor and a small battery.
The ratio of meter current to actual measured amps at FSD is the ration meter resistance to shunt resistance.
However shunt resistance that low is tricky to measure accurately. Much easier is to fit a clibrated amp meter in series with the shunt and meter. Compare readings at typical current.
A shunt can be made by using a bar or strap of stainless steel. Not mild steel as resistance is too low. Typically a few inches of bar say 1cm wide 1 mm thick with bolt holes each end mounted on something insulating. Remember the resistance rises with lenght and falls with more width.
If your shunt is for say a 100ua moving coil meter then thick ss would be right. If your shunt is for a 200mv digital display then thin strap is appropriate. good luck olewill

 

[nigelmercier]

I'd just reconnect it as is. I've got a 130A alternator, never seen more than half that.

 

[Norman_E]

Thanks everyone. A lot of good advice here. I have a feeling that the meter might have been disconnected and the shunt removed because the shunt was getting hot. If that was the case and a 100amp 50mV shunt was getting hot, then a 150amp 75mV shunt will be the answer.

 

[pvb]

Thanks everyone. A lot of good advice here. I have a feeling that the meter might have been disconnected and the shunt removed because the shunt was getting hot. If that was the case and a 100amp 50mV shunt was getting hot, then a 150amp 75mV shunt will be the answer.

With a constant 100A flowing, the shunt would only have produced 5W of heat, hardly enough to constitute a risk.

 

[Norman_E]

With a constant 100A flowing, the shunt would only have produced 5W of heat, hardly enough to constitute a risk.

I suppose it might still have got pretty hot if the heat could not escape, but as the meter was disconnected before I bought the boat I don't know why it was done. It may just have been caution when the uprated alternator was fitted to provide enough power for the inverter driven fridge.

EDIT: The white label beneath the ammeter reads "Nicht Beschalten" meaning not connected. As it is in German I infer that it dates from when the former German owners installed the inverter fridge setup.

 

[Daydream believer]

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Flash git -- that chart table is bigger than my dining table at home!!!!