Cell Phone Batteries

How is it that cell phone batteries always die at the most inconvenient of times? It's not like they have one certian lengh of time that they last and then they die, at least not for me. My phone will stay on for days and days at home and then when I go to the movie store and try to call my husband to consult which movie to watch, the cell phone dies? And now, as I'm expecting a call to come in, one that I've been expecting for days, my phone calmly announces that it won't make it. GEEZ!!!!!