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Old 30-09-2015, 07:31   #1
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Alternator Shunt and Meter

I know this has been asked and answered before, but I can't find what I'm looking for.
I desire to be able to measure the amps that I am putting into the battery from the Alternator, I assume this is an amp meter and shunt, I'd prefer digital, permanent installation on my power panel. Currently I can tell current output of my other charge sources, just not the Alternator, looking for equipment recommendations, and I assume in this case I'd need the shunt in the Pos lead from the alternator?

Thanks
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Old 30-09-2015, 08:20   #2
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Re: Alternator Shunt and Meter

Measuring the alternator output, except in a diagnostic situation, doesn't tell you much. The 12 volt system voltage reading is much more useful. However...

The shunt should go on the negative lead to the battery. So, between the battery and the -ve bus. Then you are measuring input/output from all charge sources and all loads. This however doesn't give you an exclusive alternator reading unless all other charge devices are shut off. Look up some of Dockhead's recent (last month or so) posts and you should be able to find more information on dual shunts. I guess if you want an exclusive alternator reading you should put the shunt on the +ve lead from the alternator. I believe that was discussed in Dockhead's thread as well.
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Old 30-09-2015, 08:37   #3
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Re: Alternator Shunt and Meter

Yes, I'm looking for an Amp output from the Alt to the battery, and not to be argumentative, but if your at absorption voltage, in my case 14.3 I believe, Amp output will give you a decent idea of the SOC of the bank. Plus I'm curious as to what my Alt output is.
I had intended on using my Smart Gauge to determine charge status, but have since discovered it's not nearly as accurate during charging as it is in discharging.

I'll look for Dockhead's post, thanks
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Old 30-09-2015, 12:45   #4
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Re: Alternator Shunt and Meter

I think you can do this although I have never done it myself. I have read that you will need a special version of a shunt designed to be used for the positive lead. Unless your alternator has an isolated negative/ground you can't just put a negative cable on it since some will be going through the engine block from its main ground.

Why I have seen that you need a different type of shunt is beyond me. I would think you might have to reverse the leads from a negative one but don't have the foggiest why you would need different technology.

Seems to me that you could install a digital shunt (different than a shunt for analog meters) and just reverse the leads. But dunno....
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Old 30-09-2015, 13:03   #5
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Re: Alternator Shunt and Meter

Yes, apparently I need a shunt meant for the positive side, I could have spec'd an Alternator with an isolated Neg, but didn't think I'd need it so I didn't, live and learn I guess.
I don't understand the difference in shunts either, but there must be some difference.

I'm looking for equipment recommendations, I'm sure I'm not the only one who has done this, it ought to be simple
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Old 30-09-2015, 13:24   #6
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Re: Alternator Shunt and Meter

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Originally Posted by a64pilot View Post
I don't understand the difference in shunts either, but there must be some difference.

I'm looking for equipment recommendations, I'm sure I'm not the only one who has done this, it ought to be simple
A shunt is a shunt, doesn't know what side of the circuit it's on. All a shunt is is a voltage sensor in mV that reads up in amps.

Your "challenge" is to find an appropriate mV shunt and a calibrated gauge. It should be simple...

a64, ITWMB, I'd spring all of $150 for a Victron 600, hook it up, read net voltage or turn your fridge off to read the amp output. Why? By the time you cobble something together...

Better yet, if you know what yoru fridge draws, then the math is simple.

I do this all the time with our Link 2000.

Loads other than the fridge which would "modify" your alternator output wouldn't be so large as to make much of a difference at bulk charging amperage anyway.

And any fewer connections you need to make on that AO would be better.
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Old 30-09-2015, 13:27   #7
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Re: Alternator Shunt and Meter

The only time you can really be sure how charged your batteries are is to read the total current going in and the voltage. So the amount being supplied by your alternator isn't all that relevant unless all other charge sources and loads are shut down. That is why the Smart Gauge or Battery Monitor is important. The current at the -ve terminal will give you the net current into the battery bank. Now it wont be 100% accurate because of surface charge most likely but it should get you close enough if understood and used properly. Alternator current alone tells you only that.

Edit: What Stu said...^^
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Old 30-09-2015, 14:43   #8
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Re: Alternator Shunt and Meter

Our local marine consignment place has a number of shunts sitting on the shelf. Any shunt would work placed between the Alt and battery. All a shunt does is insert a small resistance so that the voltage drop across that resistance can be measured. Here is one that I just picked off a google search.

https://www.altestore.com/store/Mete...urce=shopzilla

An amp meter then reads the voltage drop across the shunt with (in the case of the above shunt) a 50 mV reading being 500 amps through the shunt.

So in reality the actual meter is a volt meter that reads full scale at 50 mV. The legend on the meter is marked in amps to match the capacity of the shunt.

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Old 30-09-2015, 14:53   #9
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Re: Alternator Shunt and Meter

Quote:
Originally Posted by a64pilot View Post
I know this has been asked and answered before, but I can't find what I'm looking for.
I desire to be able to measure the amps that I am putting into the battery from the Alternator, I assume this is an amp meter and shunt, I'd prefer digital, permanent installation on my power panel. Currently I can tell current output of my other charge sources, just not the Alternator, looking for equipment recommendations, and I assume in this case I'd need the shunt in the Pos lead from the alternator?

Thanks
Blue Sea makes a product called the "Shunt Shifter" so you can use a shunt in the positive wiring.

As has been said the "net" current going into the battery will tell you more about SOC (at absorption voltage) than just looking at the alt output because the alt output is also feeding house loads...
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Old 30-09-2015, 14:56   #10
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Re: Alternator Shunt and Meter

I understand and agree that with total current and voltage, you can reasonably determine SOC of a bank.
Fridge draws 5 amps, all DC current draws are through a shunt, so I do know how much is being drawn.
My Outback 80 I believe will tell me it's current input and voltage as well (yet to be installed)
My 120VAC charger tells me both voltage and current.
I can tell voltage of course from my Smart Gauge, but I have no way of determining current output from my Alternator.

This is of course not a requirement, just I'd like to know alternator output is all, call it another toy if you will, my regulator will tell me if it's limiting current due to alternator or battery bank temp, but not current output.

Yes, I understand how a shunt works, it works by having an increasing although very small voltage drop as current increases, measure the voltage drop and current can be derived at, it does not measure current, it measures voltage and derives current from voltage drop across the shunt.

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?


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Old 30-09-2015, 15:03   #11
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Re: Alternator Shunt and Meter

Maine Sail, Only way that I can see I can determine "net" current going into the bank is if I can determine how much is going in from all sources, subtract house loads and determine net.
Now I can see how maybe having all input sources going into one side of a shunt and the other side connected to the bank would work, but that's not how I'm wired.
Easiest thing is to add an ammeter to the alternator, alternator output of course is not true DC, but pulsed DC, but I assume that's irrelevant?


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Old 30-09-2015, 15:29   #12
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Re: Alternator Shunt and Meter

Quote:
Originally Posted by a64pilot View Post
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?
I am an electrician

Absolutely no reason a shunt won't work on the positive side. I have one on my alternator, and I find it very useful. And it is on the positive leg because the alternator doesn't have an isolated ground. The main reason for negative connection is that a break in the shunt leads (the tiny ones to the ammeter), should one occur, is less likely to cause a short (it would have to short to a positive wire). Your ammeter leads are tied to the line that is connected to the shunt, so if your ammeter leads goes to ground you now have a short from a giant positive wire to ground (and through a tiny ammeter lead). You can protect against that with a fuse in the ammeter line, but since you are measuring mV you have to be pretty careful about the installation to not introduce errors.

No reason it can't be done, any shunt will work, any ammeter that is correct for the shunt will work, just have to be careful about the leads from the shunt.

[Edit] and yes, the shunt itself if it is not insulated/protected, as you now have an exposed positive, but there are many of those on the boat, and they all should have some sort of protection to prevent dropping a wrench and causing a short. [/edit]
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Old 30-09-2015, 16:02   #13
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Re: Alternator Shunt and Meter

The shunt is usually on the negative side if using a digital meter. Some digital meters work either way but most require a converter or shunt shifter for using in the positive side. Analog tend to work either way in my experience. I believe it's the way the digital meters read the voltage drop that causes the issue. I think an alternator output meter is usefully personally just so you have an idea what it's doing.
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Old 30-09-2015, 16:14   #14
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Re: Alternator Shunt and Meter

^^^^

that's because many digital meters get their negative power supply from the shunt leg as well. Three-wire connection; shunt +, shunt - (also used to provide the negative connection for the meter itself) and meter positive. You can get meters that do the opposite, or a shifter.
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Old 30-09-2015, 16:43   #15
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Alternator Shunt and Meter

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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