in the battery stats "android os" takes 16%, "android system" takes 14%. How can i know why?
Those percentages are from apps that use Android functions instead of their own modules (to save time/space/compatibility). The battery report bundles all battery usage from those Android functions and puts it into those two entries (depending on function). You can install an app like Wakelock Detector to see what exactly is using your battery the most in the background.
According to this post by Joe Levi at PocketNow:
“Android OS” and “Android System”
1% and 2% are nothing to be worried about, but that’s just what my device is reporting now. By the end of the day those numbers will be much different. Some users are even reporting that either of those two processes make up 30- or 40-percent of the battery usage on their phone or tablet. Something’s not right there, but what is it?
An operating system is a pretty big “thing”. It’s got tens of thousands of lines of code, acts as an abstraction layer between hardware and software, and often includes a bunch of apps and services all bundled together.
When an app wants to know where you are, it asks Android. When your battery stats are reported, it’s “Android” that used the battery to determine your location, not Facebook, Twitter, Google+, Waze, or whatever other geo-aware app you were using. Location awareness is just one example. We could talk about apps that use data from the barometer, pedometer, light sensor, magnetometer, etc,. but geo-location is probably the easiest to talk about to illustrate the concept.
Everything is behaving as it should be, but your battery report isn’t being as helpful as you’d like it to be. You want to know what apps are using more of your battery than you’d like them to, then put pressure on their developers to fix it, right? We’re not there just yet, and even with the advancements in battery reporting coming in Android 5.0 Lollipop, tracking down the power gobbling culprits isn’t quite as clear cut as we’d like it to be. Not yet anyway, but we’re making progress!