When I charge fully phone then after some time battery drain fast.
Please follow below steps may improve battery backup:
1. Enable “Power saver” (Settings > Power management)
2. Close apps that are not used (Bluetooth, WiFi, GPS, etc.)
3. Adjust sleep time and brightness (Settings > Display)
4. Boost memory when you see memory consumes is high with no app running,
5. check for background running applications, stop/disable unwanted apps, you can manage apps from auto-start manager.
Same problem with me