USB 2.0 cable plugged into USB 3.0 port for charging

[MetaDrow]

Currently I'm using a USB 2.0 cable that came with my Galaxy S3 to charge my S5 and I'm charging via USB 3.0 port on my PC. I started wondering can my S5 draw up to the USB 3.0 power spec of 900mAh even with a USB 2.0 cable if it asks to, but then wall chargers can go up to 1-2A so it should be able to. Am I over thinking this since USB devices will draw as much as it needs and as much as the output can give right?

[Tedster]

Let's look at it this way:

USB 2.0 has 5 pins. USB 2.0 is rated for 0.5 amps - so 0.1 amps per pin.

USB 3.0 has 9 pins. USB 3.0 is rated for 0.9 amps - again 0.1 amps per pin.

 

You'll need to use all pins from a standard output. What happens with the 1 or 2 amp chargers is they just pump power through. They're not standard connectors in the sense that you can't put data through them.

[MetaDrow]

Let's look at it this way:

USB 2.0 has 5 pins. USB 2.0 is rated for 0.5 amps - so 0.1 amps per pin.

USB 3.0 has 9 pins. USB 3.0 is rated for 0.9 amps - again 0.1 amps per pin.

 

You'll need to use all pins from a standard output. What happens with the 1 or 2 amp chargers is they just pump power through. They're not standard connectors in the sense that you can't put data through them.

So basically...

charging from PC - cable matters because of power standards?

charging from wall charger - use whatever is convenient

 

About the pins, isn't it 4 pins on USB 2.0,1 or 2 pins for power and the rest(+5 extra on USB 3.0) for data?