USB Cable Resistance: Why your phone/tablet might be charging slow
Tablets and smartphones are now ubiquitous, and almost universally, they rely on some form of USB connection to provide the power to charge the device. This is, in no doubt, because of a desire to reduce waste and meet EU regulations and has made it rather convenient for end users as cables and chargers are easy to find and are, to some degree, interchangeable. However, it does cause problems sometimes – namely slow or incomplete/inconsistent charging.
The Problem
When USB was conceived, the maximum current available was 500mA at 5V for a total of 2.5W. This is enough to power most smaller devices, but with smartphones and tablets, this is often only enough to power the device without any surplus left for charging.
In order to get around this, many manufacturers increased the current delivered through the same port/cable to higher levels, beginning at 700mA, 800mA, 1A, then 2A and now even 2.4A. But the devices needed to know when they were attached to a high-speed charger, and hence an array of incompatible, vendor specific communication methods utilizing voltages on the D+ and D- lines were developed, as well as a later USB Dedicated Charging Port specification.
This is why, when an iPad is attached to a regular USB port, it will display “Not Charging”, and when devices are attached to chargers of “other” products, they can sometimes charge at reduced rates or display “Not Charging”.
See full article at http://goughlui.com/2014/10/01/usb-cable-resistance-why-your-phonetablet-might-be-charging-slow/