I've heard claims that a usb 3 cable charges faster than a usb 2 cable. is this the case? I'm skeptic.
I recently bought a samsung galaxy s5 with a usb 3 charging port, but it came with a usb 2 cable. why? is it because of backwards-compatibility with other devices? nonetheless I find it to be penny-pinching not to include a usb 3.0 cable when they make a usb 3.0 phone!
I took a look at the schematics of a usb 3.0 cable. it has the same power cord as the old usb, but an extra ground. does an extra ground do anything for the charging process?
If you're talking about charging via a computer's USB connectors then yes, it will make a difference. USB 2.0 can draw 2.5W and USB 3.0 can draw 4.5W. If you're talking about charging from a wall socket then the type of cable won't matter because the power of the charger determines the power, not the connector.
In that case the power would be limited by the cable. USB 2.0 has 4 pins and I think USB 3.0 has 9. USB 2.0 cables can only make contact with the bottom 4 pins on a USB 3.0 connector so it will only be able to draw power from those pins. So it would work at USB 2.0 power and data rates.
From Wikipedia "A unit load is defined as 100 mA in USB 2.0, and 150 mA in USB 3.0. A device may draw a maximum of five unit loads (500 mA) from a port in USB 2.0, or six unit loads (900 mA) in USB 3.0. There are two types of devices: low-power and high-power. A low-power device (such as a USB HID) draws at most one-unit load, with minimum operating voltage of 4.4 V in USB 2.0, and 4 V in USB 3.0. A high-power device draws, at most, the maximum number of unit loads the standard permits. Every device functions initially as low-power (including high-power functions during their low-power enumeration phases), but may request high-power, and get it if available on the providing bus." This is when you're plugged into a computer.
If you're charging using a wall socket there's no data connection. It just puts the rated power of the charger into the device no matter what.
If you're plugging into a computer a USB 3.0 socket and cable can output 4.3W. If you're using a 2.0 wire in a 3.0 socket you can only get 2.5W because the extra 5 pins on the USB 3.0 interface are what make the higher power and faster data rates possible. Without those 5 extra pins, it's just a USB 2.0 interface. The data connection is relevant in this case because the device has to request more power from the computer.
Every part has to be USB 3.0 for 4.3W to be achieved in a computer's USB 3.0 port.
In a wall charger the cable doesn't matter because all of the power comes from 2 pins unless it also has a USB 3.0 interface.