The car I recently bought has a built in USB but only provides 500mA.
I want to use a cable like the one below to connect my iPhone to the built-in USB to play music/satnav on the car stereo, but I want my iPhone to draw a power from my powerful car charger (4.2A).
I understand that connecting two power sources together with this kind of cable is not a great idea, I'd have to modify it but disconnecting power (+) wire from one cable. Related discussions (below) say that both 'host' devices have to have a common ground. The car's USB and the car charger ground are common so this should work.
Will it work and recognise my iPhone?
Update: Jailbroken iPhone will show you what the current is during charging. It'll show around 2000mA when the phone is off. With the screen and other things on it shows over 1700mA. Using iPad charger in the screenshot.
Related, but I thought my question could possible be more specific as it's to do with cars. By reading those I think the answer to my question is yes, however I'd like a confirmation from you guys.
USB data transfer without charge
Can the power source for a USB device be different from the data source?
Answer
It should work, but you won't get more than 500mA. When a USB device, such as your iPhone, is connected to a data port, it asks the port how much current it can supply before pulling more than 100mA. Since the car doesn't know there is a 4.2A supply available, it will report '500mA'. The iPhone will then draw up to this limit from the port.
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