Think you might need your iPhone’s battery replaced? You probably do! Here are some things to considering when making that decision.
Check SETTINGS / BATTERY / BATTERY HEALTH. (iOS 11.3 and up, iPhone 6 and up).
Use an app like BATTERY HEALTH (link in comments) and check the wear level.
Getting less run time than in the past? Make sure you’re comparing apples to apples.
Does the phone shut off suddenly then come back on with a arbitrary battery percentage?
Is your phone more likely to shut down in cold weather?
Is your phone getting hot or is their smoke rolling out of it?
Has your phone been in service for more than 3 years without a battery replacement?