The USB ports in cars follow older USB standards, USB 1 and maybe USB 2- Since these standards are set, it charges your phone at a very low amperage. Since these standards were set a very very long time ago, phones, tablets and other devices which use USB to charge have dramatically changed. It is not uncommon for you to be using your phone for GPS navigation, have it plugged in and then have it die while it is being plugged in a charging.
Most newer devices require at least 1.5 amps to charge correctly or even more. There is no way to improve the existing USB port in your car to make it charge with more power, but you can use the cigarette lighter plug and get a high amperage charger.
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