A shunt resistor is used to measure electric current, alternating or direct. This is done by measuring the voltage drop across the resistor. Several parameters are important to specify a shunt resistor. Shunt resistors have a maximum current rating. The resistance value is given by the voltage drop at the maximum current rating. For example, a shunt resistor rated with 100A and 50mV has a resistance of 50 / 100 = 0.5 mOhm. The voltage drop at maximum current is typically rated 50, 75 or 100 mV. Microhm milliohm shunt resistor  MMS8420 can withstand current up to 600A, which is a good partner for the BMS of electric vehicles.  
Other important parameters include the resistance tolerance, the temperature coefficient of resistance and the power rating. The power rating indicates the amount of electric power that the resistor can dissipate at a given ambient temperature without damaging or changing the resistor parameters. The produced power can be calculated with Joules law.

 
Shunt resistors have usually a derating factor of 66 percent for continuous operation. This is defined for a run time longer than two minutes. High temperatures negatively influence the accuracy of the shunt. From 80 degrees Celsius thermal drift starts. This gets worse with rising temperature, and from 140 degrees the resistor will damage and the resistance value be permanently changed.