How Do Wireless Chargers Operate?

Every mobile gadget — from smartphones and e-readers to tablets and portable music players — needs power to function. Eventually, you'll find yourself charging them. That's when dealing with a variety of cables for different devices can really get annoying. It might leave you asking, "How do wireless chargers work?"

It's true: power can be transferred, and devices can be charged, all without the need for cables. Today, there are several products available that allow you to recharge a device simply by placing it on a charging pad. In this article, we'll break down how wireless chargers work. Power flows from the pad to the device, seemingly like magic. But it's not magic — it's science!

Tesla's Vision of Wireless Power Transfer

Nikola Tesla was a fascinating figure. Known for his eccentric genius, his groundbreaking work during the late 19th and early 20th centuries reshaped the world of electronic engineering. Tesla was a pioneer in numerous fields, including radio transmission and the development of alternating-current electricity. He also faced stiff competition from some of the era's greatest inventors, such as Thomas Edison and Guglielmo Marconi.

Tesla filed numerous patents throughout his life. One of his notable patents, granted on Jan. 18, 1902, was titled "Apparatus for Transmitting Electrical Energy." In this patent, Tesla described a device he believed could transmit electrical power from one conductor to another without the need for wires. Despite the challenges of engineering and financial difficulties, Tesla's dream of wireless energy transfer did not vanish with his passing.

The idea of transmitting power via radio waves initially showed promise. A simple example of how radio waves can carry power is a crystal radio. This basic radio includes a long antenna wire, a diode, another wire for grounding, and a crystal earphone. When connected properly, it can pick up radio waves and transmit sound without requiring a battery or other power source. The radio waves themselves provide the necessary power for operation.

The problem with using radio waves for power transfer lies in their inefficiency. Radio waves spread out as they travel, meaning only a small fraction of the energy reaches the antenna designed to capture it. However, other technologies, such as microwaves or magnetic fields, offer more promising alternatives for wireless electricity transmission. The magnetic method particularly appealed to engineers who sought a solution to eliminate the need for cumbersome charging cables.

Magnetism, Electricity, and Inductive Coupling

To comprehend how wireless chargers operate, we need to explore the connection between magnetism and electricity. This relationship powers countless electronic devices across the globe!

Let's start with an electromagnet. Creating a simple electromagnet is easy — you'll just need a battery, some insulated copper wire, and an iron nail. Wrap the wire around the nail, leaving enough wire on each end to connect to the battery. Be sure to keep the wrapping direction consistent as you coil the wire around. The more coils you make, the stronger the electromagnetic field will become.

Once your nail is wrapped in insulated wire, connect the wire ends to the battery terminals. The electricity flowing through the wire creates a magnetic field around the nail, allowing it to attract other nails. Reversing the battery connections will flip the magnet's polarity, turning what was the north end into the south, and vice versa.

If you set up a second transmitter coil and position it near the first one, you can harness the magnetic field from your electromagnet to generate a flow of electrons in the second coil. By connecting this second coil to a voltmeter, you'll notice the needle or readout shift whenever you connect or disconnect the battery wires. This happens because exposing a coil of wire to a changing magnetic field induces electricity to flow through it. The key is that the magnetic field must change; a steady field will not work as an inductor.

A battery delivers direct current, meaning electricity always flows in the same direction. However, when you connect an electromagnet to an alternating current — a circuit where the electricity reverses direction multiple times per second — the polarity of the electromagnet changes in sync with the current's direction. This creates a continuously changing magnetic field, which is ideal for inducing electricity.

Power Transfer Using Inductive Charging

The majority of wireless mobile charging systems use inductive coupling. Here's a common method:

The charging station typically takes the form of a mat or a similar flat surface. Within the mat are one or more inductive coupling coils. The mat itself is wired — you must plug it into a wall socket. Since the electricity in your home is alternating current, the mat supplies the power needed for the coils to create a changing magnetic field.

To make use of this magnetic field, your mobile device needs a special case or attachment. Some manufacturers offer devices with cases and built-in electronics for inductive coupling — for example, the Palm Pre had this feature. However, many manufacturers still produce devices that require cables or wires to charge. For these devices, you might need a special sleeve tailored to each device model, or an adapter that plugs into the charging port of your mobile device. The sleeve or adapter includes a matching coil that corresponds to the inductor coils in the mat.

Whether your device supports inductive coupling natively or requires a sleeve or adapter, the next step is to place the device on the charging mat. The inductor coils within the mat create the magnetic field that induces electricity into your device, sleeve, or adapter. This electricity then charges your device’s battery. Since no direct current flows between the mat and your device, it’s completely safe to pick up your device while it’s charging and continue with your day.

Inductive coupling is quite handy, but it does come with some limitations. The biggest issue is its short range. If a device is placed too far off-center, it may not charge properly. Some wireless charging pads attempt to address this by marking the optimal position for the device or by creating raised areas that guide the device into place, ensuring the coils are aligned for effective charging.

Although inductive coupling is the most widely used method for wireless charging, it isn’t the only option. Let's explore some other alternatives to induction.

Conductive Connections, Radio Transmissions, and Wi-Fi?

One method for wirelessly charging mobile devices takes a more straightforward approach. Conductive charging mats establish a direct electrical circuit between the device and the charging surface. The mat has conductive metal strips embedded in it, and when a device with compatible electrical contacts touches these strips, electricity flows into the device.

For this system to function, the device must either have built-in contacts in its case or be placed into a special sleeve equipped with the necessary contacts. Once the device is inserted into the correct sleeve, simply placing it on the mat ensures that the contacts align, completing the circuit and enabling charging.

It's essential to ensure the contacts on the sleeve align properly with the conductive strips on the charging surface, or the circuit won't be completed. However, conductive strips tend to be more efficient than inductive coupling, which the Wireless Power Consortium states operates at only 50-70% efficiency [source: Higginbotham]. This means that at least 30% of the power used for charging is wasted in an inductive system, even when it’s functioning efficiently.

While radio frequency power transmission isn’t very efficient, it's still possible to design a charger that can convert radio waves into direct current electricity. However, without a large antenna and a powerful broadcast source, you won't capture much electricity from the surrounding signals.