Thanks Meter 30
Join Date: Jun 2012
Location: Wrocław PL/Nurnberg DE
slowly decreasing while plugged in happens under heavy use while low power charging like 2.0 USB port or a low mah charger.
Try a more powerfull charger, turn fast charge on if you are on custom kernel and try not to use your phone to heavy.
-You should even get a notification message that your battery is draining faster then charging. Thats what happens to me while heavy usage on my AkkuCase