External shunts are used especially on boats where you have great distances between the Bat/ Alternator and the gauge. With a internal shunt you have to run large current carrying wires all the way to the cockpit and back . The greater the distance, the larger the wire, the greater chance of failure and the greater potential chance of damage. External shunts provide very low current to the meter and require very small wire.
>>unmeasurable low resistance. It does not get hot.<<
The lower the resistance, the higher the current, the higher the heat.
