AMPERAGE
Shunts are heavy-duty devices that allow us to easily measure DC current flow in a cable, without running all the current through the meter. The current flow is measured in amps. In order to measure amps, all current must flow through the shunt. Heavy-duty shunts are needed to measure high flows of current.
Only one side of the circuit Is monitored to measure current flow. Current flows, In a manner, from source to load up one cable and from the load back to source down another cable. With some meters, If you measure across both cables, the meter will read zero, as current In one cable cancels out current flow In another cable.
The source (alternator DC, battery charger DC output, campground AC outlet, generator AC, etc,) can only deliver so much current based upon Its design. All sources have an amp limit beyond which they cannot deliver. The demand of the load determines how many amps will flow In the cables (up to the amp limit of the source).
If batteries are low, high current will flow from the source. If batteries are almost full, the little current will flow from the same source, as batteries determine for themselves how much current they will take In. As batteries get full, their Internal resistance goes up, which makes it harder to get amps Into battery. That Is why a low battery may come up from 50% to 80% In a few hours, but it takes 12-24 hours or longer to go from 80% to 100%.
VOLTAGE
The voltage, on the other hand, measures the force of the current. Voltmeters can determine voltage by examining the difference between two points, like positive & negative in DC or hot & neutral In AC. Current does not flow through the meter, 60 voltmeters can be simple and small.
SHUNTS
Shunts are very old and simple technology. Just 2 bocks of metal connected with several fins. Shunts are resistors that produce a small linear voltage drop between both sides of the shunt based upon the amount of current flowing across the shunt. Shunts are calibrated for the consistent voltage drop across varying current loads.
The voltage drop is very small and can be measured In millivolts (1/100 volt). When current Is flowing In any resistor, the voltage drop, no matter how small can be measured with a voltmeter with probes on each side of the resistor. The voltmeter measures the voltage drop across the resistor, not voltage In circuit. Voltmeter uses a millivolt setting. One millivolt Is .001 volt. A voltmeter can be far from the shunt, connected to shunt with ordinary small diameter twisted pair of wires.
Shunts are usually Installed by removing the battery ground wire from the negative battery post. Connect the ground wire to one side of the shunt. Connect the new short cable from another side of a shunt to the battery ground post. Now all current in or out of battery flows through the shunt. Connecting to the ground side of the battery Is safer because If the twisted pair from the shunt to meter touches the metal ground, sparks will not be produced. When there are multiple house batteries, each battery having its own shunt would show how loads are being shared among batteries.
Shunts are manufactured with 2 specifications.
1.) Maximum amp rating
2.) Voltage drop at above amp rating
For high loads like engine starter, 500 amp rating Is useful. Keep In mind that with boost switch on, house batteries will be used to start the engine. If the voltage drop specification Is 50 millivolts, it Is easy to compute amp draw from millivolt reading.
If 500 amps readout as 50 millivolts.
Multiply millivolt reading by 10 to directly read amps. 100 amps would readout as 10 millivolts, 10 amps = 1 millivolt, 35 amps = 3.5 millivolts. Keeping the above In mind, shunts should be purchased with millivolt ratings that match amperage ratings. 200 amp-20 mIllIvolt, 100 amp-10 millivolts, etc. (Purchasing 200 amp – 50 millivolt ratings would require multiplication by 4 to know amperage.)