Sansa m240 : Battery goes out with new battery, last 30 minutes.

Hello everyone,
Well i just recently been using my Sandisk sansa M240, and i attach the battrey and everything and ready to start listening to tunes. So, i listen to music for about 30 minutes, then turn it off then head to school. I leave it in my pocket for about the rest of the school day until the last 45 minutes, turn it back on, and its says low battery already ? I have just put a new one in, so i was like okay, must be the battery or something, someone put it back in the box when done using. So i head home, get another triple A battery and listen to it for about another 30 minutes, then plug it into my computer to get some new songs. It only took like 3 minutes, took it back out, its already low battery, so im unable to listen to anything ( there is a warning its on low batery, concluding not letting you listen/ do any actions to the device ) . This is odd, id like to get a solution please, and is it the batteries or the device?device:Sandisk sansaM240 Batteries:Duracell,Konnoc