iPhone battery health explained: what the percentage really means

Battery health is the single most misunderstood number in phone buying. It is not how much charge the phone has, and it is not a countdown to death. It is a measure of maximum capacity: how much charge the battery can hold now compared with when it left the factory.

A phone at 90% battery health holds 90% of its original capacity — meaning a battery that once lasted ten hours now lasts nine. That is the entire mystery, demystified.

How to check it in 30 seconds

On any iPhone: Settings → Battery → Battery Health & Charging. The 'Maximum Capacity' figure is the number everyone quotes. Below it, 'Peak Performance Capability' tells you whether the battery can still deliver full power without the phone throttling itself.

When you buy from us, that check has already been done: every phone on Optimise Marketplace lists its battery health on the product page, so you know before you pay, not after.

What the numbers mean in practice

100–95%: effectively new. 94–85%: excellent daily performance; the difference is measured in minutes, not hours. 84–80%: fine for lighter users — Apple's own threshold for 'consider a replacement' is 80%. Below 80%: the battery is consumable-grade; the phone works, but plan a replacement.

Two useful truths: batteries degrade fastest in their first year and then slow down, and a battery replacement resets the clock entirely — an iPhone 13 with a new battery will outlast the same phone that has sat in a drawer 'saving' its original one.

Habits that genuinely slow degradation

Heat is the enemy — more than charging habits, more than anything. Avoid leaving the phone in direct sun, on a dashboard, or under a pillow while charging. Enable Optimised Battery Charging (it learns your routine and holds at 80% overnight). Do not obsess over keeping charge between 20% and 80% — it helps at the margins, but Apple's charge management already does the heavy lifting.

When replacement beats upgrading

If your phone does everything you need but dies by dinner, you have a battery problem, not a phone problem. A professional battery replacement costs from around £49 — roughly 5% of a new flagship — and takes under an hour. Get an instant fixed price for your exact model with our repair quote tool.