Estimate how long a battery will last

The job

Runtime starts as a division: capacity in mAh divided by average current in mA gives hours. Real batteries lose some charge to heat and conversion, so an efficiency factor applies; and devices that sleep most of the time stretch their calendar runtime beyond the active-hours figure.

Estimating a device

Enter the capacity (mAh or Ah) and the average draw (mA or A), then set efficiency — 85% is a reasonable default for a boosted/regulated load. If the device sleeps, set the duty cycle: the percentage of time it is actually drawing its active current.

• Active run time = capacity × efficiency ÷ load.
• Calendar time = active time ÷ duty fraction — a device awake 10% of the time lasts 10× longer on the wall clock.
• Deep sleep currents (µA) count toward the average too; estimate them honestly.

Worked example

2,000 mAh pack, 40 mA sensor, 85% efficiency: 2000×0.85÷40 ≈ 42.5 active hours. With a 10% duty cycle the calendar runtime is about 425 hours — roughly 17 days, not 42.5.

Where it stops

The estimate assumes a steady average load. Transmit bursts, motor starts, cold temperatures, and old cells all cut the real number — often noticeably. For critical deployments, measure the actual draw profile with a meter rather than trusting a nameplate figure.