Battery Life Estimator
Estimate how long a battery lasts from its capacity, average load current and a real-world derating factor.
Estimate only
- Estimated runtime
- 17h
- Runtime (hours)
- 17.0 h
- Runtime (days)
- 0.71 d
- Usable capacity
- 1700 mAh
How Battery Life Estimator works
Battery capacity is quoted in milliamp-hours, which is a charge rather than an energy: 2000 mAh means the cell can nominally deliver 2000 mA for one hour, or 200 mA for ten. Dividing capacity by average load current gives an ideal runtime, and that ideal is always optimistic.
It is optimistic because capacity is not a constant. It is measured at a specific discharge rate, temperature, and cut-off voltage, and it falls as any of those move. Draw current faster than the rating assumes and the usable capacity drops — an effect described by Peukert’s law, pronounced in lead-acid and modest in lithium-ion. Cold has a similar effect: a lithium cell at 0 °C may deliver only 70–80% of its room-temperature capacity.
Self-discharge sets a ceiling on very low-current applications. An alkaline cell loses a few percent a year, a lithium primary well under one percent, and older NiMH chemistries up to 20–30% per month. For a sensor drawing microamps, self-discharge rather than the load can be what ends the battery’s life.
The derating factor exists to fold all of this into one honest number. Applying 0.7–0.8 to the ideal runtime is a reasonable default for a well-behaved design at room temperature; harsher conditions, high pulse currents, or an end-of-life margin call for more aggressive derating.
Reference
- Ideal runtime (h) = capacity (mAh) / average load current (mA)
- Realistic runtime = ideal runtime × derating factor
- Duty-cycled average current: I_avg = (I_active × t_active + I_sleep × t_sleep) / (t_active + t_sleep)
- Energy (Wh) = capacity (Ah) × nominal voltage (V)
- C-rate = load current / capacity (1 A from a 2000 mAh cell = 0.5 C)
How to use this estimator
Enter the capacity
Take the mAh figure from the cell or pack. For cells in parallel, capacities add; in series they do not.
Work out the average current
For a duty-cycled device, weight the active and sleep currents by how long each lasts — do not use the peak.
Apply a derating factor
Start at 0.8 for a benign design, 0.7 or lower for cold operation, high pulse currents, or an aged pack.
Read the estimate
Compare the result against your target and iterate on sleep current, which usually dominates long-lived designs.
Worked examples
A always-on sensor
- Given
- 2000 mAh cell, 15 mA constant draw, derating 0.8
- Result
- ≈ 107 hours, or about 4.4 days
The ideal figure is 133 hours. The 20% haircut covers cut-off voltage, self-discharge, and rate effects.
A duty-cycled logger
- Given
- 80 mA active for 2 s per minute, 20 µA asleep, 2000 mAh
- Result
- I_avg ≈ 2.69 mA → about 25 days at 0.8 derating
Sleep current is 20 µA yet contributes under 1% of the average. Reducing the active window pays far more than optimising sleep here.
Where sleep current dominates
- Given
- The same logger waking once per hour instead
- Result
- I_avg ≈ 64 µA → roughly 3 years
Now the 20 µA sleep current is a third of the total. Below about 100 µA average, self-discharge starts competing with the load.
When to use it
- Estimating how long a battery-powered sensor or logger will run between changes.
- Comparing the impact of different sleep strategies on a firmware design.
- Sizing a battery pack to hit a required service interval.
- Sanity-checking a manufacturer’s runtime claim against your actual load.
- Deciding whether a design needs a larger cell or lower average current.
Things to watch out for
- Capacity ratings assume a specific discharge rate and cut-off voltage. Drawing current faster than rated reduces the usable capacity, sharply so in lead-acid and NiMH.
- Cold cuts capacity substantially — often 20–30% at 0 °C for lithium-ion, and far more below freezing.
- Cells lose capacity with age and cycling. A pack at 80% of its original rating is generally considered end-of-life for cycling applications.
- For microamp-level loads, self-discharge can dominate. Choose a chemistry with low self-discharge rather than simply a larger capacity.
Frequently asked questions
How do I calculate battery life?
Divide capacity in mAh by average load current in mA for an ideal runtime in hours, then multiply by a derating factor of roughly 0.7–0.8. The derating accounts for the cut-off voltage, self-discharge, temperature, and rate effects that the nominal rating ignores.
Why does my battery last less than calculated?
The nominal capacity is measured under favourable laboratory conditions. Real designs face higher discharge rates, colder temperatures, an earlier cut-off voltage, aged cells, and current peaks the average hides. That gap is exactly what the derating factor represents.
What is a good derating factor?
Use 0.8 for a moderate load at room temperature with a fresh cell. Use 0.7 or lower for cold environments, high pulse currents, an aged pack, or when you need the estimate to be a floor rather than a best case.
How do I handle a device that sleeps most of the time?
Compute a time-weighted average current: multiply each current by the fraction of time spent in that state and sum. A device drawing 80 mA for two seconds a minute and 20 µA otherwise averages about 2.7 mA — nowhere near either extreme.
Do mAh ratings compare across chemistries?
Not directly, because mAh is charge, not energy. A 2000 mAh lithium-ion cell at 3.7 V stores 7.4 Wh; a 2000 mAh NiMH cell at 1.2 V stores 2.4 Wh. Compare watt-hours when the chemistries or voltages differ.
What does C-rate mean?
The discharge current expressed as a multiple of capacity. Drawing 1 A from a 2000 mAh cell is 0.5 C. High C-rates reduce usable capacity and raise internal heating, which is why cells carry a maximum continuous discharge rating.
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