Insights / Hardware & Electronics
Power Supply Options for IoT Devices: Mains, Batteries and Energy Harvesting
Mains, primary cells, rechargeable batteries or harvested energy: how to choose an IoT device's power source from its duty cycle, location and maintenance cost.
The short answer
An IoT device can be powered in four ways: from the mains through a compact AC/DC converter, from primary (single-use) batteries, from rechargeable batteries, or from harvested energy such as light, heat or radio waves stored in a small buffer. Many products combine two, for example mains with a backup battery, or a solar cell charging a lithium-ion cell. The right choice follows from the device's duty cycle and location: how often it wakes, measures and transmits, whether a cable can reach it, and how often anyone can replace a battery.
Start from the power budget, not the battery
Phones and laptops struggle with battery life because their processors run fast and stay busy. Most IoT devices are different. Microcontrollers and radio chips built for connected devices are designed for very low leakage, and in deep sleep they draw almost nothing. A temperature sensor that wakes, measures, transmits and goes back to sleep spends most of its life idle.
So the average current, and therefore the battery size, depends mostly on how often the device wakes and how long the radio stays on, not on the peak consumption in the datasheet. Measuring those currents is a design task in itself: sleep currents in the micro- or nano-ampere range need precise instruments to measure.
- Power budget
- The device's average energy consumption over a realistic duty cycle (sleep, wake, measure, transmit, retry), set against the energy the source can deliver over the intended life or charge interval.
Mains power and backup
Stationary devices that can reach a socket or wiring, such as routers and gateways, cameras, metering units, lighting and heating controls, usually run from the mains through an AC/DC converter. The requirements are compact size, high efficiency and a stable output voltage and current; for most IoT devices the converter delivers a few watts, rarely more than about 20 W. Off-the-shelf modules cover most needs.
Mains power is usually the cheapest option for the manufacturer, but it can add cost elsewhere: cables, adapters and installation, and an awkward fit in a finished room. A smart switch, for example, has to fit its supply, controller and radio into the same wall box as an ordinary switch.
Video surveillance, access control and alarm systems also need to keep working when the mains fails. Sealed lead-acid batteries are a common backup because they tolerate being kept at full charge for long periods and deliver high peak current.
Batteries: how they work and which chemistry fits
Every electrochemical cell has two electrodes of different materials, a cathode and an anode, separated by an electrolyte in which charges can move. When a load is connected, a chemical reaction between the electrodes and the electrolyte produces an electromotive force. In primary cells the reaction cannot be reversed; in rechargeable cells it can, for a limited number of cycles.
The materials define the cell's voltage, energy density, peak current, temperature range and self-discharge, which is the slow loss of charge that happens whether or not the device draws power. For a device meant to run for years, self-discharge can matter as much as the device's own consumption.
| Chemistry | Rechargeable? | Characteristics | Typical IoT fit |
|---|---|---|---|
| Zinc-carbon | No | Low cost, wide choice of sizes, modest capacity | Low-drain, low-cost devices where replacement is easy |
| Alkaline (zinc / manganese dioxide) | No | Moderate cost, good capacity, very widely available | Remotes, sensors and controls with user-replaceable cells |
| Silver oxide (silver / zinc) | No | High capacity for its size, flat discharge voltage | Small coin-cell devices such as watches and hearing aids |
| Lithium primary | No | High energy density, wide temperature range, long shelf life | Sensors and trackers that must run for years without a visit |
| Lithium-ion / lithium-polymer | Yes | High energy density, flexible shapes, ages with time and cycles | Wearables, portable devices, solar-charged outdoor nodes |
| Lead-acid (sealed) | Yes | Heavy and bulky, high peak current, low cost per watt-hour | Backup power for security, access control and gateways |
Lithium-ion cells can be made thin and in many shapes, which suits compact devices, but they cost more than alkaline cells and lose capacity as they age. Lead-acid cells are cheap and robust but their weight and size rule them out of most devices other than fixed installations.
Energy harvesting
Harvesting sources deliver little power, but for a device that sleeps most of the time, little can be enough. Each one needs a storage element, a rechargeable cell or a capacitor, to cover the moments when the device is awake and the source is weak.
| Source | How it works | Where it fits | Main limitation |
|---|---|---|---|
| Solar (photovoltaic) | Light frees electrons in a semiconductor, producing current | Outdoor sensors, well-lit indoor devices | Output depends on light level and spectrum; nothing in the dark |
| Thermoelectric | A temperature difference across a junction produces a voltage (Seebeck effect) | Devices on warm pipes or machinery, or worn against the body | Needs a sustained temperature difference |
| Radio frequency | An antenna converts a surrounding electromagnetic field into current, as in passive RFID tags | Tags and sensors near a reader or a strong transmitter | Only useful where the field is strong; unsuitable for general outdoor use |
| Radioisotope (betavoltaic) | Decay of a radioactive isotope generates a very small, continuous current | Niche, very long-life applications | Tiny current, so a buffer battery is required; regulatory constraints |
The choice is about fit as much as physics. A watch that harvests heat from the wearer's skin works because it is always in contact with a heat source; a solar-charged watch would stop when covered by a sleeve.
Matching the power source to the device
| Device type | Duty cycle | Usual power choice |
|---|---|---|
| Energy management, metering and lighting controls | Always on, fixed location | Mains through an AC/DC converter |
| Cameras, access control and alarm panels | Always on; must survive outages | Mains plus a backup battery, often sealed lead-acid |
| Environmental sensors (temperature, humidity) | Wake, measure, transmit, sleep; slow updates are fine | Primary cells, practical even across wide areas |
| Motion and light sensors | Always listening, must react quickly | Larger battery, or solar charging a lithium-ion cell outdoors |
| Item trackers and tags | Rare, short transmissions over a long life | A replaceable coin cell; building in a rechargeable cell would add cost for little gain |
| Wearables | Frequent sensing, daily use | Rechargeable lithium-ion or lithium-polymer |
The engineering implication: power sets the product's limits
However capable the device, it stops when the energy runs out. The power decision fixes the enclosure size, the radio you can afford to use, how often data can be sent and what a year of operation costs in battery replacements or site visits. It is also hard to change after launch, because the battery compartment, charging circuit and certification are built around it.
How to choose a power source for an IoT device
Work through these in order
- Write down the duty cycle: how often the device wakes, measures and transmits, and for how long.
- Measure sleep and active currents on a prototype, including radio join time and retries.
- Decide where the device lives: can a cable reach it, and who can replace or charge a battery, how often?
- Set the target life or charge interval and check self-discharge and temperature range against it.
- If mains-powered, decide what must keep working during an outage and size the backup.
- Consider harvesting only where the source is reliable at the installation, and size the buffer for the worst day.
- Weigh size, cost, user convenience and disposal together; the cheapest cell is not always the cheapest product.
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Where this shows up in our work
ExpertiseHardware and electronics engineering for connected productsMeasuring real duty cycles, sizing the battery or supply and designing the power path and charging circuit is hardware and electronics work we own with the firmware team that sets the sleep behaviour.Where devices live in homes, battery changes and wall-box space are product decisions as much as electrical ones. See Smart Home & Consumer IoT.
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