How does a phone power a tag that has no battery?
The reader drives its own coil with an alternating current at 13.56 MHz, which produces an alternating magnetic field. The tag sits inside that field, and a voltage is induced across its own coil. Rectified, that voltage powers the chip. The tag holds no charge and does nothing at all outside the field.
What is physically inside a tag
Peel a sticker apart and you find three things: a plastic film, a flat spiral of aluminium or copper printed on it, and a grain of silicon smaller than a sesame seed bonded across the two ends of that spiral. Nothing else. No battery, no capacitor holding a charge, no clock running quietly in a drawer.
The spiral is the antenna. It is also, in a sense, the power supply.
A transformer with air between the windings
When you bring a reader close, its coil is driven with an alternating current at 13.56 MHz. A changing current in a coil produces a changing magnetic field around it. A changing magnetic field passing through a second coil induces a voltage across that coil. This is induction, and it is the same effect that makes a transformer work.
The only difference is that a transformer has an iron core forcing almost all of the field through the second winding, while here the two coils are separated by air, a phone case and whatever the tag is stuck to. The fraction of the field that reaches the tag is small, and it collapses fast as the coils separate.
This is not the same as wireless charging
Wireless charging uses the same principle at a different scale, moving watts rather than microwatts, at a much lower frequency and with coils designed to sit almost touching. The two systems interfere with each other, which is why a tag placed on a charging pad often stops being readable.
What the chip does with the energy
The induced voltage is alternating, so the chip rectifies it and regulates it into a supply rail. Only then does anything run. The chip boots, listens for a command from the reader, and answers.
All of that has to happen inside the fraction of a second your hand holds the phone still. It is fast enough that you never notice, but it is also why a tap that is too quick returns nothing: the chip needs to be powered long enough to complete the exchange, not merely to be touched.
Why it fails abruptly rather than gradually
Coupling in the near field weakens extremely fast with distance. Doubling the gap does not halve the energy reaching the tag, it collapses it.
Below a threshold the chip simply never reaches its operating voltage. There is no degraded mode, no slower read, no partial answer. The tag either wakes up and replies, or nothing happens at all. This is the single most confusing thing about NFC for people used to Wi-Fi, where a weak signal still gets you something.
It also means "almost working" is not a state that exists. If a tag reads intermittently, it is sitting exactly at that threshold, and the fix is to change the geometry rather than to try harder.
What follows from having no battery
A passive tag costs a few cents, survives years unused, needs no maintenance and cannot be switched off or drained. Those four properties are the reason NFC ended up on cards, posters and packaging rather than Bluetooth.
The price is that everything the tag does must fit within the energy the reader supplies at that instant. There is no sensor running in the background, no clock, no log of previous reads unless a battery is added, which turns the object into something else entirely.
Frequently asked questions
Does reading a tag drain the tag over time?
No. There is nothing in a standard tag to drain. The chip runs only while it is inside a reader's field and is completely inert the rest of the time. Wear comes from writing, not from reading, and only after a very large number of write cycles.
Can a tag be powered from further away with a stronger reader?
Only marginally. Regulations cap how much field a reader may produce, and near field coupling falls off so fast that even a large increase in power buys very little distance. Enlarging the tag antenna helps far more than driving the reader harder.
Why does my tag work on one phone and not another?
Phones differ in antenna size, antenna position and how much field they produce. A tag sitting close to the coupling threshold will cross it on one handset and not on another. The tag is not faulty, it is marginal.
Does the tag need to be powered to be written, as well as read?
Yes, and writing needs slightly more energy than reading, held for longer. A tag that reads reliably at the edge of its range may fail to accept a write at the same position.