iPhone battery replacement: the signs it is time, and how to squeeze more life out of the one you have

Start with the number: Battery Health and Maximum Capacity

Before you judge anything by feel, look at the number Apple gives you. Open Settings → Battery → Battery Health & Charging. The headline figure is Maximum Capacity, a percentage against the battery the phone shipped with. A new battery reads 100 percent; after a couple of years it typically settles in the high or low 80s.

Apple designs the cell to keep about 80 percent of its capacity through roughly 500 charge cycles on older models, and 1,000 on the iPhone 15 and 16 families. Once it slips below 80 percent, the phone marks the battery worn and may show a Service message — the textbook line for replacement, but a starting point, not gospel.

Here is the nuance most guides skip: two phones can both read 84 percent and behave completely differently. Capacity measures how much charge the cell holds, not how well it delivers it under sudden load — a battery with a high reading but rising internal resistance will still shut the phone off. So treat the percentage as evidence, not a verdict, and pair it with the lived-in signs below — or walk in and we will pull a full battery report free as part of an iPhone repair diagnosis.

The seven signs your iPhone battery is done

Across thousands of batteries on our bench, the same symptoms recur. Tick two or more and the cell — not iOS, not your apps — is almost certainly the culprit:

• It no longer lasts the day. The most common complaint: a phone that used to reach bedtime now begs for a charger by mid-afternoon.
• The percentage drops in jumps. It falls from 40 to 25 percent in minutes, or sticks then plunges — the gauge can no longer read a tired cell.
• It shuts down with charge to spare. The phone dies at 20 or 30 percent, especially when you open the camera or a demanding app — covered below.
• It runs hot doing little. A worn cell has higher internal resistance, so persistent warmth on light use is worth checking.
• The phone feels sluggish. iOS throttles performance to protect a weak battery from crashing, so slowdowns can be a battery signal, not an ageing chip.
• Maximum Capacity is below 80 percent or the Service message has appeared.
• The screen or back is lifting — the most serious sign of all, covered next.

None on its own proves the battery is finished — but a cluster that persists is the tell, and a free walk-in reading settles it in minutes.

Swelling: the one sign you must never ignore

Everything else on this list is about convenience. Swelling is about safety. When a lithium cell ages badly — or is overheated, overcharged by a dodgy cable, or exposed to moisture — it can produce gas internally and expand. On an iPhone that shows as the display lifting at one edge, a gap around the frame, or the phone rocking on a flat table. It is a genuine fire and injury risk — urgent, not a someday job:

We see swelling most in phones two to four years old, charged overnight on cheap bricks, or in units that took a splash months earlier. It is a straightforward fix with the right tools, but not a DIY job. If your phone has also taken water, mention it — corrosion can complicate the swap, and our iPhone water damage repair notes are worth a look first.

Sudden shutdowns and the throttling trade-off

Two symptoms are worth their own explanation, because people blame the wrong thing.

Random shutdowns with charge left. A healthy battery holds its voltage steady under load. An ageing one cannot — when you fire up the camera or a game, the current demand makes the voltage sag below what the phone needs, so iOS shuts it off to protect itself, even though the gauge read 30 or 40 percent. A fresh cell cures it.

The slowdown you can feel. iOS includes performance management: when a battery is too weak to supply peak power without crashing, the system caps performance to keep the phone stable. The same Battery Health screen shows a note when it is active, with an option to turn it off — but that risks the shutdowns above. The real fix is a new battery, which removes the limit and restores full speed. So if your phone feels slow and the battery is worn, you may simply be feeling a tired cell rather than an ageing chip. Our rundown of common iPhone faults covers how to tell battery-driven slowdowns from storage or software ones.