Battery drop from 50% to 15% every time

And after draining you also charged it
1.) without turning on the phone AND
2.) up to >80% AND
3.) in one run`as he wrote?
Then what is the result?
How long did it take? (a complete charging cycle with the original 1A-charger should last about 1.5-2h)

Maybe you can check with an app of your choice (e.g. GPS Status and Toolbox) the battery voltage in dependence of the percentage and post here some vaules for it at 80/60/55/50/40…0%? Than we can compare with a working device to check if we can look for a problem here.