I have used
Blackberry 8520, I must have to say 8520 is a good phone with full qwerty keyboard although it has Standard battery, Li-Ion 1150 mAh and its supports Stand-by Up to 408 hours & Talk time Up to 4 hours 30 min, I think you have to charge it correctly try to put it on charge up to 6 to 7 hours in night. if still its not responding just take it to your mobile provider.