A contact sensor on a window, a thermometer in the cellar, a water meter in a manhole: devices that run for years on a small battery without anyone remembering them. A smartphone, with an enormously bigger battery, does not last two days. The difference lies entirely in the design: an IoT sensor spends almost all its time doing nothing.
The principle: sleep almost all the time
An electronic device's consumption is measured in milliamps when active and can fall to a few microamps, thousandths of a milliamp, when asleep. A well-designed sensor:
- sleeps for the overwhelming majority of the time;
- wakes up on an event, such as a magnet moving away, or at regular intervals;
- reads the value and transmits it in a few milliseconds;
- goes back to sleep.
What counts is average consumption. A device that draws a lot for an instant every ten minutes and very little the rest of the time has a very low average draw. The conceptual calculation is simple: add up the energy spent in the brief active moments and the small but continuous energy of sleep.
A worked example: if a sensor uses little power asleep but transmits every ten seconds instead of every ten minutes, transmissions become sixty times more frequent, and average consumption can rise enough to cut battery life from years to weeks. Transmission frequency is the most powerful lever.
Choosing the radio protocol
The radio is usually the thirstiest component. Different protocols have very different energy costs.
| Protocol | Energy per transmission | Range | Typical use |
|---|---|---|---|
| Wi-Fi | high, slow to connect | home | mains-powered or rechargeable sensors |
| Zigbee, Thread | low | home, with mesh | battery home sensors |
| Bluetooth LE | low | a few to tens of metres | thermometers, beacons |
| LoRaWAN | low, slow transmissions | kilometres | agriculture, meters, large buildings |
| NB-IoT, LTE-M | moderate | cellular network | meters, trackers, remote sites |
LoRaWAN
LoRaWAN uses a radio modulation designed to send small amounts of data over long distances with low power use. Messages are small and infrequent, but can cover kilometres in open terrain. You need gateways that receive the messages and forward them to a network server. It is widely used for agricultural sensors, environmental monitoring and meters.
NB-IoT and LTE-M
These are variants of cellular networks designed for low-power devices. Their advantage is using operators' infrastructure without installing gateways, at the cost of a SIM and a subscription. Features such as power saving mode let the device stay registered on the network while sleeping for long periods.
The right battery
Not all batteries are equal, and the choice affects lifespan as much as the firmware does.
- Lithium coin cells such as the CR2032: tiny, suited to sensors that transmit rarely. They struggle with current spikes: with frequent transmissions the voltage sags and the device may reset.
- Lithium AA cells: more capacity, excellent cold tolerance, suited to outdoor devices or cold cellars.
- Alkaline batteries: cheap, but deliver much less in the cold and can leak at end of life.
- Rechargeable batteries: convenient for higher-draw devices, but with greater self-discharge and voltages that differ from primary cells.
Temperature matters: in a freezer or a manhole in winter, the usable capacity of many batteries drops noticeably.
Mistakes that shorten battery life
- Weak signal: a sensor far from the hub retransmits several times and uses more power. Adding a repeater node to the mesh can extend battery life more than any setting.
- Report intervals that are too short: many sensors let you set how often to send data, or to transmit only when the value changes beyond a threshold.
- Status LEDs that flash on every transmission: on some devices they can be turned off.
- Outdated firmware: some updates fix precisely these power consumption problems.
Managing dozens of battery sensors
With many devices at home, managing batteries becomes a small job in itself. A few habits make life easier:
- an automation that warns you when a battery drops below a certain threshold;
- a list with each sensor's battery type and the date it was last replaced;
- a small stock of the most commonly used batteries.
In short
- IoT sensors last for years because they sleep almost all the time and transmit for a few milliseconds.
- Transmission frequency is the factor that most affects battery life.
- Zigbee, Thread and Bluetooth LE work well at home; LoRaWAN and NB-IoT cover long distances.
- Battery type and operating temperature change real-world battery life considerably.
- A good radio signal and low-battery alerts keep management simple.
