How long should an iPhone battery last?
Apple rates iPhone batteries for roughly 500 full charge cycles before capacity drops to about 80 percent — for most people that lands somewhere between the second and third year of ownership. In the Alabama summer that timeline shortens, because heat is the single hardest thing on a lithium-ion cell. A phone left in a car on a July afternoon in Phenix City ages more in a week than in a normal month.
Once Battery Health falls below 80 percent, iOS begins throttling peak performance to prevent unexpected shutdowns. That is why an aging iPhone feels slow: the processor is being held back by the battery, and a $59 to $99 replacement usually restores full speed.