Battery Life Calculator
Estimate runtime from a battery's capacity (mAh) and your device's current draw (mA). The efficiency factor accounts for real-world losses — keep 85% for typical loads, or lower it for high-drain devices and cold conditions.
Estimated runtime
10 hr 12 min
12.0 hours theoretical × 85% efficiency
Runtime = capacity ÷ draw × efficiency. The 85% default covers voltage sag, temperature, and converter losses; drop it to ~70% for high-drain loads or freezing conditions.
How Battery Runtime Is Estimated
Milliamp-hours describe how much charge a battery holds: a 3,000 mAh cell can supply 3,000 mA for one hour, or 300 mA for ten hours. Dividing capacity by draw gives the theoretical runtime; multiplying by an efficiency factor brings it down to reality.
Formula
Runtime (hours) = (Capacity mAh ÷ Draw mA) × Efficiency Example (18650 cell powering a 250 mA device): 3,000 ÷ 250 = 12 hours theoretical 12 × 0.85 = 10.2 hours realistic Comparing across voltages? Use watt-hours: Wh = mAh × Volts ÷ 1,000
Frequently Asked Questions
How long will a 3,000 mAh battery last at a 250 mA draw?
Theoretically 12 hours (3,000 ÷ 250); realistically about 10 hours after the ~85% efficiency factor. The simple division assumes a steady draw — devices with bursts (radios transmitting, screens waking) average out somewhere between their idle and peak currents.
Why apply an efficiency factor instead of using the full capacity?
Rated mAh is measured under gentle lab conditions. In real use, voltage sag under load, converter losses, temperature, and cutoff circuits that stop before true zero all shave usable capacity. 85% is a fair default; high-drain or cold conditions justify 70%.
Can I compare batteries by mAh if their voltages differ?
No — mAh only compares batteries at the same voltage. A 3.7 V phone cell at 3,000 mAh stores nearly three times the energy of a 1.2 V NiMH AA at 3,000 mAh. To compare across chemistries, use watt-hours: Wh = mAh × volts ÷ 1,000.
Does cold weather really drain batteries faster?
It reduces deliverable capacity rather than draining charge: lithium cells can lose 20–40% of usable capacity near freezing and alkalines fare worse. The energy largely returns when the battery warms up — which is why a 'dead' phone revives indoors.