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How Battery Runtime Is Actually Calculated

The basic runtime formula is simple division, but the gap between that theoretical number and what a battery actually delivers comes down to a few predictable factors.

Basic formula: runtime (h) = capacity (mAh) Γ· current draw (mA)

A 3000 mAh battery powering a device that draws 300 mA on average would theoretically last 3000 Γ· 300 = 10 hours β€” before accounting for real-world losses.

Real capacity is always somewhat less than rated capacity

Manufacturers rate capacity under specific test conditions that rarely match real usage. Applying a realistic efficiency factor of roughly 70-85% of the rated capacity gives a more honest runtime estimate than the raw number alone.

mAh only means the same thing at the same voltage

Milliamp-hours measure current over time, not energy β€” comparing mAh across two devices with different battery voltages is misleading. Converting to watt-hours (Wh = mAh Γ— voltage Γ· 1000) allows a fair energy-based comparison across different devices.

Higher discharge current reduces effective capacity

Batteries deliver somewhat less total usable energy when drained quickly at high current than when drained slowly, an effect sometimes described by Peukert's law. A device with power-hungry bursts (like a camera flash) can drain a battery faster than the average current alone would suggest.

Age and temperature both reduce usable capacity

Rechargeable batteries lose capacity over charge cycles, and both very cold and very hot conditions temporarily reduce how much usable energy a battery can deliver, independent of its rated capacity.

Why the theoretical number is always optimistic

The mAh-divided-by-mA formula treats a battery like a perfectly efficient tank of a fixed size, but real batteries lose some energy to internal resistance, voltage sag as they discharge, and the load fluctuating rather than staying constant. A phone's average current draw also fluctuates enormously between an idle screen-off state and active use, so a single "average mA" number is itself an approximation layered on top of an already-simplified formula.

Why comparing two devices by mAh alone is misleading

A 5000 mAh battery at 3.7V stores meaningfully less energy than a 5000 mAh battery at 7.4V, because energy depends on both current capacity and voltage. Converting both to watt-hours puts them on the same footing, which is why watt-hours, not mAh, is the more meaningful number when comparing battery capacity across genuinely different devices.

Frequently Asked Questions

Why does my device never actually last as long as the calculated runtime?

Real-world runtime is shortened by internal battery inefficiency, voltage sag near full discharge, fluctuating rather than constant power draw, and battery aging β€” all factors the basic capacity-divided-by-current formula does not account for.

Should I compare batteries by mAh or by Wh?

Watt-hours (Wh) give a more accurate comparison when voltages differ, since Wh accounts for both current and voltage. mAh alone is only a fair comparison between batteries or devices that share the same voltage.