Player turns itself off with battery half full

the battery of my clip+ 8gb lasts about 5 hrs. usage mp3, no radio, no sd card, display brightness turned down a notch. i wonder if the device is defect because the battery lasts so much shorter than the promised 15 hours. this is way off. also when the battery is about half full the player turns itself off. then it reports that the battery must be charged. should i return it for exchange or can it be fixed?

i am absolutely sure it was turned off. this type of shutdown happened every time, not occasionally.