Last updated: 2026-07-02

Battery Life

A battery stores a fixed amount of charge, and a device drains it at some rate. Divide one by the other and the runtime follows — but the tidy arithmetic sits above a pile of real-world losses that a single efficiency factor keeps honest.

The runtime formula

Battery capacity is quoted in milliamp-hours (mAh): a 3,000 mAh cell can, in principle, supply 3,000 mA for one hour, or 300 mA for ten hours, or 150 mA for twenty. The current a device pulls is quoted in milliamps (mA). Dividing capacity by current gives the ideal runtime:

where $Q$ is capacity in mAh and $I$ is the average current draw in mA. The units cancel cleanly — milliamp-hours divided by milliamps leaves hours — so no conversion is needed.

Real batteries never deliver the full figure, so the estimate multiplies the ideal runtime by an efficiency factor $k$ between 0 and 1:

A factor of 0.85 (85%) is a common starting point; heavier loads and older cells warrant less.

Worked example

Take a 3,000 mAh battery driving a device that averages 150 mA, at 85% efficiency.

The ideal runtime first:

Then apply the derating:

So the battery lasts about 17 hours in practice, against a theoretical ceiling of 20. Bump the load to 300 mA and the ideal runtime halves to 10 hours, giving roughly 8.5 hours after derating.

Why real life falls short

The ideal formula assumes the cell hands over every milliamp-hour on its label at a steady voltage with nothing lost along the way. Several effects erode that:

• Regulator losses. A device rarely runs at the raw battery voltage. Converting it up or down wastes a few percent as heat.
• Self-discharge. Cells leak charge even when idle, more so when warm.
• Temperature. Cold reduces the chemistry's usable capacity; heat accelerates ageing.
• Rated-capacity optimism. The label figure is measured under gentle, standardised discharge — kinder than most real duty cycles.

The efficiency factor bundles all of these into one number. A well-matched regulator under a light, steady load might reach 85–90%; a bursty, high-current load on a tired cell can fall to 70% or lower.

The Peukert effect

There is one loss the efficiency factor only approximates: capacity shrinks as the discharge current rises. Peukert's law captures this — draw twice the current and you often get noticeably less than half the runtime, because faster discharge leaves more of the stored charge stranded. The effect is pronounced for lead-acid batteries and high-current loads, and milder for the lithium-ion cells in phones and laptops. For a load that stays roughly constant, folding it into a lower efficiency factor is good enough; for wildly varying or very heavy draw, a dedicated Peukert model gives a closer answer.

Estimating average current

The formula rewards a realistic current figure. Most devices spend the bulk of their time idle, waking briefly to do work, so a peak-current reading badly overstates the drain. Where possible, use the average over a full duty cycle — for example, a sensor that draws 50 mA for one second every minute and 1 mA the rest of the time averages just under 2 mA, not 50. Datasheets often list a typical or standby current that is closer to the useful average than the maximum rating.

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