I have a rarely used polariod pdv-0813a portable dvd player and the large battery that hooks to the bottom of the machine won't hold a charge. I used to keep it plugged to the AC adaptor with the battery attached. Because I wasn't using it I unplugged it. Recently I took it out to use it but the battery doesn't stay charged. It only plays for about 15 minutes and then the *low battery* icon pops up, within a minute the unit stopped playing.