ElectroHub

Battery Life Calculator

Divide capacity by current and you get the ideal runtime. Reality is shorter — internal resistance, temperature and discharge curves take their cut — so the calculator applies a derating factor rather than pretending otherwise.

Battery Life — t = capacity ÷ load

How long a battery lasts for a given average current draw. The derating factor accounts for real-world losses (cut-off voltage, self-discharge, converter efficiency) — 70–85% is typical.

Ideal runtime
Realistic runtime
Realistic runtime (hours)
  1. 1.Given: capacity = 2000 mAh, load = 250 mA, derating = 80 %
  2. 2.FormulaIdeal runtime = capacity ÷ loadSubstitutet = 2000 ÷ 250Resultt = 8 h (8 h)
  3. 3.FormulaRealistic runtime = ideal × deratingSubstitutet = 8 × 0.8Resultt = 6.4 h (6 h 24 min)
Common trap: mAh is a charge rating, not energy — it only compares fairly at the same voltage. And the C-rate matters: draw a small cell too hard and its usable capacity drops well below the printed mAh, so the derating factor isn't optional for high loads.

The formula

Runtime (h) = (Capacity in mAh / Load in mA) × derating factor

Capacity
battery rating (milliamp-hours)
Load
average current drawn (milliamps)
derating
0.7–0.85 typical; accounts for real discharge behaviour

Worked example

A 2000 mAh cell powering a 250 mA load, derated to 0.8.

  1. Ideal: 2000 / 250 = 8 hours
  2. Realistic: 8 × 0.8

About 6.4 hours of usable runtime.

Where you'll use it

Sizing a battery for a portable project. The average current is what matters, not the peak — a device that sleeps at 1 mA and wakes for 100 ms bursts lasts far longer than its peak draw suggests.

The laws behind it

Parts this applies to

Resistor (fixed)Capacitor (aluminium electrolytic)Capacitor (tantalum)LED (light-emitting diode)MOSFETRectifier DiodeSchottky DiodeZener Diode

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