Dual usb car charger the 31a dual usb universal car charger 12 24v has a 21a and a 1a usb port to let you keep two devices charged while youre driving gone are the days when you have to decide whether to charge your phone or your tablet or indeed any other device that can be charged by a usb port such as your sat nav ipod or mp3 player the charger is neat and compact only 65mm long with a top that measures just 32mm x 32mm this also makes it easy to swap between cars and store when