My Samsung Galaxy Note 8 is using Digi data plan to access the Internet.
Since last week, its battery was draining very fast. Battery Monitor Widget Pro showed that even when it is idle and not in use, it lost battery charge at the rate of around 3.5% per hour. Within half a day, 50% of its battery drained off from fully charged, even when not in use!
The Statistics screen in Battery Monitor Widget Pro told me that it was the Android System that drained a lot of battery, but I need more detail to nail down the root cause.
I finally found the culprit with Better Battery Stats. In its Kernel Wakelock screen, I discovered the ril-fd wakelock was keeping my tablet awake most of the time, and it hardly gone into Deep Sleep mode.
A search in the Internet told me this ril-fd wakelock is related to a feature called Fast Dormancy, and it seems that some sort of flaw in Digi base station network has caused this Fast Dormancy feature to drain off battery in my tablet.
Finally, I managed to turn Fast Dormancy off by using FastDormancy Toggle for i9300. Yes, my Samsung Galaxy Note 8 is N5100, but this app seems to work well with other Samsung mobile devices running on Android 4.x as well.
In order to use FastDormancy Toggle for i9300, your Samsung mobile device need to be rooted. After toggling the Fast Dormancy on/off, you need to restart the device and charge the battery up to 100%.
Now, My Samsung Galaxy Note 8 is back to normal, draining battery at the rate of around 0.27% per hour when not in use.
I am running on stock ROM with build number KOT49H.N5100XXDNJ2 (Android version 4.4.2). It is rooted, as I need to gain back full access to its SD card. Now, the root access also enables me to toggle the Fast Dormancy feature to get rid of battery draining problem.