Wireless Charging Creeps Forward

It’s well known that electricity can travel long distances through the air, but expanding beyond the boundaries of a wire has never seemed a practical or reliable way to power delicate electronics.

In fact, wireless power has been widely available for years. Whether this approach will be used to extend battery life isn’t entirely clear. But it is attracting renewed attention as the balance between more processing and functionality in edge devices clashes with glacial improvements in battery capacity, and as new markets such as electric cars and connected industrial applications continue to gain steam.

“I remember working 10 or 15 years ago on medical implants that had to be recharged using a cuff or coil so the power could get through and you wouldn’t have to go back to surgery,” Jeff Miller, a product marketing manager at Mentor, a Siemens Business. “Or charging the tire pressure monitoring system on a truck – you can’t get wires through a tire, and you didn’t want to take the tire off, so it’s worth looking into a different approach.”

Most applications for wireless power involve industrial, automotive, or low-end consumer products such as rechargeable toothbrushes. But increasingly it is finding a home in smartphone chargers. These are mainly based on the Qi standard for wireless charging, which relies on magnetic induction generated by a transmitter and receiver that must sit so close to each other during charging that they nearly touch.

The other major effort is to develop ways to charge the same phone in the same amount of time from several feet away, and possibly charge many other devices at the same time using magnetic fields, ultrasound, lasers, or RF in a variety of frequencies as transfer media.

“The fundamental physics of power without cables has been known for decades, so the difficulty is not so much getting it to broadcast through the air as to do it within limits of field strength, without interference, and at a cost that can make it a mass-market product,” according to Menno Treffers, chairman of the Wireless Power Consortium (WPC), which is consolidating two formerly rival technical specifications into its own Qi standard for near-field cell-phone charging.

It is very difficult to get most wireless power products to generate and actually deliver 5 watts or more that a smartphone needs to charge, Treffers said.

“It is easy to do a demo that looks as if you are charging a phone from 15 feet, but it is important to look at the numbers,” he said. “If you are only delivering microwatts to a device that wants 5 watts, that is not very useful. With magnetic induction you can get 5 watts, 2.4 kilowatts for appliances, power for laptops and drones at 60 to 100 watts. Power at a distance feeds the dream that you never have to charge, but we are nowhere near solving the problem at higher power levels.”

Wireless convenience
The desire to charge a smartphone without the inconvenience of a plug helped drive up the market for wireless-power-equipped consumer goods by 40% during 2017 to an estimated 500 million units, according to a February report from IHSMarkit. By 2022, the firm expects sales of wireless power-equipped smartphones to reach 90 million units.

The push for wireless power on smartphones started in earnest in 2014, when Starbucks installed charging pads from PowerMat, which operated on the PMA standard (now managed by the AirFuel Alliance), and McDonalds installed chargers supporting the competing Qi standard chargers in 50 of its U.K. locations. But the exposure also emphasized the split in the industry, with Samsung and AT&T backing PMA, and Philips, Qualcomm and Nokia backing the Wireless Power Consortium’s Qi.

Fig. 1: Energous’ WattUp technology. Source:  Energous

Fig. 2: Aircharge’s McDonald’s solution. Source: Aircharge

Apple’s decision last year to support the Qi standard tipped the balance of power, however, and the battle ended in a deal to integrate the two standards, which was announced Jan. 8. The IHSMarkit report predicted that the combination of Samsung and Apple phones and relief from competing standards will bring other phone makers on board and boost the number of users still further.

Power at a Distance
The other big news came the FCC, which approved two products to charge at a distance — 3 feet in one case, and up to 80 feet in the other. There are currently no commercial products designed or approved to charge cell-phone batteries from a distance of more than a couple of inches.

In December, the FCC approved an RF-based, one-to-many charging unit called the WattUp mid-Field Transmitter, to charge devices up to three feet away using RF at 915MhZ.
It also approved the 3-watt PowerSpot transmitter from Powercast, which operates on RF to charge at distances as far as 80 feet. That one uses 915MHz, which is usually reserved for ISM, and 850 to 950MHz, which is more typical of UHF RFID.

The number of both technical and regulatory issues makes it difficult to estimate when we can expect to see wireless power at a distance from commercial products, according to Victoria Fodale, who wrote the IHSMarkit report.

Fig. 3: Market growth for wireless power/charging. Source: IHS Markit

“None of the products for charging at a distance is commercial because of all the tradeoffs they need to make,” according to Phil Solis, who covers mobile devices and hips as research director for Enabling Technologies at IDC.  “It’s not a perfect solution, but it would be a big deal if you could continue to use your device as long as you’re within a few feet, even if you just trickle-charge for an hour to slow down the battery drain.”

Energy levels drop so fast with wireless power applications that it is difficult to keep the signal strong enough to be worthwhile at more than a couple of feet. Beyond that requires boosting the power of the signal up so high that it violates local broadcasting and even safety regulations, according to Treffers, who isn’t sure we’ll ever find a good balance between power, safety and efficiency.

“Charging cell phones over distance is the holy grail for the industry,” according to Charles Greene, chief operating officer and CTO at Powercast, one of two companies to receive approval from the FCC recently to sell products able to charge cellphone batteries at a distance of 5 to 10 feet.

“The physics does impose limitations,” Greene said. “The energy drops off as you move away from the source. There are ways to combat that, to focus it down, but in doing that you narrow your scope and make it harder to create a zone where you can do one-to-many charging, which is what we’re focused on.”

Powercast has adopted the Qi specification for cell-phone charging, but not at the cost of its own RF-based charging, Greene said. Instead, the company is building a Qi phone charger into its own transmitters, which it will probably launch in Q3.

“Qi is very well established, but it does have its limitations,” Greene said. “Typically it’s a one-to-one solution, not one-to-many, and it does require a power pad. We’ll be able to offer alternative solutions in combination. With a transmitter on a countertop, today we could deliver a 10% to 15% charge overnight (from a distance). We may be able to get that to 50%, which would let me put that transmitter on my nightstand or my desk and work and do continuous charging throughout the day and maybe never have to plug it in.”

There’s no reason wireless chargers can’t scale up to much higher power than they deliver now and do one-to-many charging across at least short distances, according to Morris Kesler, CTO of WiTricity, which spun out of MIT in 2007 to commercialize the research of MIT Professor Marin Soljačić into magnetic resonance wireless transfer.

WiTricity said it will deliver parking pads later this year, as an accessory to an automaker’s first high-end electric vehicle. The system is designed to scale from 3.6kW to 11kW, depending on the size of the car, at efficiency rates of up to 93% as long as the receiver under the car is within 25cm of being directly above the transmitting pad on the ground. That allows owners to park cars normally rather than making sure they are in a spot measured to the closest millimeter.

“How far you send depends on how big the transmitter and receiver are and your power level, but one of the advantages magnetic resonance gives you is the ability to do one-to-many connections,” Kesler said. “The human body doesn’t react with a magnetic field much at all, so that’s not much of a concern. If you were trying to send energy across a room, 10 meters or so, you’d probably want to do that with an antenna that would send a signal to a specific spot.”

There’s no reason to think cell phones are the only market available for wireless power, he said. The electric vehicle (EV) market is just getting started, but there are electric vehicles in every factory and warehouse, automated ground vehicles, industrial machines, medical applications and implants – almost anything that uses power and moves.

“The thing you do have to watch out for is efficiency,” he said. “If you’re going to use a kilowatt of energy at 70% efficiency, you’ll be dissipating 300 watts as heat, and you really don’t want that.”

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