Battery Life Calculator
How long will your battery actually last? Enter the cell capacity in milliamp-hours (mAh), the current your device draws in milliamps (mA), and a discharge efficiency factor that accounts for real-world losses. The calculator returns the estimated runtime in hours, days and minutes. It is handy for sizing power banks, drone packs, IoT sensors, LED projects and any portable build where you need a quick, honest estimate before you commit to a battery.
How to use the calculator
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1
Enter capacity and load
Type the battery capacity in mAh and the average current the device draws in mA.
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2
Set the efficiency
Lower it from 100% to model heat, voltage sag and converter losses. 70-85% is realistic for most packs.
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3
Read the runtime
The tool shows the estimated battery life in hours, days and minutes, updating as you type.
The formula
Battery runtime is capacity divided by current, scaled by a discharge efficiency factor:
hours = (capacity ÷ load) × (efficiency ÷ 100)
Where capacity is in mAh, load is in mA, and efficiency is a percentage. Because mAh ÷ mA cancels the milli prefix, the result is already in hours. Days are hours ÷ 24 and minutes are hours × 60.
Worked example
A 2000 mAh battery powers a sensor that draws 200 mA, with an 80% efficiency factor:
hours = (2000 ÷ 200) × (80 ÷ 100) = 10 × 0.8 = 8 hoursminutes = 8 × 60 = 480 minutesdays = 8 ÷ 24 ≈ 0.33 days
Without the efficiency factor the naive figure is 10 hours, so real-world losses cost roughly 2 hours here.
Why efficiency matters
| Efficiency | What it models | Typical use |
|---|---|---|
| 100% | Ideal, lossless (theoretical maximum) | Textbook only |
| 85% | Light load, mild temperatures | Low-power sensors |
| 75% | Mixed real-world conditions | Phones, power banks |
| 60% | Heavy load, cold, high discharge rate | Drones, power tools |
Pitfalls
- Voltage matters too. mAh ignores voltage. To compare packs of different voltages, convert to watt-hours:
Wh = (mAh ÷ 1000) × volts. - Peukert’s effect. High discharge rates reduce usable capacity beyond a flat efficiency factor, especially for lead-acid cells.
- Self-discharge and ageing. A cell rated 2000 mAh when new may deliver far less after hundreds of cycles.
- Average vs peak current. Use the average draw, not the momentary peak, or you will badly underestimate runtime.
Frequently Asked Questions
There is no single right answer, but 70-85% covers most lithium-ion and NiMH packs under normal use. Use a lower value (60% or so) for heavy loads, cold weather or high discharge rates, and a higher value (90%+) only for gentle, room-temperature loads.
Internal resistance turns some energy into heat, DC-DC converters waste a few percent, voltage sags under load so usable capacity drops, and temperature changes the chemistry. The efficiency factor folds all of that into one adjustable number.
Multiply amp-hours by the nominal voltage: Wh = (mAh ÷ 1000) × volts. A 2000 mAh cell at 3.7 V holds about 7.4 Wh. Watt-hours let you compare batteries that run at different voltages.
No. The calculation runs in your browser session and nothing you type is uploaded, saved or shared. The numbers exist only for the current page.
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