Most of the street chargers have a maximum charge output of 500mA – so what does this mean to you? The smaller the output the longer it will take to charge. What they don’t tell you is that you need a minimum of 2100mA to charge you iPad using a Car charger or a wall charger.
The USB 4-Port Car Charger has an output of 6800mA – This clever design provides you with two 2100mA on the top and two 1000mA on the side, now you can charge your iPad and mobile phone at the same time without compromising on the power output.
Refer to above PDF Data sheet