There are over four billion mobile phone users in the world, a figure that is expected to grow to five billion by the end of the decade. We’ve all become hugely dependent on this amazing tool and the feeling of it running out of battery is all too familiar: that desperate scramble before it goes gently into the night. These devices last, at best, about one full day on a charge and must spend the better part of the evening tethered to a wall. Unless you are constantly carrying your charger with you, it is difficult to keep your phone from dying over the course of a busy day.
But what if we could charge our phones conveniently without having to find a plug and carrying a charger with us at all times? With the proliferation of electric cars and electronic biomedical devices like Pacemakers, one vital piece of technology, first developed about a century ago, is said to significantly improve the way we keep our devices powered. Its called wireless charging, and it’s popping up in cafes, fast food restaurants, and IKEA furniture.
Although there are competing standards, wireless charging is power delivery from a power source to an electronic device without the need for a tethered wired connection. Inductive charging is the most popular form of wireless charging for mobile devices, and leading the way is the Wireless Power Consortium, with their Qi Open Interface standard. Using fundamentally the same technology, other groups are incorporating inductive charging tech for use in electric vehicles. Getting to the bottom of inductive charging, you need to go all the way back to the mid-1800s when Michael Farraday discovered the underlying principles of electromagnetic induction.
He discovered that, in the presence of an alternating magnetic field, an electromotive force would be produced across an electric conductor. By the late 1800s, Nikola Tesla utilized this idea and demonstrated the phenomenon of resonant inductive coupling by lighting an incandescent lamp wirelessly. By tuning the current to match the resonant frequency of the coils, the two coils would couple, providing higher efficiency power transfer.
The discoveries that stemmed from Farraday’s led to the modern electric motor and other unmeasurably important inventions. However, for inductive charging, the science proved easier than implementation. for starters, there weren’t many electronics that would greatly benefit from such technology in the days of Nikola Tesla, and due to the limitations in circuit board design and size constraints, it wasn’t until the 1990s and 2000s that the technology became viable at the consumer electronic level, where we started seeing wireless charging toothbrushes and, in 2008, wireless charging mobile phones. An inductive charger consists of only a few parts: AC current from the wall, an oscillator electrical circuit, and the transmission coil.
The transmission coil is a tightly wound copper element that, as the alternating current passes through, would produce magnetic flux. The magnetic flux density is based on things like the number of turns in the wire, the diameter of the transmission coil, the distance from the coil, and other properties, such as current. On the receiver’s end, the process is basically the same, except the opposite. In the receiver device, a coil of the same type is embedded into the charging circuit.
The alternating magnetic field is picked up by the receiving coil, and a current is induced. The AC power is passed through a power rectifier and stabilizer to convert it into DC power the phone can use to charge the battery. Both the transmitter and the receiver have electrical resonant frequencies, designed to be the same.
For low displacement distances, such as a mobile phone on a charging pad, it is actually LESS efficient to operate at the circuit resonant frequency, due to heat generation. However, for a larger displacement distance, such as a car parked over a charging pad, operating at the resonant frequency causes inductive coupling to counteract some of the displacement-related transmission losses. But there are many other factors that affect the performance of the transmission. And with no standardized design on the phones’ end yet, charging pads need ways of detecting the type of device it is charging, have multiple coil arrangements and control modules to alternate between modes.
The two big questions with inductive charging are ones we keep coming back to: Do people really care about the technology? And what are the implications of inductive charging becoming popular? Having a fully charged phone wirelessly, but nonetheless stuck to a charging pad, is barely a convenience. Teams are working within the Qi wireless standard to create devices such as the Pi Charger, which can charge up to four devices at a range of up to a third of a meter with a max power output of 10W per device. it’s completely conceivable that the office furniture of the future will have inductive charging capability built-in followed by most laptops, cell phones including wireless mice.
But consumers need to care about it, and hardware manufacturers need to see the value. But there is one industry that inductive charging will make a meaningful impact, and it happens to indirectly involve Tesla, once again. Going to the petrol station is a legitimate inconvenience, and visiting a Tesla charging station is an even bigger inconvenience. That’s why start-ups and corporations alike are beginning to shift their attention to this lucrative opportunity.
Similar to charging a cell phone on a charging pad, a transmitter would be installed in parking spaces and a receiver on the bottom of the car. Imagine your autonomous electric vehicle dropping you off for dinner, and seeking out a parking space with inductive charging. No attendant needed: the car simply aligns itself with the transmission pad, and the charging process is initiated. If these industries can make this tech convenient, there is a huge chance people will want to use it.
But is that a good thing? According to a study in 2015 conducted by the Wireless Consortium, the creators of the Qi standard, they found in real-world conditions, the Qi wireless charger had an efficiency of about 59.4%, with their competitor coming in at just 39.6%. With 30 million iPhone X sold in its first quarter, they have an impact of 415MWh per day, if charged fully once per day.
With an efficiency of only 60%, that would result in an increase of 278 MWh load on the grid, which is about 25 years’ worth of power for an average home. This is only accounting for one mobile phone, and not accounting for the other 2.5 billion smartphones projected to be in use by 2019. Just for the added load. With electric vehicles, things can get even more interesting. The Tesla Model 3 is getting between 4.5 and 5.5 km/KWh, depending on driving habits.
With a daily commute of 35km, Tesla uses 7 KWh: or almost the same power usage as a small family home. So for every EV on the road, it’s similar to adding another house on the grid. And when you introduce inductive charging it can equate to adding nearly two additional homes. National power grids can likely accommodate this growth, but local infrastructure may not be capable, or even willing, to expand their capacity for the sake of convenience to EV drivers. But that may not necessarily be true forever. Oak Ridge National Lab recently demonstrated a 20 KWh inductive charging system that operates at 90% efficiency, with power delivery up to 3 times faster than traditional wired charging.
Although it’s too early to know what the real-world wall-to-battery efficiency will look like, it’s still promising to see some of the sharpest engineers in the energy field digging deeper into the technology and beyond. Theoretically, it is possible to transfer energy over greater distances with electromagnetic waves. NASA awarded one company $900,000 for its laser-powered climbing robot. The robot had no power source onboard and managed to climb 900m up a cable suspended from a helicopter in just under four minutes, using a high powered laser to provide enough energy to solar cells on its undercarriage.
This is a fascinating technology that could have huge potential. We could have huge solar cells in space beaming to Earth or other satellites, but right now the power loss in transmission is far too great to be useful. and this is a problem that is facing current technology on earth, whether we want to acknowledge it or not. Learning about simple core concepts like this that change the world allows us to apply our knowledge to real-world problems.