I have been shopping around for a car charger for the TF300 and noticed they come in two flavors. One states it has 12V out and the other has 15V out. Will the 15V out just charge faster? Could the 15V out damage the tablet if used for a long time? Is the 12V a safer charge to leave the charge for several hours? Any thoughts or experiences with these two charging output voltages would be appreciated.