Hi
SridharmaniThank you for your asking. As other user mentioned, battery aging is a normal behavior.
1. If you have any concern, please practice the Factory Reset, enter "Safe Mode" and leave a 100%-charged phone in sleep(idle) mode in a night, then provide me the screenshot of
①Setting>Battery
②Setting>Battery>3 dots on the top"Show app usage"
# How to enter and exit "Safe Mode"
https://www.asus.com/support/FAQ/1005163/If the battery works well in "Safe Mode", please check on any disturbance from the installed 3rd party Apps.
2. Is there any zone heating up while using/idling? How long has the battery been used?
3. Kindly refer to the link below for further information of battery usage tips
[Phone/Pad] Tips for battery charging and storage | Official Support | ASUS Global[ZenFone/Pad] Power saving skills | Official Support | ASUS Global