Sudden battery drain is nearly always one identifiable app or radio, not general 'aging'. Android's battery screen names the culprit, then you act on it.
- 1Open Settings → Battery → usage since full charge. Any app above ~10% you barely use is a suspect.
- 2Restrict the culprit: tap the app → Background restriction, or uninstall it.
- 3Reduce screen drain: lower brightness, shorter timeout, dark theme on OLED.
- 4Turn off radios you're not using: GPS/Location, Bluetooth, hotspot, NFC.
- 5Check GPS abusers: Settings → Location → recent requests.
- 6Old battery? If drain is universal and the phone is 2+ years old, measure its health.
Tip: After a system update, drain for 1-2 days is normal (re-indexing). Beyond that, review the battery screen again.