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?

double USB cable

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.

iPhone 7+

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?

  • \$\begingroup\$ w/ a common ground and power splice it will work fine, and your phone will charge much faster. \$\endgroup\$
    – dandavis
    Commented Jun 7, 2017 at 19:52
  • \$\begingroup\$ @dandavis That's what I thought. The cable arrived today, and I'll check tomorrow. 👍🏼 \$\endgroup\$
    – Maciek
    Commented Jun 7, 2017 at 21:45
  • \$\begingroup\$ @dandavis it didn't work :/. Could it be because one end of this cable has only black and red wires and the other all four wires? Does it matter? Can I solve this using a chip or arduino etc? \$\endgroup\$
    – Maciek
    Commented Jun 8, 2017 at 16:54
  • \$\begingroup\$ yes. you need to specially handle the middle wires on newer iphone to get them to pull more. they use a different trick than samsung, i can't recall the details atm, but it's very simple. \$\endgroup\$
    – dandavis
    Commented Jun 9, 2017 at 2:31
  • \$\begingroup\$ Which iPhone is this? Which connector, Type-C, or Lightening? \$\endgroup\$ Commented Jul 6, 2017 at 0:12

1 Answer 1


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.

  • \$\begingroup\$ you're right about the spec, but i think only laptops and high-end phones abide. most chargers don't manage current like that, and most phones will draw at least an amp from naive chargers while charging. \$\endgroup\$
    – dandavis
    Commented Jun 7, 2017 at 19:55
  • \$\begingroup\$ On top of this the latest iPhones, especially those with bigger screens are able to draw 2A! \$\endgroup\$
    – Maciek
    Commented Jun 7, 2017 at 21:59
  • \$\begingroup\$ @dandavis I agree that many devices don't comply, but I would be surprised if that included Apple. \$\endgroup\$
    – Jon
    Commented Jun 7, 2017 at 22:12
  • 1
    \$\begingroup\$ @dandavis - the data lines used to indicate the charger capability won't be connected to the charger; instead they will be connected to the car, which should act like an ordinary USB host. Effectively the only way the phone could know that the actual supply can provide more is to try to draw more, and see that the voltage doesn't sag or protectively shut off. Since that would ordinarily cause a host like a computer to drop the connection, the phone shouldn't do that. \$\endgroup\$ Commented Jun 7, 2017 at 23:00
  • \$\begingroup\$ @ChrisStratton John was right, iphone recognised only 500mA. Can I use something like LTC4412 chip to do what I want? \$\endgroup\$
    – Maciek
    Commented Jun 8, 2017 at 17:03

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