Look, the wall charger converts from 120/220/whatever you're using AC current to 5/9/12V DC current. AC is alternating current so it goes in both directions, DC is direct current that flows in one direction. The wall plug on V's charger is designed to connect to an AC source in the range of... something similar to 110-250V. If you connected it to a DC source, it would probably be fried. Anyway, the power bank. It has USB ports and those are 5V DC. And it also has a wall socket which is 120V AC. The battery inside is 3.7V DC. So, when you connect something to the wall socket, you actually connect it to 120V AC, which is converted from the battery's 3.7V DC. And then, the V's charger converts it from 120V AC to 5/9/12 DC for the tablet.