Repeating the (unanswered) question above: I would like to have a 3.3V Arduino board on a vehicle, and the current to be measured is up to 120 amps for about 22 volts.
I understand the idea of a shunt, but I don't know how to scale up the concept to deal with the circuit. Of course, I also don't want to burn up much power in the measurement function.
You can buy shunts on eBay - typically they drop 50 or 75 mV at full load.
Or, you can buy a DC clamp ammeter attachment - uses a Hall effect sensor to measure DC current. They are used by auto mechanics.