USB charging is shrouded in mystery -- which is a bit ironic, given that almost all of us rely on USB charging of our smartphones and tablets on a daily basis. Why, for example, does an iPhone recharge quickly when you plug it into your MacBook Pro, but slowly when you plug it into your PC, or a non-Apple wall charger? Why does your smartphone charge quickly to start with, but slow down as it fills up? In this story we will answer these questions and more as we investigate the optimum methods of charging your USB devices as quickly as possible. We will also be taking a look at a new device, called the Practical Meter, which finally gives us insight into the arcane world of USB charging.