Ammeter and shunt questions

Wyowanderer

Explorer
I'll be installing a bigger (in amps, from about 45 to 130) alternator on my truck soon, and I have a question regarding the use of the ammeter in the truck.

I intend to remove all of the present charging system and start new, with 4 gauge cables throughout. I'd like to keep the present ammeter in addition to installing a quality voltmeter and maybe an idiot light.

I've read on the Blue Sky website that an ammeter can be installed in the ground wire, so long as a shunt is installed inline in the ground, and a fellow at a local electrical supply house told me that I ought to be able to use the ground cable as a shunt, so here's what I'd like to do;
I'd like to connect one end of the ammeter's two wires to the ground terminal, and one to either the engine block, the frame, or the body, including a 5a fuse for safety. Will I get more deflection at the ammeter from high, medium, or low resistance? It's a mechanical ammeter.

Does it sound like I'm trying to cause a fire in my truck?
Here's the Blue Sea page I was referring to:

http://bluesea.com/files/resources/instructions/9322.pdf

Thanks.
 
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wrcsixeight

Adventurer
What size shunt does your ammeter come with?

Remember if you put it on the battery negative cable, all the starter amps will flow through it. Can your analog gauge read upto 200+ amps?

I have a 500 amp shunt and have seen well over 120 amps on my digital meter when cranking my starter, and it is not instantaneous, so I am assuming 200 amps.

Go for 2 awg cable too.
 

dwh

Tail-End Charlie
A shunt is for a meter designed to use a shunt. The main amperage goes through the shunt, and the meter just has small wires that connect to both sides of the shunt and very little amperage goes through the meter. Since the meter knows the resistance of the shunt, it then knows how much current is flowing through the shunt.

There are different amperage ratings for shunts, and there are also different resistances. You have to have the right resistance for your particular meter, or the measurement will be off. And of course, the shunt has to be rated to handle the amperage and the meter has to be able to display it.

Shunts are precision resistors, so no - you can't just use a negative cable as a shunt. However, a shunt typically connects between the negative cable and the battery. so that the meter can measure ALL current flowing into and out of the battery (even starter amps, as wrc mentioned).

But the shunt doesn't *have* to go in the negative cable at all. You could install one in the positive line that feeds your radio if you just wanted to measure how much current the radio draws. You put it between the battery negative and the battery negative cable if you want to measure everything.

If you were using an amp counter gauge, such as a Xantrex LinkPro:

http://www.xantrex.com/power-products/power-accessories/linkpro-battery-monitor.aspx

Then you certainly would want to measure everything, since the LinkPro will remember everything that went in, and everything that went out, and give a rough estimate of how much is in the battery.

Gauges like the LinkPro are generally used for aux battery systems, such as for house batteries in an RV or for solar systems.



An "external shunt" type ammeter is a different animal than an ammeter which is designed to have the full amperage flow through it, such as a typical automotive ammeter, which has its own little shunt (resistor) inside of it - i.e., "internal shunt".
 
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Wyowanderer

Explorer
I'd like to use the stock gauge that came in the truck. It uses a length of wire as a shunt, and although you could never accuse FoMoCo of having good wiring ideas, it did work from the factory.
I'd be fine with installing a shunt in the system rather than connecting accross the ground, so my next question is this; how do I measure the ammeter to determine the right size shunt? It would be much easier to install the shunt in the ground; would there be any problem with that?
I'll post up pics of the ammeter guts tonight so you have a better idea what I'm using.
 

dwh

Tail-End Charlie
Hrmmm...I jumped the gun with last night's quicky reply just before I hit the sack. Should have slept on it first. Woke up this morning and realized that stock automotive "ammeters" generally aren't exactly either of the types I described last night (at least not the modern ones).

Duh.

Got a little more time and a fresh cup of coffee now, so any goofs can't be explained by being tired or in a hurry - they'll have to be put down to just plain stupidity. :D



So lemme see...a stock automotive "ammeter" measures the difference between the alternator and the battery. Looking at the one in my truck (76 E-250) I see it's not really an "ammeter"...what it is, is a "charging indicator". It's only marked D and C and doesn't indicate amps.

Digging out the Chilton's for that truck, I see that the gauge is tapped off the hot wire from the alternator at two points.

So okay, technically it's "shunted" by creating an extra path for current to flow through, and the meter is not getting the full current flowing though it because the "meter circuit" (wires that lead from the taps to the meter, and the meter itself) has a higher resistance than the main charging wire that the meter circuit is tapped off of.

But the gauge, on my truck at least, is not a calibrated meter - it just indicates that there is a difference, not precisely what that difference is. My truck has a 100a alternator, an engine start battery and a deep cycle aux battery connected to the engine battery with a split-charge solenoid.

Even when I run my aux battery down very low (which I do frequently), the needle on that charging indicator never gets anywhere near full deflection. Not even close; About 1/4 of the way to full deflection is the most I've ever seen it do.

If the one in your truck is like the one in my truck, then the whole lash-up is so imprecise that it doesn't matter much what you do. You should be able to just tap the charging wire from the alternator at two points (probably at both ends) and get a relative indication of alternator vs. battery on the gauge.




But what if the gauge is calibrated in amps instead of just a dumb "D...|...C" charging indicator like the one in my truck?

Well, for the meter to be accurate, the resistances have to be right. The resistance of the section of alternator charging wire between the two taps has to be a known value, and the resistance of the meter circuit also has to be a known value, and the relationship between the two resistances has to be correct.

The Chilton's I'm looking at, which covers Ford vans 61-88 doesn't indicate anything about how that is done. Probably just all the wires involved are of known length and gauge (known resistance). Change the length or size of the wires, and you alter the meter's calibration.

OR, the full amperage has to flow through the ammeter (internal shunt as in my previous post). Basically, the charge wire runs from the alternator to the meter, though the meter, and then to the battery. There is no external shunting (tap off the charging wire). That's how a lot of older vehicles ammeters were set up (and often disconnected and bypassed as a fire hazard).



(Leaving aside for the moment that the meter is going to be a bit off due to the altered resistances...)

So let's say you've got a meter that reads in amps, and it's done with the same "tap the charging wire at two points" as shown in this Chilton's.

Then you change the charging wire out from a #10 to something fat like a #4 and install a bigger alternator. Let's say the meter is calibrated to a 40a +/- range and now, with the bigger alternator and fatter wire, you could exceed that range (needle hits full deflection).

But you want to keep that gauge instead of changing it out for one with a wider range...

Well, then what you'd have to do, is alter the resistance of the ammeter circuit to re-calibrate the meter to a different range (BUT, the range wouldn't be correct in regards to the amp scale printed on the face of the meter). You could alter it so that the meter only reads half of the actual value, so if there is 30a showing on the meter, then you know 60a is actually flowing.

That would only get your 40a meter up into the 80a range though, so you'd probably have to set it up for a different ratio. Say you setup the meter to show 25% of the actual value. Then if the meter hits full deflection (shows 40a), you'd know 160a was actually flowing.

That's a bit of a kludge, but could be done.

There's a decent article here about how to add resistors to change an ammeter's operating range:

http://www.allaboutcircuits.com/vol_1/chpt_8/4.html
 
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Wyowanderer

Explorer

Wyowanderer

Explorer
If the one in your truck is like the one in my truck, then the whole lash-up is so imprecise that it doesn't matter much what you do. You should be able to just tap the charging wire from the alternator at two points (probably at both ends) and get a relative indication of alternator vs. battery on the gauge.



http://www.allaboutcircuits.com/vol_1/chpt_8/4.html

This is exactly what I was looking for.
Any reason I can't attach it to the neg. cable as I intend?
And
Will the charge indicator deflect more with more resistance, or with less resistance, i.e. more= tying the indicator to the neg. cable and the body VS. less=tying the indicator to the neg. cable and the engine block (which is at the other end of the cable)?

Thanks for your help.
 

dwh

Tail-End Charlie
Should work off the negative cable since current flows though it just the same as the positive cable.*

Though, if you want it work like factory, I don't know why you'd bother - just hook it up like it was.

I can see hooking it to the negative if you were going to use an external shunt and a real meter that accurately measures the current, but for the D...|...C doohickey I don't see a reason to bother.

Also, as wrc68 pointed out...if you put it on the negative, then it's going to catch *everything*. That means every time you hit the starter, you're likely to ram that needle all the way to the stop on the D side.

If you just hook it up like factory (on the charging wire between the alternator and battery), you're going to catch everything except the starter. Since it's just a charging indicator and not a real ammeter, it doesn't have to be exact, just close enough to give you an good idea of what's going on.



As for the resistance...jeez...I dunno. Gettin' late...might have to sleep on it. :D

Lemme see...you've got two paths - one through the main wire, and one through the meter circuit. If you were to lower the resistance of the main wire (with a bigger wire for instance), then more current would flow though that wire, so less current would flow through the meter, and there should be less needle deflection.

So I suppose more resistance in what is being measured (or less resistance in the meter circuit) is going to get you more deflection on the meter.



*(Actually, direct current flows from negative to positive, not the other way as most people think. Ben Franklin got it backwards when he figured it out and we've been using his wrong-way nomenclature ever since. The Limeys got it right with their positive ground setups.)
 

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