West Herr Auto Group answers some of questions regarding why USB charging seems slow in a car compared to when plugged into a wall outlet in a posting on its website.
Sometimes, it's a matter of the age of the technology. Many cars have USB 2.0 ports, which is technology from a few years back, the release said. Newer ports are USB 3.0, which offers a little more power, but still not as much power as a wall outlet.
Other times, the cord used to charge the phone is the culprit for slow charging. Inexpensive USB cords found in gas stations or discount stores often are not of the best quality. West Herr recommends purchasing cords directly from your mobile device's manufacturer for best results.
West Herr also recommends using USB car chargers that plug into the port that looks like an old-fashioned cigarette lighter. These ports are designed for charging only and do a better job than the extra USB ports located throughout some vehicles. For the fastest charging, the USB port should output at 2.1 amps. Check the package or information page about the charger for information about the output level before purchase.