Uneven Battery Drain in 2nd half of discharge

[mrmessma]

I have an extended battery for my GN. I will go for 3 or 4 hours of internet radio using 20% battery. By the time I get to 50% battery, I will not use the phone hardly at all, and then the battery level just drops precipitously. I kept the battery on the charger for over 12 hours (the first time) as is suggested for Li-Ion, not cycling full to dead 6x (Ni-Cd) which is bad for a Li-Ion. I also ran a Battery Calibration App without success. The overall battery life is great, as I could never imagine running all the data, sync, google location, google now, and internet radio on a single battery, but I'd like an even show of battery status. Do you think I have a faulty cell?