The OBD adapters I know get 12 volts through the 16 pin J1962 connector since it is supposed to go to the battery. I added a passthrough wire to power the bluetooth adapter (pin 9, 3-12v for aircable and many others). On my harley, it has a 4 pin connector, only 3 used, ground, switched 12v, and j1850. The problem with cars and "forget about it" is that it will still draw power - only a little, but enough you wouldn't want to leave it for a week.