If there was such a thing you'd have to leave the app running for it to even work. Battery monitoring doesn't fall under one of the 7 multitasking api's.
Though there is a ray of hope, apparently you aren't the only one who has thought of this.
"Get a cell phone that tells you to unplug it. Nokia will be rolling out new phones with audible alerts that say, "Battery is full, please unplug the charger." This feature will first appear in models 1200, 1208 and 1650 and will most likely debut in Europe."