The battery indicator for 5800xm is not linear type, mean it will hang very long for the top 7 bar but when it starting drop to 6 bar u better beware because from my experience 6 bar ~70% batt left and 5 bar ~50% 4 bar ~40% 3 bar ~30% 2 bar ~20% and 1 bar ~10%.
So if your phone left 2 bar it mean u only have ~20% juice left, I think although u off your bt/wifi but u can't be sure what backgroud apps that might trigger those wifi/bt to be on back or if u set it to silent mode (vibration on) it may be midnight got someone call u and make the phone dead. 

It is possible to install quick office and pdf reader in NK 5800 ?