A Beacon lives or dies by its power budget. Unlike a wired sensor, a Bluetooth Beacon has no grid and no technician visiting every month, so the battery decision is the single most important hardware choice. This article walks through coin-cell selection, a concrete lifetime model, and how far you can push batteryless designs with energy harvesting.
Why power is the constraint
A beacon spends 99.9% of its life asleep. The only events that matter are the advertising bursts: a few hundred microseconds where the radio draws 5-15 mA. Everything else is leakage and a low-power timer. The art is maximizing the gap between bursts without breaking detection latency.
Coin-cell selection
| Cell | Capacity | Diameter | Typical use |
|---|---|---|---|
| CR2032 | 225 mAh | 20 mm | slim tags, <2 yr |
| CR2450 | 620 mAh | 24 mm | general beacons |
| CR2477 | 1000 mAh | 24 mm | long-life, 5+ yr |
Nominal voltage is 3.0 V but the curve sags under load and as the cell depletes. Internal resistance climbs from ~10 Ω fresh to >30 Ω near end-of-life, so a beacon that transmits at 10 mA drops from 2.9 V to ~2.6 V at the tail. A boost converter stabilizes the radio at 3.0 V and squeezes out the last 20% of capacity.
Lifetime model
Average current is a duty-cycle sum:
I_avg = I_adv * (t_adv / T) + I_sleep * (1 – t_adv / T)
Example: t_adv = 0.5 ms at 12 mA, interval T = 500 ms, I_sleep = 1.2 µA.
duty = 0.5e-3 / 0.5 = 0.001
I_adv_avg = 12e-3 * 0.001 = 12 µA
I_avg = 12 µA + 1.2 µA = 13.2 µA
A CR2032 rated 225 mAh:
life = 225 mAh / 13.2 µA = 17,045 h = 1.95 yr (ideal)
derated 70% -> ~1.4 yr realistic
Doubling the interval to 1000 ms roughly halves I_adv_avg and extends life toward 3 years. The trade is slower first-seen latency and lower gateway hit probability.
Energy harvesting options
| Source | Indoor density | Notes |
|---|---|---|
| Ambient light | 10-100 µW/cm² | needs PV cell + MPPT |
| RF (WiFi/Cell) | 1-10 µW | unreliable, urban only |
| Thermoelectric | few µW per ΔT 5°C | body/pipe mounted |
| Piezo (motion) | burst mJ on press | event-driven only |
Harvested energy is buffered in a supercap (10-100 mF) or a rechargeable Li (LIR2032). The supercap survives more cycles; the Li stores more per volume.
Batteryless beacon
A capacitor-only beacon transmits only when enough charge accumulates. It wakes on harvest, advertises a few frames, then sleeps until the next threshold. You lose guaranteed periodicity but gain infinite theoretical life. This fits asset tags that move through lit doorways or near readers.
Practical checklist
- Derate capacity 30% for temperature: Li coin cells lose half their capacity below 0°C.
- Avoid deep discharge; cutoff at 2.0 V to prevent leakage.
- Use a boost converter for stable 3.0 V to the radio.
- Pick the longest interval your detection SLA allows.
- For harvesters, size the buffer for at least 3 transmit bursts.
A well-chosen Beacon with a CR2477 and a 1 s interval realistically runs 5+ years; add a small solar cell and the same unit can approach batteryless operation.