More often than not, I put my phone on charge through one of my laptop’s USB ports for hours, only to find out it has only charged 10%. Shoot! There goes my plans for the night. And mine isn’t the only device known to do this. The omnipotent iPhone also suffers from the same problem. Try charging everyone’s favorite smartphone through a non-Apple charger, or from a Windows PC and you’ll be ready to kill yourself in a few hours. So, why exactly does our phones behave like snobs and refuse to charge over USB sockets?
The reason is pretty interesting and somewhat like how Godfather Vito Corleone used to get his work done. But first, some lessons in tech.
In any USB network, there is one host and one device. Your smartphone, tablet or camera is the device and the PC or laptop is the host. While data can flow both directions, power can only flow from the host to the device.
Based on the ability to transmit power, the USB ports are classified into three kinds: Standard Downstream Port, Charging Downstream Port and Dedicated Charging Port. The first two are found in laptops and PCs but the third one is found only on the wall chargers that comes bundled with your smartphones.
A standard downstream USB 1.0 and 2.0 ports can only provide up to 500mA of amperage, which is the measure of the amount of current that can pass through the ports. The USB 3.0 counterpart’s amperage is slightly higher, at 900mA. The charging downstream, on the other hand can transmit up to 1500mA of power.
But this is only the theoretical limit up to which the ports can deliver power. But more often than not, the actual power that is transferred is much lower. How much power will be drawn is decided, or rather, negotiated by the device.
Yes you read it right. Your smartphone doubles up as a smooth-talking negotiator whenever it has to charge itself. When the device is plugged into the USB charging port or a wall charger, the USB controller in the device sits down for a chat with the USB controller present in the USB port or the charger, and decides how much power to draw based on the port/charger’s voltage. If no controller is available, the device only checks the voltage and decides how much amperage to draw, and thus, how quickly it will charge.
Now, it is completely upto the makers of our devices how much power the device will draw based on the voltage. The iPhone, for example, will only charge at full power if the right host is being used for charging (either an Apple PC, Macbook or Apple’s Wall Charger). Otherwise it will fall back to charging slowly. For Android devices too, a laptop USB port that also wants to initiate a data connection will charge the device slowly. This can be overcome when there are manufacturer drivers installed in the PC.
So, to charge your devices as fast as possible, always stick to the provided wall charger. But if you have to depend on the USB port, make sure you are connecting to the charging downstream port in your laptop/PC and not the stand downstream port. The charging downstream has higher capacity of transferring power due to its high voltage output, as compared to the standard downstream port.
Also Read: Is charging my phone/tablet with any USB charger safe for my phone?