You should replace your iPhone battery when its health drops to around 80% or when your daily use is clearly affected (whichever comes first).

Key battery health signals

• Check Settings → Battery → Battery Health & Charging; if Maximum Capacity is near or below 80% and you see a “battery significantly degraded” message, it is time to replace it.

• Replace sooner if the phone shuts down unexpectedly, feels noticeably slower, or can’t comfortably last through your normal day even if health is slightly above 80%.

Situations for immediate replacement

• Visible battery swelling (screen lifting, gap in frame) means stop using the phone and replace the battery immediately for safety.

• If the phone is more than about 2–3 years old and you charge it multiple times a day, a new battery will usually restore normal runtime even if health is in the low‑80s.

Where to get it replaced

• Using Apple or an authorized service provider ensures you get a properly specified battery and preserves water resistance as much as possible.

• DIY replacement kits are cheaper but require careful work with adhesives and small screws; plan 30–120 minutes and follow a detailed guide if you go that route.