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This question already has an answer here:

My car has 2 USB sockets that provide power and also serve as a data link for audio, etc. from phones to the entertainment system. The entertainment system also accepts a Bluetooth connection for hands free and audio, etc.

The audio connection via USB provides more features than the connection via Bluetooth, for example album art is displayed on the entertainment system and I can seek to a specific point in a track.

The downside of using the USB connection for data is that i must also use it for charging and it doesn’t provide much current.

I also have a 12v to USB adaptor in the car that provides 2.1A and charges the phone much more effectively.

My question is; is it safe make my own wiring to take power from the 12v to USB adaptor and data from the car’s sockets so that I can get the advantages of 2.1A charging and the data connection to the entertainment system?

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marked as duplicate by Michael Karas, RoyC, Michel Keijzers, Dave Tweed Mar 19 '18 at 11:14

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Provided they have the same ground, yes. A continuity tester will show you if the grounds are all connected together on the car side, but it would be prudent to connect them together in the Y connector you are making.

There are a few cases where grounds are deliberately at different levels where this will cause problems.

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If well designed they will all use the battery negative from the car as ground which means that it shouldn't be a problem.

However, it might be wiser to use a USB hub with a separate power supply. You could connect the data to your entertainment system and power the hub with your 12V to USB adapter.

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