I have a TI-nsipre that's not very old and I've really only used it once or twice. Other than that, it's stayed in a drawer in my desk untouched. When I first got it, it worked perfectly. Now it won't hold a charge and I have to keep it plugged into the USB charger and it just stays at 5% battery. Everything else if functional but I don't know why it won't charge. Does this mean I need a new battery?