Hooking up a device which is powered externally, to the OBD port of a car
I have built the device and it is working perfectly fine, there are no issues at all.

I have also added a buffer with a capacitor (10000 uF) so I could switch between the car battery and external battery seamlessly with a DPDT switch, this also works very fine.

I didn't experience any significant inrush current in my configuration.

Thank you very much for your comments.