Shunt doubt

[Daniel Freitas]

Hello people, new here.
I have a big lifeboat (Harding MPC 36) and I'm redoing the complete electric system from zero, it has a 24v system (two 12 in parallel), and I have bought a new digital ammeter (from osculati) that came with a 80 amp shunt, the thing is that in the instructions it says I must connect the shunt in the positive (+) line of the system. But I'm reading everywhere that one should wire the shunt in the negative (-), I've attached the original instructions that came in the box. Any thoughts or suggestions?

 

[noelex]

Smart shunts can be designed to fit on the negative cable, the positive cable or be OK for either option.

Most smart marine shunts are made to fit on the negative cable. This tends to make installation easier as the positive lead will normally have a fuse attached.

An 80A shunt is small for a boat your size, although having a 24v system helps .Check this is adequate before fitting.

There are some reasonably inexpensive marine battery monitors that may be a better choice.

 

[Hurricane]

Just out of interest, what is a "smart shunt"?
And why should any shunt work on either the positive or the negative?
I assumed that in a lot of installations, there might be several wires attached to the positive thus making it physically difficult to attach a shunt in serial to the wiring.
So, I assumed that it might be easier to put the shunt in the negative/ground.

I thought that a shunt is just a big resistor with a means of accurately measuring the voltage drop across the resistor (shunt).
Then, using ohms law it can calculate the current flowing.
I would have thought you could put a shunt in the positive or the negative.
I would be interested to hear why some insist on the negative.

 

[noelex]

A shunt is just a calibrated resistor, so can be attached anywhere, but most devices likely to be installed in a modern boat use smart shunts. These have additional electronics to partially process the data and then communicate this processed data with the display head.

As shunts on boats are generally connected to the minus side of the battery many smart shunts cannot be used on the positive lead. Some such as my Pico battery monitor can work on either the positive or negative lead, but you need to make some software adjustments.

If you have a device such as an old moving coil ammeter that uses a simple (non smart) shunt you can install the device on either the negative or positive lead by simply switching the leads to the meter. This does not apply to most modern marine battery monitoring equipment. If it a simple ammeter only there is a chance it uses an ordinary (non smart) shunt. If it is a digital display it needs an ordinary (non smart) shunt and generally a seperate power supply.

Hopefully the OP can post a link to the exact device.

 

[john_morris_uk]

Hello people, new here.
I have a big lifeboat (Harding MPC 36) and I'm redoing the complete electric system from zero, it has a 24v system (two 12 in parallel),

I think you mean series.

 

[Scubadoo]

The original factory fitted shunt on my boat is fitted on the postive side.

 

[BartW]

A shunt is just a calibrated resistor, so can be attached anywhere, but most devices likely to be installed in a modern boat use smart shunts. These have additional electronics to partially process the data and then communicate this processed data with the display head.

As shunts on boats are generally connected to the minus side of the battery many smart shunts cannot be used on the positive lead. Some such as my Pico battery monitor can work on either the positive or negative lead, but you need to make some software adjustments.

If you have a device such as an old moving coil ammeter that uses a simple (non smart) shunt you can install the device on either the negative or positive lead by simply switching the leads to the meter. This does not apply to most modern marine battery monitoring equipment. If it a simple ammeter only there is a chance it uses an ordinary (non smart) shunt. If it is a digital display it needs an ordinary (non smart) shunt and generally a seperate power supply.

Hopefully the OP can post a link to the exact device.

All agreed.

The reason why some smart shunts need to be connected to minus, is that the shunt cabling is designed so that the electronics gets the minus from the shunt connection, and you just have to add a positive supply wire for the smart electronics.

And indeed if you have just a ordinary analog amp meter without any electronics inside, it doesn’t matter if you bring the shunt in the pos or neg batt connection.

So yes we need to know exact type of amp meter OP has got.