A shunt is a precision resistor that allows the safe measurement of potentially high electric current. Placed in series (inline) with a current-carrying conductor, a shunt’s low resistance creates very little voltage drop compared to other items in the circuit, so it consumes almost no energy. Measuring the voltage across the shunt gives the amount of voltage drop across that precise resistance. Those amounts are plugged into the Ohm’s law equation to give an accurate computation of current flow in amperes through the circuit containing the shunt.
Shunts are rated by their maximum ampacity and the corresponding voltage drop in millivolts across them. A 500 A / 50 mV shunt will have a 50-millivolt voltage drop when 500 amps are flowing through it. The most common battery monitor shunts are 500 A / 50 mV and 100 A / 100 mV. Typically they are installed within the DC disconnect enclosure in series with the negative conductor, between the batteries and the inverter. But shunts can be placed anywhere that current needs to be monitored. Some newer monitors have multiple inputs for multiple shunts, so they can monitor a variety of inputs and outputs.
Ohm’s law, voltage (E) = current (I) x resistance (R), and its sibling equations (I = E ÷ R and R = E ÷ I) explain that if a 50 mV (0.050 V) voltage drop occurs at a current flow of 500 A, the resistance across the shunt equals 0.0001 ohms:
R = 0.05 V ÷ 500 A = 0.0001 ohms
The resistance of this shunt remains constant (0.0001 ohms), so the monitor can at any time compute the circuit amperage by plugging into its internal calculations the voltage drop across its shunt. For example, how much current is flowing if the monitor measures a 10 mV drop across the above shunt?
I = 0.01 V ÷ 0.0001 ohms = 100 A