The two signs that settle it
Maximum Capacity below 80%, in Settings → Battery → Battery Health & Charging. That is Apple's own threshold for a consumed battery, and it is where most people describe the phone as no longer lasting the day.
Performance management is active. If Peak Performance Capability reports an unexpected shutdown and that iOS has applied performance management, the phone is being deliberately slowed to stop it dying under load. That is not a software problem and no setting fixes it.
Other symptoms worth acting on
An iPhone that shuts down suddenly at 20 or 30% charge. One that drops ten or fifteen percent in a single moment. One that will not turn on unless plugged in. And any physical swelling — a screen lifting away from the frame, or a phone that no longer sits flat.
Swelling is the one to act on immediately rather than schedule. Stop charging it and get it looked at.
What it costs
Apple's out-of-warranty battery service is a fixed price per model, and it is a fraction of a new phone — typically well under a fifth. AppleCare+ covers it at no charge when capacity is below 80%. Apple's own page lists the current price for each model.
Third-party repair is cheaper, sometimes considerably. The trade-off is below.
Genuine versus third-party parts
A battery that iOS cannot verify as genuine produces an Important Battery Message in Settings and, on many models, no battery health data at all — Maximum Capacity and cycle count simply stop being reported. The phone works, but you lose the ability to monitor it.
Apple's own service and Independent Repair Providers use genuine parts and keep the reporting intact. A reputable independent shop using genuine parts is a reasonable middle ground; the very cheapest options generally are not.
Is it worth it on an older iPhone
Usually yes, and by a large margin. A battery replacement on a four or five-year-old iPhone that still receives software updates transforms it, for a small fraction of replacement cost. The question to ask is not the phone's age but whether it still gets iOS updates.
Our compatibility checker answers that for your model, and our support lifespan data shows how many versions each model receives. A phone with two more years of updates ahead of it is worth a new battery; one already at its last version is a closer call.
Before you hand it over
Back up first — a repair can involve a restore, and you do not want that to be the moment you discover your last backup is six months old. Our backup check guide takes five seconds.
Turn off Find My before service if asked to, and know your Apple Account password. A phone with Activation Lock on cannot be serviced by most providers.
Frequently Asked Questions
When should I replace my iPhone battery?
When Maximum Capacity is below 80%, or when Peak Performance Capability reports that performance management is active. Either alone justifies it.
How much does an iPhone battery replacement cost?
Apple charges a fixed out-of-warranty price per model, typically well under a fifth of a new phone. AppleCare+ covers it free below 80% capacity.
Will a third-party battery cause problems?
A battery iOS cannot verify shows an Important Battery Message and usually stops reporting health data entirely. The phone still works.
Is replacing the battery worth it on an old iPhone?
Usually yes, if the model still receives iOS updates. Check your model's remaining support before deciding.