That doesn't really make any sense unless the charger you used is faulty. Any USB charger should output ca. 5-5.5v. You could also invest in an Inverter and use your regular 110v (if in N America) or 240v (if in Europe) charger.
The tablets usually require a higher current, my chargers are rated at 2 Amps at 5 VDC. It's possible the car adapter wasn't putting out enough current.