On iPhone, go to Settings → Battery → Battery Health & Charging. You’ll see a percentage — 100% is new, and Apple recommends replacing the battery when it falls below 80%.

On most Android phones go to Settings → Battery → Battery Usage. Some Samsung devices have a built-in “Battery Protection” setting. For detailed stats you can also dial *#*#4636#*#* in the phone app.

Replace your battery when it drops below 80% health or if you notice your phone randomly shutting down, significant performance throttling, or battery draining in less than half a day with normal use. A battery replacement typically costs $30–$80 and gives your phone a new lease on life.

The best practice is to keep your battery between 20% and 80%. Avoid charging to 100% every night or letting the battery drain to 0%. Use the original charger or a certified one, and avoid wireless fast-charging as a primary method if battery longevity is a priority.

If your phone is less than 3 years old and the only issue is battery degradation, a battery replacement is almost always more cost-effective than buying a new phone. If your phone is 5+ years old and also has screen damage or performance issues, upgrading may make more sense.