Electronic components are designed for a maximum and a minimum working voltage. The chips in consumer devices generate heat. It may be the case that on battery powered equipment the internal circuits have a standby or shut-down period to conserve the internal battery.
When using an external power pack or power supply, the manufacturer has anticipated that the device will be used continuously. By reducing the voltage to the minimum working voltage, there is less heat generated for the same workload.
It may be an interesting exercise to compare the heat generated, by touch, between internal battery use and external power pack or power supply. That information may be useful to those members who shoot continuous photos on an external power supply to determine the long term reliability of their camera.
