Alternator Shunt and Meter

[Cadence]

Originally Posted by a64pilot

I was off doing a little research, and it appears that you are all correct, that is an analog ammeter doesn't care which side it's on, but since the sense wires are hot if the shunt is on the + side they should have circuit ptotection, digitals seem to be the ones that require the shunt "shifter".
As usual "marine" ammeters, at least brand name ones seem to have quite a large increase in price over non marine ones.
I believe I will buy a Blue sea analog as it will match my current ammeters on my panel, I don't mind a needle and realize just because a digital may display a .1, it isn't necessarily any more accurate.
One thing I'm curious is at what engine RPM do I have to hold for X current? I don't know, it may be that my 140 amp alternator may only be making 50 amps at 1,000 RPM, and until you measure it, how do you know?


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Besides the analog meter won't drive you crazy fluctuating on minute changes.

[Stu Jackson]

Originally Posted by a64pilot

..........

I'll likely go the Blue Sea analog route as I believe they match what's already there and cost is pretty reasonable.

But Frankly's post has me thinking, why not put a shunt in the neg side, and if you could find a 200 amp ammeter that shows both discharge and charge, then of course you would know the total number of amps either going in or out of the bank? You know the kind of meter where O is at the top, but the needle swings left for discharge, and right for charge.
Right now I have an ammeter that came with the boat that shows discharge only, I was going to add one that showed charge only, but logically it would seem one meter should be able to show both?

a64,

Why do you think there would be anything coming OUT of the alternator negative?

IIRC, your goal was to find out the total amps being provided by the alternator.

The only place I know of that you can find out what's going OUT of your bank is at the bank negative.

This assumes that you battery banks have one ground to the engine and that the alternator negative is a separate ground from the alternator to the block, which is the way many boats are wired because the batteries are further away from the engine block than the alternator so the alternator negative is wired directly from it to the block.

The Blue Sea ammeter on the positive side sounds like what you're looking for, taking the good advice about the wiring as expressed in earlier posts.

Good luck.

[evm1024]

The confusion builds.....


Analog meters vs Digital meters.

The typical analog meter has fully isolated inputs (both + and -) which allows the shunt to be placed in the positive or negative lead of the alternator (or battery bank).

The typical digital meter has the negative input referenced to the negative power input for the meter. This prevents the shunt from being placed inthe positive lead of the alternator (or battery). To do so would cause a short from the negative side input from the shunt (which is at + voltage) to the power ground.

The more expensive meters have fully isolated inputs which allows the shunt to be placed in the positive or negative side.

Battery powered digital meters do not get power from the ships batteries and thus even though they may have the negative input referenced to the battery negative side it is not a problem because the meters ground is isolated from the ships ground.

Alternator negative

Remember that electricity travels in a loop. Thus power comes "out" of the alternators positive lead ( and the shunt can be placed there) and returns to the alternator through the alternators ground terminal. Most alternators we have have the negative terminal connected to teh alternator case and thus to the ships ground through the engine. With an Isolated ground alternator you can put the shunt on the negative output with the other side of the shunt to the ships ground.

Regards

[Tikka]

It is not surprising , there is not much about measuring current on alternators.
Of corse there is a reason for that.
Measuring current is quite simple, as inserting shunt resistor / 10 or 100 miliohms and with any voltmeter in milivolt range / measure milivolts, they directly corresponding amp.
Reason is simple when you inserting shunt you also provide lugs for shut connection and just that even when properly done add to circuit more miliohms.
All modern alternators have very sensitive voltage / current controller.
Total voltage drop, alternator adds to tha bat. voltage and will be less charge.
Your battery will not be change fully.
In boat enviroment, miliohms through shunt connection slowly increasing,
and charging will be affected more and more.
In fact 100 miliohms shunt is out of question ...voltage drop is too high
10miliohms is the OK considering another 10+ miliOhms is through lags .

[Tikka]

By the way, there is the way measuring alternator current without introducing shunt.
But I leave it as homework for enthusiastic.

[Maine Sail]

Originally Posted by Tikka

It is not surprising , there is not much about measuring current on alternators.
Of corse there is a reason for that.
Measuring current is quite simple, as inserting shunt resistor / 10 or 100 miliohms and with any voltmeter in milivolt range / measure milivolts, they directly corresponding amp.
Reason is simple when you inserting shunt you also provide lugs for shut connection and just that even when properly done add to circuit more miliohms.
All modern alternators have very sensitive voltage / current controller.
Total voltage drop, alternator adds to tha bat. voltage and will be less charge.
Your battery will not be change fully.
In boat enviroment, miliohms through shunt connection slowly increasing,
and charging will be affected more and more.
In fact 100 miliohms shunt is out of question ...voltage drop is too high
10miliohms is the OK considering another 10+ miliOhms is through lags .

50mV X 500A shunts are pretty standard for the typical battery monitor. At 100A of charge current you are losing 0.01V across the shunt or about 10mV....

Most poorly made terminations and undersized wire are causing multiples more than that, heck I measure drops of 20-30X that pretty much daily even on factory systems untouched by a previous owner or poor electrical tech......

Any good alternator/regulator will use a voltage sense circuit to correct for voltage drop so the 0.01V should not make any real difference. Heck even at 500A you are losing just 0.05V across the shunt....

[svlamorocha]

Originally Posted by a64pilot

I don't understand why a shunt must be on the Neg side, but most specifically state that they must be, so therefore it seems logical that the manufacturer understands the limitations of their product? I'm no electrician, just a jack leg mechanic that has limited experience with things electrical is all, it would seem to me that a shunt could be either on POS or Neg side, perhaps the Neg limitation is due to not being insulated?

Good question. I will have a go.

A plain shunt that has only 4 wires connected to it (two fat to carry the alternator's current and two thin two measure milivolts across the shunt) has no way of knowing if it is installed in pos or neg wire. If you get an ammeter for automotive use that has a shunt (as opposed to the useless ones that require you to run the current through the back of the instrument) you will get one of these.

Some fancy shunts such as the one in my battery monitor also have a wire that goes to the other pole of the battery (to power the actual battery monitor, connected to shunt by an Ethernet cord that carries 12V pos and neg for the monitor's own power, delta milivolts across the shunt and I think also a pair of voltage sensing wires) hence I would not flip polarity of those.

If you cannot use the fancy shunt of the Victron BMV series then you might prefer to get the other brand (do not recall name) that uses a standard shunt. I bet Maine Sail has a good article on this.

Even in the first case, convention seems to be that shunts belong in the negative wire. I bet that this is because that makes it easy to avoid having other wires that bypass the shunt. In many boats there is more than one wire going to the positive terminal, hence you would have to change that to install the shunt in the positive side.




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[Cadence]

Originally Posted by Tikka

By the way, there is the way measuring alternator current without introducing shunt.
But I leave it as homework for enthusiastic.

A clamp on will. Not practicle however unless they make one allowing a remote meter?

[a64pilot]

Originally Posted by Cadence

A clamp on will. Not practicle however unless they make one allowing a remote meter?

I had thought about an inductive one also, but for some reason I don't think they are as accurate.


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[Cadence]

Originally Posted by a64pilot

I had thought about an inductive one also, but for some reason I don't think they are as accurate.


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They probably are with 2/0 wire. Old school says a shunt and an analog meter that could care if it was on the + or - lead.
Sounds like the OP had decided on that. Even matches his other gauges.
If you remember the match books you remember cars all had an amp meter.

[a64pilot]

Originally Posted by Cadence

If you remember the match books you remember cars all had an amp meter.

Lotsa airplanes still do, actually I think you need both a volt meter and an ammeter to give you all the necessary info and a circuit breaker in the field wire of the Alt, once in a very long while it seems at least internally regulated Alt's just go wide open so to speak and a CB in the field wire you can turn the alt off.
I think maybe ammeters were common in cars that had generators?


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[Saltyhog]

Originally Posted by a64pilot

I had thought about an inductive one also, but for some reason I don't think they are as accurate.


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No inductive current sensing on the DC output of an alternator, as they sense AC current only. The clamp on current meters that measure DC current use a hall effect device, not inductive coupling. I had wondered if that would be a viable solution here, but I gather they are in general not very accurate or precise. I think you're still better off with a shunt.

[Cadence]

Originally Posted by a64pilot

Lotsa airplanes still do, actually I think you need both a volt meter and an ammeter to give you all the necessary info and a circuit breaker in the field wire of the Alt, once in a very long while it seems at least internally regulated Alt's just go wide open so to speak and a CB in the field wire you can turn the alt off.
I think maybe ammeters were common in cars that had generators?


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Yep, generators and you always had a set of brushes around and some emery paper for the commutator. I thought the biggest problem with alternators has been a diode going out? Buy one rebuilt. A shined up case and a new diode pack

[a64pilot]

I'm sure diodes going out is common, but an alt that is putting out max current, could cause a fire, so it's good to have a way to turn it "off"
I've only had one diode failure that I can remember, and again going on memory I think the symptom was the "battery" light was on, with the car off?


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[svlamorocha]

Originally Posted by a64pilot

I'm sure diodes going out is common, but an alt that is putting out max current, could cause a fire, so it's good to have a way to turn it "off"

+1. I like your breaker idea. Of course, you need an engine ammeter or voltmeter at the helm (as in some Leopard cats)to find there is a problem . For me, this is independent of the battery monitor at the nav station.


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