Charging without wires possible but at a price

Wireless charging may one day replace plugs and wires similar to how Wi-Fi and Bluetooth have modernised personal communication. Wireless charging with inductive coupling uses an electromagnetic field that transfers energy from the transmitter to the receiver.

Wireless charging works well with mobile phones, digital cameras, media players, gaming controllers and Bluetooth headsets. Other potential applications are power tools, medical devices, e-bikes and electric cars.

Transferring power by wireless is not new. In 1831, Michael Faraday discovered induction and stated that electromagnetic forces can travel through space. In the late 1800s and early 1900s, Nicola Tesla began demonstrating wireless broadcasting and power transmission.

Early experiments in Colorado Springs in 1899 led to the Wardenclyffe Tower in New York. Tesla was adamant to prove that electrical power could be transmitted without wires.

The futuristic tower was designed and partially built in 1901 when Tesla had the vision of world communications from this centre. The complex was conceived as a transAtlantic radio telephony transmitter and for general broadcasting.

It was also intended to demonstrate a ‘wireless’ method of transferring power. The tower, at almost 60 m tall, was surmounted by a cupola but it and the supporting laboratories and workshops were never completed due to financial problems.

The complex was blown up in 1917 because the Americans thought it was a war hazard.

It was not until the 1920s that public broadcasting began and Europe built massive AM transmitters with signal strengths to penetrate many countries. The transmitter at Beromünster in Switzerland could have transmitted at 600 kW, but legislation on electrosmog and protests from the local population limited the power to 180 kW. Smaller FM stations have since replaced these large national transmitters.

How does wireless charging relate to radio transmission? Both models are similar in that they transmit power by electromagnetic waves. Wireless charging operates in a near field condition in which a primary coil produces a magnetic field that is picked up by a secondary coil close by.

The radio transmitter works on the far field principle by sending waves that travel through space. While the receiving coil of the wireless charger must capture most of the energy generated, the receiving antenna of the radio needs only a few µV to raise the signal above the noise level and receive clear intelligence when amplified.

Wireless charging is classified into three categories: radio, inductive and resonance. Radio will serve low-power devices operating within a 10 m radius of the transmitter to charge batteries in medical implants, hearing aids, watches and entertainment devices.

It can also activate advanced RFID chips through resonantly enhanced induction. The transmitter sends a low-power radio wave at a frequency of 915 MHz (frequency of microwave ovens) and the receiver converts the signal to energy.

The radio charging method is closest to a regular radio transmitter in that it offers high flexibility but has low power capture and exposes people to electrosmog.

Most of today’s wireless chargers use inductive charging featuring a transmit and receive coil close together. Electric toothbrushes were one of the first devices to use this charging method and mobile phones are the largest growing sector to charge without wires.

To retrofit an existing mobile phone for mobile charging, simply attach a ‘skin’ that contains the receiver and provides interconnection to the charger socket. Many new devices will have this feature built in.

For larger batteries such as those in electric vehicles, resonance charging, or electro dynamic induction, is being developed. This works by making a coil ring. The oscillating magnetic field works within a 1 m radius. The distance between transmit and receive coil must be well within the 1/4 wavelength (915 Mhz has a wavelength of 0.328 m).

Currently, resonance charging in trials can deliver about 3000 W at a transfer efficiency of 80-90%.

The success of wireless charging was behind adapting a new global standard and the WPC (Wireless Power Consortium) accomplished this in 2008. With the ‘Qi’ norm, device manufacturers can now build charger platforms to serve a broad range of compatible Qi devices.

The first release limits the power to 5 W and works like this:

While in ready mode, the charging mat sends signals that sense the placing of an object. Detection occurs by a change in capacitance or resonance. The mat validates the device for WPC compatibility by sending a packet of data by modulating the load with an eight-bit data string.

The receiving device awakens and responds by providing the signal strength. The mat then sends multiple digital pings to identify the best positioning of the placed object.

Only then will service begin. During charging, the receiver sends control error packets to adjust the power level.

The charge mat only transmits power when a valid object is recognised. With no load or when the battery is fully charged, the mat switches to standby mode. The transmit and receive coils are shielded to obtain good coupling and to reduce stray radiation.

Some charge mats use a free moving transmit coil that seeks the object placed above for best coupling; other systems feature multiple transmit coils by engaging only those in close to the object.

Inductive charging is not without its disadvantages. The California Energy Commission (CEC), Level V, mandates that AC adapters meet a minimum efficiency of 85%; Energy Star, Level V, requires 87% (European CE uses CEC as a base).

Adding the losses of the charger circuit to the AC adapter brings the overall efficiency for a hardwired charger to about 70%. Wireless charging has a transfer efficiency of 70-80 %; coupled with its own AC power conversion the overall charge efficiency hovers between 60 and 70%.

In addition to efficiency losses, the wireless charger includes the ‘readiness’ mode to identify the placement of an object, a feature that adds to power consumption.

Charger manufacturers, including Cadex Electronics, make great efforts to meet regulatory requirements. Losses incurred through less efficient charge methods go against the government-backed Energy Star program and exceptions may need to be made to allow more energy use to support convenience.

With roughly one billion chargers on standby or in charge mode, the extra power consumed is significant. The number of mobile phones is estimated at over five billion in the world. In 2008, 3.2 billion power supplies were manufactured globally and most are plugged into the main drawing power.

Lost energy turns into heat and a wireless charger can get quite warm during charge. Any temperature increase to the battery causes undue stress and batteries charged on wireless devices may not last as long on a mat than on the regular plug-in charger.

It should be noted that the heat build-up only occurs during charging. The Qi wireless charger will cool down when the battery is fully charged.

WPC was very careful when releasing Qi. The first version has a power limit of 5 W. A medium-power version of up to 120 W is in the works but this norm must meet stringent radiation standards before release.

There are health concerns because the devices operate close to human activity at a radio frequency ranging from 80-300 kHz. Some stations transmit at 915 MHz.

Electromagnetic energy from radio towers, mobile phones, Wi-Fi, routers and now wireless charging is categorised as non-ionising radiation and is believed to be harmless. Ionising rays from X-rays, on the other hand, have been shown to cause cancer.

As the number of non-ionising devices increases, citizens begin to question safety. Regulatory authorities are waiting for evidence and will only impose restrictions if a health risk can be scientifically proved.

Meanwhile, parents object to schools installing Wi-Fi, and homeowners protest about electric meters that communicate data without wires. Radiation from wireless chargers may be seen as harmless because they do not transmit intelligence.

In most cases, the household radiation is low enough not to worry about but it is the field strength and proximity to the source that could add to potential harm.

Charging EVs without plug and cable offers the ultimate in convenience as the driver simply parks the vehicle over a transmit coil. Engineers talk about embedding charging coils into highways for continuous charging while driving or when waiting at traffic lights.

While this is technically feasible, cost, efficiency and radiation issues at these higher powers are major challenges.

At a transfer efficiency of 80-90%, 10-20% of the power is lost. This reflects in a substantial energy cost to the user and should be calculated as a decrease in drivable distance per watts consumed.

Applied to a large vehicle population, this goes against the efforts to conserve energy. Daimler’s head of future mobility, Prof Herbert Kohler, says that inductive charging for electric vehicles is at least 15 years away and cautioned about safety. The potential radiation of EV charging is higher than Wi-Fi or talking on a mobile phone. It could also endanger people wearing a pacemaker.

Besides low efficiency and radiation concerns, wireless charging offers decisive advantages in industry. It allows safe charging in a hazardous environment where an electrical spark through charge contacts could cause an explosion or where heavy grease, dust and corrosion would make electrical contacts impractical.

Wireless charging also helps when multiple insertions would wear out the battery contacts too quickly. There is, however, a cost premium and this is especially apparent in custom devices that cannot take advantage of cost reductions through mass production.

Currently, a wireless charging station will cost roughly 25% more than a regular charger. A 25% premium also applies to the receiver. If the portable device cannot be charged with the battery installed, as is possible with a mobile phone, then each battery would need its own receiver and the battery pack would bear the added cost.

Unless wireless charging is necessary for convenience or environmental reasons, charging through battery contact continues to be a practical alternative.

By Isidor Buchmann