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I want to keep a device in always on situation in the car.

I do not want the device to drain the car battery.

The car is used at least once in two days and the current consumption of my device is not that huge < 500mA .

Wiring :

I plan to keep the OBD port's +12V disconnected, connect the OBD GND to my device's GND and the data lines to my device then power my device with an external battery. . I will use the 12v DC output of it.

Example of an external battery : https://www.amazon.de/dp/B07P6V7SZH/ref=sr_1_19?keywords=power+akku+generator&qid=1581438260&sr=8-19

Wire the external battery such that it's + line connects my device and - line connects to the OBD GND which is now shared with my device's GND.

Do you see any issues with that wiring ?

Thank you very much.

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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.

From my experience it is quite safe to build a circuit like this. I didn't experience any significant inrush current in my configuration.

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