Why does fast charging make a phone hot, and does it hurt the battery?

Fast charging makes a phone warm because pushing more current through a battery generates more heat inside it. Running current through a battery produces internal heat, and the more current, the more heat; charging fast means a lot of current. Whether that hurts the battery depends on how well the heat, and the cold, are kept in check. A fast charger is therefore not just a big power supply; it is a power supply that keeps an eye on temperature.

Engineers have found several ways to go faster without cooking things. One is to raise the voltage rather than the current. Qualcomm's Quick Charge increases the output voltage from the USB charger, which lets more power (watts) travel through the cable without excessive heating, because for the same power a higher voltage means a lower current and so less resistive loss. Quick Charge can exceed the 7.5 watts possible in the older USB standard. Some rivals do the opposite: OPPO's VOOC supplies higher current without raising the voltage and relies on thicker wires to avoid overheating. Quick Charge 2.0 even added an optional feature that splits the power into two streams to reduce phone temperature.

The other half of the story is the battery. High charging rates need the charger to monitor voltage and temperature, and only some battery types can take them. If a lithium-ion battery is charged too fast or too cold, lithium metal can plate on the negative electrode, and the resulting dendrites can short-circuit the cell and cause ignition. Heat is also hard on the battery in the long run: degradation is faster at higher temperatures, and prolonged heat reduces battery life.

So does fast charging hurt the battery? It is a trade of speed against heat and wear. Lithium-ion cells are normally charged in two phases: first a constant current, which fills them quickly, then a constant voltage while the current gradually falls, which is why the last stretch is slow.