Why does my phone drop from 30% to 0% suddenly?

Because the percentage is an estimate, not a measurement, and on a degraded battery that estimate becomes unreliable at exactly the point it matters.

How the percentage is calculated. There is no fuel gauge inside a battery. The device infers remaining charge from voltage, current draw and a model of the cell's expected behaviour. Lithium-ion cells have a notably flat voltage curve through the middle of their range — voltage barely changes between roughly 80% and 20% — so the estimate in that region relies heavily on the model rather than on direct evidence.

Why it goes wrong:

Battery ageing. As a cell degrades, its actual capacity falls and its internal resistance rises. The device may still be calculating against something closer to the original capacity, so its estimate diverges from reality — and the divergence appears as a sudden collapse when the voltage finally drops off the end of the flat curve.

Voltage sag under load. Higher internal resistance means voltage drops sharply when the phone demands current — opening the camera, a game, a location fix. A degraded battery can read 30% at rest and hit the shutdown threshold the moment it is asked for power. This is why it often happens during a demanding task.

Cold. Low temperatures raise internal resistance temporarily, producing the same effect in an otherwise healthy battery. Charge frequently returns once warm — the energy was never gone.

Protective shutdown. The phone switches off with charge remaining, to avoid over-discharging the cell and causing permanent damage. It is protecting the battery, not failing.

What to check and do:

Battery health in settings — iOS reports maximum capacity, Android varies by manufacturer. Below roughly 80% is generally considered degraded.

Recalibration — a full discharge then an uninterrupted full charge — can improve the estimate, though it does not restore capacity.

Replacement is the real fix, and it is far cheaper than a new phone.