A phone cannot measure how much charge remains in its battery. It estimates, and the estimate is built from indirect signals that drift.
There is no fuel gauge inside a cell
A lithium cell has no float or dipstick. Nothing inside it reports a quantity of stored energy that could simply be read out.
What can be measured is voltage across the terminals, current flowing in or out, and temperature. Every displayed percentage is derived from those three.
The derivation involves a model of how the specific cell chemistry behaves, which means the number is a calculation and not an observation.
Voltage maps onto charge unevenly
Cell voltage falls as charge is used, so voltage can indicate remaining capacity. The relationship is not a straight line.
Across the middle of the range, voltage changes very little for a large change in charge, making it a weak signal exactly where most of the capacity sits.
Near empty and near full the curve steepens, which is why readings at the extremes tend to move faster and feel less stable.
Counting current fills the gap
Between the useful voltage regions, the system tracks how much current has flowed out and subtracts it from a known starting point.
This is accurate over short spans but accumulates error, since small measurement inaccuracies add up over hours and no correction arrives on its own.
The gauge periodically resynchronises against voltage when the phone reaches a full charge or a very low state, which is when a sudden jump becomes visible.
Load and temperature move the reading
Drawing heavy current pulls voltage down temporarily. A phone under sustained load can appear to have less charge than it does at rest.
Cold has a similar effect, raising internal resistance so the cell delivers less usable power until it warms, sometimes producing an abrupt shutdown.
Neither case reflects lost energy. The charge returns to the reading once the load drops or the temperature recovers.
Ageing shifts the reference point
Total capacity declines as a cell ages, so a full charge stores less than it once did while still displaying one hundred percent.
Software tracks this decline and rescales the percentage against current capacity rather than original capacity, which keeps the display meaningful.
The consequence is that percentage becomes a less reliable proxy for hours of use over a device's life, even when the gauge itself is working correctly.