Battery Life Calculator
Capacity + load current → estimate how long the battery lasts.
What it does: Estimate how long a battery will run under a given load.
When to use it: When designing battery-powered devices, sizing battery capacity, or estimating runtime for a low-power project.
Disclaimer: This result is a reference estimate. For actual production, refer to the device datasheet / local regulations as authoritative.
MEANS About — hours (efficiency factor —). Set the factor to 1 to see the theoretical maximum.
⚠ This result is a rough estimate; the actual runtime is affected by the discharge curve, temperature, self-discharge, load fluctuation, and battery health — refer to measurements / the battery datasheet as authoritative.
No history yet. Each calculation is automatically saved to this device.
How to use the battery life calculator
Enter capacity → enter load → adjust the factor and read the runtime.
- 01
Enter the battery capacity
Check the mAh printed on the battery (e.g. an 18650 is about 2500–3500, a 9V alkaline about 550). Accepts
2000or2.2Ah→2200. - 02
Enter the average load current
The average current the device draws (mA); for intermittently active devices use the average, not the peak.
- 03
Adjust the efficiency factor and read the runtime
Default 0.8 (allows for real-world losses); set 1.0 to see the theoretical maximum. The result is given in days/hours/minutes.
Common battery capacities
For comparison when estimating; the actual figure is whatever is printed on the battery.
| Battery | Nominal capacity | Nominal voltage |
|---|---|---|
| AA alkaline | ~2000–3000 mAh | 1.5 V |
| AAA alkaline | ~850–1200 mAh | 1.5 V |
| 9V alkaline | ~550 mAh | 9 V |
| 18650 Li-ion | ~2500–3500 mAh | 3.7 V |
| CR2032 coin cell | ~225 mAh | 3 V |
| Phone battery | ~3000–5000 mAh | 3.7 V |
Common nominal ranges for each battery type (manufacturer specs vary).
Common questions, answered in 3 minutes
Why multiply by 0.8 by default instead of just dividing?
In reality battery capacity can't be fully used, regulators have losses, and temperature and ageing eat into capacity, so multiplying by an efficiency factor < 1 is closer to measured results. To see the theoretical maximum, set the factor to 1.
Should I enter the peak or average load?
Enter the average current. Many devices only hit full load occasionally, so using the peak greatly underestimates runtime; the average consumption over one duty cycle is more accurate.
What if my capacity is given in Wh?
Convert first: mAh ≈ Wh ÷ battery voltage (V) × 1000. For example, 11.1Wh at 3.7V → about 3000 mAh. For comparisons across different voltages, using Wh directly is recommended.
Is it normal for the estimate and the measurement to differ a lot?
Yes. The discharge curve, temperature, self-discharge, whether the load is constant, and battery health all matter; this tool only gives an order-of-magnitude estimate — measure for the precise value.
What should I watch for with high-current discharge?
At high current the effective battery capacity drops (the Peukert effect), so lower the efficiency factor (e.g. 0.6–0.7).
Standards and sources referenced by this tool
| Item | Value / Formula | Source |
|---|---|---|
| Runtime formula | t = (mAh / mA) × η | Q = I·t current integration |
| Efficiency factor η | Empirical derating | Engineering estimate, not a standard value |
The result is a rough estimate; for the actual runtime, refer to measurements / the battery datasheet as authoritative.