Qi wireless charging works on the principle of inductive transfer, or more specifically, resonant inductive coupling. A resonant transformer provides the electrical energy and directs it to the receiving device to charge a battery wirelessly.
The Qi wireless charging method is generally restricted to very short distances; typically, the distances for effective charging are measured in centimeters. Newer methods seek to increase effective distances up to several dozen feet using more advanced methods. In general, Qi wireless charging uses fairly low wattage transmissions in order to maintain safe operations.
Qi wireless charging typically works through the use of a pad or horizontal plane that devices can rest on to be charged. This method promises much more convenient charging than the traditional wall outlet methods, because users don't need to carry around cords or cables. Some of the other benefits of Qi wireless charging are that the charging tools can be encapsulated in plastic or other materials to make them safer, whereas traditional cords can get frayed and expose users to electrical hazards. A group called the Wireless Power Consortium maintains more up-to-date information about Qi wireless charging, how it works and how it is engineered in new consumer devices on the market.