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

3_tons
Explorer III
Dec 05, 2018
Using fluid flow as an analogy, a shunt is to electrical what a flow orfice (a two sided )sensor tap) is to the measurement of fluid flow. The amperes rating is akin to the maximum expected gallons (or barrels) of flow...

3 tons
BFL13
Explorer II
Dec 05, 2018
MEXICOWANDERER wrote:

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

Where does the big lug go from the buss bar to the shunt then? You are supposed to add up all the ampacities of the "branches" that go on the bar, and then have one fat wire from the buss to the next thing.

At least the 500a shunt bolt can take a 300a lug from the inverter neg plus a 100a charger's lug, and some little ones. Got to stack some somewhere!

I thought it was just on a battery post that Mex didn't like stacking lugs.
CA_Traveler
Explorer III
Dec 05, 2018
Here's my 500A shunt. It's not recommended to install a shunt like this near flooded batteries.
MEXICOWANDERER
Explorer
Dec 05, 2018

Never a good idea to stack cables on a shunt, use....
BFL13
Explorer II
Dec 05, 2018
Another thing the 500a Trimetric shunt "does" is act as a negative buss bar. Then only need one lug to battery neg post.
Big_Katuna
Explorer II
Dec 05, 2018
For optimum accuracy the shunt should be kept between 20-80 C. If it gets hotter the resistance can drift. That’s for Manganin shunts.

Riedon makes shunts up to 10,000 A.

I used to install 1600A, 52VDC, three phase battery charges. Usually three in parallel.
The batteries were five ft tall. 2V. For telephone offices.
MrWizard
Moderator
Dec 05, 2018
oops sorry. 75 millivolt NOT 75 ohms

brain fart
MEXICOWANDERER
Explorer
Dec 05, 2018
I was fortunate enough to find a 500 ampere shunt rated at + - 8 ppm at 50 Mv.

Gold plated everything.

It rests in a vacuum thermal sealed bag with my array of NIST traceable 1 watt resistors. And silica gel. With my 6.5 digit calibrated bench meter I can pretty much guarantee that I can calibrate amp hour meters.
ScottG
Nomad
Dec 05, 2018
There's a slight resitance across a shunt as power goes through it that creates a different voltage on each side of it. This difference in voltage is directly proportional to the amount of current that's going through it.
So as an example, 10 millivolts (.010V) of drop across the shunt is shown on the meter as 10 amps. 20 millivolts = 20 amps, etc.

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.
time2roll
Nomad
Dec 05, 2018
Needed because the ammeter is really a voltmeter.
The shunt creates a calibrated voltage drop and "ammeter" reads the voltage drop.