My battery dies very quickly I've had this issue a couple of months ago too but then I could fix it by turning off an option at my WiFi settings.

Now the problem is back but it my previous solution doesn't work.

I own a Samsung Galaxy S5 and I've had it for about a year now. It runs on Android 4.4.2. At some point the battery started to die really fast, and I don't have a clue what the reason could be. When I check the battery usage, it says "Android-System" uses about 30-40% of my battery, but I don't have a clue where this service comes from or how to kill it.

Has anybody got a solution for this?
Sounds like you could use a factory reset. This will reload your operating system. After you do that I would load applications one by one and see if any of your apps are causing the battery drain.

Here is a guide on how to factory reset your phone:

http://www.digitaltrends.com/mobile/factory-reset-galaxy-s5/