Here's a few related threads that may help
http://forums.androidcentral.com/sam...m-i-think.html http://forums.androidcentral.com/sam...ife-note3.html
From what I've learned by owing Android phones, there are many variables that determine and influence battery life. Using GSAM or similar app to understand what processes, features, or functions are consuming battery is the best approach once you've determined in parallel that poor cellular connectivity, heavy screen time, and faulty apps are not the primary sources.
As for Android System consuming 18.2%, you can drill down further to see which process(es) are consuming the most of the 18.2%. I wouldn't worry too much about that value, though, as it's probably normal for the OS to experience utilization fluctuations given what your phone is doing at the time you take a measurement. Keep in mind that the snapshot values may just be a peak in resource utilization.