Technicians who advise "never leave your phone plugged in overnight" misunderstand modern microcontrollers. Modern BMS chips (like Qualcomm PMICs or Apple custom silicon) physically disconnect current or drop into micro-amperage trickle maintenance as soon as the cell hits target voltage. You cannot overcharge a functioning smartphone.
If engineers actually permitted lithium cobalt oxide cells to absorb 100% of physical capacity, the voltage would exceed 4.35V–4.45V, precipitating metallic lithium plating, dendrites, and rapid SEI layer degradation. The BMS displays 100% while keeping physical reserve headroom around ~95-97%.
The "drain to 0% before charging" rule originated from Nickel-Cadmium (NiCd) and NiMH batteries that suffered crystal memory effects. In modern Lithium-Ion cells, draining to absolute 0% causes copper foil dissolution and catastrophic degradation. Modern BMS will force shut down long before true zero.
High charging current produces internal heat (P = I²R). Sealed smartphone cells undergo reversible gas evolution during fast charging; manufacturers design hermetic casings that reabsorb volatile byproducts during the cool-down CV tapering phase.