iPhone Battery Health: How to Read It Properly

Published August 16, 2026 · Last reviewed September 2, 2026

The three numbers on that screen

• Maximum Capacity — how much charge the battery holds now, as a percentage of when it was new. This is the number that predicts your screen-on time.
• Cycle Count — how many full charge cycles the battery has been through. Two half-charges count as one cycle, so this is usually lower than people expect.
• Peak Performance Capability — a message, not a number. It tells you whether iOS has had to slow the processor to prevent unexpected shutdowns on a worn battery.

Apple designs recent iPhone batteries to retain up to 80% of original capacity at 1,000 full charge cycles under normal conditions; older models were rated to 80% at 500 cycles. Either way, 80% is the design endpoint rather than a failure point — the phone keeps working below it, with less runtime.

What the number actually predicts

Maximum Capacity maps roughly linearly onto runtime. A battery at 82% gives you about 82% of the screen-on time it gave when new. If a new phone comes off the charger at 07:00 and reaches 20% at 21:00, the same phone at 82% capacity reaches 20% around 18:30. That is the whole effect — and it is why the number matters more than any settings change once it drops far enough.

Below roughly 80%, a second effect can appear. A worn battery cannot always deliver the current spike a fast processor demands, so iOS may reduce peak performance to avoid a sudden shutdown. That is the throttling people remember from the news, and it is disclosed on the same screen. It only applies to batteries that have degraded significantly or already caused an unexpected shutdown.

How to decide

Action

Read Maximum Capacity, then ask whether the phone actually fails to cover your day.

Why it helps

Two people with the same 79% battery can have completely different experiences depending on how heavily they use the phone. The percentage is context, not a verdict.

Verification

You can state both a number and a symptom, for example 'at 78% it dies by 4pm'.

Action

If capacity is above 85% and the phone still dies early, diagnose drain instead of replacing anything.

Why it helps

At that health level the battery is not the constraint. A background app, a poor cellular signal, or a runaway process is.

Verification

The per-app battery breakdown names a specific app with disproportionate background activity.

Action

If capacity is below 80% and the shortfall is real, book a replacement.

Why it helps

A replacement restores full runtime for a fraction of the cost of a new phone, and typically buys another two years of ordinary use.

Verification

After replacement, Maximum Capacity reads 100% and Cycle Count resets.

Action

If a Service or Unknown Part message appears, note which one before acting.

Why it helps

Service means degradation; Unknown Part or Important Battery Message means the battery could not be verified as genuine, typically after a third-party repair. They call for different responses.

Verification

You can quote the exact wording of the message on the screen.

What actually wears a battery out

Lithium-ion cells age through chemistry, not through use in the way people imagine. The three things that measurably matter:

1. Heat. Sustained high temperature is the largest single factor. Charging inside a hot car, gaming while charging, or leaving the phone in direct sun does more damage than any charging habit.
2. Time at a high state of charge. A cell held at 100% for long periods ages faster than one cycling in the middle of its range. This is exactly what Optimized Battery Charging and the 80% charge limit are for.
3. Full cycles. Each complete discharge-and-recharge contributes wear, which is what Cycle Count records.

Notably absent: charging overnight, topping up frequently during the day, and using the phone while it charges in a cool room. None of these are meaningfully harmful on a modern iPhone.

The two settings worth having on

Optimized Battery Charging learns your routine and holds the charge at 80% until shortly before you normally unplug, reducing time spent at full charge. On iPhone 15 and later there is also an 80% Limit option, which never charges beyond 80% at all — useful if you rarely need a full day's charge and want to slow ageing, at the cost of about a fifth of your runtime.

How it works

An iPhone battery is a lithium-ion cell whose usable capacity falls as its internal chemistry degrades. iOS measures this continuously by comparing the energy actually delivered during discharge against the cell's rated design capacity, then reports the ratio as Maximum Capacity. Because the estimate is derived from observed behaviour rather than read from a sensor, it can move by a percent or two after an iOS update or a period of unusual usage — a jump of one point is noise, not a fault.

Cycle count is measured in cumulative full discharges rather than in plug-ins. Using 40% of the battery today and 60% tomorrow records one cycle, not two. This is why a phone charged several times a day can have a far lower cycle count than its owner expects, and why frequent topping up is not the problem folklore says it is.

The performance management behaviour has a specific trigger. A degraded cell has higher internal resistance, so when the processor demands a sudden current spike, the voltage can sag far enough that the device shuts down to protect itself. Rather than let that happen, iOS smooths the peaks by limiting maximum processor frequency — but only after a battery has degraded substantially or has already caused an unexpected shutdown. The state is disclosed on the Battery Health screen, and replacing the battery removes it.

Worked example

A three-year-old iPhone. Battery Health reads Maximum Capacity 79% with a message that the battery's health has significantly degraded, and Cycle Count 842. The owner reports the phone reaching 15% by mid-afternoon, where it used to last until bedtime.

Both halves of the test are satisfied: the number is below the 80% threshold, and there is a real symptom that matches it. At 79%, the phone delivers roughly four-fifths of its original runtime, which is exactly the gap described.

The arithmetic is straightforward. A battery replacement costs a fraction of a new phone and restores the full day; the phone is otherwise fine, still receives iOS updates, and has adequate storage. Spread over the two further years the phone will plausibly serve, the replacement is far cheaper per year than a new device amortised over five. The owner books a replacement.

The verification afterwards is unambiguous: Maximum Capacity 100%, Cycle Count 1, and the phone finishing the day at 40% instead of 15%. Compare that with a second phone in the same household at 91% capacity and a similar complaint — there, replacing the battery would have changed almost nothing, and the real cause turned out to be a podcast app downloading constantly in the background.