Set a phone on a charging pad and the battery starts to fill, with no plug and no metal contacts. Here is the puzzle: a phone battery can only store direct current, but the trick that moves energy through the air only works with current that keeps changing. So the energy has to switch forms several times on its way from the wall to the battery. This article follows it step by step.
One common mix-up first: the outlet in your wall does not supply direct current. It supplies alternating current. The small adapter you plug into it is what makes direct current.
Electric current is a flow of electric charge through a wire.
A current in a coil of wire makes a magnetic field around it. If that current keeps changing, the field keeps changing too. Put a second coil nearby, and the changing field pushes charge around the second coil: a current appears in it even though the two coils never touch. This is called electromagnetic induction, and it is the same idea that makes transformers work.
The key word is changing. A steady current makes a steady field, and a steady field induces nothing. That is why a DC current in the pad's coil would deliver no energy to the phone, and why the pad has to make AC again. The faster the field changes, the more voltage it induces in the second coil.
The pad and phone also talk to each other during charging. The phone sends small signals back through the same coils to ask for more or less power, and the pad adjusts.
Because induced voltage depends on how fast the field changes, a field that flips about 100,000 times a second induces far more voltage than one that flips 60 times a second, about 1,700 times as fast. That lets a thin, coin-sized coil with a few turns of wire do the job. At household 60 Hz you would need far more wire and a heavy iron core to move the same power, like the bulky transformer bricks of older electronics, and nothing that size fits in a phone.
Only the part of the field that passes through the phone's coil does useful work. If the phone sits off-center, or a thick case adds distance, less of the field reaches the receive coil. The pad then has to push harder to deliver the same power, more energy is wasted, and charging may slow down or stop. Basic pads work only across a few millimeters of gap, which is why they ask you to put the phone right on top.
Every step loses a little energy as heat: the adapter, the inverter, the resistance of the coil wires, the part of the field that misses the receive coil, the rectifier and the charging circuit. A battery also warms up as it fills. A cable skips the coil-to-coil step, so it wastes less.
How much less depends on the setup. In one independent test of a phone with a 12.7 watt-hour battery, a USB-C cable used about 18 watt-hours from the wall for a full charge, so roughly 70% of the energy reached the battery. A well-aligned magnetic wireless charger used about 23 watt-hours, roughly 54%, and a badly aligned pad did much worse and kept the battery hotter. Heat is the main reason to care: batteries age faster when they are kept warm.
Pads also watch for stray metal. A coin or key left on a pad can heat up in the field, so certified pads check how much power goes missing and shut off if something other than a phone seems to be soaking it up. This is called foreign object detection.
Qi (pronounced "chee") is the shared wireless charging standard published by the Wireless Power Consortium, an industry group. It is why one brand's pad can charge another brand's phone. For the common Qi power profiles the operating frequency is typically in the range of 87 to 205 kHz.
MagSafe is Apple's version, introduced with the iPhone 12, which adds a ring of magnets around the coils. The magnets carry no power. Their only job is to snap the phone into the exact spot where the coils line up.
Qi2 is the newer version of the standard. Its Magnetic Power Profile is based on the magnet design Apple contributed, so certified magnetic chargers from any brand line up the same way. It runs at a fixed frequency of about 360 kHz. Some phones described as "Qi2 Ready" have no magnets built in and need a magnetic case to complete the ring.