Scope and assumptions
Battery % = current charge ÷ rated capacity × 100. It measures charge, not runtime. Illustrative.
The number in the corner of your screen looks like a countdown, and it isn't. When it reads 50%, the battery is half full — half its stored charge remains — but that says nothing about how many minutes you have left. Open a game and a 50% phone can die in forty minutes; leave it in airplane mode and the same 50% can last a day and a half.
Confusing "how full" with "how long" is the single most common misreading of a battery indicator.
A ratio, not a clock
Battery % = current charge ÷ rated capacity × 100
Rated capacity is the design maximum printed on the cell — 3,000 mAh, 4,000 mAh. Current charge is what's actually stored right now. The division gives a share of the tank, and the tank's size is fixed by the hardware.
| Capacity | Charge remaining | Indicator |
|---|---|---|
| 3,000 mAh | 1,500 mAh | 50% |
| 3,000 mAh | 750 mAh | 25% |
| 4,000 mAh | 1,000 mAh | 25% |
The last two rows both read 25%, but the 4,000 mAh phone is holding 250 mAh more. Percentage compares each phone to itself, never across devices.
Why the last slice disappears fast
Two separate effects stack up near empty.
The first is load. Discharge isn't linear. Streaming, navigation, gaming, and cold weather pull current harder, and the final stretch is often the most demanding because the device is doing the same work with less voltage headroom.
The second is estimation. The gauge infers charge from voltage and current, both of which move under load. Near empty, a small voltage sag can translate into a large percentage jump.
So the bar is honest about charge and imprecise about remaining time. The last 20% isn't shorter — it's just measured with a noisier ruler while being spent faster.
The tank shrinks too
Lithium cells lose capacity over hundreds of cycles. A phone that once held 3,000 mAh might hold 2,600 mAh after two years, and "100%" now means 2,600. The percentage is always relative to current capacity, so an older device at 50% genuinely holds less energy than a new one at 50%.
Where the reading goes wrong
Treating the percentage as minutes remaining; assuming the discharge curve is a straight line when it bends under load; forgetting the ×100 and reading 0.5 as the answer; comparing percentages across devices with different capacities; trusting a quick top-up to 100% when the cells may be unevenly balanced; and ignoring temperature, since cold raises internal resistance and makes both the reading and the runtime sag.
FAQ
Why does my phone jump from 20% to 0% so quickly? Two causes: the drain rate rises under heavy load, and the fuel gauge's estimate of remaining charge drifts. Some devices also recalibrate by shutting down early.
Does charging to 100% every day damage the battery? It accelerates chemical ageing compared with staying in the 20–80% band, but modern devices manage this automatically. Heat matters more than the top-up itself.
What does mAh actually measure? Milliamp-hours — charge capacity. A 4,000 mAh battery stores more charge than a 3,000 mAh one, so 50% of each is a different absolute amount.
Why do cold batteries lose percentage so suddenly? Low temperatures raise internal resistance, so voltage sags under load and the gauge reports less available charge. Capacity often recovers once the device warms up.
Sources: Battery University, state of charge; electronics guides.
Disclaimer: Illustrative. Real runtime varies with usage and age. Informational only.
Sources and references
- Battery University — battery state of chargehttps://batteryuniversity.com/