A. Wipe Cache Partition. Steps to perform. 1) Turn off the device. 2) Press and hold the Volume Up key and the spower button. 3) When the Android logo displays, release all keys. 4) Press the Volume down key to navigate and select "Wipe cache partition". 5) Press Power key to select. When the wipe cache partition is complete, Select Reboot System. 6) Press the Power key to restart the device.
B. Reset App Preferences. Settings >Apps> Tap on Top Right 3Dots and select Reset App preferences. This can Help you little bit more. But if unfortunately it won't work then last option is
3. "Factory Data Reset" but before that take proper backup.
3. one of the main reasons is even you close apps but they uses ur battery and running in background unnecessary, see image and read the statement. 2nd due to huge notification enable. for e.g ; Google has huge notification which we don't need at all see image just scroll down image and check how many notification it does have. So do some more settings to save more battery, which consume unnecessarily.