Car voltage is higher to charge the battery. Many car accessories have a decently good tolerance to voltage since it is a bit necessary. 13.5v to 16v is intentional. The way I understand it, the alternator will pulse, for example, twice the voltage at 50% duty cycle to equal the correct voltage On PSUs, voltage is commonly set intentionally high so that the voltage droop under load stays within spec. Take two identical power supplies, one set for 12v and one set for 12.2v, the former will drop out of "normal" 12v range as defined by the ATX specification slightly before the latter. Voltage regulators keep the voltage constant. Like W1zzard said, logic on the battery/phone will kill the power before causing physical damage.