The Off-Key Bard
Technology

How Does Wireless Charging Work Through Glass and Plastic?

Or: why your phone can charge without a cable, and why a few millimeters can make a difference. The glass isn't conducting anything. It's just staying out of the way.

Or: Why Your Phone Can Charge Without a Cable, and Why a Few Millimeters Can Make a Difference

Set a smartphone on a wireless charging pad and something faintly unnatural happens. There's no cable, no exposed contact, no visible connection of any kind, and yet the battery starts filling, sometimes within a second. The energy appears to pass straight through the phone's glass back, through a protective case, and through the plastic skin of the pad itself. It looks as though electricity has learned to travel through solid objects.

It hasn't. Electricity isn't flowing through the glass the way it flows through copper wire. The pad creates a changing magnetic field, and a component inside the phone turns energy from that field back into electrical current. The principle is electromagnetic induction, understood since the nineteenth century and the same physics behind generators and transformers. Wireless charging is an old idea, shrunk until it fits under a pane of glass.

A Changing Field Makes a Voltage

Run current through a wire and a magnetic field forms around it. Coil the wire, and the fields from each turn reinforce one another into something stronger and more concentrated. Now the interesting half: a changing magnetic field can induce a voltage in a nearby conductor. Britannica's explanation of Faraday's law of induction states it plainly, that a changing magnetic field induces a voltage in a circuit, and credits Michael Faraday's experiments in 1831. He found that a current appeared in a nearby conductor whenever an electromagnet's field was switched on or off, and that moving a permanent magnet in or out of a coil did the same. James Clerk Maxwell later wrote the law mathematically, tying the induced voltage to the rate at which magnetic flux changes.

A wireless charger puts that to work with two coils. The pad contains a flat coil fed by electronics that supply rapidly alternating current. Alternating current reverses direction continuously, so the magnetic field around the coil rises and falls with it, an oscillating field in the space just above the pad. The phone carries a second coil near its back. Set the phone down and its coil sits inside the oscillating field, which induces an alternating voltage across it. Nothing has to conduct through the glass. Glass and plastic are insulators that don't meaningfully block that kind of field, so they simply occupy the gap between two coils that talk through the field they share.

The closest everyday relative is a transformer, where one coil's changing field induces voltage in another and energy crosses without any direct electrical connection. A wireless charger is a transformer with the core sawn in half and the halves given a gap, so you can lift one off whenever you like. That freedom costs something. A real transformer keeps its magnetic energy tightly channeled between its windings; a charger has to work across a case, a glass back, and a bit of air.

Why the Phone Is Not a Receiver for Everything

If magnetic fields can induce current, why doesn't a pad charge every object on it? Induction needs a suitable conductive structure, and geometry, material, and orientation all matter. The phone has a coil designed for this field, plus electronics that regulate the result. A lump of plastic has neither. Metal objects do interact with the field, but not helpfully: they develop unwanted currents and warm up, which is why the pad is a controlled coupling between two designed systems, not a broadcaster of electricity into the room. It isn't Wi-Fi, either. Ordinary phone pads use near-field coupling, where the two coils sit close together and the transfer depends on the local magnetic field rather than a wave traveling across a room. That's also why your phone can't charge from the other side of the apartment: coupling weakens quickly as separation grows, and sending meaningful power over distance is a different and harder engineering problem.

The coil's output also can't go straight into the battery. It's alternating voltage, and a lithium-ion battery wants carefully controlled direct current. So the phone has a rectifier to convert the alternating current into direct current, regulation circuitry to smooth and condition it, and a battery-management system to decide how the cell should actually be charged. The field carries the energy; the electronics make it usable.

The pad and phone talk to each other, too. A charger can't assume every object on it wants the same power, and a phone's acceptable intake shifts with temperature, charge level, and alignment. Under the Qi standard, developed by the Wireless Power Consortium so chargers and phones from different companies cooperate, the receiver can send control information back by modulating its electrical load, and the transmitter adjusts accordingly. The pad isn't blasting maximum power and hoping for the best; it's in a negotiation.

Geometry Always Wins

Because everything depends on how much of the transmitter's field passes through the receiving coil, alignment is physics, not a software setting. Centered coils couple strongly. Slide the phone sideways and the receiving coil leaves the strongest part of the field, less flux threads through it, and transfer drops. The charger can nudge voltage or frequency to compensate, but only within limits, which is why a phone that looks squarely placed can still charge slowly or not at all if its internal coil sits offset from the one beneath it. Different phones put their coils in different places, so one pad can feel flawless with one device and fussy with another.

The Wireless Power Consortium's own explanation of magnetic chargers makes a point that's easy to miss. The magnets in systems like MagSafe and Qi2 only align the phone; they carry no power at all. Charging still happens through the two coils across the magnetic field. The static magnets don't disturb the alternating field, and by guaranteeing correct geometry they improve efficiency. The same page notes that being off by even a centimeter slows charging, with the lost energy turning into heat, and that Qi2's Magnetic Power Profile standardizes the magnet ring's dimensions and holding force so chargers and phones from different brands fit together.

Distance matters just as much. The coupling that makes everything work weakens as the gap grows, and these systems are designed around a very small one. That's why a phone may charge fine bare and struggle in a thick case. The case isn't electrically fighting the charger; it's pushing the coils apart. A few extra millimeters is a large fraction of a gap designed to be tiny. Thin plastic or silicone usually passes, while bulky rugged cases, embedded metal plates, rings, and wallet attachments can weaken coupling or interfere with the field outright. A metal plate meant for a magnetic car mount can even get warm from the currents the field induces in it.

Convenience Has a Heat Bill

Wireless charging generally loses more energy than a comparable wired connection because power must be transferred between two separate coils through a magnetic field. Some of that energy is lost as heat in the coils and surrounding circuitry. The amount of energy wasted depends on the charger's design, power level, and how accurately the phone's receiving coil aligns with the charging pad. Poor alignment can reduce efficiency further, increasing heat and sometimes slowing the charging process. This is why newer wireless charging systems increasingly use magnetic alignment to help position devices correctly and improve power transfer.

Heat matters because lithium-ion batteries dislike it. Battery University's guidance on prolonging lithium-based batteries says plainly that elevated temperature, meaning above roughly 30 degrees Celsius, hastens permanent capacity loss, and that the worst case is keeping a fully charged battery hot. Its storage figures show the pattern: after a year at 40 percent charge, estimated capacity is around 96 percent at 25 degrees but 85 percent at 40. Wireless charging doesn't inherently ruin a battery. A well-controlled system manages temperature, and phones monitor themselves, throttling or pausing charging when warm. But a pad buried under bedding, a hot sunny windowsill, a misaligned phone, or an incompatible accessory all push conditions the wrong way. When the phone slows down or pauses on a warm pad, that's usually the system protecting itself, not a fault.

Charging also slows near full regardless of method. Lithium-ion charging runs in stages, tapering current as the cell approaches its limit, so a pad advertised at a given wattage won't deliver that continuously. Watts describe the rate of energy transfer, not battery size, and a higher-wattage adapter doesn't force more power into the phone, since the pad, the standard, the receiver, and the battery-management circuitry each impose their own ceilings.

The Coin Problem

Anything conductive caught between pad and phone is a hazard. A changing magnetic field induces currents in metal, and resistance turns those currents into heat, so a coin or key left on a pad could get dangerously warm under unfavorable conditions. Qi chargers therefore include foreign-object detection, watching for power disappearing somewhere it shouldn't and cutting back or stopping if something looks wrong. That's a reason to prefer certified equipment, and to keep keys and coins off the pad even though the charger is trying to protect you.

Old Idea, Wide Reach

Inductive charging predates smartphones. Electric toothbrushes have used it for years, since a sealed handle with no exposed contacts suits a wet bathroom, and the toothbrush and base fit together in a fixed way, so alignment is easy. Smartwatches use small magnetic pucks for the same reason. Some phones can even run in reverse and act as the pad for earbuds, spending their own battery to do it, so it's a convenience rather than free energy. Nothing here creates energy: the pad draws from a wall adapter, converts some of it to field, loses some on the way, and the battery receives what's left. Scale the idea up and you get wireless charging for electric vehicles, with larger coils, bigger gaps, and much stricter demands on alignment, efficiency, and safety, and researchers have explored charging cars while they drive. The physics is similar; the engineering gets hard fast.

The Bard's Take

Wireless charging looks like electricity passing through solid glass. What's actually happening is quieter. Alternating current in a copper coil makes a magnetic field that rises and falls. A second coil inside your phone intercepts part of it, and the changing field induces a voltage. A rectifier and a battery-management system turn that into something the cell can safely take. The glass and plastic don't conduct a thing. They just stay out of the way while two coils trade energy through a field.

Everything inconvenient about the technology follows from that picture. Coils must line up, so magnets help, though they only aim the energy and never carry it. The gap must stay small, so a thick case can quietly cut the power. Losses become heat, so pads and phones throttle themselves. Metal in the field gets warm, so chargers watch for it. The underlying physics is nearly two centuries old; the implementation, with standards, negotiation between devices, and constant temperature monitoring, is thoroughly modern. You set the phone down, current reverses inside a coil, a field rises and falls, another coil answers, and a battery starts to fill. Michael Faraday would recognize every step. He'd just be astonished we fit it under a pane of glass.

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