Home automation
Sensor battery life calculator
Duty-cycle arithmetic for a battery sensor: capacity, sleep current, transmit current and how often it reports, into an average draw and an expected life — and the split between sleeping and transmitting, which is what decides whether reporting less often would help at all.
The question this answers: “The box said two years and it died in four months. Where is the charge actually going?”
Result
- Duty cycle
- 0.0067%
- 0.06 s awake every 15 min.
- Average draw
- 12 µA
- Usable capacity
- 2000 mAh
- Expected life
- 6945 days
- About 228.1 months.
- Share of the draw spent transmitting
- 16.7%
- Where the budget actually goes, which decides what is worth changing.
Worth knowing about this answer
- The answer is only as good as the sleep current, which is the input people take from a datasheet and the one a real board rarely achieves. A regulator, a pull-up resistor or an LED left populated will dominate everything else here, and measuring the assembled node beats reading the chip’s figure by a wide margin.
- Nothing here models the cell. Capacity falls in the cold, coin cells cannot supply a transmit pulse at low temperature however much charge is left in them, and self-discharge is not accounted for at all — which matters most for exactly the long lives this tool is used to check.
- The usable proportion is your choice and is an input for that reason. A node that stops reporting when the voltage sags is done long before the cell is empty.
- Sleep is 83% of the average draw. Reporting less often will barely help; reducing the quiescent current is the only thing that will.
- Above about five years the arithmetic outruns the chemistry: self-discharge, which is not modelled, becomes the thing that ends the battery rather than the load.
Show the working
- Duty cycle
0.06 s ÷ 900 s0.0067% - Average current
(30 mA × duty) + (10 µA × the rest)12 µA - Usable capacity
2500 mAh × 80%2000 mAh - Life
2000 mAh ÷ 0.012 mA166676 h
Duty cycle: 0.0067%. Average draw: 12 µA. Usable capacity: 2000 mAh. Expected life: 6945 days. Share of the draw spent transmitting: 16.7%
The time awake per report divided by the reporting interval gives a duty cycle. The average current is the transmit current weighted by that duty cycle plus the sleep current weighted by the rest of it. Usable capacity is the nameplate figure times the proportion you expect to get, and life is that divided by the average draw. The transmit share of the average is reported separately, because it is the figure that says whether reporting less often would change anything.
- A node with two states — asleep and reporting — and a fixed cost for each. A device that wakes to listen, retries a failed transmission or runs a radio in a mesh that expects it to route for others has a third state this does not model, and mesh routing in particular can cost more than reporting does.
- That the sleep current is the assembled board’s rather than the microcontroller’s. A regulator, a pull-up or a populated LED will dominate every other figure here.
- A constant capacity. Nothing models temperature, self-discharge or a coin cell’s inability to supply a transmit pulse when cold.
- It cannot tell you a particular sensor will last a particular time. It tells you what the load implies, and the difference between that and reality is the part the datasheet did not mention.
- Above about five years the arithmetic outruns the chemistry — self-discharge ends the cell rather than the load, and this does not model it.
- It gives no charging guidance, no cell-chemistry advice and no view on which battery to buy. Those are properties of a specific cell and belong to its manufacturer.
- A result is not a fire-safety assessment. Lithium cell handling, charging and disposal are covered on the page and by the manufacturer, not by a runtime figure.
Runs entirely in your browser — nothing you type is sent anywhere. Last reviewed 2026-08-19. More on Home automation.