A full guide to wireless charging, including how it works and the best models to buy

• Wireless chargers work by creating a magnetic field that your phone, watch, or other device absorbs to gain energy.
• When you place a device on a wireless charging pad, a small coil in the device receives and harvests energy from the magnetic field, and uses it to power the battery.
• Wireless charging is a hassle-free way to charge your phone, but the technology currently has a few drawbacks — for instance, it's usually slower than using a charging cable.

Smartphone batteries don't last for long, and charging cables, which can break after a few months of use, can be annoying.

Wireless charging can be a convenient alternative - you simply set it and forget it. But how does it work, exactly?

While wireless charging may seem like a recent invention, its origins date back more than 100 years to the famous Serbian-American inventor, Nikola Tesla.

In this guide, we'll tell you more about how wireless charging works and what chargers are the best.

How wireless charging works

In the late 1800s, Nikola Tesla successfully transmitted electricity through the air. He used a process called resonant-inductive coupling, which works by creating a magnetic field between a transmitter (which sends electricity) and a receiver (which receives the electricity) to power light bulbs in his New York City laboratory.

A few years later, he patented the Tesla coil - a tower with a coil at the top that shot bolts of electricity. Tesla had much grander visions of a wireless power grid, but these dreams were never realized.

The same basic principle of inductive charging applies to smartphone wireless charging.

An electromagnetic coil, the induction coil in a charging base, creates a magnetic field and is basically an antenna to transmit a field of energy. A second smaller coil in the phone receives and harvests the energy and its circuitry converts it back to usable energy for the battery.

In recent years, wireless charging technology has come a long way: Charging speeds have increased and many designers have unified their technology under the Qi (pronounced "Chee") standard so products work with multiple phone brands and models.

Phones aren't the only thing using wireless charging technology; medical implants like pacemakers can also be recharged wirelessly.

Drawbacks of wireless charging

Wireless charging does have its drawbacks. Wired charging is faster and more efficient, but the further away the receiver is from the transmitter, the less energy it'll receive from a magnetic field. Phones need so much energy that the charging distance is only a few millimeters.

Josiah Hester, an assistant professor of Computer Engineering at Northwestern University in Chicago, said the bigger the coil, the more energy it can send out, but that wouldn't be very efficient. The field is strongest at its center.

"This field drops off really quickly," he said. "It still has a little bit of energy after two centimeters, five centimeters. At 10, it's just like, nothing. If you had a coil the size of a desktop, you could probably hold it a foot above the desk, but you'd be using a lot of power. That's the big problem with wireless charging in general. The research is trying to increase the efficiency of charging, or make our phones more energy efficient."

While this makes wireless charging limited for phones that use a lot of power, it isn't as challenging for smaller devices like Radio-frequency identification (RFID) tags that use less power. The difference in energy between the two, if you thought about them in terms of mass, is like an African elephant versus an ant, Hester said.

"Wireless charging and the energy efficiency of computation are both going up, so we're taking advantage of those two things at the same time," he said. "What I could do for the same amount of energy 20 years ago, I could do way more now, so you don't need as large a battery."

In other words, although wireless chargers are relatively weak now, they'll get better every year.

How to pick a wireless charger

There are many wireless chargers on the market at different prices and levels of quality.

Business Insider contributor Simon Hill recently ranked a few of his favorites after testing about 40 of them.

• Hill writes that the Moshi Otto Q ($39.95) is the best overall charger on the market. The charger has a sleek design that looks good on a countertop or side table, and charges at top speeds. There's also a rubbery pad on the bottom that keeps it from slipping if accidentally bumped. The device also has foreign-object detection, and will turn off if a piece of metal is put between it and your device - this will protect your devices from damage.
• The RAVPower Fast Wireless Charging Pad ($16.98) is a great budget option. While it's not as pretty as the Moshi Otto Q, it still delivers top charging speeds, foreign object detection, and a rubber pad to prevent slipping. But if you have a thick case or PopSocket, this charger may not be strong enough for you.
• The most luxurious option isn't a charging pad, but the Bezalel Altair ($64.98) upright charging stand. Heavy-duty and carved from aluminum, the stand will charge your phone in portrait or landscape mode. The Bezalel Altair can also charge through cases up to 5mm thick, but unfortunately will not detect foreign objects while charging.

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