iPhone Battery Health, Explained

iOS reports a maximum capacity percentage. It is one of the few genuinely informative numbers a phone gives you about its own condition, and it is widely misread.

What the number means

It is the current maximum charge the battery can hold, as a percentage of what it held when new.

At 100 percent, the battery holds what it was designed to hold.

At 85 percent, a full charge gives you 85 percent of the runtime you had on day one.

Below 80 percent, iOS considers the battery consumed. Apple's own warranty threshold sits here, and the phone will suggest service.

The number only decreases. It is not a reading you can improve. Reports of it rising are measurement recalibration, not recovery.

What the number does not tell you

It does not predict sudden shutdowns directly. A battery can show 88 percent and still fail under load if internal resistance has risen. If the phone shuts down at 30 percent in cold weather or while using the camera, the battery is failing regardless of the percentage.

It does not account for your usage. An 85 percent battery in a phone used lightly is fine. The same battery in a phone used for navigation and video is not.

It is an estimate. It is derived from charge cycles and behaviour rather than measured directly, and it can drift.

Peak performance capability

Alongside the percentage, iOS reports whether the battery can supply the current the processor demands.

If it reports that performance management has been applied, the phone is deliberately slowing down to prevent shutdowns. This is the feature that caused considerable controversy, and it is now disclosed and can be disabled.

If it appears, the battery needs replacing. Turning off performance management removes the slowdown and reintroduces the shutdowns.

What actually degrades a battery

In rough order of impact:

Heat. The dominant factor by a wide margin. A phone left on a car dashboard, charging under a pillow, or used for navigation in direct sun while charging will lose capacity measurably faster. Heat during charging is worse than heat alone.

Sitting at 100 percent. Lithium cells age faster held at full charge. This is why both platforms now delay the final portion of charging overnight — the phone reaches around 80 percent, waits, and completes the charge shortly before you wake.

Deep discharges to zero, repeatedly. Occasional is fine; habitual is not.

Charge cycles, which is simply use. A cycle is a cumulative full charge, so two half charges make one cycle.

Fast charging, modestly. The effect is real and much smaller than heat.

What does not matter

Charging overnight. The phone stops at full and manages the rest. This was a real concern with battery chemistries from decades ago.

Charging to less than full, or topping up during the day. Partial charges are fine and arguably preferable.

Using the phone while charging. Fine, unless it gets hot.

Third-party cables and chargers, provided they are properly certified. Cheap uncertified chargers are a genuine hazard, for reasons of fire rather than battery longevity.

Letting it fully discharge to "calibrate". Not applicable to modern batteries and mildly harmful.

Closing apps to save the battery. Does nothing for battery health and slightly increases power use.

Practical habits that help

Keep it out of heat. Do not leave it in a hot car, do not charge it under bedding, and remove a thick case if the phone gets hot while charging.

Enable optimised charging so it does not sit at 100 percent overnight.

Charge when convenient rather than following a rule. The habits that matter are about heat, not about hitting a particular percentage.

When to replace

Below 80 percent capacity, if the runtime bothers you.

Any sudden shutdown at a non-zero percentage, regardless of the reported percentage.

Performance management has engaged.

Visible swelling — a screen lifting at an edge, a case that no longer sits flat. Stop using it and get it serviced immediately. A swollen lithium cell is a fire risk.

A battery replacement costs a fraction of a new phone and restores the device to full runtime. For a phone that is otherwise working, it is usually the correct decision, and it is frequently deferred by people who assume the whole phone is worn out.

Third-party replacements

Independent repair is legitimate and often cheaper. Two things to know: iOS may display a message noting the battery cannot be verified as genuine, and battery health reporting may become unavailable depending on the part used.

Neither prevents the phone working. Whether that trade is acceptable is your call, and worth asking about before the work rather than after.