A charger running at 5V and 2A creates 10W, a fast charger using 9V and 1.67A creates 15W, and so on. In fact, instead of using 9V and 1.67A (to provide 15W), VOOC uses 5V and 4A (to provide 20W). However, there is one more thing you should know. The battery of a smartphone is charged in two stages.