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🌡️ Coin-cell thermometer that lasts months
“Can a coin cell run a temperature sensor for a year?”
Yes, if the whole design is built around sleeping. A coin cell holds very little energy, so the board sleeps almost all the time, wakes briefly to measure, broadcasts the reading over Bluetooth Low Energy (BLE), and sleeps again.
What to buy
| Part | Why this one | Rough cost |
|---|---|---|
| nRF52840 board (e.g. Seeed XIAO nRF52840) | Bluetooth Low Energy with very low sleep current: the best fit when battery life is the hardest rule. | $10–15 |
| SHT40 temperature and humidity sensor board | Accurate, small, and uses very little power between readings. | $3–8 |
| CR2032 holder and cells | About 220 mAh per cell. That's tiny, which is why every microamp matters. | $2–4 |
| Capacitor, 100 µF or more | Across the cell. It covers the short current peaks of each radio burst, which a coin cell handles badly. | $0.50 |
Prices are rough ranges, not quotes. Check the current price and stock before you order.
What to install
Arduino with Seeed's nRF52 board package and the Adafruit Bluefruit library: take a reading, put it in a BLE advertisement (a short broadcast any nearby phone can hear, no pairing needed), then sleep. Use the BTHome format and Home Assistant shows the reading automatically; otherwise the free nRF Connect app shows the raw broadcast.
The honest battery maths
Battery life ≈ capacity ÷ average current. A CR2032 holds about 220 mAh.
| Average current | Rough life, before losses |
|---|---|
| 10 µA | about 2½ years |
| 25 µA | about 1 year |
| 50 µA | about 6 months |
| 100 µA | about 3 months |
Then take off a real margin: cold, the cell's own slow self-discharge, and the radio's current peaks all cut the real life. The average is set mostly by the board's sleep current and how often it wakes.
Build it in three steps
- Read the sensor on USB. Connect the SHT40 over I²C and print the temperature.
- Broadcast, then sleep. Put the reading into an advertisement every minute or two, and sleep in between. Check that your phone sees it.
- Measure before you trust the maths. Run it from the coin cell and measure the sleep current with a multimeter on its µA range, in series with the cell. Then use the table above with your real number.
What catches people out
- The board, not the chip, sets the sleep current. Power LEDs, voltage regulators and USB chips on a dev board can draw more than the chip itself.
- Not WiFi. A WiFi board like the ESP32 pulls hundreds of milliamps while connecting. A coin cell can't deliver that, and the board resets. BLE is the right radio here.
- Cold shortens it. In a fridge, or outdoors in winter, a coin cell delivers noticeably less.
- Every wake-up costs. One reading a minute is plenty for room temperature. One every 5 seconds empties the cell far faster.
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