Battery Life Calculator
Estimate how long a battery will run a device. Enter capacity in mAh and average load in mA, pick an efficiency factor (100%, 85%, or the conservative 70% rule of thumb), and get runtime in hours and minutes plus the pack's energy in watt-hours.
Example: with Battery capacity (mAh) 3000 · Average device draw (mA) 250 · Efficiency factor 85% — typical well-designed electronics · Battery voltage (V) 3.7 → Estimated runtime: 10.2 hours (10 h 12 min).
- In days0.42 days
- Battery energy11.1 Wh at 3.7 V
Computed by the calculator below using its default values. Change any input to see your own numbers.
Runtime (h) = mAh ÷ mA × efficiency. The 0.7 factor is a widely used engineering rule covering converter losses, temperature, and battery aging.
The mAh-over-mA estimate and why it needs a haircut
A milliamp-hour rating means the battery can nominally supply that many milliamps for one hour. Divide capacity by your device's average draw and you have the ideal runtime: 3,000 mAh ÷ 250 mA = 12 hours. Real batteries fall short — voltage converters waste a few percent, capacity fades with age, cold saps output, and heavy loads extract less total charge (the Peukert effect). Multiplying by 0.85 fits healthy consumer electronics; the classic 0.7 factor is the safe engineering answer when you have to promise a number.
The single hardest input to get right is average draw. A sensor that sleeps at 0.1 mA and wakes to 100 mA for one second a minute averages under 2 mA — use the duty-cycle average, not the peak.
How it’s calculated
Runtime (hours) = capacity (mAh) ÷ average load (mA) × efficiency factor. Factors: 1.0 theoretical, 0.85 typical electronics near room temperature, 0.7 conservative rule of thumb. Energy (Wh) = mAh × V ÷ 1,000, with 3.7 V as the lithium-ion nominal default. Days = hours ÷ 24.
Assumes a constant average draw and full usable capacity — high discharge rates, cold, aging cells, and early low-voltage cutoffs all shorten real runtime.
Typical device current draws (approximate)
| Device | Average draw |
|---|---|
| LED indicator | ≈ 20 mA |
| Arduino Uno, active | ≈ 50 mA |
| ESP32 with Wi-Fi transmitting | ≈ 160-260 mA |
| Smartphone, screen on | ≈ 300-800 mA |
| Raspberry Pi 4, idle | ≈ 600 mA |
Manufacturer datasheets and published bench measurements; real draw varies with duty cycle and settings.
Common mistakes
- Using peak current instead of average — a device that sleeps most of the time averages far below its spec-sheet maximum.
- Mixing units: a 2 A load is 2,000 mA, not 2.
- Ignoring the cutoff voltage — devices shut down before the battery is chemically empty, stranding some capacity.
- Comparing packs of different voltages by mAh; watt-hours (mAh × V ÷ 1,000) is the fair yardstick.
Frequently asked questions
What is the battery life formula?
Battery life in hours = capacity in mAh ÷ load in mA, multiplied by an efficiency factor. Example: 3,000 mAh ÷ 250 mA × 0.85 = 10.2 hours.
Why multiply by 0.7 or 0.85?
Converter losses, temperature, aging, and cutoff voltages mean you never harvest the full label capacity. 0.85 suits healthy consumer electronics; 0.7 is the conservative engineering convention.
My load is in watts — how do I convert?
Current in mA = watts ÷ volts × 1,000. A 2 W device on a 5 V supply draws 400 mA. Use the battery voltage if there is no converter in between.
Does mAh measure energy?
No — it measures charge. Energy is mAh × voltage ÷ 1,000, in watt-hours. A 3,000 mAh pack at 3.7 V holds 11.1 Wh; at 12 V it would hold three times more.