What the percentage actually measures
Battery Health in Settings is maximum capacity, expressed against the capacity the battery had when it left the factory. At 100 percent the cell holds what it was built to hold. At 85 percent it holds 85 percent of that.
It is not a measure of how good your battery is at delivering power, and that distinction is where most of the confusion comes from. A cell can read a healthy number and still be failing, because capacity and the ability to deliver current under load are two different things.
The number Apple treats as end of life
Apple rates an iPhone battery to hold 80 percent of its original capacity after 500 full charge cycles, and treats 80 percent as the end of the rated life. Below that, Settings starts showing a service message.
Most people notice the difference well before 80. Somewhere in the mid to high 80s a phone stops making it through a full day, and that is usually the point where replacing the cell makes the phone feel new again.
When the number is lying to you
The symptom that matters more than the percentage is a phone that shuts down with charge still showing. If your iPhone dies at 30 percent, or drops from 40 to 1 in a step, the cell is no longer delivering current under load. That happens at readings well above 80, particularly in cold weather.
The other tell is a phone that only works plugged in. That is a battery at the end, not a charging fault, though the two look identical from the outside and are worth a free diagnostic rather than a guess.
- Shutting down above 20 percent
- Charge dropping in steps rather than smoothly
- Only running while plugged in
- The screen or back lifting away from the frame
- Getting noticeably hot doing nothing
Why a swollen battery is different
If the screen is lifting at one edge, or the phone rocks on a flat table, stop charging it. A swollen cell is a chemical failure and the swelling does not reverse. The risk is not dramatic, but a split cell in a pocket is worth avoiding, and the pressure will eventually crack the screen from underneath.
This is the one battery fault we treat as same-day regardless of how busy the bench is.
What happens to the number after a replacement
iOS shows a service notice instead of a health percentage for any battery Apple did not fit itself, including genuine Apple cells fitted outside their network. It is a message about provenance, not a fault, and the battery works normally.
If seeing the percentage matters to you, say so before we start and we will talk through the options. Most people care about the phone lasting the day.