It will charge but not hold charge

My Toshiba Thrive tablet will say it is charging, it will have the light and all, but it will not hold the charge. It has done it before and out of the blue it started working again. I didn't think anything of it until it happened again only this time it was taking longer to working again. I want to know whats wrong with it before it becomes permanent. It is older and to my knowledge the battery has never been replaced. Some help would be appreciated.