I heard that charging USB devices using PC is slower than using AC sockets. If that is true, is there a way to achieve the same speed using PC or is it standard for motherboards to limit USB current to a certain threshold with no way around that?
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2 Answers
I heard that charging USB devices using PC is slower than using AC sockets.
That depends.
AC chargers are free to offers as much power as they are designed for.
(Which in theory could even be less so do check before you buy).
Nicely behaving USB on the other hand can only offer up to 500mA for USB2 or 900mA for USB3. *1
Then there are some devices which do not adhere to the standards. E.g. the apple air which uses USB2 to power an external DVD writer with 3x legal power.*2. And many laptops and some desktops and even the monitor in front of me have a 'charging port' which can provide more power. Or provides power even when the device is turned off.
But adhering to standards: No, max 500mA for USB2*3.
*1Can offer. But do not need to. Imagine a semi normal 14 USB2 port desktop. It is reasonable to expect that it will not deliver up to 500mA on all 14 port simultaneously. That would be 7 ampere flowing though the motherboard in addition to normal power.
For this reason an USB device will get 1 bin of power (100mA for USB2) and may ask for more power. It may or may not get it.
*2But only if it detects the right DVD player. If does not do so for generic devices.
*3And as written, max 900mA for USB3, though you might need the right USB3 cable for that. IIRC plugging an USB2 cable into an USB2 port should not draw more power than the cable is designed for. And USB2 cables only need to be designed for up to 500mA.
There are some motherboards around which supply a higher current for this purpose. Gigabyte sells MBs with this technology under the name "on/off-charge". Asus calls it Ai Charger.
But this applies only to newer motherboards. Older ones will deliver only reduced power for charging.
