A phone that needed one charge a day and now needs two has a number attached to it, and the number is on the phone. Reading it correctly saves you from replacing a battery that is fine, and from nursing one that is finished.
Where the number is
Open Settings, tap Battery, then tap Battery Health. The figure at the top is Maximum Capacity. Apple documents this screen in its page on iPhone battery and performance.
Apple describes maximum capacity as the battery capacity relative to when the phone was new. A battery holds less as it ages chemically, which shows up as fewer hours between charges. Apple also notes that a new phone can read slightly under 100 percent, depending on how long it sat between manufacture and activation.
What counts as worn
Apple states the design targets. Batteries in iPhone 14 models and earlier are built to keep 80 percent of their original capacity after 500 complete charge cycles under ideal conditions. For iPhone 15 models the figure is 80 percent after 1000 cycles.
Those are design targets and not promises about your phone, because ideal conditions are not the ones a phone lives in. They are useful as a yardstick: a two year old phone well under 80 percent is ageing faster than Apple planned for, and that is worth knowing before you spend anything.
When it says Service
Apple is direct about this one. If Service appears next to Battery Health, the advice is to consider replacing the battery to restore full performance and capacity. That word is the phone telling you the chemistry has degraded past the point where the software can hide it.
Until then, a capacity figure on its own is not an instruction. A phone at 85 percent that gets you through the day does not need anything.
When the battery is not the problem
Three cases look like a worn battery and are not. A phone that drains fast only in one building is hunting for signal, which costs more than any app. A phone that got noticeably worse the week an update arrived is usually reindexing in the background, and that settles in a few days. And a phone that is hot while idle has something running, which the Battery screen will name if you scroll to the list of apps below the health figure.
Checking those first costs nothing. Replacing a battery that was never the problem costs the price of a battery.
If the menu is not where this says
Battery Health has moved once: on newer software it is Battery Health and Charging, and it holds the charging options as well as the capacity figure. If you see no Battery Health entry at all, the phone is on software old enough that Apple had not added the screen yet, which by itself tells you the battery has had a long time to age.
If the number says the battery is fine and the phone still feels slow, the thing to check next is how much software it is still getting, because that decides whether any of this is worth fixing. When a phone stops getting updates is the same arithmetic on the Android side, with the manufacturer's own dates.
Last updated October 4, 2026