Battery Impedance Is Your Worst Enemy
So, what exactly happens when a battery gets older. Well, its impedance increases – a combination of resistance and other important chemical factors that affect how easily the battery can transfer current over to the iPhone’s other hardware. As the battery struggles to transfer power, voltage drops and the battery may, eventually, even endanger the phone.
However, Apple has designed iOS to be aware of battery impedance and bring up notifications if it looks like a battery is dying. iOS is also designed to mitigate current and software operation when it looks like the battery is in trouble – which has had unpleasant ramifications, like the iPhone slow down scandal that caused Apple to drop all its battery replacement costs.
You can have your iPhone tested at an Apple Store and choose to have it replaced at any time, which may be especially worth it if you have charged 500 cycles or more. Check your iPhone battery’s health in Settings > Battery > Battery Health.