eli5: why can iphones be so quick to charge to 80% and then slow down, whilst apple watch charges quickly and doesn’t havw to slow down
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To be fair, I am not sure as to whether the iPhone charges to 80% capacity or 100% capacity.
Lithium batteries have two primary methods of charging–constant current or constant voltage (there is actually a third for deeply discharged batteries that we will ignore).
Up to about 80% capacity, lithium batteries are charged by constant current. The battery will be charged at its or the chargers maximum current (which ever is lower).
Once you reach 80% capacity the charging circuitry will change to a constant voltage–the voltage across the batteries terminals is held at the terminal voltage (typically 4.2 or 4.3V for single cell lithium batteries). The voltage of the actual cell is less than this voltage so a current flows into the battery. This current increases the cell voltage until the cell voltage equals the applied voltage. The battery has an certain internal resistance, so thanks to ohms law the current must decrease as the cell voltage approaches the applied voltage. This is why it takes longer.
As a general rule, the first 80% will take about as long as the remaining 20%.
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