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A wireless charging technology from Energous Corporation, San Jose, CA, Devices must have the WattUp receiver chips built in, and a variety of sizes are available. Because the antennas are not coils, there is more location flexibility when designing products. Near field transmitters can plug into a USB port for contact charging or within a few millimeters, while software-controlled midfield and farfield transmitters work up to three and 15 feet respectively. WattUp trickle charges the batteries so that radiation power is kept to a minimum.

There are numerous use cases for WattUp technology; for example, all-in-one computers with a transmitter at the base of the monitor and receivers in the keyboard and mouse. Rechargeable hearing aid batteries as well as AA, AAA, C and D cells are also candidates. Energous licenses the chip designs to device manufacturers, and near field and midfield products are expected in 2018. Farfield devices are envisioned for 2019. See wireless charging.

Automatic Home Charging
Energous envisions a time when its products automatically charge most portable devices in the home. (Image courtesy of Energous Corporation.)

wireless charging

Transferring electrical power to a portable product by placing it in a cradle or on a flat charging surface. There is no cable to plug in. Wireless charging generally takes longer than wired charging, but it can be convenient. For example, in an office, a smartphone can be dropped onto a desktop charging pad throughout the day when not being used. The Wireless Power Consortium and the AirFuel Alliance govern the major protocols (see Qi wireless charging and AirFuel). Another addition to wireless charging are the products from Energous (see WattUp).

Closely Coupled Induction (Qi and AirFuel)
Operating at kilohertz frequencies, the Qi and AirFuel Inductive (formerly PMA) technologies create a magnetic field between the transmitting coil in the power source and the receiving coil in the target device. Devices must be within approximately 7 mm of the charging pad. Depending on the number of transmitting coils in the charger, a phone might have to be moved around a bit to align the coils and establish a connection. Qi is the most widely used.

Closely coupled charging can cause nearby metal objects to become uncomfortably warm, which is why there is a warning notice on automobile console chargers to not place metal objects such as keys and coins on the pad.

Loosely Coupled Induction (Resonant Coupling)
Operating at megahertz frequencies, the transmitter and receiver coils are highly tuned, and devices can be an inch or more away from the charger. The greater distance is advantageous in automotive and under-table implementations. However, the farther away the two coils, the longer it takes to fully charge.

AirFuel Resonant (formerly Rezence) and Qi 3D are examples of Loosely Coupled chargers. Non-certified loosely coupled Qi chargers are also available. See fast wireless charging, Qi wireless charging and AirFuel.

No Metal Contacts
An early approach to wireless charging, the toothbrush is dropped into its charging base. Using inductive transfer, power is transferred without a metal plug and socket.

Charging in a Vehicle
Many models of GM vehicles include a wireless charging mat on the center console, supporting both Qi and PMA methods. The ridges in the mat allow air to flow, because the center console is over the driveshaft and absorbs heat.

Add-On Wireless Charging in the Car
In 2013, iOttie debuted the Easy Flex system for certain smartphones. The dashboard mount (top) is wired to the car's DC power outlet, and an antenna coil (bottom) is lined up with the phone's internal battery terminals.

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