How does a device know how much power it can draw from a USB port?

Question

I know my iPhone is charged twice as fast if I use the dedicated charger instead of a USB port. How does the iPhone know it's connected to a charger? Is this functionality some part of the USB standard or is it Apple-specific? Would the iPhone also recognize a special charger port on my motherboard?

Answer 1

It isn't the Iphone that detects what type of charging method is being used and then decides how much current to draw.



When connected to a USB port it will draw the maximum amount of power the USB port can supply. Although USB 2 ports are generally rated at 5 volts [not watts as another answer says] there are three kinds of port: "Downstream", "Downstream Charging" [the one your'e interested in] and "Dumb".



In the case of USB 2 the Battery Charging Specification [for a downstream charging USB port]allows a device to draw a maximum of 500mA [it is higher with USB 3 900mA]



This limit is why the ipod will charge more quickly from the mains and also why some external hard drives and optical drives that do not have their own power supply often come with USB "Y" cables so they can connect to two USB ports at the same time to draw the power they need to operate correctly.



if you are that interested in USB battery charging standards this article may interest you: http://www.extremetech.com/computing/115251-how-usb-charging-works-or-how-to-avoid-blowing-up-your-smartphone

Answer 2

The best way to explain this, i hope, is to use an analogy.

Consider that the iphone or the ipod is like a water bottle that has run dry and needs to be refilled.

If you use a drinking straw size hose to fill it it will take much longer to fill than if you use a garden hose size hose to fill it. As long as the pressure out of the end of either hose is the same. The larger hose is capable of passing more water.



The voltage of the USB port is the same 5 volts if it is from the computer USB port or the dedicated USB power supply. It is like the pressure at the end of the hose.

The USB port on the computer can pass .5 amps of current.

The USB port on the dedicated power supply can pass 1 amp of current.

So the current is like the water and it is filling up the battery.

The dedicated power supply should take half as much time because it is passing twice as much current.

Some dedicated power supplies can pass 1.5 amps and higher.

My blackberry rapid charger is 1.8 amps and It rocks, I love it. Waaaayyy faster than plugging into a computer port.

So that explains why one is faster than the other but not why computer usb ports are low amperage.

It is because the USB ports on the computer are just soldered on to the circuit board in the computer. The circuit board has paper thin layers of copper to pass the current through. If the computer didn't limit the current through the thin copper it would overheat and burn up. It would be very fast, more like a pop and then the connection would be broken and never work again.

The dedicated power supplies use wire or thicker/wider circuit board traces that can take the higher amperage.

The other side of this is that the battery can only be charged so fast. It can't charge in seconds if you send it 100 amps. It has a maximum absorption of the current being sent to it.

As you experienced, the absorption of the iphone is higher than what the computer port can send to it.

So the iphone needn't know that is connected to a charger, it's just absorbing everything it is being sent and the charger can send more.

And if your motherboard had a special charger port, one that had more amperage it would absorb more and therefore charge faster.

Alot of those external USB hard drives have a USB cable that Y's out to two ends to plug into two USB ports on the computer and just the one end on the hard drive. Because the hard drive need more amperage than one USB port can supply. If you had that kind of cable for your iphone it should charge twice as fast.

Answer 3

Yes, it's part of the USB standard.



Specifically, if the port is powered, it will send power. There is an acceptable range. So, some devices send very little power. Others send the maximum (you can look the number up yourself).



The iPod then accepts that amount of power. Poorly made devices may not be able to accept the entire range. Even a high-end device, such as an iPhone, fail to work at the low end of the range. But no harm.



What's bad is when the device can't accept the high end. These are devices that tell you NOT to connect it to a computer, but to ONLY use their charger.

Answer 4

the USB standard is 5 watts maximum ...