Ambient IoT: battery-free devices explained

Ambient IoT is the name for connected devices that need no battery. They collect tiny amounts of energy from their surroundings, store it for a moment, and use it to take a reading or announce that they are there. The aim is a device cheap and small enough to put on every parcel, pallet or shelf, with nothing to charge or replace. This guide explains how such devices work, what the standards bodies are doing, and where the idea runs into physics. It sits with the sensors and hardware guides, and pairs with powering IoT sensors, which covers devices that do have a battery.

What counts as ambient IoT

The term covers devices that run on harvested energy and use so little power that harvesting alone can keep them going. Most have no battery at all; some have a small capacitor or a tiny rechargeable cell that stores harvested energy between uses. They share three traits:

• Power in the microwatt range. Far below what an ordinary Bluetooth or cellular radio needs.
• Very simple electronics. Often a single chip with an antenna, sometimes printed on a label.
• Intermittent operation. They work when there is enough energy, and wait when there is not.

The idea is not new. Passive RFID tags, the kind in shop security labels and warehouse tags, and NFC tags in transport cards already work without batteries, powered by the reader’s signal. Ambient IoT extends that to devices that can sense things, carry more data and be read by networks that already exist, such as cellular base stations and Bluetooth gateways.

Where the energy comes from

• Radio waves. A dedicated reader or “exciter” sends a signal that the device harvests, or the device scavenges energy from existing Wi-Fi, cellular or broadcast signals. Dedicated sources give far more energy than scavenging.
• Light. A small solar cell works indoors under artificial light as well as outdoors. This already powers many calculators and some sensors.
• Heat. A thermoelectric element turns a temperature difference, such as between a hot pipe and the air, into a small current.
• Movement and vibration. Pressing a switch, a door closing or machine vibration can generate enough energy for one message. Battery-free wall switches that power themselves from the press, such as those built on EnOcean modules, are a long-established example.

The amounts are tiny, so everything in the device is designed around doing as little as possible for as short a time as possible.

How backscatter works

A normal radio generates its own signal, which takes more power than harvesting can supply. Most ambient IoT devices use backscatter instead: they reflect a signal that something else is already sending, and change how much they reflect to encode data.

A reader sends a steady radio wave. The device switches its antenna between two states, reflecting more or less of that wave back. The reader sees the reflection flicker in a pattern and decodes it as ones and zeros. The device never powers a transmitter, so it can communicate with a tiny fraction of the energy a normal radio needs.

The weakness is range. The reader’s signal has to travel out to the device and the faint reflection has to travel back, losing strength both ways. That keeps typical backscatter links at meters to tens of meters, depending on the reader’s power and the antennas.

The 3GPP approach: ambient IoT in Release 19

3GPP, the body that writes the cellular standards, studied ambient IoT in Release 18 and wrote the first device type into Release 19. The goal is battery-free devices that cellular networks, or readers built on cellular technology, can talk to.

• Device 1, the Release 19 device, targets a peak power use of about one microwatt, stores harvested energy, and sends data by backscatter on a carrier wave supplied by the network. 3GPP describes its complexity and power as orders of magnitude lower than NB-IoT and LTE-M.
• Device 2a and 2b, planned for Release 20, add a reflection amplifier for more range (2a) or a simple active transmitter (2b).

The first scenarios studied are indoor: taking inventory of tagged items in a warehouse or factory, and sending simple commands to devices indoors. Release 19 targets the licensed spectrum operators already use, so in principle a mobile operator could offer item tracking without new spectrum. Products built on it will take time; standards come years before networks and devices.

The Bluetooth approach

A second route uses Bluetooth Low Energy. Battery-free Bluetooth tags harvest energy, mostly from radio signals, and send standard Bluetooth advertising packets that any Bluetooth gateway, and in principle any phone, can hear. Their appeal is that the readers already exist in huge numbers. The Bluetooth SIG has published its work on how Bluetooth supports ambient IoT, and companies sell such tags for supply-chain tracking, often with temperature sensing for cold chains.

Where ambient IoT is used

• Supply chain and logistics: tags on pallets, cases and returnable containers that report where they are as they pass readers.
• Cold chain: tags that log temperature so a shipper can see whether food or medicine stayed in range.
• Retail: shelf and stock tracking, building on the RFID already used in clothing.
• Smart buildings: battery-free switches and simple sensors that never need a battery change, which matters when a building has thousands of them.
• Factories: tracking tools, parts and work in progress without fitting batteries to each item.

At home, the most common ambient IoT devices today are self-powered switches. Battery-free versions of door sensors and other simple sensors exist but are less common.

The limits

• Little data. An ID number and perhaps a reading. No images, no streams, no firmware updates of any size.
• Short range. Meters to tens of meters from a reader for backscatter devices, far less than LoRaWAN or cellular.
• Readers are needed. Someone has to install and power the readers or exciters, and that is where most of the cost of a system sits.
• Not always on. A device with no energy cannot report. Systems must cope with tags that are silent for a while.
• Limited security. With so little power, strong encryption is hard. Designs vary, and it is worth asking what protection a given system uses. The IoT security basics still apply to the readers and the network behind them.

Ambient IoT against battery-powered sensors

For most monitoring jobs today, such as a temperature sensor in a cold room or a level sensor in a tank, a battery sensor is still the right tool, and the IoT sensors guide and powering IoT sensors guide show how to choose and power one. Ambient IoT makes sense when the number of items is so large that batteries are impractical, or when the device is so cheap that it is thrown away with the packaging.

For the hardware side of connected devices, read what an IoT device is made of and browse the other sensors and hardware guides.