What do a shunt do , anyway ?

Most precision Manganin shunts are either 50 or 100 MILLIVOLTS

From 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 EXAMPLE

A 100 amp gauge rated for 100mv shunt
A 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, 200

Pass 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.

MrWizard wrote:
the meter it self won't stand up to the 'amps' being pulled
can't carry amps thru the meter

the shunt mounts at the battery and your charger and load wires connect to the shunt

the shunt is a Heavy and calibrated resistor, usually 75 ohm
the 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

This 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.

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

the meter it self won't stand up to the 'amps' being pulled
can't carry amps thru the meter

the shunt mounts at the battery and your charger and load wires connect to the shunt

the shunt is a Heavy and calibrated resistor, usually 75 ohm
the 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