Yes, it is. Car chargers do not provide a consistent flow of electricity from the car to your handset. The result is much worse than a slow charge - this irregular charging pattern can cause your phone's battery to swell. This will not only result in needing to replace the battery, but could cause damage to the rest of the device as well. In fact, USB chargers connected to computers should be avoided for the same reason. Always try to charge your phone from a wall plug - alternatives should be used for emergencies only.