Compaq Armada 7400 Reference Manual page 47

Connect the 1-4 pin adapter to the cigarette lighter plug , then to
the 4-pin aircraft receptacle
Leaving the computer connected to the vehicle with
the optional Automobile/Aircraft Adapter for an extended period
of time when the vehicle is off may deplete the vehicle battery.
Depletion occurs regardless of whether the computer is turned on
or off.
.