How can I diagnose the battery issue? My phone's OS is cyanogenmod 12.1 with Android version 5.1.1.

Recently I found a battery drain. I set up the airplane mode but after a night the battery went from 100% to 41%. I viewed Settings->Battery and found that the sum of battery usages of all apps was much lower, approximately 20%. I need to know how to fetch a verbose log for the battery usage in order to diagnose the issue.

Thanks very well.
Don't know about the verbose log but for the battery drain issue, i may suggest you to try the 'Battery Calibration' app(requires device to be rooted). I own a Samsung galaxy S3, had similar issues but found significant improvement after following the steps detailed in the app.