A shunt is necessary in order to measure amps and amp-hours with the TriMetric battery monitor. A shunt is an accurate, very low resistance resistor which is placed "in line" with the wire carrying the current to be measured. With the TriMetric, it is usually placed in the negative wire from the battery bank, such that all the current going into the battery (charging) or out (discharging) must pass through it. Connected in this way it will be set up to monitor "net" amp-hours in and out of the battery. (It also could be placed in series with the negative wire coming from a solar array--or other charging source--in which case it would measure only the solar array current, if that were desired--to show total solar "amp hours" production.) The shunt needs to be placed near the batteries; since these wires carry very high currents the wires from the batteries must be kept short to minimize electrical losses.
When current flows through the shunt, a small voltage is developed across the shunt which is proportional to the current flow. The TriMetric battery monitor accurately measures this very low voltage and converts it to the "amps" reading on the meter. The resistance, which is the ratio between the voltage across the shunt and the current flowing through it, is a constant for any particular shunt--and is one of its important parameters. For most substances, resistance is usually described in "ohms", however for some reason shunts are described by the voltage drop that occurs with a certain current: for example: "50 millivolts at 500 amperes." However the meter only cares about the ratio-which is the resistance--so a 500Amp/50 millivolt shunt is equivalent, from the TriMetric's view, to a 250Amp/25 millivolt shunt. They both have a resistance of 0.1 milliohm, or one ten thousandth of an ohm.
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