Hi, there,
Main reason for battery draining is non system Apps which contains ads , installed in the device.
Activate safe mode and check the problem persists,
To activate safe mode,
Press & hold the Power button.
In the box that opens next touch & hold Power off button
You wll see a dialogue "Reboot to safe mode " in the box. Tap OK.
Your device will start in safe mode. You'll see the words "Safe mode" in a box at the bottom.
To cancel safe mode
Restart device.