This is the cheapest phone charger for your car that i could find. It comes with an lcd display that displays the battery voltage of your car when you turn ON the ignition. It also displays the voltage and the current at which your phone is charging. When the battery voltage of your car reaches below 12V, the lcd starts flashing and lets you know that the battery voltage is low. The charger can output a maximum of 3.1A. This charger is a dual usb port charger with lights and lcd display.
