For Samsung Tablet/phablet owners, one of the most frustrating problem they encounter regarding charging the device is the not-charging icon. It happens when users plug their tablets into non-Samsung power sources and a not-charging icon is displayed at the notification bar. The icon looks like a green battery with a white thunderbolt and a red cross "X" on it. So what is going on?
The origin of the problem dated back in the 90's when the USB 2.0 specification was defined. The specification allows a device to draw, from a USB port, a maximum of 500mA (or 0.5A *) which was more than enough back then. Then more and more portable devices appeared and the 5V voltage on the USB port was perfect to charge a rechargeable battery. So hardware manufacturer started to use the USB port to charge their mobile devices. A mobile phone with a 700mAh battery can be charged in couple hours.
Then came the smart phone age. As mobile devices became bigger and more power-hungry, 500mA was not quite enough. For example, The Samsung Galaxy S4 has a 2600mAh battery and Galaxy Tab Pro 10.1 has a 8200mAh battery. So hardware manufacturers design their wall charger to look like a USB port but it can actually provide more than 500mA current, for example maximum 1A (1000mA) or even 2A (2000mA). Using this method, the mobile device can still be backward-compatible to a computer USB port and have slower 500mA charging.
To distinguish between a 500mA USB port and a 2A wall charger, each manufacturer has their own methods. The basic process involves matching specialized signalling and voltages on the data lines on the USB. So when a device is plugged to a port, the device will monitor the data lines to check whether they matches the requirement. If yes, the device knows that it is a wall charger and drain maximum of 1A/2A from it. If not, it will treat it as a standard USB port and limit the current it drains.
Each manufacturer has it own signalling and voltage requirement. Therefore, when a user plug a Samsung device into an Apple charger, the device will limit its current drawn to below 500mA because the signalling does not match Samsung's requirement.
For Samsung mobile phone like Galaxy S4, the 500mA is still acceptable and can charge the battery at a slower rate. But for tablet, most of the 500mA will be used by the tablet screen, its processor, its speaker, etc. And a very small portion (or almost none) of the 500mA goes into the battery. So it will take a very long time to charge the battery full. Therefore, it such case, the tablet will display a not-charging icon.
The problem complicates more when most after market chargers are designed for Apple. These charges still claim "Samsung compatibility" because they will still work with low power Samsung devices, e.g. a low-end smart phone, and provide <500mA charging. But when a user plug a Samsung tablet to this charger, the tablet does not charge and display the evil little red cross on the charging icon.
* A USB device can draw the 500mA maxium current under a numbers of requirement.