A phone that reaches half charge in minutes takes far longer to complete the remainder. That deceleration is deliberate, and it follows directly from how lithium cells behave as they fill.
Charging runs in two distinct stages
The first stage holds current constant and lets voltage rise. This is where charging is fastest, because the cell can accept a high rate without its voltage exceeding safe limits.
Once voltage reaches the ceiling, the second stage holds voltage constant and current falls away as the cell fills. The rate declines steadily, and the last portion takes disproportionately long.
Published fast-charging figures almost always describe the first stage, which is why the headline time to a partial charge is far more impressive than the time to full.
The physical limit is lithium plating
Charging moves lithium ions into the negative electrode, and they need time to settle into its structure. If current is too high for the state of charge, ions accumulate on the surface instead.
That deposited metal is not recoverable and permanently reduces capacity. In severe cases it can form structures that compromise the separator between electrodes.
The risk rises as the cell fills, because there is less room remaining for ions to enter. This is the direct reason current must be reduced as charge increases.
Heat constrains everything else
Charging generates heat through internal resistance, and high temperatures accelerate the chemical reactions that degrade a cell over its life.
Phones therefore monitor temperature continuously and reduce charging current when it rises, which is why charging while gaming or in direct sun is noticeably slower.
Case materials and thickness affect how quickly that heat escapes, so identical charging hardware performs differently depending on the physical design around it.
Higher wattage changes where the heat is
Delivering more power at the same voltage means more current and more heat inside the phone. Raising voltage instead moves some conversion loss into the charger.
Splitting the battery into two cells charged in parallel is another approach, since each handles half the current and produces less heat individually.
Software manages the last portion separately
Because keeping a cell at full charge accelerates ageing, many phones pause below full and complete the charge shortly before the user typically wakes.
Optional limits that stop charging short of capacity trade usable runtime for slower degradation, which suits devices that spend long periods connected to power.
Both behaviours reflect the same underlying fact: the final portion of a charge is the least efficient to add and the most costly to hold.