How can a passive RFID tag reply without a battery?

A thin identification tag can reply even though it contains no battery. The illustrated tag uses a near-field arrangement, like a passive NFC tag: a reader coil and a nearby tag coil exchange energy and information through a changing magnetic field.

Alternating current in the reader coil produces that field. The field induces a voltage in the tag coil. A rectifier, a circuit that converts an alternating electrical input into a one-direction supply, helps power the chip. The tag is not creating its own energy; the reader supplies it.

The reader also sends a command. A powered, selected tag can answer using stored information. Instead of operating an independent battery-powered transmitter, it changes the electrical load connected to its coil in a controlled pattern. This is load modulation. Coupling between the coils lets the reader detect those changes and decode the reply.

The energy path and the information path therefore have different roles. Adequate coupling and a suitable command are assumed for the pictured response. Weak coupling may leave too little power to operate, and a powered chip does not reply to every possible signal. Other RFID systems use different frequencies and signaling arrangements; this explanation is specific to the stated near-field mechanism.