There is a huge variance, then, between normal USB 2.0 ports rated at 500mA and dedicated charging ports which range all the way up to 2100mA. This leads to a rather important question: If you take a smartphone which came with a 1A wall charger, and plug it into a 2A iPad charger, will it blow up?

In short, no: You can plug any USB device into any USB cable and into any USB port, and nothing will blow up — and in fact, using a more powerful charger should speed up battery charging.

The longer answer is that the age of your device plays an important role, dictating both how fast it can be charged, and whether it can be charged using a wall charger at all. In 2007, the USB Implementers Forum released the Battery Charging Specification, which standardized faster ways of charging USB devices, either by pumping more amps through your PC’s USB ports, or by using a wall charger. Shortly thereafter, USB devices that implemented this spec started to arrive.

Fast forward to 2013 and the HTC One. It is equipped with a USB 2.0 port and when no data is transferred it will accept a charge up to 1A with a minimal standard of 500mA. When you buy a charger that says 2A output, this normally means a max of 2A for the charger. When it comes to charging the phone, the phone will only pull enough power it can from the charger and not more.