Updated
Quick fix
Open Settings > Battery to see which apps use the most power, update or limit those apps, lower screen brightness, and turn on Low Power Mode (iPhone) or Battery Saver (Android). Check battery health; older batteries simply hold less.
Battery drain usually comes from one or two apps, the screen, or a weak signal, and both iPhone and Android show you which.
Step by step
Check battery usage by app
On iPhone and Android, open Settings > Battery to see which apps used the most battery. An app you barely use near the top is a red flag.
Update or remove the worst offenders
Update the app, sign out and back in, or uninstall it if you don’t need it.
Lower the screen’s power use
The screen is usually the biggest drain. Turn on auto-brightness, shorten the auto-lock or screen timeout, and use dark mode if you like it.
Limit background activity
On iPhone, review Settings > General > Background App Refresh. On Android, check each app’s battery setting (names vary by manufacturer).
Turn on power saving
Use Low Power Mode on iPhone or Battery Saver on Android when you need the battery to last.
Watch weak signal areas
A phone searching for signal uses a lot of power. In a building with poor cell coverage, use Wi-Fi and Wi-Fi Calling if your carrier supports it.
Update and restart
Install the latest system update, then restart the phone.
Check battery health
On iPhone, open Settings > Battery and look at battery health. Many Android phones show battery information in Settings or the manufacturer’s care app. A worn battery can be replaced by the manufacturer or an authorized shop.