How wireless power transfer can help make the world’s lithium go further
As pressure on the world’s lithium reserves increases, wireless power transfer simplifies charging and enables designers to use lower-capacity batteries and create new types of smart devices.
Lithium is the basis for today’s most energy-dense battery technologies for mass-market use. About 0.3 grams of lithium are needed for every Amp-hour of energy. A smartphone with a battery rated at about 2000mAh contains a little over half a gram of lithium. About two million kilograms of the metal will be needed to build the 3.4 billion smartphones Business Insider predicts will ship in 2020. This is only a small proportion of world lithium production, which currently stands at about 92 million metric tons per year.
Today, much of the world’s lithium is produced from lithium-chloride deposits harvested at large salt flats in South America and China. About 750 tons of brine is needed to yield one ton of lithium. At the current level of demand, known reserves could last for over a hundred years, but battery-powered electric vehicles (EVs) could change this dramatically.
A small EV like the Nissan Leaf contains about 4kg of lithium in its 24kWh battery, and vehicles with larger batteries contain proportionately more. Some say there may only be enough lithium for the next 20-50 years, if EV ownership were to become pervasive worldwide. There are other pressures on world lithium reserves, too, including plans for multi-MWh Electronic Storage Systems (ESS), and the tens of billions of IoT devices predicted to be connected to the Internet over the coming years. Many of these can be expected to run from lithium batteries, which, although small, could have a significant cumulative effect.
At this rate, sustainability is in doubt and the commodity price of lithium could increase if shortages become apparent. In the future, every smart object and electronic gadget - from the tiniest IoT sensor to the most prestigious EV - may need to make do with batteries that are considerably smaller than current design trends tend to use.
Unless a breakthrough is achieved in battery technology, the smaller batteries will have reduced capacity, leading to demand for more frequent recharging. This could present a catalyst for widespread adoption of wireless power transfer to charge batteries without need of a connector and cable. Wireless charging has been proposed for smartphones, but so far has failed to become ubiquitous.
Wireless power principles
Wireless power transfer relies on electromagnetic coupling between power-transmitting and receiving coils. Energy transfer can occur by inductive or resonant coupling of the transmitting and receiving sides. A chargeable device containing a receiver coil with power-conversion and management circuitry is simply placed on a charging mat containing a power-transmitter coil array. Both the device and charger are capable of negotiating the required power, depending on the receiving battery’s state of charge. Theoretically, multiple devices can be charged simultaneously when placed within the active charging area. So far, the development of wireless power transfer has been predominantly aimed at smartphone use, to enable owners to top up handset batteries throughout the day, in vehicles, cafes, workplaces, transport hubs, hotels or other locations. It can also provide a convenient and cable-free means of charging multiple devices from a single wall outlet when at home.
The two dominant standards for wireless smartphone charging each incorporate specifications for inductive and resonant coupling. The Wireless Power Consortium is promoting the Qi specification, while the AirFuel Alliance formed by the merger of the Alliance for Wireless Power (A4WP) and the Power Matters Alliance (PMA) is behind the AirFuel specification. Some regard the positioning of these two standards as having caused confusion, leading to slower adoption of wireless charging. As the two specifications have evolved, both extending to incorporate inductive and resonant power transfer, dual-standard charging products have begun to enter the market.
Now, as the IoT drives the emergence of many new smart “things” that need to be recharged, and with the prospect of smaller batteries for sustainability, wireless charging could help overcome some of the challenges involved with recharging small devices frequently.
Wireless connections are not subject to limitations on mating cycles, which can strongly influence the reliability of devices designed for use with wired connections.
Figure 1 shows the circuitry needed in both the transmit and receive networks in wireless charging systems.
In addition, product designers can eliminate any need to provide a port such as a micro USB socket or 2.5mm coaxial jack in the side of the enclosure for connecting the charging cord. This brings several advantages, including simplifying the design of the enclosure and saving bill of materials (BOM) costs by eliminating the mechanical mountings for the charging socket. An aperture-free enclosure can also be sealed more easily thereby helping to simplify splash-resistance or water-proofing.
In addition, enclosures can be made smaller and thinner, and smart functionality can be added to objects in form factors that are otherwise too small to accommodate a charging connector.
Moreover, since wireless charging saves laborious connection and disconnection of the charging cable to each device individually, routing charging is easier for users. In some applications, such as industrial process-monitoring equipment, charging could even be automated using a simple mechanism to bring a power transmitter and battery-powered smart sensors close together at regular charging intervals.
Wireless charging implementation
Wireless power transfer can be as energy efficient as charging via a cable, when the losses of the various systems are compared accurately.
To maximise the efficiency of wireless charging, coils need a variety of properties such as low equivalent series resistance (ESR) and high Q factor. Molex has teamed up with NuCurrent®, a specialist in antenna and circuit design for wireless power, to create the PowerLife™ family of wireless charging coils that deliver low ESR and high Q in compact dimensions. Their small size, including an extremely low-profile down to 0.24mm, makes them suitable for use with equipment such as hearing aids, wearable electronics, smartphone accessories, PC peripherals, gaming systems, low-power appliances, and many other small objects. A choice of flexible or FR4 substrates maximise freedom to locate the antenna in the most suitable position, and single- or multi-frequency capability allows broad compatibility with industry standards or proprietary frequencies. Various sizes and configurations are offered, including receiver, transmitter/receiver and single- or multi-frequency operation with optional support for NFC connection.
TDK also has a range of ultra-thin and flexible coils, in transmit, receive, and transmit-receive configurations, and supporting WPC Qi, AirFuel, or both specifications. The range includes a triple-coil transmit antenna, which ensures maximum efficiency and freedom to position the device being charged.
The TE Connectivity ARISO contactless system extends the benefits of contactless connectivity for power, data and signal transmission to applications including challenging industrial scenarios. With no need for connectors to be mated, the ARISO system allows enhances freedom of movement in factory automation or robotics, and helps overcome traditional limitations on connector cycle life, reliability and effects such as corrosion. In addition, ARISO connections can be easily setup or disabled on the fly.
Wireless power transfer has, arguably, failed to entice smartphone users away from conventional chargers and cables. Although products such as pads and phone sleeves have been in the market for some time, wireless charging has not yet become ubiquitous.
Sustainability could present a persuasive argument to go wireless, if typical battery capacities become smaller in response to rising global demand for lithium. While consumer acceptance may drive widespread user familiarity and economies of scale, wireless charging presents important opportunities to increase the performance and functionality of a much wider variety of products, from hearing aids and wearable medical monitors, to smart sensors used in monitoring and controlling buildings, industrial processes, infrastructure and the environment.