I was looking at a meter and it said shunt required , Why ?

the meter it self won't stand up to the 'amps' being pulledcan't carry amps thru the meterthe shunt mounts at the battery and your charger and load wires connect to the shuntthe shunt is a Heavy and calibrated resistor, usually 75 ohmthe load amps passes thru the shunt, the meter measures the voltage drop across the shunt, to give you the reading of the power amps being used

75 ohms? I thought an external meter shunt was fractional ohms.

MrWizard wrote:the meter it self won't stand up to the 'amps' being pulledcan't carry amps thru the meterthe shunt mounts at the battery and your charger and load wires connect to the shuntthe shunt is a Heavy and calibrated resistor, usually 75 ohmthe load amps passes thru the shunt, the meter measures the voltage drop across the shunt, to give you the reading of the power amps being usedThis is essentially correct, except a shunt's resistance is nearly always far, far lower than 75 ohms. Many shunts are sized to have a 75 mV drop at their full current; for a 100A shunt, Ohm's law dictates that the resistance is 0.75 milliohms or, equivalently, 750 microohms. A 10A 75mV shunt would have a resistance ten times higher (7.5 milliohms).Many ammeters that don't have an external shunt have an internal shunt and operate on the same principle. The main advantages of a separate shunt are that you don't need to route heavy wires through the ammeter itself if it's removed from where the current you're measuring is located, and the meter doesn't need to have suitably heavy connectors (and space for them) or to worry about the power dissipation inside the meter. A 100A 75mV shunt dissipates 7.5W at full scale, which is plenty enough heat to worry about in a small enclosed meter case.

Most precision Manganin shunts are either 50 or 100 MILLIVOLTSFrom one side to the other a decent handheld digital multimeter will work as a gauge to measure voltage drop this amperage passing across the shunt.Lets do A REAL EASY EXAMPLEA 100 amp gauge rated for 100mv shuntA 100 MILLIVOLT shunt.Many handheld meters have a particular setting that has a limit of 200 MILLIvolts let's start out by switching the dial to millivolts, 200Pass some stable amps across the shunt like headlights or heater blower.Polarity matters so if you get the meter red and black mixed up there will be a - sign before the numbers. Got it?Let's use headlights. Touch the leads across the shunt.The meter reads 13 millivolts.This calculation is simple because it's a 100 amp meter hooked to a 100 millivolt shunt.13 millivolts here translates to THIRTEEN AMPS.Muse on that and then gently include kickers like the 75 millivolt shunt you have. A bit confusing because a 100 amp meter rated for 75 millivolts needs a 75 millivolt drop of voltage across the shunt to equal 100 amps. Let's stop there.

Needed because the ammeter is really a voltmeter.The shunt creates a calibrated voltage drop and "ammeter" reads the voltage drop.

What do a shunt do , anyway ? | Good Sam Community

theoldwizard1
Explorer II
Dec 06, 2018
CA Traveler wrote:
Here's my 500A shunt. It's not recommended to install a shunt like this near flooded batteries.

While "not recommended to install a shunt like this near flooded batteries" a shunt needs to be a close as reasonably possible to the battery. Keep it clean (Baking soda bath a couple times a year) and coated with some kind of grease/oil (lanolin grease/oil like Fluid Film) and it will last forever, even mounted on top of a flooded lead acid battery.
wanderingbob
Explorer II
Dec 06, 2018
WOW , I am the OP , I guess that I should just follow the directions on the meter that I ain't bought yet !
MEXICOWANDERER
Explorer
Dec 06, 2018
The environment of a switchboard room is considerably friendlier than the bay of an RV.
Yes I am fussy

Yes I will disassemble a shunt. Resize the holes for SAE fasteners
Screw in SILICON BRONZE studs then silver solder the back side of the shunt. S/B flat washers, lock washers and nuts.

Apply a drop of DeOxit beneath the milli amp wires
Or for my own use SOLDER the milliamp wires direct to the blocks.

Then assemble with silicone dielectric grease.

For me it pays off. Absolute dead-on accuracy for decades.

(Got better things to do than rush an important work just in time to run in the house and catch "Judge Judy".
Sam_Spade
Explorer
Dec 06, 2018
wanderingbob wrote:
I was looking at a meter and it said shunt required , Why ?

And after all of those replies.............
Assuming that you were looking at a multi-function meter:

It will function just fine for all of it's functions EXCEPT reading high currents, for which an external shunt is required.

Typically, that means volts, ohms and amps up to 10 just with the meter itself.

And....you can't just get any old shunt. You need one with a resistance value that matches the meter calibration.
If you want to read high currents, that is.
Sam_Spade
Explorer
Dec 06, 2018
Duplicate deleted.
deltabravo
Nomad
Dec 06, 2018
https://www.youtube.com/watch?v=IFhoOXdS_bY&
Big_Katuna
Explorer II
Dec 06, 2018
deltabravo wrote:
MEXICOWANDERER wrote:

Never a good idea to stack cables on a shunt, use....

I stack cables on the shunt all the time.

I used to work for a company that sold amp hour meters, and we never told people not to stack multiple cables on a shunt.

It’s ok to stack terminals if some thought goes into it.
The higher amp terminal should be on the bottom and size matters. You want to minimize high current transitions between terminals.
time2roll
Nomad
Dec 06, 2018
DrewE wrote:
For smaller currents, there are actual analog DC ammeters that don't measure the voltage across a shunt. They have a coil driving the meter that's wound with a few turns of heavy wire, and respond to the magnetic field that's directly proportional to the current. (Indeed, analog voltmeters are arguably really ammeters as well, as it's the current through the coil in the meter that drives the movement.)
I thought it was an internal shunt. Learn something new every day.
deltabravo
Nomad
Dec 06, 2018
MEXICOWANDERER wrote:

Never a good idea to stack cables on a shunt, use....

I stack cables on the shunt all the time.

I used to work for a company that sold amp hour meters, and we never told people not to stack multiple cables on a shunt.
DrewE
Explorer II
Dec 05, 2018
ScottG wrote:

My professor liked to ask his unwitting class how many types of meters there are. His answer was one; the volt meter.
The amp meter is actually a volt meter with different markings.

With a shunt type meter, that's pretty much true.

For smaller currents, there are actual analog DC ammeters that don't measure the voltage across a shunt. They have a coil driving the meter that's wound with a few turns of heavy wire, and respond to the magnetic field that's directly proportional to the current. (Indeed, analog voltmeters are arguably really ammeters as well, as it's the current through the coil in the meter that drives the movement.)