I have not yet bought my AutoPi dongle but I just wanted to share this idea.
I want to use a Pi3A+ together with a 7 or 8 inch touchscreen in my car.
Since the screen has to be powered by the Pi, the combination can use as much as 2.5A according to the manuals!
I don’t want this power coming from my OBDII port…
This is the solution I came up with:
- A 12 to 6V DROK car power converter
12V to 6V Converter, DROK Car Power Converter DC 9V-22V 12V to DC 6V Buck Voltage Regulator 3A - A switching circuit. It is very small and could be incorporated in the RPi3 adapter (hint, hint :-))
Switching between battery and external power sources
The components used only cost about 3$. - Both 5V connections on the RPI3 adapter should be cut offcourse. This way only the RPi3 and display will be powered by the carbattery directly.
The 5V power from the AutoPi is used as the battery source. The 6V from the converter is used as the external power source. The RPi3 and touchscreen are connected as load. The diode in the 6V circuit makes the voltage drop to 5.3V, the RPi3 should operate on that just fine.
So when the car is off, the power from the AutoPi is used. When you switch on the car, the power for the RPi3 and touchscreen comes directly from the battery. The AutoPi is still powered from the OBDII port.
What do you think, will this work?
Update:
The OBDII specification (SAE J1962, article 5.3) states that pin 16 should be able to supply a minimum of 4A. In article 5.4 it says that it is recommended that the vehicle manufacturer provides circuit protection.
For my car, a 350Z if you didn’t already get that 
, I read about power loss on the OBDII connector. And nobody can find a blown fuse.
So the OBDII port should be able to supply the power for a AutoPi, RPI3 + touchscreen setup, but I am going to play it safe and try to use this circuit.