iPhone battery health explained: what the percentage means and when to replace it

Open Settings on any iPhone, tap Battery, then Battery Health, and you will see a single percentage. On a refurbished phone that number gets more attention than the camera, the storage or the colour, and with good reason: it is the clearest sign of how much life is left in the part that wears out first. This guide explains what the figure means, how to read it, and what to expect from a refurbished iPhone.

What “Maximum Capacity” measures

Every lithium-ion battery loses a little capacity each time it is charged and discharged. iOS tracks this and shows it as Maximum Capacity: the amount of charge the battery can hold today compared with when it was new. A phone at 90% can store nine tenths of the energy it did on day one, so it runs for roughly nine tenths as long between charges under the same use.

The figure is not a measure of speed, signal or camera quality. A phone at 85% works exactly like a phone at 100%; it simply needs charging a little sooner.

How to check it on the phone

• Open Settings → Battery → Battery Health (on newer iOS versions the menu is called Battery Health & Charging).
• Read the Maximum Capacity percentage.
• Below it, iOS shows whether the battery still supports Peak Performance Capability. A battery in poor condition can trigger performance management, which slows the phone to prevent shutdowns.
• On iPhone 15 and later, the same screen also shows the cycle count, the number of full charges the battery has been through.

What counts as a good figure?

Apple designs the batteries in iPhone 14 models and earlier to keep 80% of their original capacity after 500 complete charge cycles, and iPhone 15 models after 1,000 cycles. Apple treats 80% as the point at which a battery is considered worn and eligible for a service replacement. Working from that:

This is why our battery health standards set a minimum of 85% for every refurbished iPhone we sell. Phones in the Premium cosmetic grade usually come with 90% or more.

What happens when a battery is below the minimum

When a phone arrives at our workshop with a battery under 85%, we fit a new battery before the phone is listed. The replacement is a new third-party battery, not an Apple part. It is tested in the same way as the rest of the phone and it is covered by the same 12-month warranty. Because the part is not made by Apple, iOS will show it as “Unknown” under Settings → General → About, and the Battery Health screen may say it cannot verify the battery. We explain those messages in detail in our article on the “Unknown Part” message.

Five habits that keep a battery healthy

• Avoid heat. Charging in direct sun or under a pillow ages a battery faster than anything else.
• Keep it between roughly 20% and 80% when convenient. Full discharges and constant 100% charging both add wear.
• Use Optimised Battery Charging (Settings → Battery → Battery Health). iOS learns your routine and pauses at 80% overnight until you need the phone.
• Use a decent charger. Any certified USB-C or Lightning charger is fine; very cheap uncertified ones can run hot.
• Update iOS. Battery management improves with software updates.

Battery health and the warranty

Every refurbished iPhone from Rainbowmobile ships with at least 85% battery health unless the product page says otherwise, and battery health is shown on the listing whenever it is available. If the battery drops below 80% within 12 months of delivery, it is covered by our warranty: see the returns and warranty policy for the full terms. You can also read how we test every phone on our condition grades page.

Common questions

Is 85% battery health good for a refurbished iPhone?

Yes. At 85% a phone still comfortably lasts a day of typical use, and it is well above the 80% point at which Apple considers a battery worn. It is the minimum we accept for every phone we sell.

Does battery health go down quickly?

A typical battery loses around 1% of capacity every 25 to 50 cycles, depending on heat and charging habits. Most people see a drop of a few percentage points a year.

Can battery health be reset or faked?

The figure is stored in the battery’s own controller and read by iOS. A new battery starts at 100%, so a high figure on an older phone usually means the battery has been replaced, which is not a problem as long as the seller says so.

Why does my phone say it cannot verify the battery?

iOS shows that message whenever the battery is not an Apple part. It appears on phones with a third-party replacement battery and does not stop the phone or the battery working normally.