How to power up OBD device without ignition to vehicle?

I'm not real sure what your asking but if your just trying to power the tool on the bench:

-If your scan tool has an external plug for power then you simple feed it 12VDC.

-If your tool does not have external power supply port then you can power it using 12VDC with + clipped to pin 16 and - clipped on pins 4 & 5 inside the tools obd2 connector.

I would make a male/female adapter to go between the port and the obd device.

That way you can supply the 12v needed and not have to get to any of the pins with clips that can come off. Will be much more secure.

Yes, possible but not a good idea.

Yes you could power up the OnBoard Diagnostics device, but unless there is power in the engine control computer you won’t be able to communicate with the data bus system. Without data from the engine computer you wont be able to see any inputs or read stored codes.

Now you could cut the power lead wire in the OBD II harness ( to ensure you are not getting power from the vehicle,) and then power up the vehicle (so you can get data) but the vehicle ground circuit must me linked to your portable OBD II power supply. (Why would anyone want to change a circuit like that?)

OBDII readers with internal batteries include control circuitry to preclude problems from key on power. I’m pretty sure the OBD devices use only 5v (or less) power internally, so voltage / current flow matters.

This is really not a good idea.

Whats the goal here? What exactly is the concern for such a proposal? If you are worried about battery juice throw a trickle charger on the vehicle while you are working.

I was thinking of doing this to be able to run my allstate driving monitor and my performance tuner at the same time on a obd2 splitter, I have seen alot of peaple say the splitter can't keep steady power to two devices at the same time. So I figured i would tap into one side of the splitter and feed power to it. Even if I have to somehow reduce the power to 5 volts