Updated October 2026 by the Go Mobile repair team
Quick answer
A phone battery usually needs replacing when it drains fast, shuts off at 20 or 30 percent, makes the phone slow, gets hot or only works plugged in. On iPhone, check Settings > Battery > Battery Health. On Samsung, use the battery diagnostic in the Samsung Members app. A swollen battery is a safety issue: stop charging the phone and get it checked right away.