vivo mobile phone charging on the truck radio is particularly slow how to do? I got a $5 car with the same slow charger, and I burned out my Samsung before?

The current of the car radio is relatively low, so it is slow to charge. The five yuan car charger should be used as little as possible, it is not safe, and do not want to be cheap. Look at the output of the car charger, 2.1A can be