The SeaFET V2 has separate internal power terminals for the alkaline cells vs. an external voltage source. If a voltage is present on the external power input and above a nominal minimum input voltage, the input voltage from this source will be boosted to 15.5 volts supply to the main electronics and will overcome the internal alkaline battery source. If external voltage is not present, then replaceable internal alkaline D cells will be the power source. So, the external input voltage can actually be lower that your internal battery voltage, and the SeaFET V2 will still default to the external power source.
The SeaFET V2 will always default to the external power if a nominal voltage is present on this input. This is different from many other Sea-bird product that automatically select an internal/external power source based on which has the higher input voltage.