What did I do since I detected the issue:
1) I tried changing for a new battery and for a battery in other i9082 that works perfectly
2) I tried restoring the original stock ROM and recovery
3) I tried flashing different custom ROMs (I love CM11) and different Recoveries (I prefer Clockworkmod)
4) I tried all kind of battery calibrators
5) I followed as many tutorials as available for cleaning the battery stats (even deleting the file in data/system), even those in youtube for other phones and ROMs
It seams to be the same issue described originally by Saurabhrajoria at the beginning of this discussion, but I cannot find any other space where to find a definitive solution.
I tried the same steps for you: With 50%, the phone hangs up.
What if I restart it with 70% it back to 20%.
I suspect we have problem with the battery level sensor.
I bought another phone.