Wireless charging has been almost done since the end of the 19th century, when electricity pioneer Nikola Tesla demonstrated magnetic resonance coupling - the ability to transmit electricity through air by creating a magnetic field between two circuits, a transmitter and a receiver.
But for almost 100 years it was a technology without many practical applications, apart from some electric toothbrush models.
Today, about half a dozen wireless charging technologies are in use, all aimed at disconnecting cables from smartphones and laptops to kitchen appliances and cars.
Wireless charging is encroaching into the healthcare, automotive and manufacturing industries as it promises increased mobility and advances that can allow small Internet of Things (IoT) devices to receive power several feet away from a charger.
The most popular wireless technology now used relies on an electromagnetic field between two copper coils, greatly limiting the distance between a device and a charging pad. It is the type that Apple has incorporated into the iPhone 8 and iPhone X.
How wireless charging works
According to David Green, a research manager at IHS Market, broadly, there are three types of wireless charging. There are charging pads that use tightly-coupled electromagnetic induction or non-radiative charging; Charging bowls or through-surface types of chargers that use a loosely-coupled or radiative electromagnetic resonant charge that can charge a few centimeters; And uncapped radio frequency (RF) wireless charging that allows a trickle charging capability at a distance of several feet.
Both tightly coupled inductive and loosely-coupled resonant charging operate on the same principle of physics: a time-changing magnetic field induces a current in a closed loop of the wire.
This is how it works:
A magnetic loop antenna (copper coil) is used to create a oscillating magnetic field, which can cause current in one or more receiver antennas. If the appropriate capacitance is added so that the loop resonates at the same frequency, the amount of current induced in the receiver increases. It is the resonant inductive charge or magnetic resonance; This enables power transmission at greater distances between the transmitter and receiver and increases efficiency. The shape of the furrow also affects the distance of power transfer. The larger the coil, or the more coiled, the greater the distance it can charge.
For example, in the case of smartphone wireless charging pads, copper coils are only a few inches in diameter, severely limiting the distance over which electricity can travel efficiently.
But when the coils are large, more energy can be transferred wirelessly. That Strategic Strategy, a company formed from research at MIT a decade ago, has helped the pioneer. It licenses loosely-coupled resonant technology for everything from automobiles and wind turbines to robotics.
In 2007, MIT physics professor Marin Soljačić proved that they can transfer electricity over a distance of two meters; At that time, power transfer to that location was only 70% efficient, meaning that 60% of the power was lost in translation. Soljačić introduced WiTricity later that year to commercialize the technology, and since then its power-transfer efficiency has increased greatly.
In WiTricity's car charging system, large copper coils allow for efficient power transfer - up to 25 cm - 25 cm in diameter for the receiver. The use of resonance transmits a high level of power (up to 11kW) and high efficiency (over 92% end-to-end), according to WiTricity CTO Morris Kesler. WiTricity also adds capacitors to the capacitor loop, which increases the amount of energy that can be captured and used to charge the battery.
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