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My girlfirend has just installed a new CD Player on her car that comes with a USB port to connect external devices(Flash card, external hard drive, etc). My question is based due the fact that I told her that if she want to recharge her Android smartophone on her car, she must buy a 12V car adapter to USB(which is very expensive on Brazil). Now, with the new CD Player, I'm wondering if she can use it's USB port to recharge her cellphone with the USB cable. It is feasible? There is any limitations on recharging an Android smartophone on another types USB ports(I mean from another host not only the CD player)?

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Would be interesting to know if it works. Please do let us know if/when she tries this. –  Chahk Nov 16 '11 at 19:08
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3 Answers 3

up vote 14 down vote accepted

There're two flavors of USB ports - powered and unpowered. Unpowered ports can't supply power to the devices and are rather rare. You identify them easily - connect the smartphone to them and see if it starts charging. If it starts charging the port is powered and you're okay.

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If the USB port supports USB sticks then it has power to recharge a smartphone. The only question is how much power it has. The USB protocol includes what I would call a "power handshake". Host (radio) and client (smartphone) communicate with each other and the host specifies how many milliampere the client is allowed to use.

If the radio has USB high-power support you will get up to 500mA, without only up to 100mA.

BTW: A plain USB power adapter usually supports up to 1000mA. I assume a simple car 12V-to-USB-adapter would also support that much power.

Therefore even if charging via radio works, charging will take significantly longer. How many milliamperes are used for loading can be seen on a large number of phones by the Battery Monitor Widget or similar apps.

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I usually charge mine from the USB port of my TV decoder (that's always powered on) so I think there will be no problems.

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