It's almost twice as fast charging my phone directly from the power plug compared to if I charge my phone from the computer's USB port. Why? Isn't the charge the same or is it that USB has much lower voltage, and therefore it is much less energy per time unit?


closed as off-topic by Nick Alexeev May 3 '16 at 0:09

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  • \$\begingroup\$ USB is mostly 5V, but USB devices negotiate their current draw with the USB port. Default is 100ma and they can request 500ma, 1000ma or 2000ma. "Power plugs" are high current, and it's common for computer USB ports (especially laptops) to be only 500ma. iPads indicate this state by saying "Not Charging" even though they are. \$\endgroup\$ – Harper May 3 '16 at 6:17

In my opinion, it's maybe because the computer's motherboard limits the current when charging through the USB. In details, there are two phases when charging a battery. One is constant current phase, which is fast and powerful and happened at the beginning. The other one is constant voltage phase, which is slow and happened during the ending phase of charging, but this help to keep battery full. So when charging via a charger, the optimized method is applied, which is const-current first and then const-voltage. However, when charging via USB, the computer limits its voltage to 5V, which forced a const-voltage phase. This is only my experience and guess, and I'm glad to hear other answers from you brilliant guys.

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    \$\begingroup\$ One more thing, the issue is not about voltage. The USB has nearly the same voltage (about 5V) as the charger, the issue is the current. Computer motherboard cannot support huge current. Laptops are worse (Mine supports less than 1A). \$\endgroup\$ – Tom Tao Lu May 2 '16 at 22:38
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    \$\begingroup\$ 5v is above the maximum voltage for a standard lithium batter, the phone has an internal charger that deals with the constant current/constant voltage stuff. The difference between a computer and a wall plug is the current available. \$\endgroup\$ – Gorloth May 2 '16 at 22:48

USB charging on modern phones will be limited by the phone. If it detects a standard USB host port, most phones will self limit themselves to 500 mA, even if the USB port can provide more. If it detects a dedicated charging port, it will pull more. Depends on how the port is wired. Apple and the USB spent differ.


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