I bought a replacement battery from ifixit and installed it, then I followed the instructions: "Calibrate your battery...

So the solution for me was setting the hibernation mode to 25 instead of 3 by executing following command: `sudo pmset -b hibernatemode 25` Now the macbook goes into hibernation every time it reaches 5% and my work gets saved. This hibernation mode is "deep" and it saves al the RAM content as an image to the disk.