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
This question appears to be off-topic. The users who voted to close gave this specific reason:
- "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Nick Alexeev
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.
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.