Gsam battery monitor https://play.google.com/store/apps/details?id=com.gsamlabs.bbm&hl=en
Accubattery - allows you to have calculation of possible capacity remaining on your battery. https://play.google.com/store/apps/details?id=com.digibites.accubattery&hl=en
In the past I had problem with one Samsung S4. Happened that during 4 hours it drained from 90% to 22% in my pocket. Problem seemed to be an application that wanted constant connection to internet at the time. I just deleted it. It can be because of turned on Bixby or something of similar caliber.
