Why does my phone die at 20% battery?

Your phone doesn't measure charge directly — it estimates it from voltage, and that estimate is calibrated for a healthy battery. As a cell ages, its voltage curve changes shape, so the same real charge level can read as a higher percentage than what's actually left.

The result is a phone that seems to drop from 20% to 0% almost instantly, or shuts off entirely while still claiming charge is left. This is one of the clearest signs of a battery that's due for replacement, separate from how many hours it lasts per charge.

A capacity test tells us the real number in milliamp-hours, not the phone's estimate — that's what we check before recommending a swap rather than going by the percentage alone.