Battery life is, at its simplest, how much charge you have divided by how fast you use it. Capacity in milliamp-hours (mAh) divided by load current in milliamps (mA) gives hours. Two refinements make the estimate realistic: a derating factor, because you never get 100% of the rated capacity, and an average current that accounts for low-power sleep modes — the single biggest lever in battery-powered design.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: battery datasheet discharge methods and Peukert's law references.
The battery-life formula
Keep the units consistent: capacity in mAh and current in mA give hours. For amp-hours and amps, multiply both by 1000 first (1 Ah = 1000 mAh). The derating factor (typically 0.7–0.9) trims the rated capacity to what you can actually use down to your circuit's cut-off voltage.
Worked example — a wireless sensor
Scenario: A sensor wakes for 0.5 s every 10 s (5% duty), drawing 50 mA awake and 10 µA (0.01 mA) asleep, from a 2000 mAh battery at 85% usable.
If it stayed awake continuously at 50 mA, the same battery would last only 2000 × 0.85 / 50 = 34 hours. The sleep mode multiplies battery life roughly twenty-fold — which is why minimising both the sleep current and the active time dominates low-power design.
Frequently Asked Questions
Life (hours) = capacity (mAh) × derating ÷ average load (mA). A 2000 mAh battery at a steady 100 mA and 85% usable lasts 2000 × 0.85 ÷ 100 = 17 hours.
Average = active × duty + sleep × (1 − duty). 50 mA active for 5%, 10 µA asleep ≈ 2.5 mA, so a 2000 mAh battery lasts ≈ 680 hours (28 days).
Rated capacity is a lab figure. Real runtime drops with high discharge rate, temperature, cut-off voltage, self-discharge and ageing. A 70–90% factor is realistic; use the low end for high current or cold.
Multiply by 1000: 1 Ah = 1000 mAh, so 2.5 Ah = 2500 mAh. Convert load amps to mA the same way so units match.
Yes — most chemistries give less usable capacity at high rates (Peukert effect for lead-acid, resistance losses for lithium). Near the rated C-rate, lower the derating factor.