stupid sony ericsson phone

My phone kept dying more frequently the past few days T_T Yesterday alone it kept dying for dunno how many times, whenever I wanna check the time or msg and took it out from my pocket, it's dead. It's not as if there's no battery left, it's still at least 50% full. This morning, after I charged for one whole night, it dieded again mid way when i was checking the calendar >.< I need to send it to repair dy...