Why does my computer not draw power from the battery?

My PC has stopped drawing power from the battery and will only draw power from the Power Cord jack. When plugged in, the computer will detect the battery and say its charging, but as soon as I remove the power cord, my computer turns off like the power supply has been cut (battery still in computer). I saw the Jack replacement topic and I might have to do that to my computer, but what about the computer being unable to draw power from battery. Also, when I remove the battery it is warm, like it's been used. Plus, please excuse the grammar, the computer works plugged in with the power cord and without the jack.