Shunt type current measurement is a pretty simple method. It is effectively just measuring the voltage drop across a known resistance, and applying Ohm’s law (V=IR) to calculate the current. But, in practice, there are a lot of issues with this. First and foremost is that higher voltage drops are easier to read, but the higher the voltage drop across the sense resistor, the more power is wasted to heat in the sense resistor, and the more heat, the larger the resistor needs to be. It is possible to gain some current data from a sense resistor with a maximum voltage drop of 100mV, but expensive, sensitive, and highly accurate instrument amplifiers are required, and it still may not be possible to tell the difference between, say 3/4 of the maximum current and 13/16 of the maximum current. So to use shunt type current measurement you either have to waste significant power in an expensive and large sense resistor or rely upon expensive instrument amplifiers which can easily be sensitive to thermal or other sources of electrical noise. But that’s not all. Shunt type current measurement also inherently suffers from non-linearity due to temperature changes because no resistor material has constant resistivity with respect to temperature.