Why does my battery die when I leave my key in the

You have a bad battery. When you leave the key in the on position you are activating the ignition system which uses current to charge the coil. A good battery can stand the strain for quite awhile, the fact yours dies after a few minutes means the battery is bad. Replace it.. Also note its not a good idea to leave the ignition in the on position for more than a few moments when you start the car. Use the accessory position if you need to have the CD player working etc.

Take the battery to Sears or another large auto parts store and ask them to check the condition .They do it for free. I think its got a problem.