iPhone battery health explained: Maximum Capacity, the 80% limit and when to replace

TUREFF• •0

Every lithium-ion battery wears out, and iPhones show you how far yours has gone. Understanding what the numbers mean helps you decide whether to change your charging habits, replace the battery or simply stop worrying.

Where to find it

Open Settings, then Battery, then Battery Health (on recent versions of iOS it is labelled Battery Health and Charging, with charging options on a separate screen). You will see Maximum Capacity and, below it, a note on Peak Performance Capability.

What Maximum Capacity means

Maximum Capacity is how much charge your battery can hold now compared with when it was new. At 90%, a full charge holds about nine-tenths of what it did on day one, so a full battery lasts proportionally less time. It falls gradually with use and age. A drop of a few percent in the first year is normal. The number is an estimate and can move up or down slightly as the phone recalibrates, so do not read too much into a single change.

Peak Performance Capability

As a battery ages it struggles to deliver sudden bursts of power. If an old battery causes an unexpected shutdown, iOS may apply performance management to prevent it happening again, and this screen will say so. If you see that message, or a note recommending a service, a battery replacement usually restores normal performance.

Cycle count, manufacture date and first use

On iPhone 15 and later, the battery screen also shows the cycle count, the battery's manufacture date and its first use date. On earlier models these are not shown in Settings. One cycle means using 100% of the battery's capacity in total, not necessarily in one go. Using 60% one day and 40% the next counts as one cycle. Apple says the iPhone 15 models are designed to keep 80% of their original capacity at 1,000 cycles, compared with 500 cycles for earlier models.

The charging settings

Optimised Battery Charging: available on all recent iPhones. The phone learns your routine, for example charging overnight, and waits at 80% before finishing the charge shortly before you usually unplug. It reduces the time the battery spends at 100%, which is when wear is fastest.
Charging limit (iPhone 15 and later): you can cap charging at 80%. Depending on your iOS version, you may also be able to choose a higher cap in steps between 80% and 100%. With a limit set, the phone stops charging at that level and resumes if it drops a few percent below. Apple notes it will occasionally charge to 100% to keep its battery estimates accurate.
None: the phone charges straight to 100%.

Is the 80% limit worth it? It reduces wear, but you give up a fifth of your daily battery. If you rarely get through a day on a full charge, the limit costs more than it saves. If you finish most days with plenty left and plan to keep the phone for years, it is a reasonable choice.

Charging habits that actually matter

Heat is the biggest enemy. Do not leave the phone in a hot car or in direct sun, and take it out of a thick case if it gets hot while charging. Gaming while fast charging adds heat too.
Avoid long spells at 0% or 100%. Topping up through the day is fine; there is no need to run the battery flat.
Use reputable chargers and cables. Apple's own or certified accessories from known brands.
Fast charging is fine; the phone manages it. It is heat, not speed, that does the damage.
Storing a phone long term: leave it at about half charge, switched off, somewhere cool.

When to replace the battery

As a rule of thumb, a battery below about 80% of its original capacity is considered worn, and the Battery Health screen shows a service message when iOS judges it significantly degraded. Consider replacing it when:

Maximum Capacity is below about 80%.
The phone shuts down unexpectedly or the percentage jumps around.
It no longer lasts the day for your normal use.
The battery screen shows a service recommendation.

A replacement by Apple or an authorised service provider uses genuine parts and keeps battery health reporting working properly. A battery fitted elsewhere may show as an unknown part. Check the current replacement price for your model on Apple's support site, and whether AppleCare+ covers it.

Signs of a real problem

A swollen battery (screen lifting, case bulging) is a safety issue. Stop charging the phone and have it looked at promptly. What is your Maximum Capacity after a year or two, and which charging setting do you use? Share in the comments.