Estimates how long a battery-powered device will run from the battery’s rated capacity and the device’s current draw.
How it works
Battery capacity is multiplied by an efficiency factor accounting for real-world losses, then divided by the device’s current draw to give runtime.
What this does not include
This does not include variable power draw over time — many devices don’t draw a constant current, so treat the result as an estimate under a steady load.
How to use this calculator
- Enter the battery’s capacity, an efficiency estimate, and the device’s current draw.
A worked example
A 3,000 mAh battery at 85% usable efficiency, drawing 500 mA: runtime = (3000 × 0.85) ÷ 500 = 5.1 hours.
A 5,000 mAh battery at 90% efficiency, drawing 1,000 mA: runtime = 4.5 hours.
What the variables mean
| Variable | Meaning |
|---|---|
| Capacity | Rated battery capacity, in milliamp-hours (mAh) |
| Efficiency | Usable percentage of rated capacity, since real batteries rarely deliver 100% |
| Current draw | How much current the device actually pulls, in milliamps |
Edge cases worth knowing
The efficiency factor exists because rated capacity is rarely fully usable — internal resistance, temperature, and discharge curve all reduce real-world runtime below the label figure.
Zero current draw makes runtime undefined (effectively infinite) — a device that draws no power never depletes the battery, so the calculator declines to show a meaningless result.
Frequently asked questions
Why isn’t efficiency 100%?
Voltage-conversion circuitry and battery chemistry both introduce real-world losses — a battery rarely delivers 100% of its rated capacity as usable output.
Where do I find a device’s current draw?
Manufacturer specifications often list current draw directly, or it can be estimated from a device’s power rating and operating voltage (current = power ÷ voltage).
Does battery age affect this estimate?
Yes — batteries lose capacity over their lifespan, so an older battery will typically underperform its original rated capacity compared to this calculation.
