first, install something like GSAM: https://play.google.com/store/apps/details?id=com.gsamlabs.bbm&hl=en
or better battery stats (BBS): https://play.google.com/store/apps/details?id=com.asksven.betterbatterystats&hl=en
charge to full, unplug, and let it do its thing for a day and see what is...